Civil Engineering Reference
In-Depth Information
6.1 General Procedure of a Probabilistic Risk Analysis
The first comprehensive study performed to determine the risk of LWRs by
probabilistic methods, the US Reactor Safety Study WASH-1400 [
1
], was
published in 1975. Similar studies were performed later also in other countries,
such as Germany [
2
].
The risk, R
m,i
attributed to a type i accident initiated by a type m event (e.g., leak
in a primary coolant pipe) in a reactor plant can be described in a simplified way by
this relationship:
R
m
,
i
¼
F
m
,
i
DC
m
,
i
ð
Þ
where
-F
m,i
is the annual frequency of occurrence of a type i reactor accident initiated by
a type m event,
-C
m,i
is the amount of radioactive material, expressed in Bq, released into the
environment from the reactor outer containment during a type i accident initiated
by a type m event,
- D is the damage to the population resulting from the release C
m,i
of radioactivity.
D depends on a number of other environmental parameters, such as atmospheric
conditions, population distribution, etc. (Figs.
6.1
and
6.2
).
The annual frequency, F
m,i
, of occurrence of an accident is determined in
detailed probabilistic analyses applying event tree and fault tree methods
[
1
-
7
]. In those studies, the failure probability of all relevant components of a safety
system is taken into account (Fig.
6.2
). In determining the radioactivity release,
C
m,i
, the sequence of accident events must be assessed as a function of time in the
reactor core, the pressure vessel, and the surrounding containment. This then results
in the radioactivity, C
m,i
, (fission products, activation products and actinides),
released into the environment from the containment after containment failure.
Subsequently, meteorological data and models of atmospheric diffusion and aerosol
deposition are used to determine the radioactivity concentration and the radiation
dose to which individuals in the environment of the plant are exposed, counter-
measures being taken into account. Finally, health physics data (Chap.
4
) are used
to determine the probability of disability or death as a result of the exposure dose.
6.2 Event Tree Method
An accident sequence is started by an initiating event, e.g., a leak in a pipe in the
primary coolant system. The safety system of the reactor reacts to this initiating
event, and the consequences of the sequence of accident events are controlled,
provided that the safety system functions sufficiently well.
