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:
ould be noted t h a t under certain conditions a reactor system may be a t a high power level with a low decay heat condition. 3. Location o f event - I n many instances the location o f the event i s defined. For example, an event consisting o f a f a i l e d open turbine bypass valve has the location defined since it i s a specific valve failure. However, f o r some events such as pipe breaks, the location i s not defined and could have an impact on the progression o f the event. I n the case i n which location i s not defined, a l l locations that could be s i g n i f i c a n t should be considered. Each location should then be eliminated by logical argument, bounded by consequences associated with another location, or treated as a separate event. 4. Magnitude o f event - Many o f the i n i t i a t o r s can occur t o various degrees. For example, a LOCA can range from a very small break t o a f u l l g u i l - l o t i n e pipe break. Break sizes should be examined t o i d e n t i f y categories o f sizes t h a t lead t o similar system conditions. I n the case o f the LOCA event, special consideration should be given t o the i d e n t i f i c a t i o n o f break sizes t h a t could lead t o loop flow stagnation. The larger-sized LOCAs t y p i c a l l y do not contribute t o PTS r i s k since the pressure cannot be maintained because o f the large flow out o f the break. 3.2.3 D e f i n i t i o n o f Potential Transients Resulting from Each I n i t i a t o r After the complete set o f s i g n i f i c a n t i n i t i a t o r s has been defined, event trees are required t o i d e n t i f y potential sequences resulting from each i n i t i a - tor. The development o f the event tree headings and branches should be done i n a consistent and logical manner. This was done i n the ORNL studies (Refs. 1, 2, and 3) by using what have been called system state trees. These trees define the potential states o f each plant system o f interest conditional on specific