PROBABILISTIC
SAFETY ASSESSMENT:
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The objectives of nuclear
safety In constructing a facility as complex as a nuclear power plant, engineers
must comply with a number of stringent regulations aimed at limiting
the risks inherent in this type of installation, primarily the possible
release of radioactivity. These regulations are applied throughout the
lifetime of the facility, i.e. from the design and construction stages
to the operating phases and final decommissioning. They embody the principal
concern of all those involved with the plant, from construction engineers
to operators or regulators: nuclear safety. Nuclear safety has three objectives, namely to:
Pursuing these objectives enables those concerned to achieve the overall
goal of nuclear safety, namely to protect man and his environment by
limiting the release, under any circumstances, of the radioactive materials
that the facility contains; in other words, ensuring the containment
of radioactive materials.
Nuclear safety management uses two basic strategies to prevent releases
of radioactive materials, notably in the event of an incident:
This analytical procedure has been widely used throughout the world
in the design of nuclear reactors for the purpose of generating electricity.
It attempts to ensure that the various situations, and in particular
accidents, that are considered to be plausible, have been taken into
account, and that the monitoring systems and engineered safety and safeguard
systems will be capable of ensuring the containment of radioactive materials.
The deterministic approach is based on the two principles referred
to earlier: leaktight barriers and the concept of defence-in-depth.
Defence-in-depth consists of taking into account potential equipment
failures and human errors, so that suitable preventive measures may
be applied, and of making provisions for the installation of successive
devices to counter such failures and limit their consequences. It consists
of several successive stages (or levels), hence the term "defence-in-depth":
Some countries make provision for a fourth level of safety consisting
of what are known as ultimate measures, designed to provide protection
against severe conditions under which defences at the three levels described
above prove inadequate. Nuclear facilities are designed so that the risks associated with their
operation are within acceptable limits for both the public and the environment.
There is no precise definition, however, of what constitutes an "acceptable
risk"; it is basically a subjective notion. In its simplest form,
risk denotes the level of uncertainty associated with an individual's
given action. The acceptance of risk is generally governed by the degree
to which it is considered to be relatively improbable and of limited
consequence. In a nuclear facility, as in any industrial plant, risk assessment
distinguishes between the potential hazards that might be encountered
in the absence of any protective measures, and the residual risks that
will still remain despite the measures taken. The problem lies in assessing
the latter, since there is no way of ensuring that they have been completely
eliminated. The concept of event probability and its associated consequences was
rapidly incorporated into safety analysis procedures, by taking account
of the fact that the probability of an accident must be inversely proportional
to the severity of the potential consequences for the public and the
environment. This approach may be represented schematically in a probability/consequence
diagram (known as a "Farmer curve"), which sets out acceptable
and prohibited domains (Figure 3).
The question that the analyst asks himself when performing a risk assessment
is which accident conditions should he take into consideration and to
what level of probability should he pursue his analysis. As the use
of probabilistic risk analysis became more widespread, the safety authorities
asked design engineers to introduce appropriate measures whenever such
analyses indicated that the probability of an event occurring that might
potentially have unacceptable consequences for the public and the environment
was sufficiently high. Thus for example, some safety authorities stipulated that the overall
probability that a nuclear reactor would be the source of an accident
with unacceptable consequences must remain less than 10-6 a year. But
what does this figure actually mean? It means that the theoretical probability
of an event occurring amounts to once in a million years, which is equivalent
to the chances of winning a lottery in which there are 1 million tickets.
PROBABILISTIC SAFETY ASSESSMENT (PSA) The deterministic approach to the design of nuclear reactors was rapidly
supplemented by the development of probabilistic studies, referred to
more commonly as PSAs. Historically, these assessments were originally developed in order
to calculate the probability of external events such as an aircraft
falling onto a given target. PSA techniques were subsequently used to
develop scenarios for hypothetical accidents that might result in severe
core damage, and to estimate the frequency of such accidents. The first
study of this kind carried out in the United States was published in
1975 (Rasmussen report) and provided the first assessment of the potential
risk of core damage for two power reactors. The accident in 1979 at the Three Mile Island plant generated renewed
interest in this type of study. One of the recommendations made after
the accident was that probabilistic analysis techniques should be used
to supplement conventional safety assessment procedures for nuclear
power plants, and that probabilistic objectives should be developed
in order to facilitate the determination of acceptable safety levels
for nuclear facilities. A large number of generic and plant-specific PSA studies (over one
hundred to date) have been carried out or are currently in progress
in those OECD countries currently operating nuclear plants. These studies
are of interest not only in determining the absolute value of the risk
of damage to the reactor core, but also for the information they can
provide about the various components of this risk and their relative
weighting. Lastly, the accident at Chernobyl in 1986 revealed the potential consequences
of failure to manage nuclear power plant safety, and lent greater urgency
to the need to develop PSA applications in the areas of safety management
and accident prevention. PSAs can be used to calculate the probability of damage to the core
as a result of sequences of accidents identified by the study. With the development of this type of analyses, PSAs can now also be
used to assess the size of radioactive releases from the reactor building
in the event of an accident, as well as the impact of such releases
on the public and the environment. These studies are referred to as
level 2 and level 3 PSAs respectively (level 1 corresponding to the
assessment of the risk of a core damage). Level 2 analyses have been
performed, or are planned, in most NEA countries in view of their importance
in determining accident management strategies and identifying potential
design weaknesses in reactor containment buildings. Level 3 analyses
are used for emergency planning. The results of these analyses can therefore identify not only the weaknesses
but also the strengths with regard to the plant's safety, and thus assist
in setting priorities and focusing efforts on the points identified
as the most sensitive in terms of the contribution they can make to
improving the safety of facilities. Indeed, it is this type of assessment
that is most commonly carried out, given that its use as an "analytical
tool" was rapidly recognised as its most important aspect. A PSA is an analysis that is used during both the design and the operating
stages of a nuclear plant to identify and to analyse every possible
situation and sequence of events that might result in severe core damage.
A typical PSA involves:
A PSA generally comprises:
Just as the deterministic approach has its limitations, so too does
probabilistic assessments. These are due to the fact that the results
of a PSA invariably contain uncertainties arising from three main sources:
In view of these uncertainties, the assumptions on which PSAs are based
are designed to ensure sufficient safety margins. It is worth noting
that the uncertainties are not intrinsic to PSAs, but may generally
be attributed to lack of detailed knowledge. Indeed, one of the benefits
of conducting PSAs is that they can identify areas about which we need
to learn more. Despite these uncertainties, the assessment of both the strengths and
the weaknesses of the safety features can clearly suggest ways of improving
both the design and operation of nuclear facilities. Probabilistic safety
analysis has thus become an important supplement to deterministic analysis
in checking the safety level of a facility and improving it by identifying
design weaknesses. In addition to assessing the safety of a plant at
a given point in its lifetime, such applications have also demonstrated
the usefulness of PSAs in other areas and a certain number of programmes
are already being developed which hint at future applications. The development of probabilistic analysis has resulted not only in
an increase in the number of assessments carried out, but also and more
importantly in expansion of their scope of application. A study published
in 1989 by the OECD Nuclear Energy Agency entitled Probabilistic Safety
Assessment in Nuclear Power Plant Management demonstrated the benefits
afforded by PSA in the management of safety in nuclear power plants.
The conclusions set out in the study were based in particular on the
example of one utility that considered the use of PSA to be an integral
part of the daily activities of its organisation. The experts who drafted
this report considered that the use of PSA as an instrument of safety
management in nuclear power plants offers immediate benefits to those
who implement PSA techniques in the design and operation of their plants,
and for all those endeavouring to enhance the safety of nuclear power
plants. According to the authors, the implementation of PSA will reduce
the frequencies of severe incidents and accidents and will thus be of
benefit to the nuclear industry as a whole. A new report published by the Nuclear Energy Agency and entitled Living
Probabilistic Safety Assessment for Nuclear Power Plant Management describes
recent developments in the use of PSA. Recent applications of PSA techniques
have demonstrated their unique ability to assess alternative configurations
or engineering modifications that could be made to existing facilities.
It has thus become apparent that PSA techniques could be successfully
used in the assessment and management of safety-related operations and
of the engineering modifications routinely made to nuclear power plant
systems. In order to keep track of such modifications, however, they
have to be incorporated in a procedure for regularly updating the PSA
model in order to ensure that the latter accurately reflects the current
configuration of the facility. This process constitutes the "living" PSA programme, whose
basic element is a well-structured, well-documented, reviewed, highly
detailed and specific study of the plant. To ensure that this study
remains "living", it is periodically updated to reflect all
relevant plant changes, thereby monitoring the safety level of the plant
over time. While a PSA provides a safety profile of a plant at a given time, a
"living" PSA programme monitors and influences changes in
this safety profile as a function of time. This ability to monitor the
impact of design and procedural changes on the safety profile of the
plant, and to influence changes that improve safety, makes a "living"
PSA programme a powerful tool with which to support decisions that affect
plant safety and to foster understanding between the utility and the
safety authorities. After a period of cautiousness, the performance of PSAs has started
to become more widespread and many applications are pending or have
already been developed. Further development, however, will require a
better understanding of the current limitations of PSA techniques notably
the significant uncertainties that still remain. Countries that are
actively implementing PSAs are currently endeavouring to reduce these
uncertainties by improving their models and the reliability of their
input data. These limitations should not necessarily curb the use of
PSAs, provided that adequate allowance is made for them in the safety
assessment. If PSAs continue to be used by operators, constructors and safety authorities
to assess the design of their installations, then we can safely predict
that their use as an instrument of plant management will increase substantially
for the daily management of nuclear power plants under both normal and
accident conditions. It is in this respect that PSAs meet the overall
objective of nuclear safety and thereby constitute an indispensable
tool for assessment and dialogue between the various actors responsible
for the safety of nuclear installations.
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