Failure Knowledge Database / 100 Selected Cases

Conceal troubles at nuclear power plants
Aug. 29th, 2002, Pref. Fukushima and Niigata
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
Periodic inspections of nuclear power plants approximately once per year are required by law. From thelatter half of the eighties to the nineties, the Tokyo Electric Power Company, Limited (TEPCO) conducted
self-inspections during the periodic inspections at the Fukushima first nuclear power plant, the Fukushimasecond nu clear p ower p lant, an d the Kash iwazaki Ka riwa n uclear po wer p lant. Although th ey fo undcracks i n t he core s hroud which is i nstalled inside t he rea ctor pres sure c ontainer, t he eval uation, repair ,record and report to the government were inadequate.
After that, it was discovered, TEPCO, the Chubu Electric Power Company, Incorporated (Chuden) andthe Tohoku E lectric Power Company, Inc orporated d id no t rep ort fa ilures to th e government while theyfound cracks in the Primary Loop of Recirculation system (PLR) piping at the nuclear power plant.
In addition, it was also discovered that the Japan Atomic Power Co., Ltd. didn't report the failures to thegovernment while they observed cracks at the core shroud.
A large number of cracks were found in the equipments of m any nuclear power plants, although, plantowners did not report these facts to the government and the cracks were repaired secretly or left withoutrepaired.

When this fact was finally appeared to the general public, a strong fear concerning the safety ofnuclear power plants arose.
In response to these incidents, the electricity enterprises law was am ended. The integrity evaluation
system for nuclear power plants was introduced and the rules on fitness-for-service for nuclear power plantcomponents prepared by Japan Society of Mechanical Engineers (JSME) are going to be utilized.
1. Component
The structure of the core shroud is shown in Fig.1. Many short metal cylinders are welded and form along tubular structure.

It is sup ported vertically by shroud su pport.

The core shrou d serv es to su pport

nuclear reactor core (fuel assembly), direct flow of coolant water around the reactor core, and hold waterover the reactor core when an accident is occurred.
The structure of the PLR (Primary Loop of Recirculation system) piping is shown in Fig. 2.

The PLRpiping is p iping w hich circulates t he h igh tem perature pure w ater i n a reac tor pressure vess el through apurifying facility, and has connected also with a condenser.
2. Event
As a rule for desi gn a nd c onstruction of nuclear p ower plan t, t here i s a not ification (N o. 50 1) w hichdefines technical standards of nuclear components.
This notification used not only as a technical stand for the design and construction, but also used as a1

Failure Knowledge Database / 100 Selected Cases

technical sta ndard for t he in-service inspection (ISI). In ot

her words, t his m eans t hat m anufactured

performance should be maintained during the in-service period.
At the time of m anufacture, a wel ding defe ct oft en arises.

When the defec t is de tected by aninspection, repair and/or exchange is a fundamental principle. Therefore, the inspection standard for t hedesign and construction rule is re quired only qualitative judgment of flaws. (judge existence of flaw, flaw
size is not concerned).

The radiographic testing (RT) is dominant as the inspection method for the designand construction rule (during pre-service period).

In-service period, weld defects are supposedly not exist

and fatigue crack and/or stress corrosion cracking (SCC) are frequently occurred. The ultrasonic testing is
dominant as the insp ection m ethod for the in-service p eriod.

Regarding to th e d esign and construction

rule, when the flaws are det ected by the in-service inspection, repair and/or exchange is basically required.
This judgment doesn't take into account the difference in the method of RT and UT and the difference offlaw size and/or flaw nature.
It is na tural result that cr acks ar ise aft er the s ufficient time passes duri ng in- service per iod.

Byevaluating flaw size a nd flaw growth, continuous operation is permissible without repair a nd/or exchange
for a certain period.

However, this evaluation and permission is basically unaccepted. The repair and/orexchange are basically required.
A l arge nu mber o f st ress co rrosion crack ing were foun d at bo th th e co re sh roud and the PLR piping,although, the evaluation, repair, record and report to the government by companies were inadequate. Froma viewpoint of engineers who participates in these matters, causes are considered as follows;(1) Since applying n on-destructive i nspection sta ndard of the design a nd c onstruction ru le to thein-service inspection is unreasonable, they ignored the standard.
(2) They had controlled material and environment to prevent SCC occurrence.

Therefore, occurrenceof SCC was outside assumption for them and they hid the fact which SCC occurred.
3. Course
On A ugust 29th, 2 002, a N uclear an d In dustrial Safet y A gency (N ISA), Ministry of Econ omy, Tradeand In dustry (METI), Ja pan announced the i nvestigation resu lt of ficially t hat t he TEPCO ha d fou nd t heSCC duri ng the self-i nspections d uring 1986s-1990s (General El ectric Int ernational In c. t ook char ge),although, the record and the report to the government had been inadequate.
triggered by provision of i nformation t o METI o n J uly, 2 000.

This investigation was started

On Sep tember 20t h, 2002, i t bec ame

apparent that the TEPCO, Chuden and the Tohoku Electric Power Company didn't report the SCC detected
at PLR piping to the government.

In addition, on September 25th, 2002, it was also discovered that theJapan Atomic Power Co., Ltd. didn't report the failures to the government while they observed cracks at thecore shroud. The interim report about these incidents was compiled by NISA on October 1st, 2002.
4. Cause
Judgment of t he e ngineer w ho in volves is no t th e true caus e. Th e problem is the or ganization w hichapproves t he judgment w ithout ch eck. In t he pres ent c ondition re formed fro m safety e nsuring by aregulation system to safety ensuring by self-imposed control, exact quality assurance and risk management
2Failure Knowledge Database / 100 Selected Casesis required. The essential cause of conceal troubles at a nuclear power plants is lack of a quality assurance
system and risk management.
5. Immediate Action
In order to prevent concealing, the subcommittee was established in the nuclear insurance and the safety
department committee.

They developed following preventative measures;○ Reliability ensuring for the safety activity by companies.
○ Establishment of a quality assurance system for the safety activity by companies.
○ Strengthening of the company's efforts to comply with the rules.
○ Cl arification and transparency of regulatory system operation.
○ The improvement of a report system operation.
○ Publication and sharing of information about small troubles.
○ Address for recovering the public trust to the safety regulation○ Reexamination of the safety regulation system and operation.

6. Countermeasure
Some of abo ve-mentioned prev entative measures w ere alre ady im plemented.

It is nota ble t hat t heintegrity evaluation system was introduced on October, 1, 2003.
Figure 3 ex plains a flow of the int egrity evaluation syste m. The el ectricity ent erprises law wasamended and the periodical op erator ins pection a nd the i ntegrity e valuation sy stem for nucle ar pow erplants were i ntroduced. I t is the i nnovative sy stem th at the sy stem util izes privat e-standard (JSMEstandard).
○Periodical Operator Inspection
The pr esent self-inspection perform ed by thecompany was positi oned as “ Periodical Operat orInspection” on t he s tatute. In aperiodic al operator ins pection, i t is obliged t o d o a n ondestructive

inspection (Ultrasonic test ing).

Rules o n Fitness-for-service f or Nucl ear Powe r Plants by JSME (2002edition) is utilized as a concrete inspection method.
○Integrity Evaluation
When th e c racks we re de tected during the pe riodical operat or inspection, crac k growt h s hould be
predicted and the integrity of the system should be evaluated.

The equipments evaluated by the integrity

evaluation are the c ontainer and piping bel onging to the a pparatus which c onstitutes nuclear react orpressure bo undary (cl ass-1 equipments) an d a co re sh roud (Re actor core s upport structure).

Although,

PLR piping made by SUS316 is excluded from the integrity evaluation equipments, it will be included into
the equipments because the reliability of an ultrasonic testing is confirmed.
Items of the integrity evaluation are flaw modeling, allowable flaw, crack growth analysis, and failure
analysis.

These e valuations w ere co nducted by utilizing r ules on fitness-for service for nuclear power

plants by JSME.

3

Failure Knowledge Database / 100 Selected Cases○Evaluation results
The d etails of a matter of record and t he report m atter to a government are defi ned.

Although, theresult of the integrity evaluation is an object of a report, allowable flaw does not need to report.
○Establishment of a related private-standard
Although the above was re vision of the regulation sy stem, apart fro m this, t he pr ivate -st andard wasenacted as a countermeasure against lack of a quality assurance system.

As a countermeasure against lackof a qu ality a ssurance sy stem which is the esse ntial c ause, "Qua lity Assurance R egulations Co ncerning
Safety at Nuclear Power Stations (JEAC4111-2003)" was specified newly.

This regulation uses ISO 9001

as a base, replaces a quality by a nuclear safety, and specifies top management's role.
Furthermore, a gui deline fo r the ul trasonic crack si zing used as t he base of i ntegrity eva luation w asadded t o the "Guideli nes for the ultras onic test ing during i n-service ins pection of li ght-water react or
nuclear power plant (JEAG4207-2004)".
7. Knowledge
It seems that the regulations were made by accidents. However, the reality is different. Necessity of ruleon fitness for service has been acknowledged by concerned parties and they started preparing regulations
from 1993 and it w as est ablished from JSME (Japa n Socie ty of Mechanical E ngineers) in 2000. Theregulations would be referred to the technical standards of the government, but concealing of troubles was
discovered just before authorization.

That is, operation by the standard of the design and construction rule(not by the rule on fitness for serv ice) causes troubles, and the trouble accelerated practical use of the ruleon fitness for service.

Originally, standard is established in order to prevent accidents.

standard is not established or changed if an accident does not occur.

However, in fact,

All systems in Japan will not change

without experiencing serious damage.
Only the way to modify such systems is to form specific safety guidelines as a law and practical use ofa priv ate-standard.

Safety ensuri ng can not be re alized o nly by l aws.

As far as the production fie ld,

companies sh ould m ake stand ards t hemselves and sh ould obey t he stan dards. This is the b est way t oensure the safety.
8. Primary Scenario
01. Poor Value Perception
02. Difference in Culture
03. Difference of principle
04. Organizational Problems
05. Inflexible Management Structure
06. Information communication insufficiency
07. Misjudgment
08. Narrow Outlook
09. Standard defectiveness4

Failure Knowledge Database / 100 Selected Cases

10. Usage
11. Operation/Use
12. Core shroud
13. Usage
14. Maintenance/Repair
15. Inspection
16. Failure
17. Fracture/Damage
18. Stress corrosion cracking (SCC)
19. Loss to Organization
20. Social Loss
21. Confidence losing
22. Damage to Society
23. Change in Perception
24. Atomic energy infidelity

5Failure Knowledge Database / 100 Selected Cases

The symbol which shows
the part of a weld line

Reactorpressure vessel

ShroudShroud support

Fig. 1 Structure of a reactor core shroud.

6Failure Knowledge Database / 100 Selected Cases

Reactorpressure vessel

Outlet valve ofa recirculation pump
Recirculation pump

Inlet valve ofa recirculation pump
Fig. 2

Structure of a Primary Loop Recirculation System.

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Failure Knowledge Database / 100 Selected Cases

Electricity
Enterprises Law
The 55th article,
section 1
Obligation of regularinspection

Regular Inspection(Ultrasonic Testing)JSME
FFS Code 2002
Edition<In-service
Inspection>

Integrity Assessment○The First Step Flaw Evaluation<Allowable Flaw>

Electricity
Enterprises Law
The 55th article,
section 3
Obligation ofintegrity
assessment
Electricity
Enterprises Law
The 39th article,section 1
Conformity duty of atechnical standard

Electricity
Enterprises Law
The 55th article,
section 3
Obligation of record,storage and report

Fig. 3○The Second Step Flaw
Evaluation<Crack growth analysis><Failure Analysis><Acceptance Criteria>○The measure based on anassessment result<Continue Operation><Continue Operation with
Applying Both General & SpecificInspection Program><Repair or Replacement>

JSME
FFS Code 2002
Edition<Structural
integrity
assessment>

Assessment Result<Record・Store><Report>Integrity Evaluation System for Nuclear Power Plants.

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