Manual on Radiological Safety in Uranium and Thorium Mines ... Safety Standards/Safety_Series_0… · FOREWORD Uranium mining and milling industries are growing fairly rapidly in

M an u a l on Radiological S a fe ty

in U ran ium and Thorium M in es and M ills

S p o n s o r e d b y t h e

I A E A a n d I L O

This publication is no longer valid Please see http://www-ns.iaea.org/standards/

MANUAL ON RADIOLOGICAL SAFETY IN URANIUM AND THORIUM MINES AND MILLS


SAFETY SERIES No. 43

MANUAL ON RADIOLOGICAL SAFETY

IN URANIUM AND THORIUM MINES AND MILLS

SP O N SO R E D B Y T H E IN TE R N A T IO N A L ATOM IC E N E R G Y AG EN CY

AND TH EIN TE R N A T IO N A L L A B O U R O R G A N ISA TIO N

IN T E R N A T IO N A L A TO M IC E N E R G Y A G EN CY V IEN N A , 1976


M A N U A L ON R A D IO L O G IC A L S A F E T Y IN U RA N IU M AND TH O RIU M M IN ES AND M IL L S

IA EA , V IEN N A , 1976 STI/PUB/449

ISBN 9 2 - 0 - 1 2 3 1 7 6 - 8

Printed by the IAEA in Austria September 1 976


FOREWORDUranium mining and milling industries are growing fairly rapidly in m any countries

and this trend may continu e in the future with the increasing demand fo r nuclear fuel. A similar trend is also noted in thorium mining and milling industries, although these do n o t y e t represent a significant part o f the nuclear industry. T h e problem s o f radiological hazards in this com p onen t o f th e nuclear fuel cycle have received considerable attention because radioactiv ity is present in these m ines and mills - although recently it has been recognized th at such problem s can also be significant in non-radioactive m etal mining, because o f the pervasive distribution o f uranium and thorium in the earth ’s crust. The radiological hazards result prim arily from the exposure to airborne radioactiv ity and, to a lesser degree, from external radiation. T h e inhalation hazard is o f m ore concern in underground and deep-pit mining o f radioactive ores than in open-cast mining. I t is particularly significant in uranium mining operations, which are m ostly underground, while it is relatively less significant in thorium mining, which in general is a surface operation . T h e inhalation hazard in mills is o f concern only at the concen trate areas.

F o r radiological p ro tection in m ining and milling o f nuclear m aterials nothing short o f com p letely adequate m onitoring and control should be provided, particularly as there has been som e evidence o f occu pational illness in uranium mining. T h e introd uction o f e ffic ien t control measures depends primarily on the co rre ct knowledge o f th e radia tion situation , especially on the accurate determ ination o f the con cen tration o f airborne radioactiv ity. Particularly in underground uranium m ines, e ffic ien t control o f radon and its daughters in the mine atm osphere is a som ew hat d ifficu lt task.

T h e IA E A and IL O , attaching great im portance to these problem s, have prepared the present m anual with the help o f a panel o f experts, which m et in Vienna from 2 to 6 Ju ly 1 9 7 3 , for the guidance o f those responsible fo r the regulation, m anagem ent and operation o f uranium and thorium mines and mills. T he manual follow s and supplem ents IA E A Safety Series N o .26 , “ R adiation P rotection in the Mining and Milling o f R adioactive O res” , which was prepared at a jo in t IA EA /ILO m eeting o f exp erts in 1965 and issued by the IL O as a jo in t pu blication with the IA EA in 1 9 6 8 . T h e present pu blication , planned as com plem entary to the previous on e, is intended as an operating m anual with p articu lar em phasis on im plem entation m ethods. T h eoretical con cep ts and technical term s are held to a m inim um and practical m atters are em phasized. I t is anticipated th at, in addition to its prim ary audience, this manual will have som e application at m ines and mills unrelated to the nuclear industry since, as stated above, radiological hazards can ex ist there also.

T he scope o f this manual is lim ited to occupational hazards in m ining and milling, but in Section 3 liquid, solid and gaseous wastes produced by the various processes are


identified, although n o t treated in detail. F o r the m onitoring and contro l o f radioactive waste, the reader is referred to the IA E A Code o f P ractice on the M anagem ent o f W astes from the Mining and Milling o f Uranium and Thorium O res, published at the same tim e as the present book as Safety Series N o.44.

T he IA E A and IL O wish to express their thanks to all the panel participants who to o k part in the preparation o f this manual. Thanks are especially due to Mr. A .J . Breslin, chairm an o f the panel, who, as a consu ltan t, to o k great in terest in rewriting a large part o f the manual. The final com p ilation o f the m aterial was the responsibility o f Mr. J.-U. Ahmed o f the A gency’s Division o f N uclear Safety and Environm ental P rotection . G ratitu de is expressed to the Trades Union Congress o f the United Kingdom anil the M iners’ International Federation for their valuable com m ents.


CONTENTS

1. IN T R O D U C T IO N ............................................................................ ...................................................... 1

2. A D M IN IST R A T IV E R E S P O N S IB IL IT IE S .............................................................................. 2

2 .1 . O rgan ization .................................................................................................................................. 22 .2 . N otification , registration and licensing.............................................................................. 32 .3 . General duties o f em ployers and w orkers........................................................................ 3

3 . R A D IA TIO N H A Z A R D S.................................................................................................................. 4

3 .1 . G e n e ra l............................................................................................................................................. 43 .2 . Uranium m ines.............................................................................................. ................................ 5

3 .2 .1 . A irborne radioactivity .......... ................................................................................. 53 .2 .2 . External rad ia tio n ..................... : ................................................................................ 73 .2 .3 . Surface co n ta m in a tio n ............................................................................................ 73 .2 .4 . Radioactive w aste..................................................... ................................................. 7

3 .3 . Uranium m ills ................................................................................................................................ 83 .3 .1 . A irborne rad ioactiv ity .............................................................................................. 83 .3 .2 . External rad ia tio n ...................................................................................................... 83 .3 .3 . Surface co n ta m in a tio n ............................................................................................ 93 .3 .4 . Radioactive w a s te ...................................................................................................... 9

3 .4 . Thorium m ines....................... ...................................................................................................... 93 .4 .1 . A irborne rad ioactiv ity ................................................. ............................................ 93 .4 .2 . External rad ia tio n ...................................................................................................... 103 .4 .3 . Surface co n ta m in a tio n ............................................................................................ 103 .4 .4 . Radioactive w a s te .................................................... ................................................. 10

. 3 .5 . Thorium m ills................................................................................................................................. 103 .5 .1 . Airborne rad ioactiv ity .............................................................................................. 103 .5 .2 . External rad ia tio n ....................................................................................................... 113 .5 .3 . Surface contam in ation ....!....................................................................................... 113 .5 .4 . Radioactive w a s te ....................................................................................................... 11

4 . S T A N D A R D S .............................................................................. ............................................................ 12

4 .1 . Dose limit for occupational exp osu re.............................................................................. 124 .1 .1 . Occupational exp osu re .................... ........................................................................ 12(a) Rate o f dose accu m u lation .................................................................................... 13(b ) Planned special exp osu re............................................................... ........................ 13(c ) Radiation p rotection o f w ork ers........................................................................ 144 .1 .2 . Sum m ary.......................................................................................................................... 14

4 .2 . Surface co n ta m in a tio n .............................................................................................................. 14


4 .3 . A irborne hazards........................................................................................................................... 154 .3 .1 . Uranium and thorium d u s t ........................................................................... ....... 154 .3 .2 . Radon and radon daughters................................................................................... 164 .3 .3 . Thoron and thoron daughters............................................................................... 18

5. M ON ITO RIN G P RO G R A M M E....................................... .................................. ............................ 19

5 .1 . External rad ia tio n .................................................................................................................••••• 195 .2 . Inhalation hazards........................................................................................................................ 2 0

5 .2 .1 . General co nsid eration s............................................................................................ 2 05 .2 .2 . Radon and radon daughters................................................................................... 225 .2 .3 . Thoron and thoron daughters............................................................ ................. 225 .2 .4 . Ore d u s t......................................... j ................................................................................ 225 .2 .5 . C oncentrate d u st.......................................................................................... .............. 23

5 .3 . C alculation o f individual exp osures................. ......................................................... ........ 235 .4 . Surface co n tam in atio n .................................................... ..................... ; ................................ 23

6 . M O N ITO RIN G M ET H O D S............................................................................................................... 24

6 .1 . Radon and th o ro n ........................................................................................................................ 2 46 .1 .1 . Radon m easurem ents with the scintillation cham ber............................... 2 46 .1 .2 . Tw o-filter m ethod fo r radon ......... ..................................................................... 276 .1 .3 . T w o-filter method fo r th oron ................................................ .............................. 29

6 .2 . Radon and th oron daughters................................................................ : ............................... 316 .2 .1 . Kusnetz and m odified Kusnetz m ethods for measuring

radon daughter working lev e l...................... ................................................. 316 .2 .2 . Tsivoglou and m odified Tsivoglou m ethods for

measuring the working level and the concentrationso f RaA, R aB and R a C ......................................................... : ................ ;.................. 33

6 .2 .3 . R olle m e th o d ......................................!......................................................................... 356 .2 .4 . Additional m ethods fo r measuring radon d au gh ters................................ 39(a) Alpha sp ectroscop y ................................................................................................... 39(b ) A utom atic in stru m en ts............................................................................................ 39(c ) Measurement o f uncom bined fraction o f radon d au gh ters.................. 4 06 .2 .5 . Measurement o f thoron daughters........................................... .......................... 4 06 .2 .6 . Measurement o f m ixed radon and th oron d au gh ters............................... 41

6 .3 . M easurement o f long-lived alpha e m itte rs ..................... ................................................ 426 .4 . Measurement o f external rad iation ..................................................................................... 4 46 .5 . M easurement o f surface co n tam in atio n ......................... ................................................ 4 4

6 .5 .1 . Equipm ent and p ro ced u res.................................................................................... 4 46 .5 .2 . Assessment o f surface contam ination levels and p recau tion s............... 45

6 .6 . Personnel m on itorin g ..................................................................................... ........................... . 456 .6 .1 . Dose m eters fo r external rad ia tio n ..................................................................... 45


6 .6 .2 . Personal samplers for airborne radionuclides............................................... 46(a) Personnel dose m eters for radon and radon daughters........................... 47(b ) Persona] sam plers for dust (long-lived rad io n u clid es)............................. 476 .6 .3 . B ioassay ............................................................................................................................ 48

7. C O N T R O L M E A S U R E S ..................................................................................................................... 49

7 .1 . C ontrol measures in m ines....................................................................................................... 497 .1 .1 . M echanical v en tilation .............................................................................................. 507 .1 .2 . Suppression and confinem ent o f radiation source..................................... 507 .1 .3 . Personal protection and jo b ro ta tio n ................................................................ 517 .1 .4 . Air cleanin g..................................................................................................................... 51

7 .2 . C ontrol measures in m ills ........................................................................................................ 517 .2 .1 . C o n fin em en t.................................................................................................................. 527 .2 .2 . M echanical ventilation ............................................................................................... 527 .2 .3 . W orking practices and housekeeping................................................................ 527 .2 .4 . Personal hygiene and protective c loth ing ........................................................ 537 .2 .5 . Protection from external rad iation .................................................................... 53

7 .3 . Respiratory p ro tec tio n .............................................................................................................. 53

8. M ED ICA L C O N T R O L ......................................................................................................................... 54

8 .1 . Genera]................................................................................................................................................ 548 .2 . Medical exam in ations.................................................................................................................... 548 .3 . Pre-em ploym ent exam ination ................................................................................................ 55

8 .3 .1 . Pre-em ploym ent h is to ry ........................................................................................... 558 .3 .2 . Details o f the pre-em ploym ent exam ination ................................................. 558 .3 .3 . Conclusions o f the pre-em ploym ent exam ination ...................................... 56

8 .4 . Periodic medical exam in ation s............................................................................................. 578 .5 . Medical exam ination at term ination o f em ploym ent

and fu rther m edical fo llow -up............................................................................................... 578 .6 . Special procedures o f medical supervision...................................................................... 578 .7 . Post-illness precau tion ............................................................................................................... 588 .8 . Medical advice to mine m anagem ent.......................... : .................................................... 588 .9 . Personal health re c o r d s ............................................................................................................. 58

8 .9 .1 . Medical re co rd s .................................................. .-........................................................ 588 .9 .2 . Radiation exposure record s.................................................................................... 58

A PPEN D IX I. Physical properties o f relevant radionuclides.......................................... 63

A PPEN D IX II. Equations for radon daughter concentration(Tsivoglou m ethod) ............................................................................................. 68

A PPEN D IX III. Factors fo r use in the tw o-filter m ethod....................................................... 69


A P PE N D IX IV . Precautions to be observed in the co llection o f air sam ples............. 72

A P PE N D IX V. Medical contraindications for em ploym ent in uraniumand thorium mining and m illin g .................................................................... 73

R eferen ces......................................................................................................................................................... 75

Recom m ended b ib lio g rap h y .................................................................................................................. 77

G lossary .............................................................................................................................................................. 78

List o f particip an ts.................................................................... .................................................................. 81


1. INTRODUCTION

Uranium mining is unique in the nuclear industry in th at it is the only com p onent o f the nuclear production cy cle th at has associated with it a significant incidence o f occu pational illness. Uranium can be mined safely but it is clear from evidence that the radiological environm ent o f underground uranium m ines is quite hazardous unless appropriate contro ls are applied. Indeed, the hazard is n ot lim ited to uranium m ines, investigations having shown that the same radiological constituents th at have caused lung cancer am ong uranium m iners, i.e . radon and radon decay products, occu r in o th er types o f underground m ines and, in som e instances, also in su ffic ient con cen tration to cause occu pational illness.

The reasons fo r th e con trast betw een uranium m ining and the o th er phases o f nuclear processing are varied, involving national and local regulatory policies, the situation o f m any m ines in rem ote geographical areas and th e w ell-entrenched m ining traditions which influenced th e m anagem ent and working practices in the young uranium m ining industry. By nature, m ines are n ot easily susceptible to atm ospheric control. Moreover, atm ospheric m onitoring, a vital ad junct to control, is d ifficu lt in uranium m ines because o f adverse environm ental cond itions and the peculiar radiological properties o f the hazardous gases and dusts.

Present m onitoring and control technology is adequate fo r m aintaining safe mining environm ents b u t it lags behind o th er technologies applied in the nuclear industry because it has received relatively little support fo r research and developm ent. T here is a d efinite need fo r im provem ents in all aspects o f m onitoring and control.

Uranium m illing has been substantially free o f occu pational illness because, during its b rie f h istory , environm ental contro l has been generally satisfactory . Y e t, logically , m ining and m illing m ay be covered jo in tly because o f a considerable num ber o f com m on aspects. T he radiological hazards are essentially identical (although differing in relative m agnitudes), mines and mills are o ften situated tog ether geographically, and supervision and authority are o ften the same. A nother com m on characteristic is th at bo th are m ore nearly descendants o f earlier technologies than part o f the m odem ‘nuclear industry’ and its recognized innovations. However, mills are m ore am enable than mines to conventional con tro l m ethods, and maintaining safe occu pational environs has n ot been especially difficu lt.

Thorium mining and milling does n o t yet represent a significant part o f the nuclear industry. Thorium mining, to th e ex ten t th at it is perform ed, is relatively hazard-free because it is largely done by open-pit m ethods. B u t because th e radiological hazards and m onitoring and con tro l techniques are similar, thorium mining and milling is included in this docum ent together w ith uranium mining and milling.

This m anual describes the significant radiological hazards in uranium and thorium m ines and mills and the current techniques o f m onitoring and contro l. T he reliability and com pleteness o f the techniques, especially with regard to m onitoring, are quite variable because som e are new and relatively untried. In fac t, fo r som e types o f m easurem ent, certain m ethods are m erely suggested, never having been tested in actual

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working environm ents. This lack o f practical field exp erience is m ore pronounced in th e case o f techniques fo r thorium mines and mills w hich are so few in num ber. But in all cases, an e ffo rt has been made to describe the best available technology and to ind icate the degree o f confid ence w ith w hich the techniques m ay be used.

Providing a firm guidance fo r m onitoring and con tro l o f uranium and thorium ore processing at this tim e is com plicated by the uncertain status o f applicable M axim um Perm issible C on centrations (M PC). The International Com m ission on Radiological P ro tection (IC R P ) recom m ended MPCs fo r b oth radon and th oron in 1959 [ 1 ] but those values are being reviewed by an IC R P task group and are su b ject to revision. Moreover, whereas the 1 9 5 9 MPCs are expressed in units o f radon and thoron concen tration , th e new values m ay be in units o f the concen tration s o f the respective decay products (radon and th oron daughters). A d ifficu lty imposed by the uncertainty concerning units is th at th e techniques fo r measuring radon or th oron d iffer from the techniques fo r m easuring the decay products. T o m eet this problem , techniques for both kinds o f m easurem ent are described.

Thus, the m ining and m illing phases o f the nuclear industry are undergoing transition w ith respect to b o th technology and standards, w hich m ay hasten the obsolescence o f portions o f .this manual. B u t the preparation o f a reference w ork cannot be delayed ind efin itely while the industry continues to expand.

2. A D M IN IS T R A T IV E R E S P O N S IB IL IT IE S

2 .1 . Organization

T he op erator o f a uranium or thorium mine or mill should apply the same sound m anagem ent principles (e.g. forecasting, planning, directing and controlling) to the provision o f safe and healthy working cond itions that are applied to the production o f ore or concentrates. The m anagem ent’s policy o f integrating safety and operations should be clearly stated in w ritten form .

The m anager o f a m ine is responsible fo r the health and safety o f the workers. He should em ploy qualified persons who can adequately m onitor and assess radiation hazards and forecast, plan and im plem ent the necessary contro l measures th at will adequately p ro tect the w orkers; he should also allot adequate funds and m anpower to execu te these fu nctions.

An im portan t aspect o f m anagem ent responsibility is a thorough investigation o f working conditions. This manual describes principally the instrum entation and procedure fo r assessing th e nature and ex ten t o f p otentia l radiation hazards that m ay be encountered.

In opening a new m ine it is especially im portant to m ake a thorough study o f all p ertinent factors so th at adequate funds and facilities can be provided as soon as the m ine is put in to operation.

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The operating rules should include instructions regarding safe working practices that will p ro tect all personnel from hazardous exposure to radioactiv ity. A ll regular and new em ployees should be made fam iliar with the necessity and reasons for com plying w ith such practices. It is the responsibility o f the manager to m ake sure that the supervision is adequate fo r this purpose and th at su ffic ient inspection is provided.

2.2. Notification, registration and licensing

The com p etent authority should be n otified o f the onset and cessation o f the production , processing and handling o f uranium and thorium ores. I f required, these operations should also be licensed by th e com p etent authority in the light o f th e radiation hazards involved. N otification and licensing may n o t be required fo r handling solid natural radioactive m aterials containing less than 0 .0 5 w t% o f the elem ents uranium or thorium . The licensing authority should require the applicant to dem onstrate in advance th at he is capable o f conducting his proposed mining and milling activities in accordan ce w ith the national laws or regulations relating to occu pational and public safety and health.

An applicant fo r a licence to m ine or mill uranium or thorium ores should be able to prove th at he has qualified personnel, adequate facilities and equipm ent and adequate operating procedures. In particular, the applicant should provide detailed inform ation on the organization o f the m ine or m ill, the qualifications o f th e principal o ffic ia ls, the location o f the m ine or m ill in relation to nearby inhabited areas, the location o f nearby sources o f w ater supply, the proposed ventilation system s and the proposed radiation p rotection organization and facilities. The applicant should also provide detailed inform ation on the proposed m ethods o f dealing with liquid, solid and airborne wastes.

2.3. General duties of employers and workers

All plants, equipm ent and working processes should com ply w ith applicable official laws o r regulations pertaining to occu pational safety and health and radiation protection .I f the o ffic ia l laws or regulations are n ot applicable, the em ployers should in all cases prepare and im plem ent th eir own rules to ensure the appropriate level o f safety and health. The em ployers should provide the w orkers w ith personal protective equipm ent and protective cloth ing, as necessary fo r safety.

The em ployer or his designated representative is responsible fo r com plying w ith all rules and regulations to ensure the p ro tection o f the w orkers against hazards arising from radioactive substances.

Considerations o f prod uction or urgency o f w ork should in no circum stances be placed before the safety and health o f the workers.

T he em ployers should m aintain and periodically inspect m ines, workings, buildings, plants and equipm ent, and should organize the w ork so as to p ro tect th e w orkers from risks o f accid ents and illness. When m achines, appliances, vehicles or o th er equipm ent are acquired or put in to service, th e em ployers should ensure th at they conform to

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applicable safety and health regulations or, i f there are n o respective regulations, th at they are safe fo r routine use.

The em ployers should provide such supervision as w ill ensure that the workers perform their w ork under safe conditions. W ork that is done jo in tly by a num ber o f persons requires concerted action , adequately supervised to prevent hazards to life or health.

The em ployers should arrange fo r instru ction o f the w orkers on the potential hazards encountered in the mining and m illing o f uranium and thorium ores, and on the precautions to be taken to prevent occu pational accidents and diseases. In particular, newly engaged, uneducated and untrained workers should be properly instructed and adequately supervised.

The em ployers should prom inently post copies o f the special operating procedures th at should be follow ed to m aintain safe working conditions. These copies should be p ro tected in such a w ay that they are resistant to w eather and norm al wear. Whenever appropriate, the em ployers should also provide the w orkers with w ritten instructions and notices relating to safety and health at w ork. Foreign w orkers should, as far as practicable, be provided w ith instructions and notices w ritten in th eir own language.

W ithin the lim its o f th eir responsibilities, the w orkers should do everything in th eir pow er to m aintain th eir own and their w orkm ates’ safety and health. The w orkers should m ake proper use of, and properly m aintain, all safeguards measures, safety devices and o th er appliances provided fo r th eir p ro tection or the p rotection o f others.

No w orker, unless duly authorized, should tam per w ith safety devices or other appliances provided for personnel p ro tection , or in terfere with any procedures adopted to prevent occu pation al accidents or diseases. T he w orkers should m ake themselves acquainted w ith and observe scrupulously all safety and health instructions pertaining to their w ork. T hey should refrain from careless or reckless practices th at are likely to result in accidents or illness.

Before beginning w ork, the w orkers should exam ine th eir working places and the equipm ent th at they are to use and should prom ptly report to the responsible person any defect liable to cause danger. Use o f the working place or equipm ent should n ot be perm itted until the d efect has been remedied.

T he em ployers should establish a reliable checking system to ascertain th at all m em bers o f a sh ift are accou nted fo r at the close o f w ork.

3. R A D IA T IO N H A Z A R D S

3 .1 . G eneral

In uranium mines the radiological hazard is prim arily from exposure to airborne radionuclides and to a lesser degree from external radiation. The airborne radionuclides consist o f radon and short-lived radon daughters, w hich con stitu te the m ost serious hazard, and long-lived alpha em itters, w hich are present in th e m ine atm osphere in the

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form o f dusts. T he con cen tration s o f airborne radionuclides are very m uch greater in underground mines than in open-cast operations w here th e inhalation hazard is usually o f m inor concern. M ost uranium mining operations are underground, although open-cast mining is conducted to som e exten t.

In uranium mills the hazards due to radon and its daughters are far less pronounced than in underground m ines but airborne radioactive contam ination in th e form o f long- lived radionuclides is likely to result from the physical processing o f ore. Sim ilarly, airborne uranium dust usually occurs in concen trate areas and warrants contro l.

Thorium mining in general is a surface operation and th e radiation hazards are m ainly extern al in origin. In the thorium mills, however, dusts produced during the physical separation o f placers constitu te inhalation hazards.

3 .2 . Uranium mines

3.2 .1 . A irborne rad ioactiv ity

(a) Radon an d its daughters. The hazards due to airborne radiation in uranium mines result from the inhalation o f radon and its daughters and from long-lived alpha em itters present in the m ine atm osphere in the form o f dusts. T he prim ary airborne radionuclides are 222R n and its short-lived daughters 218Po (R aA ), 214Pb (R a B ), 214B i (R aC ) and 214Po (R aC '). As 222Rn is an inert gas, it passes freely in to and ou t o f th e lung with m inim al uptake by the respiratory system . On the o th er hand, radon daughters deposit p referentially in the respiratory tract, th e exact location depending on their particle size. C onsequently, the radiation dose to the respiratory system due to th e alpha decay o f inhaled radon daughters is 2 0 tim es greater th an th at due to the decay o f radon itse lf in the lungs. The m agnitude o f the dose depends on the concentration o f th e daughter products in the inhaled air and the particle size distribution o f the dust to which the daughter products are attached (and the fraction o f uncom bined daughter products) as well as physiological param eters.

S ignificant increases in lung cancer noted am ong uranium miners are believed to be the result o f excessive radiation exposure o f th e lung to radon daughter products.The reported concen tration s o f radon in uranium mines in the U SA , during the years w hen the exposures leading to lung cancer occurred, were from 3 7 pCi/1 to 59 0 0 0 pCi/1 o f air in d ifferent mines and at d ifferent locations w ithin the sam e m ine [2 ]. Characteristically , the concentrations o f airborne radon and radon daughters are highly variable am ong mines and w ithin a single mine.

Radon, the daughter product o f 226R a, decays by alpha em ission w ith a half-life o f 3 .8 2 days. It is liberated in to the m ine atm osphere m ainly by diffusion through the ore body. U nlike ore dust, w hich is produced only during m ining operations, radon em anation is continuous. A less frequent source o f radon is w ater coursing through the ore body, dissolving radon gas to a considerable ex ten t and releasing it to the mine atm osphere in th e process o f dripping from roofs or walls. C lean drives and barren regions o ften show unusually high concentrations o f radon due to this phenom enon.

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Radon daughter products have relatively short half-lives, a characteristic which results in a rapid increase o f th eir concentrations when radon is released to the air and, conversely, in a very rapid decay when the daughters are separated from the air as in air sampling. Thus, tim e becom es a m atter o f critical im portance both in th e application o f ventilation fo r atm ospheric contro l and in m onitoring. V entilating air must be used prom ptly b efore high concen tration s o f radionuclides can build up, and air samples o f radon daughters m ust be analysed soon a fter co llection before the radioactivity becom es undetectable.

One o f the critical radiological variables in m ine atm ospheres is th e ratio o f the respective concen tration s o f radon and its short-lived daughters. This ratio , usually expressed as R n : R aA : R aB : RaC, changes rapidly with th e age o f th e ventilating air as it courses through the m ine and varies trem endously with location and tim e. In relatively clean air the daughter fractions are low com pared to radon and approach unity in old air (in stagnant locations). T he determ ination o f this ratio provides useful inform ation abou t the sources o f radon and the e ffects o f ventilation and o th er m odes o f control.

Radon daughters are heavy m etals and som e aspects o f their in teraction with atm ospheric dust particles, ions and condensation nuclei are n ot yet fu lly understood. These atom s originally ex ist singly in the air but in a short tim e they attach themselves to atm ospheric aerosols and mine walls. T he removal o f the radon daughters from the air space by deposition on the mine walls is term ed ‘plate-out’. T hose radon daughters w hich becom e attached to atm ospheric aerosols follow the behaviour o f these particles or nuclei. As m ost o f the particles in th e m ine atm osphere are sub-m icron in size, essentially all o f the radon daughters are respirable.

The single atom s, know n as uncom bined radon daughters, are considered to deposit preferentially in the upper passages o f the respiratory tract where m ost m iners’ lung cancers develop. This poin t was recognized by the International Com m ission on Radiological P ro tection (IC R P ) [ 1 ], which introduced a fac to r fo r th e uncom bined fraction in the form ula fo r the m axim um perm issible con cen tration o f radon in air (S ectio n 4 .3 .2 ) .

U ncom bined fractions o f radon daughters have been measured in Fran ce [3 ] and the U nited States o f Am erica. George and H inchliffe [4 ] observed th at the uncom bined fraction o f radon daughters ranged from 0 .0 0 2 to 0 .1 2 , m ore than h a lf o f th e values being less than 0 .0 3 . Higher fractions were found in fresh ventilating air. An inverse relationship was noted betw een uncom bined fraction and particle concentration w hich, in the m ines studied, ranged betw een 3 X 10 3 cm -3, near ventilation inlets, and m ore than 1 0 7 cm -3, in m ain haulage ways and w orking stopes.

Som e o f th e physical properties o f radon and its daughter products are given in Appendix I.

(b ) L ang-lived radionuclides. Uranium ores contain all elem ents o f the 238U and 235U fam ilies. From the view point o f internal contam ination the long-lived alpha- em itting nuclides 238U, 234U, 230T h , 226R a and 210Po are significant. Mining operations

6


consisting o f drilling, blasting, slushing, etc. produce airborne dusts containing these long-lived nuclides w hich, in m ost ores, are close to radioactive equilibrium .

The long-lived radionuclides are generally m aintained considerably below the applicable m axim um perm issible concen tration s by the ventilation that is required fo r the control o f radon and radon daughters. Typical gross alpha concen tration s are in the range o f from 2 to 10 dis/m in-m 3 [5].

3.2.2. External radiation

Uranium miners are exposed to external beta and gamma radiation em itted from the ore bodies. Radon daughters plate out on mine surfaces and m iners’ clothings, thereby creating additional but usually m inor sources o f extern al radiation. T he gamma radiation rate will n o t vary over w ide ranges except in mines having occlusions o f fairly rich ore. The extern al radiation levels in m ost mine areas are less than 1 mR/h, but in th e p rox im ity o f excep tionally rich ore bodies the radiation levels m ay be as high as 2 0 mR/h and occasionally 1 5 0 mR/h or even higher. G enerally , external radiation does n o t pose a significant problem in uranium mines.

3.2.3. Surface contamination

Because o f the nature o f operations in uranium mines, dust deposits on surfaces, depending on the operational m ethods adopted and the w etness o f the m ine, b u t norm ally it is n o t hazardous excep t possibly as a source o f air contam ination . However, surface contam in ation can assume significance w here high-grade ore is extracted . Equipm ent from the m ine should be checked fo r contam ination before it is released outside fo r m aintenance.

3.2.4. Radioactive waste

(a) Liquid waste. M ine w ater, w hich contains uranium , radium and dissolved radon, poses only an ind irect hazard in the mine by contributing to airborne radon concentration. O nce outside the m ine it can be a source o f environm ental contam ination .The w ater should be properly channelled and, if necessary, co llected fo r treatm en t prior to disposal. This w ater can be conveniently disposed along with the tailings from the mill.

(b) Solid waste. Ore is the feed for the mill, w hereas any substandard batches ( o f a grade which is n ot econ om ical fo r uranium recovery) are norm ally used as backfill m aterial in the mine. Hence, solid w astes are not a problem in uranium mining.

(c) Gaseous waste. T he air in uranium mines contains uranium, radon, traces o f o th er uranium daughter products, dust and fumes from blasting and diesel engines. In underground m ines, m echanical ventilation dilutes these contam inants su ffic ien tly to

7


avoid occu pational hazards. Thus, spent air usually can be discharged safely above ground where it is fu rth er diluted and rapidly dispersed. Open-pit operations rely exclusively on natural ventilation fo r dilution and removal o f gaseous wastes. However, under inversion cond itions the level o f airborne contam ination m ay reach a significant level.

3 .3 . U ranium mills

3.3.1. Airborne radioactivity

In uranium mills, radon and its daughters usually present only a m inor inhalation hazard com pared to ore and uranium dusts, although significant radon concen tration s may occu r near ore storage bins and crushing and grinding circuits.

Typical dusty operations are ore crushing and grinding and final product preparation. During the initial stages, viz. crushing and screening, the long-lived airborne radionuclides U, 230Th, 226Ra, 210Po, tend to be in equilibrium , but during subsequent operations this equilibrium is necessarily disturbed and the concen tration s o f individual radionucldes must be measured fo r the assessment o f hazards. A fter leaching o f the crushed ore, m ost o f the radionuclides excep t uranium rem ain with the w aste cake, w hich a fter filtration

is sent fo r tailing treatm ent. Thus airborne uranium is predom inant in the filtration , precip itation and recovery section since the solutions and solids handled here are rich in uranium. In tailing treatm ent areas the airborne radionuclides are predom inantly ionium ( 230Th), radium, and polonium .


The exposure o f workers in uranium mills to external beta and gamma radiations is generally com parable to th e exposure o f workers in uranium mines but it m ay be significantly higher in certain locations. Iranzo [6 ] has furnished an exam ple o f the extern al radiation hazard in a uranium m ill w ith com prehensive details o f radiation levels and personnel exposures. In his exam ple, m ost mill workers are reported to receive less than 20% o f the annual dose lim it o f 5 rem/a. The radiation levels in plants range from 0 .0 2 —0 .9 mR/h (co n ta ct with incom ing ore) to about 1.2 mR/h (co n ta ct w ith final concen trate).

These levels vary from m ill to mill depending on th e grade o f ore, type and grade o f con cen trate , and type o f process, but generally, external radiation hazards assume significance m ainly in the final stages o f precipitation , filtration , con cen trate packing and storage. U nder unusual conditions, interm ediate o r final products m ay accum ulate in pipes and/or tanks and give rise to local radiation fields. In addition to this, the feed ores o f su fficiently high grade can create significant radiation intensities.

Freshly separated uranium is prim arily an alpha em itter, b u t as daughter products build up, bo th beta and gamma activities also build up. In abou t 2 4 days 50% o f the equilibrium beta-gam m a activity is reached. Thus, in th e product storage area the radiation levels w ill increase w ith th e tim e o f storage o f the product.


In som e cases, concen trate o f pitchblende has been reported to give rise to radiation fields up to 4 0 mR/h. Readings betw een 2 0 and 3 0 mR/h are n o t uncom m on in such cases. Readings o f up to 10 0 mR/h have been reported fo r concentrates o f uranothorianide m ixed in th e Malagasy R epu blic, b u t a considerable fraction o f this could be due to the thorium co n ten t o f these ores.


Surface contam ination , i f n o t controlled by proper containm ent and regular housekeeping, can contribu te to airborne activity through resuspension. This is a significant problem m ainly where concentrates are handled such as in th e precip itation , filtration , weighing and packing areas. Tailings treatm ent areas are also susceptible to significant surface contam ination .


Tailings from uranium m ills contain m ost o f th e radioactivity originally present in th e ore. It has been reported [7 ] th at significantly greater quantities o f 226R a are dissolved in. the alkaline leaching process than in acid leaching. Bu t subsequent steps in acid leaching mills result in higher concentrations o f dissolved 226R a in w aste stream s. These liquid w astes contain concentrations o f soluble 226R a and 210Pb at levels w hich can present a p otentia l hazard o f excessive internal contam ination . Dissolved uranium and thorium are n o t usually present in any significant concentrations. T he solid w aste w hich is accum ulated in tailings piles em its m ore radon to the atm osphere than to the surrounding soil because o f its radium conten t, and winds m ay resuspend radioactive dust from the surface o f dry piles.

3 .4 . Thoriu m mines


Thorium is found m ostly as m onazite in placer deposits in India, Ceylon, Australia, Canada and som e oth er parts o f the world. As the m ajority o f thoriu m m ining is by open-pit m ethods, the radiological problem s, particularly inhalation hazards, are relatively small com pared to underground uranium mining. In Canada thorium occurs as a by-product o f uranium. Inhalation hazards arise m ainly from dusts produced during the physical separation o f the m ineral constituents o f placers. The m ethods used in dry operations are m agnetic and e lectrosta tic separation and separation by wind tables, w hich are very dusty. U nlike uranium, thorium m ay be present in high concentrations in m ine dust, as the w orking ore grade is generally high. Thus the assessment o f hazards should include an assessment o f thorium and its long-lived daughter products in the w orking atm osphere, in addition to thoron. T he dose delivered to th e lung from breathing an atm osphere con-

9


taining th oron and its daughters will arise principally from : (i) the decay o f thoron and ThA in the airways o f the lung, and (ii) the deposition and subsequent decay o f inhaled daughter products.


External radiation hazards arise from bo th beta and gamma radiation em itted by M sT hll, 228Ac (1 .0 M eV gamma radiation and 1 .2 , 1 .7 , 1 .9 and 2 .2 M eV beta radiation), ThC, 212Bi (2 .2 5 MeV beta radiation) and T h C ", 208T1 (1 .8 M eV beta and 2 .6 MeV gamma radiation). External radiation levels m ay be less than 1 mR/h excep t in the handling o f m onazite bags where the dose rate m ay exceed 2 mR/h. However, the radiation levels increase with increasing ore grade. F o r exam ple, in th e Malagasy Republic, where ore contains up to 40% Th and up to 30% U, the radiation levels from co n tact with ore lump m ay be as high as 100 mR/h.


As in uranium mines, ore dust deposits on surfaces in am ounts depending on the operational m ethods used and on the wetness o f the m ine; norm ally , it is n ot hazardous excep t possibly as a source o f air contam ination . However, clothing contam ination may be a m ore significant source o f exposure than in uranium mines because o f the m ore pronounced beta and gamma em itters in thorium ore. Possibly, this has n ot yet received critical evaluation. Equipm ent from the m ine should be checked fo r surface contam ination before it is released outside fo r m aintenance.


Presum ably, the prod uction and hazards o f wastes are analogous to those at uranium m ines, but detailed inform ation has n o t been obtained.

3 .5 . Thorium mills


Thoron and ore dust may lead to exposure in ore storage areas. Thorium dust con cen tration s above perm issible values m ay occu r during certain physical separation processes. Ore crushing processes and drying o f the concen trate cake also give rise to dust and should be assessed fo r th oron and its daughters and fo r thorium .

Thorium ores usually contain very small am ounts o f uranium, and although the typical ratio o f thorium to uranium is 2 0 :1 , 222Rn and radon daughters m ay occu r in significant air con cen tration s along with 220Rn and thorium in the initial chem ical treatm en t areas o f the plant.

10



Extern al radiation associated w ith the physical treatm en t o f m onazite depends on the grade and quantity o f the ore. In a recent survey o f such a facto ry , Sunta et al. [8 ] have recorded radiation levels o f from 0 .3 mR/h to 6 mR/h. In th e m onazite stores and filling area, however, the radiation levels w ere observed to be high ( 1 0 —2 0 mR/h). But even so, personnel exposures in the plant were w ithin the annual dose lim it o f 5 rem and about 80% o f the persons received exposures o f less than 3/10 o f th at value.

The chem ical treatm en t o f thorium m inerals involves tw o fractions: th e thorium fraction (consisting o f 232T h and 228T h from the thorium series, and 234T h , 230T h , 231Th and 227Th from the uranium series) and the non-thorium fraction (consisting o f 228R a,224Ra and o th er daughters from the thorium series, and 226Ra w ith daughters from the uranium series). Owing to the short half-life o f 228Th and 228R a, the daughter activities will build up in b oth fractions in a relatively short period. T h erefore, the external radiation in processing and handling thorium com pounds varies w ith th e degree o f chem ical separation, th e con cen tration o f constituents o f thorium and non-thorium fractions, and the tim e interval betw een each step in th e processing.

R adiation levels in a m onazite processing plant were reported to vary betw een 0.1 mR/h and 8 mR/h, bu t rad iation levels o f from 18 to 35 mR/h were found at the stores (m on azite and products). All personnel exposures in this p lant were less than 3 R/a, and abou t 70% w ere less than 1 R/a.


In the physical processing o f m onazite sand, the dust generated during the operations settles down on surfaces. In the chem ical processing o f m onazite, however, bo th surface and personnel contam in ation assume significance. T he p otentia l fo r surface contam ination is greater in areas w here concen trates and waste cakes are handled. In addition to contributing to airborne activity, surface contam in ation also becom es a source o f external radiation due to daughter prod uct activities.


Thorium ore, m onazite, is essentially an orthoph osph ate o f rare earths, thorium and uranium. As such, there is no significant problem o f liquid w aste in mining or in m ineral processing plants using physical m ethods.

However, the liquid effluen ts from the chem ical processing o f m onazite contain the decay products from th e uranium and thorium series. T he gross alpha and gross beta activities in these effluents are at con cen tration s o f abou t 10~5 MCi/ml. A bout 20% o f this is a B a S 0 4-carried fraction . Because o f suspended and to ta l solid load in the effluents, they are allow ed to pass through settling tanks, the clear overflow from w hich, a fter suitable dilution, can be released to nearby recipient w ater bodies at concen tration s less than the MPC fo r unidentified nuclides.

11


Milling o f thorium gives rise to solid w aste m ainly in the form o f concen trate cakes. O ther item s like filter c loth , etc. contribute insignificant volumes to the to tal solid waste. These contain daughter prod uct activities in addition to small am ounts o f uranium and thorium . Gross alpha and gross beta activities in these wastes are o f the order o f 10 “2 ptCi/g to about 1 mCi/g, thus w arranting careful procedures fo r safe disposal.

4. ST A N D A R D S

The lim its o f radiation exposures are given in the IA E A Basic S afety Standards for Radiation P rotection [9 ]. These lim its have been developed on the basis o f th e recom m endations o f the International Com m ission on Radiological P ro tection (IC R P ) [2 , 10]. The recom m ended dose lim it for occu pational exposure, derived from extensive scien tific and tech nical investigations and from years o f experience w ith the practical problem s o f radiation p rotection , represent an international consensus o f opinion regarding measures necessary fo r safety in the situations to which these standards apply.

4.1. Dose limit for occupational exposure

It is emphasized th at th e recom m ended values are m axim um values; the ICRP recom m ends th at all doses be kept as low as reasonably achievable and th at any unnecessary'^exposure be avoided.

It m ust be realized th a t som e tissues and organs o f the body are m ore sensitive to radiation than others. F o r this reason, n o general single value is laid dow n; the dose lim it fo r occu pational exposure depends on w hether the w hole body or only a part o f it is irradiated and, in the latter case, w hich organs are included in the field o f radiation.

Doses resulting from natural background and from m edical exposures are excluded from the values recom m ended by the IC R P 1.

4.1.1. Occupational exposure

Whole body, gonads, red bone marrow 5 rem in a y ear2Thyroid, bone, and skin o f the whole body 3 0 rem in a yearHands, forearm s, fe e t and ankles 75 rem in a yearO ther single organs3 15 rem in a year

1 Medical exposure refers to the exposure of patients in the course of medicalnot to the exposure of the personnel conducting or incidentally associated with such procedures.

In most cases of X-, beta or gamma irradiation, the dose equivalent in rem is approximately equal to the tissue dose in rad. For gamma radiation, the exposure in roentgen and the air dose in rad are usually somewhat higher than the whole-body or organ dose.

3 The bronchial mucosa of the lung is taken to be a single organ and the absorbed dose should therefore not exceed 15 rem in a year.

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(a) Rate of dose accumulation

(1 ) A ccording to the IC R P [1 0 ] recom m endations, in a period o f a quarter o f a year, up to one-half o f the annual dose lim it fo r occu pational exposure m ay be received. These values, rounded o f f upwards to th e n ext w hole num ber, are:

Whole body, gonads, red bon e marrow 3 rem in a quarterSkin o f the whole body, bon e and thyroid 15 rem in a quarterHands, forearm s, feet and ankles 4 0 rem in a quarterAny o th er single organ 8 rem in a quarter

(2 ) I f necessary, the quarterly quota m ay be received as a single dose, bu t this is undesirable. In certain circum stances it will be ju stifiab le to perm it the quarterly quota to be repeated in each quarter o f the year, provided th at the to ta l dose accum ulatedat any age over 18 years does n ot exceed 5 (N -18) rem , where N is the age in years.

(3 ) Previous exposure unknow n: It shall be assumed th at the w orker has received th e currently recom m ended dose lim it fo r occu pational exposure in each year o f any

given period .4(4 ) Persons starting work at an age o f less than 18 years: T he dose shall n o t exceed

5 rem in any one year under the age o f 18, and the dose accum ulated up to the ageo f 3 0 shall n o t exceed 6 0 re m .5 No w orker under the age o f 16 shall be engaged in w ork involving ionizing radiation or in underground m ining operations.

(5 ) Exposure o f wom en o f reproductive capacity : Women o f reproductive capacity should only be occu pationally em ployed under cond itions where th e exposure o f the abdom en is lim ited to 1.3 rem in a quarter, corresponding to 5 rem in a year, delivered at an even rate.

(6 ) Exposure o f pregnant wom en: T he IC R P [1 0 ] recom m ends th at when a pregnancy has been diagnosed, the dose accum ulated during the remaining period o f the pregnancy should n ot exceed 1 rem.

(b ) P lanned special exposure

Som etim es it m ay be necessary to perm it a w orker or a few workers to receive doses in excess o f the recom m ended quarterly limits. However, n o single contribu tion should exceed tw ice the annual dose lim it, and in a lifetim e th e to ta l dose received from such exposures should n o t exceed five tim es the annual perm issible dose. A t the same tim e, planned special exposures should not be perm itted in the follow ing cases:

(1 ) I f the addition o f the intended dose to the w orker’s accum ulated dose exceeds the am ount determ ined by th e form ula D = 5 (N -18) rem , w here N is the age in years;

4 Furthermore: If the dose received is not known due to the loss of a film badge or because of any other reason, it shall be assumed that the individual received the full quota permitted for the period. The dose should be recorded as such in the Health Register.

s Radiation work at an age of less than 18 years is not recommended.

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(2 ) I f th e w orker has received, in the previous 12 m onths, a single exposure or intake o f radioactive m aterial with a dose com m itm ent in excess o f the quarterly qu ota ;

(3 ) I f the w orker has previously received abnorm al exposures in excess o f five tim es the annual dose limit.

Doses received from these exposures should be recorded together w ith those received from norm al exposures.

(c ) Radiation p ro te c tio n o f workers

(1 ) The IC R P [ 10] recom m ends th at workers who m ight receive single doses in excess o f 3/10 o f the annual dose lim it fo r occu pational exposure should be subjected to special health supervision.

(2 ) F o r w orkers w orking under cond itions where the possibility o f their being exposed to doses exceeding 3/10 o f the annual dose lim it fo r occu pational exposure is only rem ote, individual m onitoring and special health supervision are n o t required.

4.1 .2 . Sum m ary

T he dose lim its fo r occu pational exposure are as follow s:

Organ or tissueYearlymaximum(rem)

Quarterlymaximum(rem)

Weeklyaverage(mrem)

Hourly average for 40-h week (mrem)

Whole body, gonads, red bone marrow 5 3 100 2.5

Skin of the whole body, bone and thyroid 30 15 600 15

Hands, forearms, feet and ankles 75 38 1500 38

Other single organs 15 8 300 8

4 .2 . Su rface contam in ation

Areas in w hich uranium (o r thoriu m ) chem ical concen trates are handled in the dry state should be kept separate from o th er parts o f the mill and should be su b ject to restriction o f entry.

There are n o internationally accepted values for the lim its o f skin and surface con tam ination by alpha em itters. T he lim its used in d ifferent countries are found to vary by an order o f magnitude. It m ay be noted th at such perm issible lim its also depend on

14


the rad io toxicity . However, fo r the purposes o f this m anual, the perm issible levels for skin and surface contam in ation by uranium or thorium are as follow s: skin = 1 (T 5 f id /c m 2, surface = 1CT3 juCi/cm2.

The lim it o f surface contam in ation on walls, floors, benches, cloth ing, etc. is 1 0 "3 fiCi/cm2, which is equivalent to 3 .5 mg/cm2 (U -nat) or 9.1 mg/cm2 (T h -n at). Such an am ount o f surface contam in ation is readily visible as dirt and does n ot require a radiation survey m eter fo r detection .

4 .3 . A irborne hazards

Exposure lim its are given for uranium and thorium dust, fo r radon and radon daughters and fo r th oron and th oron daughters.

4.3.1. Uranium an d thorium du st

T he m axim um perm issible concen tration s in the air o f w orking areas recom m ended by IC R P fo r insoluble and soluble natural uranium, respectively, are 6 X 1 0 “n juCi/cm3 and 7 X 1 0 " u /uCi/cm3. 6 Because o f the very small difference in the tw o MPC values the value fo r insoluble dust is o ften applied uniform ly regardless o f the chem ical form o f the uranium.

The MPC o f soluble dust is set by IC R P on the basis o f the chem ical to x ic ity o f uranium, the m ore lim iting criteria th an radiation dose, and is equivalent to th e MPC o f 0 .2 mg U-nat/m3 recom m ended by th e A m erican C onference o f G overnm ental H ygienists [1 1 ] and the U K N ational Radiological P ro tection Board [1 2 ] , When a soluble uranium com pound is inhaled, it transfers quickly from the lung to the bloodstream and a large fraction is rapidly excreted in the urine — m ore than h a lf w ithin 2 4 hours. A lm ost no uranium can be found in the faeces b u t a significant fraction remains in the kidneys w ith resultant chem ical damage.

Internal contam in ation can be m onitored by urinalysis (see Section 6 .6 .3 ) . F u rth erm ore, the characteristic uranium in jury to the kidney w ith resultant album inuria is a very sensitive and reliable ind icator o f chem ical poisoning.

Thorium M PCair= 3 X 1 0 '11 Ci/cm3. 6 However, severe e ffects have occurred in som e cases w ith less than the ICRP m axim um perm issible body burden, as reported in a study o f thorium body burdens follow ing thorotrast m yelography [1 3 ],

The MPCair o f ore dust is d ifficu lt to establish because it m ust take in to accou nt the state o f equilibrium o f each elem ent o f the uranium or thorium chain. However, in a first approxim ation , the conservative value o f 3 X 10~n yiCi/cm3, w hich is the low est value fo r the MPC o f any elem ent o f the chains, can be used. (Applying M P C ^ =3 X 1 0 '“ ixCi/cm3 for thorium allows fo r th e fa c t th at there is a large am ount o f airborne m aterial serving as a carrier fo r the thoriu m .)

6 The curie value of natural uranium is considered to correspond to 3.7 X 1010 dis/s from 238U,3.7 X 1010 dis/s from 234U, and 1.7 X 108 dis/s from 235U. The curie value of natural thorium is considered to correspond to 3.7 X 1010 dis/s from 232Th and 3.7 X 1010 dis/s from 228Th.

15


4.3.2. Radon and radon daughters

The dose to th e trachea and large bronchi where m iners’ cancer tends to develop is a ttribu ted by som e investigators to the fraction o f radon daughters unattached to dust or condensation nuclei. C onsequently, the IC R P has established a form ula fo r the maximum perm issible concen tration in air o f radon and, indirectly , its daughters, which takes the degree o f a ttachm en t into consideration [2 ], This approach is, however, at present under review. T he M PCajj- fo r a 4 0-h ou r w eek is recom m ended by ICRP as 3 X 10 -6/( 1 + 1 0 0 0 f) juCi/cm3, where f is the fraction o f the equilibrium am ount o f R aA ions which are unattached to nuclei. A value o f f = 0.1 was adopted by IC R P fo r an average atm osphere, giving M PCair= 3 X 1 0 '8 juCi/cm3. However, th e uncom bined fractions o f radon daughters measured in six U S m ines gave f-values betw een 0 .0 0 2 and 0 .1 2 , w ith m ore than h alf less than 0 .0 3 ; the la tter would lead to a higher M P C ^ [4],

Under practical cond itions in mines, the radon daughters are seldom in equilibrium , either w ith the parent radon or with one another. S ince the energy values o f the radiation from the radon daughters d iffer from each other, and since tissue damage can be related to th e to ta l absorbed energy, it follow s th at the possible danger o f these radionuclides is m ore satisfactorily described by th eir potential a-energy than by the concen tration in m icrocuries. C onsequently, a new con cep t was introduced [1 4 ] o f assessing the radon daughter concen tration s in m ines in relation to a so-called W orking Level (W L) o f1.3 X 1 0 s MeV/1, this being the sum o f the a-energies released by the decay o f RaA ,R aB , RaC and RaC' w hich are in equilibrium w ith 1 0 0 pCi radon (T able I). One WL therefore represents any com bination o f th e short-lived radon daughters in one litre o f air that will result in the ultim ate em ission o f 1 .3 X 10 s MeV o f alpha energy, taking n o accou n t o f the radon.

T A B L E I. C O M PO SITIO N O F W O RKIN G L E V E L A T RAD ON D A U G H TER EQ U IL IB R IU M

NuclideUltimate a-energy per atom (MeV)

Total a-energy (MeV/100 pCi)

Fraction of total a-energy(MeV)

Radon Excluded None None

RaA 13.68 0.134 X 10s 0.10

RaB 7.68 0.659 X 10s 0.52

RaC 7.68 0.485 X 10s 0.38

RaC' 7.68 0.000 X 10s 0.00

1.278 X 10s 1.00= 1.3 X 10s

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The establishm ent o f a m axim um permissible value may now be based on a pulm onary dosim etric m odel or on epidem iological data. In the first approach the radiation dose to the lung is calcu lated and related to the IC R P perm issible doses.F o r instance, Harley and P asternack [1 5 ] have found fo r an equilibrium atm osphere o f 1 0 0 pCi/1 each o f RaA , R aB and RaC a lung dose rate o f o nly 2 .8 rad/WL-year, and4 .3 rad/WL-year fo r a non-equilibrium atm osphere, which is m ore representative.Although several such calcu lations have been made (e.g. Hamard et al. [1 6 ] ) , uncertainties regarding critical dose, critical tissues, im portant entities am ong the radon daughters — to nam e b u t a few o f the m any quantities inexactly know n at this tim e and required to precisely define a satisfactory dosim etric m odel - d irect tow ards an exam ination o f the available hum an data. C onsequently, the IC R P has appointed a Task Group on Radon, Thoron and th eir Daughters which is engaged in th e preparation o f a report on radon standards and m axim um perm issible levels.

In the m eantim e, studies o f increased lung-cancer m ortality have been carried out, as reported fo r fluorspar m iners in Canada, iron ore m iners in the U nited Kingdom , tungsten, fluorspar and lithium m iners in Czechoslovakia and underground w orkers in tw o Swedish mines [1 7 ] . As in the case o f the original so-called ‘ Bergsucht’ am ong underground miners in the Krusne H ory (Erzgebirge) [1 8 ] , the miners had been occu pationally exposed to high levels o f radon. In contrast, no increased m ortality was detected in South A frican gold/uranium miners exposed to low er levels o f radon [1 9 ],

In n one o f the instances m entioned was it possible to study how the increased m ortality is related to exposure. This, however, was done on a fu rth er group o f underground miners, working in the uranium ore mines o f the C olorado Plateau in th e USA. Much inform ation has been published on a group o f 3 3 6 6 w hite and 7 8 0 non-w hite uranium miners, but a m ajor u n certainty affecting th e conclusions lies in the assessment o f th e individual exposures. An additional difficu lty in interpreting th e results arises from th e fact th at m ost o f the m iners included in the study were cigarette sm okers, which apparently results in a synergistic e ffect. It has lately been postulated th at the presence o f thin areas o f m etaplasia in the bronchial epithelium could result in the occu rrence o f bronchogenic cancer in main stem bron chi [2 0 ] . Fu rtherm ore, it has been noted th at the observations made on the uranium m iners are d ifficu lt to recon cile w ith the results o f the A tom ic Bom b Casualty Com m ission Study.

While the new IC R P report is anxiously awaited, national authorities have accepted various m axim um perm issible values based upon the existing IC R P recom m endations or, m ore frequ ently , expressed in term s o f exposure rate in W L and/or integrated exposure in W orking Level M onths (W LM ); these values range over an order o f m agnitude. Although it has been concluded “ ... that the unit W L is neither appealing nor appropriate as a dose rate e s t im a to r ... fo r the b iologist o r epidem iologist who studies th e risk involved during exposures to radon daughter prod ucts” [2 1 ] , the U S Federal Radiation C ouncil has recom m ended, and the U S Environm ental P ro tection Agency has specified, a reduction in perm issible radon daughter levels in uranium mines from January 1 9 7 1 , such th at no em ployee shall be perm itted to receive an exposure o f m ore than 4 WLM in a year.

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C onsequently, there are tw o MPC values currently being applied in the contro l o f occu pational exposures to radon daughters, the IC R P value o f 3 X 1 0 “8 11C i/cm3 (3 0 pCi/1) and 4 WLM per year w hich, expressed as an average concen tration , is 0 .3 WL. F o r an atm osphere containing an equilibrium m ixture o f radon and its daughters, the tw o MPCs are identical because 1 W L is equivalent to 1 00 pCi/1 each o f R aA , R aB and RaC, as indicated previously. B u t equilibrium concen trations rarely occu r in practical circum stances, and 0 .3 WL is m ore likely to be associated w ith radon concen trations o f the order o f 10 0 pCi/1 than 3 0 pCi/1. T herefore, the MPC recom m ended by ICRP tends to be m ore restrictive by an approxim ate fac to r o f three.

4.3.3. Thoron an d thoron daughters

Thoron and its daughters have received m uch less atten tion than the radon species because there are few underground m ines w here airborne activity can accum ulate to hazardous concen tration s [2 2 ]. There are n o epidem iological studies to provide evidence fo r an exposure/risk relationship and, in consequence, standards o f exposure are based on analogies w ith radon and on dose calculations.

The IC R P [2 ] presents an equation fo r calculating M P C ^ fo r th oron w hich is sim ilar in con cep t and form to the equation for radon MPCajj- (S ectio n 4 .3 .2 ) . The equation fo r a 40-hour-w eek occu pational exposure to thoron is 6 X 10~6/(1 + 4 0 0 0 0 f) , in w hich the uncom bined atom s o f T hB con stitu te a fraction , f, o f the equilibrium n um ber o f such atom s. IC R P adopted a value o f 1 / 2000 fo r f as typical fo r ordinary u nfiltered air. Su bstitu ting th a t value fo r f in the equation results in MPCajr fo r thoron =3 X 1 0 “7 juCi/cm3. The equation im plies th at the dom inant dose o f consequence is due to ThB.

IC R P [2 ] also presents an M P C ^ value fo r T h B w hich is 2 X 1 0 “8 i i d / c m 3. Presum ably, the sam e considerations th at led to M P C ^ fo r thoron were applied in calculating M P C ^ fo r T h B , although this is n ot exp licitly m entioned in the ICRP report.

Since the hazard from thoron is predom inantly attributable to T h B , w hich occurs with th oron in all practical situations, it is perm issible to apply the M P C ^ value fo r T h B as the standard o f contro l for b oth radionuclides. This elim inates the n6ed o f th oron m onitoring fo r which a sim ple, reliable technology has n ot y e t been developed.

Because o f the very short half-lives o f th oron (5 5 .3 s) and T hA (0 .1 5 s) compared to T h B (1 0 .6 h), dilution ventilation is relatively ineffective fo r these radionuclides b u t it can reduce th e concen tration o f T h B by a large factor. Thus, in som e atm ospheres, th e con cen tration o f th oron m ay exceed th at o f T h B b y orders o f magnitude.This situation is restricted to places w here clean ventilating air is continuously available at th e source and therefore it could be m anifested in mills but n ot in mines. Duggan [2 3 ] has cautioned th at the dose from th oron itse lf m ay be com parable to th a t o f T h B in cases o f extrem e disequilibrium . C onsequently, in mills it would be prudent to verify th e exclusive use o f the value fo r ThB-MPCajr- Duggan describes a procedure fo r m onitoring th oron and th oron daughters separately.

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5. MONITORING PROGRAMME

The principal ob jectives o f m onitoring are to evaluate occu pational exposures w ith respect to accepted standards and to provide data needed fo r adequate control.F o r th e latter, m onitoring can serve the follow ing fu nctions:

(a ) D etection and evaluation o f th e principal sources o f exposure;(b ) Evaluation o f the effectiveness o f control equipm ent;(c ) D etection o f anom alies in operation;(d ) Prediction o f the e ffec t o f fu ture operations on contam in ation levels.

The degree o f hazard presented by different avenues o f exposure influences the relative emphasis on d ifferent types o f m onitoring. In uranium mines, the radon daughters con stitu te the dom inant hazard and require continual m onitoring. N ext in order o f im portance are external radiation and then ore dust. The order is probably the same in thorium m ines but inform ation on this is lacking.

In mills, the order o f hazards varies w ith the particular phase o f the process (see Section 3). In the ore storage, grinding and classifying areas, ore dust presents the dom inant source o f exposure, radon daughters occu r in low to m oderate concentrations, and extern al radiation is m inimal. In prod uct areas, con cen trate (uranium or thorium ) dust is the principal source o f exposure although external radiation intensities may be high enough to warrant surveillance; radon daughters are insignificant or absent.At interm ediate processing areas, b o th external radiation and airborne radionuclides are d etectab le but tend to be o f m inor im portance.

C onducting the m onitoring programme in a mine or a m ill is a full-tim e occupation fo r at least one man and o ften requires a s ta ff o f several men. Individuals assigned to this work should be observant and carefully trained because, fo r m axim um u tility , environm ental m easurem ents should be accom panied by notation s o f working conditions, w orking practices and effectiveness o f con tro l equipm ent, inform ation that m ay be needed fo r proper in terp retation o f the m easurem ents and fo r determ ining means o f reducing exposures.

A separate, clean w orkshop or laboratory is needed fo r the calibration and maintenance o f instrum ents and fo r the analysis o f samples by m eans o f counting, chem ical m ethods, or both . Preferably, the room for this purpose should be external to the m ine or m ill so as to reduce contam ination . Calibration facilities should be provided fo r determ ining the flow rate in air sampling instrum ents as well as for determ ining the response and efficien cy o f radiation survey instrum ents and counters.

5 .1 . E x tern al radiation

The general standards fo r extern al radiation are universally accepted and do not present any difficu lties regarding their application in m ines or mills.

In m ines, the exposure levels depend essentially on the grade o f ore. W here the ore assay is very low, it is su ffic ient to measure the gamma radiation levels with survey

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instrum ents in all working areas and to determ ine an approxim ate mean exposure from the values. The m easurem ents should be m ade at least on ce a year. However, the frequency should be increased i f the estim ated m ean exposure seems likely to exceed 1.5 R per year, w hich can occu r at ore assays in excess o f 1.5 to 2 parts per thousand.In th at case, individual exposures should be measured every quarter, preferably by means o f personnel dose m eters. Bu t an acceptable alternative is to calcu late individual exposures from ore surveys and individual working schedules by m eans o f the m ethod described in Section 5 .3 .

In certain m ines where the ore assay is very high and where careful contro l o f w ork assignments is necessary to m aintain exposures within the prescribed limits, exposures m ay exceed 5 R per year and the m iners should be provided with dose m eters th at are read at least m onthly.

It should be noted th at radiation dose m eters are vulnerable to high hum idity and dust and should be protected while being worn by miners. This is especially true for film s, which should be sealed in plastic envelopes. Certain therm olum inescent d etectors are su b ject to fading and o th er undesirable e ffects and m ay require covers i f they are n ot provided w ith som e p rotection in their inherent design.

In mills, it is convenient to m onitor radiation exposures with individual dose m eters th at are read at least quarterly. Persons w ho are likely to receive significant exposures are those working at places where large am ounts o f ore or concen trate are stored. In the case o f exposures approaching the recom m ended lim its, area m easurem ents with radiation survey instrum ents should be m ade to locate the dom inant sources o f exposure and to verify the effectiveness o f corrective action.

In b o th m ines and mills it may be a local policy to m onitor the exposure o f visitors and non-radiation workers who are occasionally present in controlled areas. Personnel dose m eters (see Section 6 .6 .1 ) are used fo r this purpose. .

5 .2 . Inh alation hazards

5.2.1. General considerations

In m ines, virtually all air m onitoring is by grab-sampling m ethods, i.e . samples are co llected m anually in short tim e-periods. A few autom atic area m onitoring instrum ents have been developed fo r mines b ut n one have been put in to general use. Sim ilarly, a num ber o f personnel dose m eters fo r radon daughters have been reported b u t n one have w ithstood the test o f rou tine operation in mines. Grab samples indicate radionuclide concen tration s only at the tim e and location o f co llection and, consequently, sampling must be repeated periodically wherever average concen tration s m ust be known.

G eneral air (G A ) sampling, one type o f grab sampling, is th e accepted m ode o f m easurem ent fo r radon or radon daughter exposures and fo r determ ining general levels

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o f radon or radon daughters in mines. A GA-sample is collected at a fixed position in a working place (stope, room , drift, station, e tc .) so as to represent the air con cen tration in the area. With regard to exposure evaluation, it has been found th a t a sample collected within a few m etres o f a m iner usually represents his exposure ju st as well as a sample co llected in his im m ediate proxim ity although there are special circum stances in which this generalization does n ot hold, as in the vicinity o f a ventilation outlet. Fu rtherm ore, GA-samples m ay adequately ty p ify exposures at a location even i f the m iner is absent because concen tration s o f radon are n ot significantly altered by local mining activities excep t blasting and breaking rock.

Ore dust concen tration s fo r the m ost part are dependent on m ining operations, and a m iner’s exposure to ore dust m ay be strongly influenced by his p rox im ity to radiation sources. C onsequently, air samples should be co llected very close to the m iner to m easure his exposure accurately. B u t ore dust concentrations are norm ally low enough in m ines so th at this precau tion is unnecessary.

Air m onitoring in mills is also prim arily by m eans o f grab samples b u t personal air sam plers can be used fo r measuring individual exposures to ore dust and concen trate dust. M ost airborne contam inants in mills derive from localized sources, resulting in sharp con cen tration gradients in the vicinity o f p oints o f release. C onsequently, the location o f grab samples fo r exposure m onitoring is m ore critical than in m ines and a greater frequency o f repeated m easurem ents is required because o f greater variation. G eneral air samples m ay be co llected to estim ate average radionuclide concen tration s within working areas, b u t breathing zone (B Z ) samples are required to measure the exposures o f mill w orkers at dusty jo b s such as em ptying dust co llectors, filling product drums, and processing ore and con cen trate samples. BZ-samples are co llected as near as possible to the nose o f the w orker w ithou t obstructing his m ovem ents. T hey are not required i f personal air samplers are used fo r m onitoring exposures but th ey are still useful fo r m easuring the relative contribu tion to a w orker’s daily exposure from individual, dusty operations.

In bo th m ines and mills, concen tration m easurem ents are required fo r exposure evaluation and fo r the design or alteration o f con tro l measures, principally ventilation. O ften a given m easurem ent serves b o th purposes b u t the m onitoring programme must provide fo r the separate objectives. N orm ally, an exposure is determ ined fo r each w orker by m eans o f samples obtained in each o f the areas th at he occupies and, additionally fo r m ill workers, at each dusty operation during a sh ift. F o r this purpose, samples m ay be co llected at any tim e but should be representative o f actual working conditions. F o r engineering purposes, additional samples m ay be required th at can be used specifically fo r evaluating the effectiveness o f ventilation and o th er con tro l measures.

In m ines, som e significant sources o f radon, such as leakage from old workings, are n ot readily apparent but can be detected by m onitoring. Sharp increases either in radon con cen tration or in radon daughter concen tration along an air course are indications o f an influx o f contam ination . A brupt d ifferences in radon daughter ratios are also sensitive indications o f radiation sources.

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5.2.2. Radon and radon daughters

Exposure to radon daughters by inhalation is the dom inant occupational health hazard in uranium m ines and warrants the greatest proportional e ffo rt in the m onitoring program m e. Since IC R P standards are expressed in term s o f radon concen tration , it is the practice in som e countries prim arily to m easure radon, with due allow ance fo r concentrations and uncom bined fractions o f radon daughters as prescribed by IC R P (see Standards, Section 4 .3 .2 ) . O ther countries have adopted an exposure standard expressed in term s o f radon daughters, the w orking level m onth, such th a t only m easurem ents o f radon daughters are necessary. E ith er approach is satisfactory.

In mills, radon and radon daughters con stitu te a secondary source o f exposure and are confined m ainly to ore storage, grinding and classifying areas. G eneral air samples should be

Manual on Radiological Safety in Uranium and Thorium Mines ... Safety Standards/Safety_Series_0… · FOREWORD Uranium mining and milling industries are growing fairly rapidly in

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