Nuclear Safety BNEN 2012-2013 Intro William D’haeseleer.

Nuclear Safety

BNEN 2012-2013 Intro

William D’haeseleer

Well-known nuclear accidents

TMI Harrisburg PA 1979


Chernobyl Ukraine 1986


Dai-ichi Fukushima Japan March 11 2011

Serious nuclear accidents

U* H3* Pu* Mo* Sr* Cs* Co* Xe*

I*

Serious nuclear accidents

Inhalation

.

fallout

wash out

Intake

Cloud radiation

Reactor accident vs nuclear explosion

• Because limited enrichment in U-235

• Because of confinement of fuel

Impossible that nuclear reactor would explode like a nuclear bomb


BombBomb

• Basically pure U-235 or Pu 239 (>90%)

• Establish super-critical mass via chemical explosion

• During “implosion”, a fast build up of an exponential chain reaction

• Inertia “helps” nuclear chain reaction

• Correct timing very important

Nuclear Bomb - principle


BombBomb

• Then suddenly release of massive amount of energy ~ 1012 Joule in short time

• Huge power pulse leads to– Enormous temp rise (> 106 °C)– “vaporization” & ionization of matter– Huge fire ball


BombBomb• Then suddenly release of massive amount

of energy ~ 1012 Joule in short time• Huge power pulse leads to …

– Enormous pressure wave (over several km)– Creation very strong EM pulse/waves

leads to fires, fire burns, etc

– Release big neutron flux– Release of fission products & actinides


BombBomb

• Immediate victims due to– Pressure shock wave– Vaporization & fires– Direct / Acute irradiation


Reactivity accident in reactorReactivity accident in reactor

• Energy release much smaller, – even in most sensitive cases, in case of

super-prompt-criticality– About ~ factor 103 à 106 less release of

energy over “longer” period of time Much much smaller “power” release



• Chain reaction cannot continue to grow exponentially– Feedback mechanisms cfr Doppler – “flying apart” of structural material



No possibility for– Fire ball, vaporization & E&M pulse– Pressure wave,…


Reactivity accident in reactorReactivity accident in reactor: : ChernobylChernobyl

• original “power excursion” of nuclear origin (over-criticality)

• But the chemical explosion / steam explosion

• Then establishment of graphite fires in the moderator

• Large release radio-isotopes

• Totally different sort of consequences!

Safety philosophy in NPPs


Fundamental safety principleFundamental safety principle:

At the time of the design, during construction & start up, and during operation,

There should “never” be a release of a large amount of radioactivity that may harm the public


“Defense in depth” philosophy

=

Use of multiple successive barriers to prevent release of radioactivity to

environment

Russian doll principle

1

2

3

Three subsequent physical barriers


Three safety levels

to avoid that none of the barriers is compromised as result of abnormal occurrences such as equipment failure, human error or natural phenomena


• Three safety levels 1) “Design for maximum safety in normal

operation and maximum tolerance for system malfunction. Use design features inherently favorable to safe operation; emphasize quality, redundancy, inspectability, and testability prior to acceptance for sustained commercial operation and over the plants lifetime”


Examples:

• Negative reactivity coefficients

• Use only radiation-resistant materials


• Three safety levels

2) “Assume that incidents & mishaps will occur in spite of careful design, construction and operation. Provide safety systems to protect operators and the public or minimize damage when such incidents occur”


Examples:

• ECCS to cope with LOCAs

• Electric emergency power supply (diesels, batteries)


• Three safety levels 3) “Provide additional safety systems as

appropriate, based on evaluation of effects of hypothetical accidents, where some protective systems are assumed to fail simultaneously with the accident they are intended to control”

For unforeseen events or events with very small probability: DBA


Example:

• Assume ECCS fail:– Core melt…– Release radioactive material in reactor building– Containment must be leak free– Sprinkler system in containment building

• Reduce p & T• Condense volatile isotopes (e.g., 131I)

– Also ventilation & filtration in intermediate region


Which accidents for DBA?

“How safe is safe enough?”

Rule of thumb USAEC (1973)

“An event with a frequency > once/1000 years need not be taken into account in the design”


Which accidents for DBA?“How safe is safe enough?”

By year 2000, estimated that 1000 reactors operational in USA

With extra safety factor of 10

Probability limit 10-7/reactor-year


Which accidents for DBA?

“How safe is safe enough?”

If P>10-7/reactor-year:

Necessity to perform safety analysis with the most pessimistic safety assumptions, to maximize the consequences

Additional safety concepts

• After TMI accident (lessons learned)

• Now incorporated in all new “passive” or “inherently safe” designs– Fail safe– Full proof– Walk away safe– Forgiving


• Fail safe– If, after shortcoming of an important

component, the installation can be brought back into a safe state

• Fool-proof– If it remains safe w.r.t. whatever human

intervention (even with bad intentions) – safety locks –


• Walk away safe– If the installation can be left alone for a

“reasonable” time, after having been brought to a safe state in the beginning of the accident

• Forgiving– If reactor “tolerates” a late or an erroneous

human action, without giving rise to an accident.

Typical accident classes

• Reactivity accidents transient behavior

• Lack of cooling transient behavior

• Fuel manipulation (reloading, maintenance)

• Site-related accidents (earth quakes, air plane crash)

Typical accident classes

Example: LOCA

• After shut down reactor still produces heat ~ 7-10% of thermal heat at t=0

• Heat due to fission products

• Heat decays exponentially

• LOCA = breach in primary coolant circuit

• May lead to dry cooking of reactor core melt down

LOCA – safety systems

PRA

• Risk = Probability x Consequences

• Via ‘event trees’ and ‘fault trees’, probability of occurrence can be evaluated

WASH 1400

WASH 1400

Actual & Predicted accidents

Actual & Predicted accidents

“Accepted accidents”

• Deadly annual fatalities car accidents – About 1000+ in Belgium– Roughly 30 000 à 50 000 in USA – About 1.2x106/a worldwide (Ref WHO)


• Deadly annual fatalities car accidents – About 1000+ in Belgium– Roughly 30 000 à 50 000 in USA – About 1.2x106/a worldwide (Ref WHO)


• Deadly annual fatalities car accidents – About 1000+ in Belgium

– Roughly 30 000 à 50 000 in USA

– About 1.2x106/a worldwide (Ref WHO)

• Deadly victims due to airplane crashes worldwide ~ 1000 à 2000/a

• Industrial accidents…construction sector…• Largest accident in energy sector: failure of Hydro

dam system in China 1975: 171,000 victims Ref http://en.wikipedia.org/wiki/Banqiao_Dam. See also Savacool (later)

http://en.wikipedia.org/wiki/Banqiao_Dam


• Deadly victims due to CO poisoning in B ~ 100/a

Deadly victims developed world ~10-5/a 104 victims/a

• Ghislenghien 2004 28 deaths• …

TMI accident 1979

TMI

• PWR plant - LOCA type

• Origin: failure feedwater pump secondary

• LOCA through PRV’s pressurizer

• Operators turned off ECCS (focused on level presurizer)

• Health consequences:– Average absorbed dose ~ 0.01 mSv– Max off site dose ~ 0.8 mSv

Chernobyl accident - RBMK

Chernobyl accident

• RBMK reactor, graphite moderated, water cooled

• BWR in pressure tubes

• RBMK has a positive void coefficient

Chernobyl accident

• Irony: safety experiment April 25 utilizing kin en turbine generator electricity primary pumps

• Operators forced go ahead test

• Incredible “trespassing” all safety rules / turned off emergency signals / last SCRAM signal turned off

• In region unstability… pos react coeff…

Chernobyl accident

• Reactor power up by factor 100 in 4 s

• Steam explosion

• Lifting up roof from reactor building

• Ignition fire in graphite moderator

• Fission products “ejected” into the air (up to 1 km high)

TMI & Chernobylradioactivity releases

TMI & Chernobylradioactivity releases

217 USA

215 SU

45 FR

22 CH

21 UK

Consequences Chernobyl - WHO



Chernobyl forum

Chernobyl forum

• Conclusions:– 50 emergency workers died of acute radiation

syndrome (ARS)– 9 children died of Thyroid cancer– 3940 people to die in most contaminated

areas according to LNT– Total ~ 4000 deaths

Dai-ichi Fukushima accident Japan – see separate slide show -

INES scale

INES scale

Nuclear Safety BNEN 2012-2013 Intro William D’haeseleer.

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