Civil Engineering Reference
In-Depth Information
Chapter 6
Probabilistic Analyses and Risk Studies
Abstract
The first comprehensive study to determine the risk of LWRs by prob-
abilistic methods was the US Reactor Safety Study WASH-1400 in 1975. Similar
studies in other countries, e.g. Germany, followed. The methodology starts with the
event tree analysis followed by the probabilistic analysis. This is continued by an
analysis of the radioactivity release for the different accident sequences. Subse-
quently meteorological data and models for atmospheric diffusion and aerosol
deposition are used to determine the radioactivity concentration and radiation
dose to individuals in the areas around the plant. Countermeasures can be taken,
e.g. evacuation or relocation, to lower the radioactive exposure of the population.
Finally the results of event tree and fault tree analysis for different PWRs and
BWRs (presently operating and more recent (future) designs) are presented. In
addition, the results of reactor risk studies in the USA (WASH-1400) and in
Germany are reported and discussed.
Modern Light Water Reactors (LWRs) have up to four redundant cooling systems,
four redundant emergency cooling systems, four redundant emergency power
supplies etc. as well as diverse emergency power supplies (diesel engines, steam
turbines, gas turbines etc.). Severe core damage can occur when, e.g. three out of
four of all four emergency cooling systems fail.
Failure of all of these redundant safety systems ultimately will cause a core
meltdown accident. In probabilistic risk studies, such a failure of all systems is
assumed conservatively, without accounting for the fact that the operating team
could e.g. repair the failed components in due time.
Experience has shown that any component of specific safety systems can fail
within a given period of time. In that case, event tree diagrams (fault tree analyses)
and assignment of individual probabilities of component failure can be employed to
compute the failure of major single systems and the overall probability of an
accident sequence (probabilistic safety analysis, PSA).
