Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
9.6.4.3. Use for safety issues - examples
Several specific safety issues have been merging for the past ten years. Most of them
have been assessed with the help of PSA among other aspects. Here are some examples.
Shutdown risks
In the late eighties, both deterministic studies (based on operational events) and PSAs
raised the importance of risk during shutdown phases of PWR plants. In France, the PSA
results of 900 and 1300 MW plant series justified implementation of several
modifications in that field: automatic water make-up systems (using ECCS pumps) and
anti-dilution devices (against heterogeneous dilution) have been added to the plant, EOPs
have been completed in these states. Then, specific and detailed models dedicated to
shutdown states have been issued to demonstrate the efficiency of these measures. Now,
these models are being integrated in the renewed versions of the reference PSA models.
Combustible gas control
As part of the US NRC staff’s program to risk-inform the technical requirements of 10
CFR Part 50, the staff identified 10 CFR 50.44, "Standards for Combustible Gas Control
System in Light-Water-Cooled Power Reactors," as a regulation that warranted revision.
The NRC completed a feasibility study that evaluated the combustible gas control
requirements against risk insights from NUREG-1150 and the IPE program. The study
concluded that combustible gases generated from design basis accidents were not
risk-significant for any LWR containment types. Specifically, combustible gas generated
from severe accidents was not risk-significant for boiling water reactor (BWR) Mark I
and II containments, provided that the inerted atmosphere was maintained; for BWR
Mark III and pressurized water reactor (PWR) ice-condenser containments, provided that
the required igniter systems were operational, or for PWR large dry containment because
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