Document: NRC Regulatory Guide
Document ID: da2e0703-3488-44b0-b6d0-089aac7cae3d
Document Type: regulatory_guide
Title: Format and Content of Plant-Specific Pressurized Thermal Shock Safety Analysis Reports for Pressurized Water Reactors
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740028.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.154
CFR Part: 
CFR Title: 

Content:
manner. Failure of a component to perform its required function could lead to PTS considerations. Thus, it is necessary to assign a failure and successful operation probability for each component on a per-demand basis. These probabilities can be obtained by estimating the number of failures observed within a period of time, combined with an estimate of the number o f demands expected within that same period, or by developing fault trees. If no failures have been observed and no other information is available with which to estimate a fai 1 ure-on-demand probabi 1 i ty , a standard statistical method can be used to develop a probability. As with all event trees, the probability associated with a particular branch is conditional on the prior branches in the sequence. Questions of conditional probabi 1 i ty should be careful ly considered before a fai 1 ure probabi 1 i ty i s assigned. The potential for coupled or common cause failures within a system or between systems should be examined in the analysis. Careful consideration should be given t o increasing the f a i l u r e potential o f a component, given the f a i l u r e o f one o r more components o f the same type i n the same system o r i n other systems being subjected t o the same environment o r f a u l t causes. As additional components o f a p a r t i c u l a r type are postulated t o f a i l , the proba- b i l i t y f o r the next component o f the same type t o f a i l should increase. Based on the ORNL analysis, a simplified approach would be t o assume t h a t the f a i l u r e p r o b a b i l i t y o f the second component, given t h a t the f i r s t component has f a i l e d , might be as high as 0.1. The t h i r d component might be assumed t o f a i l w i t h a 0.3 probability, given the f a i l u r e o f two identical components. One could then assume that, a f t e r the f a i l u r e o f three components o f the same type, a l l remaining components o f t h a t type i n the same o r i n other