Toward Post-Fukushima No. 14

14. How Safe is Safe Enough?
Antti Vuorinen
Director General, Emeritus, STUK Finland

How safe is safe enough is a question repeatedsituations;providedwithsufficientqualifiedoften, when talking about safety of nuclear power

resources. At utility level personnel of nuclear

plants; but most often left without clear answer. In

utility from the top management to the operating

the following is an attempt to highlight some key

and maintenance workers have to understand theelements, which has to be kept on mind and taken

importance of prioritizing always the safety before

care, when attempting to achieve high level of

economy or other interests and take the actions

safety in nuclear technology.

correspondingly.

Especiallypersonsinkey

positions have to understand the importance ofEvery incident and accident at nuclear power

independent

qualityassuranceandtotakeplant gives a reason to raise this question. It does

necessary actions for fulfilling it. To be effective

not matter, what the reason of the abnormal

safety culture needs continuous development andbehavior of the facility was. Is the design safe? Isfollow-up.

the quality of the plant adequate? Are theoperating personnel properly qualified? Is there a

What are the safety objectives?

authority?

Nuclear safety is finally culminated to theGenerally speaking, we have to ask at first: Is thequestion of radiological safety of personnel of thesafety culture of the country and the operating

plant and people in the environment; as well as thecompany at acceptable level?

radiological contamination of the environment.

qualified

independentregulatory

Technical safety objective is to prevent the release

What are the essentials of good safetyculture?
Responsible

persons

ateverylevelof radioactive materials from the reactor to theenvironment. To achieve this goal, heat transfer

mustfrom the reactor core has to be assured in all

acknowledge and understand the quality and thecircumstances. However, unexpected disturbances,magnitude of nuclear risks and commit to act inincidents and even accidents may happen on theresponsible way. The commitment has to beplant. The attempt is to keep the number of all

manifested clearly and openly. At state level, therekind of abnormalities to the minimum. And providemust be an up-dated nuclear safety regulation

the plant with the protection systems to assure

assigning clearly the responsibilities of nuclear

safe shut-down, core cooling, monitoring plantutilities, licensing system and the full-powered

conditions and if needed mitigating possible

independent regulatory authority as well as on-radiological consequences to the minimum.

and off- site rescue organization for emergency
1

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Toward Post-Fukushima No. 14

In plant technology, it means high quality,
verifiedbycomprehensive

qualityrescue officials. The utility is obliged to arrange

assurance

immediately an information service for rescue

program, utilizing defense in depth principle,organization, for international collaborators andincluding multiple-barrier protection, redundantfor public needs. Ageing of the plant and thesystems working with diverse principles as far as

knowledge of personnel is an important challenge.

possible. The plant and its system must be tested

That requires special emphases in planning andand analyzed using best estimate and probabilistic

fulfilling maintenance and retraining personnel.

approaches to verify plant behavior in normal,transient and accident conditions to make sure

The leading principle in radiation protectionthat plant meets the preset design objectives. The

with dose limit system is so- called ALARA-

special values of probabilistic analysis are thatprinciple: Radiation doses should be kept “As low

these provide information about the internal

as reasonably achievable”! In practice, it means

reliabilities, revealing possible weaknesses. Thus itneed for optimization. The leading target of

may enhance towards balanced system.

nuclearsafetyshouldbeSAHARA-principle:“Safety as high as reasonably achievable”! It
Modern plant is running utilizing automatedmeans that in addition to the well-proven solutions

systems in normal and into certain extend ineven the latest achievements of science andabnormal conditions also. However, the role ofnuclear technology should be utilized, whereoperator, shift supervisor and safety engineer isapplicable. For improving nuclear safety level, one

crucialessential way is learning, not only from major

inprocedures

unexpectedforsituations.

unexpected

Operation

situations

andaccidents but from minor incidents too, at home.

diagnostic, event based or symptom oriented, aids

Learning

are essential to restoring safe conditions on theconsideration about the need for corresponding

plant. Extreme, natural phenomena, such as

back-fitting of the plant, procedures, operator

storms, earthquakes, tsunamis etc. may cause

training etc. Decisions on what changes are reallyloads exceeding design values of the plant. At least

improvements are often difficult. Probabilistic cost

partly, this problem should be taken as an extra

benefit analyses may be here helpful. However,margin of certain components and systems. Suchwhen it is a question of major accident with a very

components are for example: primary circuit,

low probability and possibility for very severe

especially reactor vessel and containment. Systems,

consequences, the uncertainties of analyzes are too

which need very special analyses and judgment

big for an analytic decision. That is why theare: Diagnostic system for abnormal situations,pragmatic approach is preferable. All practicable

electric power supply, heat removal from primarymeasures to prevent major radioactive leakages

circuit, water supply, ultimate heat sink, pressure

from nuclear power plant must be taken.

shouldbe

followedby

carefulrelief-valves of primary system and environmental
radiation monitoring system.
Responsibility

andauthorization

August 2011
forallprotective and mitigating actions on the plant must
lay on the operating shift. Responsibility forpossible rescue actions off the plant lays on the2Copyright © JAPAN NUS CO., LTD. All Rights Reserved