Ancrages.pdf

For Official Use NEA/SEN/NRA/WGOE(2015)1 Organisation de Coopration et de Dveloppement conomiques Organisation for Economic Co-operation and Development 04-May-2015

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_____________ English text only NUCLEAR ENERGY AGENCY

Committee on Nuclear Regulatory Activities

Working Group on Operating Experiences

Technical Note on Operating Experience:

Pre-Stressed Anchor Bolts Non-Compliances

JT03375546

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ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENT

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COMMITTEE ON NUCLEAR REGULATORY ACTIVITIES

The Committee on Nuclear Regulatory Activities (CNRA) shall be responsible for the programme of

the Agency concerning the regulation, licensing and inspection of nuclear installations with regard to

safety. The Committee shall constitute a forum for the effective exchange of safety-relevant information

and experience among regulatory organisations. To the extent appropriate, the Committee shall review

developments which could affect regulatory requirements with the objective of providing members with an

understanding of the motivation for new regulatory requirements under consideration and an opportunity to

offer suggestions that might improve them and assist in the development of a common understanding

among member countries. In particular it shall review current management strategies and safety

management practices and operating experiences at nuclear facilities with a view to disseminating lessons

learnt. In accordance with the NEA Strategic Plan for 2011-2016 and the Joint CSNI/CNRA Strategic Plan

and Mandates for 2011-2016, the Committee shall promote co-operation among member countries to use

the feedback from experience to develop measures to ensure high standards of safety, to further enhance

efficiency and effectiveness in the regulatory process and to maintain adequate infrastructure and

competence in the nuclear safety field.

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The Committee shall maintain an oversight of all NEA work that may impinge on the development of

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The Committee shall focus primarily on the regulatory aspects of existing power reactors, other

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examine any other matters referred to it by the Steering Committee. The Committee shall collaborate with,

and assist, as appropriate, other international organisations for co-operation among regulators and consider,

upon request, issues raised by these organisations. The Committee shall organise its own activities. It may

sponsor specialist meetings and working groups to further its objectives.

In implementing its programme the Committee shall establish co-operative mechanisms with the

Committee on the Safety of Nuclear Installations in order to work with that Committee on matters of

common interest, avoiding unnecessary duplications. The Committee shall also co-operate with the

Committee on Radiation Protection and Public Health and the Radioactive Waste Management Committee

on matters of common interest.


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FOREWORD

The NEA Committee on Nuclear Regulatory Activities (CNRA) believes that sharing operating

experience from the national operating experience feedback programmes are a major element in the

industrys and regulatory bodys efforts to ensure the continued safe operation of nuclear facilities.

Considering the importance of these issues, the Committee on the Safety of Nuclear Installations (CSNI)

established a working group, PWG #1 (Principle Working Group Number 1) to assess operating experience

in the late 1970s, which was later renamed the Working Group on Operating Experience (WGOE). In

1978, the CSNI approved the establishment of a system to collect international operating experience data.

The accident at Three Mile Island shortly after added impetus to this and led to the start of the Incident

Reporting System (IRS). In 1983, the IRS database was moved to the International Agency for Atomic

Energy (IAEA) to be operated as a joint database by IAEA and NEA for the benefit of all of the member

countries of both organisations. In 2006, the WGOE was moved to be under the umbrella of the Committee

on Nuclear Regulatory Activities (CNRA) in NEA. In 2009, the scope of the Incident Reporting System

was expanded and re-named the International Reporting System for Operating Experience (although, the

acronym remains the same).

The purpose of WGOE is to facilitate the exchange of information, experience, and lessons learnt

related to operating experience between member countries. The working group continues its mission to

identify trending and issues that should be addressed in specialty areas of CNRA and CSNI working

groups.


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TABLE OF CONTENTS

1. Introduction ................................................................................................................................................ 9

2. Description of the issue ............................................................................................................................ 11

3. Observations and regulatory significance ................................................................................................. 15

4. Root causes ............................................................................................................................................... 17

5. Regulatory actions and licensee corrective actions .................................................................................. 19

6. Conclusions .............................................................................................................................................. 21

Appendix A: ASN Presentation ..................................................................................................................... 23

Appendix B: WGOE Questions And Country Responses ............................................................................. 29

WGOE QUESTIONS ................................................................................................................................ 29 CANADA .................................................................................................................................................. 29 CZECH REPUBLIC .................................................................................................................................. 30 FINLAND .................................................................................................................................................. 30 GERMANY ............................................................................................................................................... 31 INDIA ........................................................................................................................................................ 31 JAPAN ....................................................................................................................................................... 31 SPAIN ........................................................................................................................................................ 32 SLOVAKIA ............................................................................................................................................... 32 SLOVENIA ............................................................................................................................................... 32 SWEDEN ................................................................................................................................................... 32 SWITZERLAND ....................................................................................................................................... 32 UNITED KINGDOM ................................................................................................................................ 32


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1. INTRODUCTION

The WGOE is a network of operating experience experts that allow countries to reach out on specific

issues to get timely feedback on practices and experience in other countries. The primary purpose of this

type of technical note is to capture the exchange of information for an emergent operating experience

challenge from a specific country and its resolution. Sometimes, these issues are resolved in such a manner

that they do not rise to the level of an International Incident Reporting System (IRS) report. This technical

note captures the operating experience.

In 2008, the presentation in Appendix A was given to the WGOE members to introduce the issue.

Following this presentation, questions were distributed to WGOE members to better understand

international experience and actions and to inform about ASNs plan of action. The questions and

responses of different WGOE members are gathered in Appendix B.


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2. DESCRIPTION OF THE ISSUE

The nineteen French nuclear power plants (NPPs) currently in operation are very similar in their design

and construction (PWR type). They each comprise from two to six units, which in total amounts to 58

reactors. Each reactor includes a nuclear island, a conventional island, water intake and discharge

infrastructures, and potentially a cooling tower.

The nuclear island mainly consists of the reactor vessel, the reactor coolant system, the steam generators

and the circuits and systems ensuring reactor operation and safety. Various support function circuits and

systems are also associated with these systems: primary effluent treatment, ventilation and air-

conditioning

These main systems or circuits are equipped with materials which are often attached to concrete slab with

anchor bolts. They are fixed by steel rods through the concrete slab. These rods are prestressed, i.e. under

tension during assembly, to prevent slippage and detachment of the support under the effect of forces

applied under normal or accidental situation, particularly in case of earthquake or pipe break.

Approximately 1,000 anchor bolts are used in a French reactor.

Figure 1: Anchor bolt description

In the early 1990s, several non-compliances were discovered:

- anchor bolts undersized comparing to their conception (ex: anchor bolts of the borated water tank

of the security injection system),

- stress corrosion cracking (SCC), which may cause the failure of the anchor bolt,


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- and loss of tension, which is caused by delayed strains in concrete (such as shrinkage and creep

phenomena).

Following are two examples to illustrate the extend of the issue.

Steam generator anchor bolts

Figure 2 : Steam generator anchor bolts

Each steam generator of the French reactors is equipped with four self-locking devices. Each device is

fixed to the concrete by four anchor bolts and designed to get locked during an accident situation without

hindering the possible displacements of the steam generator in normal operating situations.


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Originally, these steam generator anchor bolts of 900 MWe CP01 reactors were not protected against

corrosion. Given their steel characteristics and their tension value, they are listed as being susceptible to

failure by SCC.

In order to cancel this risk, the 48 original anchor bolts of each reactor (except one) have been replaced by

protected against corrosion by a zinc layer, by the end of 2011.

Reactor vessel support anchor bolts

The concrete support of the reactor vessel is equipped with 18 to 19 devices which aim at preventing

horizontal displacement of the reactor vessel during earthquake. Each device is composed of 8 anchor

bolts.

Anchor bolts of the 900 MWe CPY2 reactors (except one 4-unit NPP) are affected by tension loss and

failure by SCC. The following diagram shows the evolution over time of the number of broken anchor

bolts discovered in the three most affected reactors.

1 CP0 reactors : four reactors at Bugey (reactors 2 to 5) and two reactors at Fessenheim

2 CPY reactors : consisting of another twenty-eight 900 MWe reactors, that can also be subdivided into CP1 (eighteen

reactors at Blayais, Dampierre-en-Burly, Gravelines and Tricastin) and CP2 (ten reactors at Chinon, Cruas-

Meysse and Saint-Laurent-des-Eaux)

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Figure 3 : reactor vessel support


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3. OBSERVATIONS AND REGULATORY SIGNIFICANCE

These deficiencies could impact the operability of safety materials during accidental situations, particularly

in case of earthquake or pipe break.


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4. ROOT CAUSES

Initially, no maintenance was supposed to be performed on the anchor bolts during plants lifetime. Moreover, after tensioning an anchor, the prestressing force decreases due to the effects related to

behaviour of the steel and concrete. These effects were under-estimated during the conception.

Both conception deficiencies (loss of tension in anchor bolts) and construction/operation deficiencies

(SCC) are responsible for these non-compliances.


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5. REGULATORY ACTIONS AND LICENSEE CORRECTIVE ACTIONS

Since 2003, for each component equipped with anchor bolts, a specific file has been created in order to

gather information concerning:

- the conception plan and the precise dimension of the anchor bolt, - the minimum tension required, - the maximum tension allowed (to prevent plastic deformation) - the last prestressed tension applied,

Moreover, the loss of tension due to concrete shrinkage and steel relaxation is calculated in order to

estimate the date of the next prestressing operation. In 2009, the first results of these investigations

concluded the necessity to perform a tightening of 51,000 anchor bolts among the 60,000 in total every ten

years.

In 2010, ASN issued a position letter requesting particularly EDF to forward a preventive periodic

maintenance program and the demonstration of the exhaustiveness of its strategy. The aim was to ensure

that all anchor bolts were considered.

In 2011, a specific maintenance program has been issued by EDF in order to carry out the following

periodic checks:

- compliance with the conception design, - no failure of the anchor, - visual control of concrete (no crack and burst), - visual control of the anchor to detect corrosion, - sufficiency of prestressed tension and repair if necessary.

Concerning rupture due to stress corrosion cracking, anchor bolts have been repaired. Two main methods

are used:

- substituting the damaged anchor by another one protected against corrosion with a zinc layer, - injecting a wax between the metallic sheath and the anchor in order to protect the anchor from

moisture.

ASN ensures that periodic checks and maintenance work are actually performed in accordance with the

requirements through worksite inspections, checks during the reactor outages and periodic technical

meetings with EDF.


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6. CONCLUSIONS

Conception conformity should always be checked during plants lifetime. Such checking should be performed on site (walkdowns) and should also consist in reviewing the documentation.

ASN considers that actions undertaken by EDF to determine the applicable baseline requirements for each

component equipped with anchor bolts are indispensable to identify eventual non-compliances. Moreover,

the preventive maintenance programme provides a specific framework to carry out examinations and

repairs. However, the non-compliances identification method and the sufficiency of the preventive

maintenance program are currently being evaluated by ASN and its technical support, IRSN, and a position

letter should be issued in the near future.


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APPENDIX A: ASN PRESENTATION


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APPENDIX B: WGOE QUESTIONS AND COUNTRY RESPONSES

WGOE QUESTIONS

Q1 - Are your NPP equipped with pre-stressed anchor bolt?

Q2 - If yes, did the operators in your country face any problem with these equipments? How did they deal

with it?

Q3 - Do the operators take into account delayed strains in concrete to ensure anchor bolt but also reactor

containment structure (...) integrity?

CANADA

Q1 - are your NPP equipped with pre-stressed anchor bolt?

Pickering NGS Vacuum Building (VB) used similar post-tensioned rods in the concrete structure to tie the

roof slab to the wall. The anchor rods used in this case were not through bolts but they were embedded into

the wall of the VB.


with it?

Currently, the operator is developing the plan and the procedures for the inspection and testing for this type

of post-tensioned rods. Inspection of the pre-stressing system is part of the periodic inspection program

requirements for containment structures in NPPs in Canada.

Q3 - do the operators take into account delayed strains in concrete to ensure anchor bolt but also reactor


Yes, the effect of degradation of the concrete structure, including shrinkage and creep, will be expected to

be considered by the operators in the inspection and testing methods selected, and in the evaluation of the

inspection results for the post-tensioned rods. The periodic inspection program prepared by the operator is

required to be submitted to the regulator for review and acceptance.

Contact : James Mok, M. Sc., P. Eng.

Specialist/Spcialiste

Operational Engineering Assessment Division | Division de l'valuation technique de l'exploitation

Directorate of Assessment and Analysis | Direction de l'valuation et de l'analyse

Canadian Nuclear Safety Commission | Commission canadienne de sret nuclaire

Ottawa, Canada K1P 5S9

[email protected]

Telephone | Tlphone 613-996-7338 / Facsimile | Tlcopieur 613-943-1818

Government of Canada | Gouvernement du Canada


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CZECH REPUBLIC

Q1: No, our NPPs are not equipped with pre-stressed anchor bolts use at Czech NPPs. We use different

types of anchoring usually bolts fixed in concrete.

Q2: Not applicable.

Q3: Changes in the quality of concrete are taken into account in the program of in-service periodical

inspections. Delayed strains are considered.

Quality of concrete is assessed mainly for the containment building at the Temelin NPP and for so called

confinement at Dukovany NPP.

You can find some information on Temelin NPP containment structure in the IRS report No. 7982.

FINLAND

Olkiluoto 1 and 2

There are no pre-stressed anchor bolts in Olkiluoto 1 and 2 units. Even machine foundations are

anchored by common grouted through bolts.

Olikiluoto 3

In Olkiluoto 3 unit steam generators and main circuit pumps as well as certain main stream piping

anchorage have been and will be done by pre-stressed bolts. Design solutions cover total life time

(60+5 years) of the plant by taking into account long period behaviour of structures and materials.

Such phenomena are ex. relaxation of bolts and creeping of concrete. Corresponding maintenance

program will be inspected during commissioning inspections after which STUK is able to give

further information.

Loviisa 1 and 2

Below is a drawing of the Loviisa reactor containment structure that shows (detail G) how the pre-

stressed anchor bolts are used for fastening the steel containment into the concrete base plate. The

bolts are re-tightened and inspected via ultra sonic testing periodically (4years) for

possible deficiences. The outer bolts are demountable and can be replaced according to the

inspection program. So far (= 33 years of operation) no major problems has been noticed with the

bolts.


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Additional information considering Loviisa NPP:

Jouko Turpeinen, Manager, International Programs

Fortum Power and Heat Oy, Loviisa Power Plant

POB 23, 07901 Loviisa, FINLAND

Tel: + 358 10 4554733 (direct) or + 358 50 5446840 (mobile)

Fax: + 358 10 4554435

Email: [email protected]

GERMANY

German NPPS do not have this type of anchors. No associated events.

INDIA

Q1 - Are your NPP equipped with prestressed anchor bolt ?

In Indian Pressurized Heavy Water Reactor (PHWR) design, pre-stressed anchor bolts are not in use. For

heavy equipment, like DGs, compressors, transformers etc, cast in place J bolts or grouted anchor bolts are

used. For lighter equipment like control panels, cabinets etc, embedded plates are used to weld the

equipment base.

However, the hanger supports for calandria and steam generators at older generation Rajasthan and Madras

atomic power station units 1&2, are pre-stressed with external springs. The spring tension and hanger pre-

stressing is checked periodically.

Q2. If yes, did the operators in your country face any problem with these equipment? How did they deal

with it?

No problems with existing anchors.

Q 3, do the operators take into account delayed strains in concrete to ensure anchor bolt but also reactor

containment structure integrity?

In Indian NPPs, no pre-stressed anchor is used on containment structure. Reactor

containment structure is provided with specially designed embedded openings for routing pipes, power

cables and I&C cables etc.

JAPAN

Response to French question regarding pre-stressed anchor bolt (Japan)

By: Shigeo TAMAO (JNES)

Q1 - are your NPP equiped with pre-stressed anchor bolt?

No.

Q2 -If yes, did the operators in your country face any problem with these equipment?

How did they deal with it?

NA

Q3 -Do the operators take into account delayed strains in concrete to ensure anchor bolt but also reactor



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In Japan anchor bolt is not pre-stressed. The only concern of delayed strain in concrete is the relaxation of

tendon in PCCV. But tendons are applicable to re-stress if needed.

SPAIN

No associated events.

SLOVAKIA

The nuclear power plants (NPPS) do not have this type of anchors. No associated events.

SLOVENIA

The answers to the questions of the pre-stressed anchor bolt are:

1. Answer: NPP Krsko is not equipped with the pre-stressed anchor bolts.

2. Answer: N/A.

3. Answer: N/A.

SWEDEN

NPPS do not have this type of anchors. No associated events.

SWITZERLAND


It is not very common but we have some.

Q2 - If yes, did the operators in your country face any problem with these equipments ? How did they deal

with it ?

No problems, but the corresponding equipment is listed in the periodic maintenance program. E.g. for pipe

supports a perodic control (readjustement) of the force is requested. - Problematic is the use of so called

expansible anchors. There is a phenomena that reduces pre-stress within very short time.



The mentioned phenomena are known. See also Q2.

UNITED KINGDOM

Response from UK Magnox South's 5 sites


A1 - Yes, Liebig anchor bolt installations to brackets civil structures at Dungeness A and Bradwell. They

were installed well after original construction but over 10 years ago. MXS civil personnel also have

experience of these bolts at nuclear power stations now operated by other licenses which inform us, but not

directly referred to here.


with it?


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A2 - Installation of Liebig anchors consists of drilling and under cutting the bolt hole, inserting the bolt

assembly, torquing the bolt and re-torquing it more than 24 hrs later. There have been very few installation

problems which are revealed before or during bolt torquing. Appropriate training and management of the

operatives has been sufficient to deal with such matters. The brackets are subject to routine inspections

which show the bolts remain in good condition, for some bolts these inspections include torque checks and

these too have been satisfactory for over 10 years.


containment structure (...) integrity ?

A3 - Yes, delayed strains in the concrete and in the bolt are taken account of by re-torquing the bolt after

more than 24 hrs. This is supported by tests that we are aware of, the manufacturer's recommendations and

our operational experience in A2. The bolts referred to contribute to reactor building structures but not to

reactor containment structures."

Response from UK Magnox North's Wylfa NPP

Reactors at Wylfa Power Station have Liebig Anchors securing the Superheater Header Restraints and load

cell readings have been taken of about 30 of them over the last 10 years to check that the anchors retain

95% of their design load. The anchors monitored are M24 and M36 with the majority being M24 with a

design loading of 132kN. A general summary of characteristics of these anchors, from a quick review of

the graphs plotted from the load cell readings, is that initial load is lost over the first 2 to 3 months and

after about a year general linear behaviour is established with losses averaging about 5% of the design

load, with a maximum of say10%, occurring over the following 9 years. If you think the detailed

information from the monitoring would be helpful to yourself or your French colleagues please would you

request this directly from Wylfa."

I have received additional information below from one of the NII civil engineering specialists.

The main points have been picked up by the French colleagues i.e Stress Corrosion Cracking and Creep. I

would also make the following observations in relation to prestressed anchorages:

1. The maintenance regime should be well documented.

2. The design should allow for checking pre-load and re-stressing if required.

3. Continuous monitoring may be appropriate (cf rock anchorages).

4. If practicable the design should allow for inspection of the anchorages and the tension system (bar, or

strand etc.)

5. Through section systems are best with a positive anchorage at each end.

6. Blind anchorages where the remote end cannot be inspected require careful design:

a) Cast in plate give a better bearing generally with less anchorage creep problems.

b) Drilled holes where the anchorage is formed by mechanical means or by grouting are more

prone to creep. Hole should be undercut.

COMMITTEE ON NUCLEAR REGULATORY ACTIVITIESFOREWORDTABLE OF CONTENTS1. INTRODUCTION2. DESCRIPTION OF THE ISSUE3. OBSERVATIONS AND REGULATORY SIGNIFICANCE4. ROOT CAUSES5. REGULATORY ACTIONS AND LICENSEE CORRECTIVE ACTIONS6. CONCLUSIONSAPPENDIX A: ASN PRESENTATIONAPPENDIX B: WGOE Questions and Country ResponsesWGOE QUESTIONSCANADACZECH REPUBLICFINLANDGERMANYINDIAJAPANSPAINSLOVAKIASLOVENIASWEDENSWITZERLANDUNITED KINGDOM

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