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:
ean I (or average) "best estimate'' surface RTpTS of 210°F. The staff used a "2-sigma" -L value (spread between "best estimate" and "upper limit") of 60°F;* thus the screening criterion expressed in terms of RTpTS, which, by definition, i s this upper limit value, was selected a t 210 + 60 = 270°F. For axial weld and plate materials, Figure 1 gives a through-wall crack frequency of about 5 x per reactor year a t 210°F, which corresponds w i t h an RTpTS of 270°F. For circum- ferential welds, the same frequency i s believed to be bounded by an RTpTS of approximately 300°F (Ref. 4). The Commission concluded that the PTS-re1 ated risk a t any PWR i s acceptable so long as the RTpTS values remain below the specified screening criterion. I t was realized that there are many unknowns and uncertainties inherent in the probabilistic calculations; thus it was w i t h deliberate intent that conser- vative assumptions such as those stated above were made. The expectation was that the true risk a t any plant due to PTS events would in all likelihood be considerably below that derived from Figure 1 and would therefore be acceptable. Also contributing to the belief that the real PTS risk a t any given plant was lower than that resulting from the analysis in Reference 4 was the belief that many of the generic plant assumptions made i n Reference 4 (e.g., material properties, system performance, crack distribution) would prove to be overcon- servative for analysis of a specific plant and that the resulting plant-specific analysis, when performed, i s likely t o result in a reduced prediction of PTS risk. If the plant-specific PTS analyses submitted by licensees in accordance with § 50.61 using the methodology described in this guide (or acceptable equi- valent methodology) predict that the PTS-related, through-wall crack penetration mean frequency will remain less than 5 x per reactor year for the requested period of continued operation, such operation would be acceptable to