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1.2.3 Positioning of barriers
To limit the risk of releases, several barriers are
placed between the radioactive substances and the
environment. Barriers must be designed to have a
high degree of reliability and must be monitored to
detect any weaknesses or failures. There are three such
barriers for pressurised water reactors: the fuel cladding,
the boundary of the reactor primary system, and the
containment (see chapter 12).
1.2.4 Deterministic and probabilistic approaches
Postulating theoccurrenceof certainaccidents andverifying
that, thanks to the planned functioning of the equipment,
the consequences of these accidents will remain limited,
is known as a deterministic approach. This approach is
simple to apply in principle and allows an installation
to be designed (and its systems to be sized) with good
safety margins, by using so-called “envelope” cases. The
deterministic approachdoes not, however, lead to a realistic
view of the most probable scenarios and does not rank
risks satisfactorily, since it focuses attention on accidents
studied with pessimistic assumptions.
The deterministic approach therefore needs to be
supplementedby an approach that better reflects possible
accident scenarios in terms of their probability, that is to
say the probabilistic approach used in the “Probabilistic
Safety Assessments” (PSA).
Thus for nuclear power plants, the level 1 Probabilistic
Safety Assessments (PSA) consist in establishing event
trees for each “initiating event” leading to the activation of
a safeguard system (level 3 of defence in depth), defined
by the failure (or the success) of the actions provided for
in the reactor management procedures and the failure
(or correct operation) of the reactor. The probability of
each sequence is then calculatedbased on statistics on the
reliability of systems and on the rate of success of actions
(including data on “human reliability”). Similar sequences
of events that correspond to the same initiating event are
grouped into families,making it possible todetermine the
contribution of each family to the probability of reactor
core meltdown.
Although the PSAs are limited by the uncertainties
concerning the reliability data and the approximations
in themodelling of the facility, they consider a broader set
of accidents than the deterministic assessments and enable
the design resulting fromthe deterministic approach to be
verified and supplemented if necessary. They are therefore
to be used as a complement to deterministic studies and
not as a substitute for them.
The deterministic studies and probabilistic assessments
constitute an essential element in the demonstration of
nuclear safety that addresses equipment internal faults,
internal and external hazards, andplausible combinations
of these events.
To be more precise, the internal faults correspond to
malfunctions, failures or damage to facility equipment,
including as a result of inappropriatehumanaction. Internal
or external hazards correspond to events originating inside
or outside the facility respectively andwhich can call into
question the safety of the facility.
Internal faults include for example:
•
loss of the electrical power supplies or the cooling systems;
•
ejection of a rod cluster control assembly;
•
rupture of a pipe in the primary or secondary system
of a nuclear reactor;
•
reactor emergency shutdown failure.
With regard to internal hazards, the following inparticular
must be considered:
•
flyingprojectiles, notably those resulting fromthe failure
of rotating equipment;
•
pressure equipment failures;
•
collisions and falling loads;
•
explosions;
•
fires;
•
hazardous substance emissions;
•
floods originating within the perimeter of the facility;
•
electromagnetic interference;
•
malicious acts.
Finally, external hazards more specifically comprise:
•
the risks induced by industrial activities and
communication routes, including explosions, hazardous
substance emissions and airplane crashes;
•
earthquakes;
•
lightning and electromagnetic interference;
•
extreme meteorological or climatic conditions;
•
fires;
•
floods originating outside the perimeter of the facility;
•
malicious acts.
1.2.5 Operating experience feedback
Operating Experience Feedback (OEF), which contributes
to defence in depth, is one of the essential safety
management tools. It is based on an organised and
systematic collection and analysis of the signals emitted
by a system. It should enable the acquired experience to
be shared (for implementation of preventive measures
in a structure that learns from past experience). A first
goal of Operating Experience Feedback (OEF) is to
understand, and thus ensure progress in technological
understanding and knowledge of actual operating
practices, so that whenever pertinent, a fresh look can
be taken at the design
1
(technical and documentary).
As OEF is a collective process, a second goal is to share
the resulting knowledge, by memorising and recording
the anomaly, the lessons learned from it and how it
1. Technical and documentary design means all the designs
of the components of the working activity: design of the machine,
its operating procedure, its maintenance, how work is organised
in relation to this machine, etc.
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CHAPTER 02:
PRINCIPLES AND STAKEHOLDERS IN THE REGULATION OF NUCLEAR SAFETY AND RADIATION PROTECTION
ASN report on the state of nuclear safety and radiation protection in France in 2015