Document: NRC Regulatory Guide
Document ID: a2b67b51-f5fe-4c86-a879-f0e439601f7f
Document Type: regulatory_guide
Title: Format and Content of Report for Thermal Annealing of Reactor Pressure Vessels
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740052.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.162
CFR Part: 
CFR Title: 

Content:
perties of reactor vessels degrade progressively when exposed to neutron radiation during service, re sulting in a loss in fracture toughness and ductility. To maintain adequate toughness and preclude nonductile failure in the vessel, a number of mitigating actions are taken during the operating life of a reactor: periodic changes are made in the pressure-temperature (P-T) limits required to preclude nonductile fracture of the materials during startup and cooldown, limitations are placed on the reduction of Charpy upper-shelf energy to maintain an adequate margin of safety against duc tile fracture, and additional restrictions are placed on toughness properties by screening criteria imposed to avoid vessel failure from pressurized thermal shock. If neutron radiation embritdement becomes so se vere that the required margins cannot be maintained, 10 CFR 50.61 and Appendix G to 10 CFR Part 50 permit the application of a thermal annealing treat ment to recover the toughness properties of the vessel materials, which would avoid premature retirement of the reactor pressure vessel. Thermal annealing, the heating of the reactor vessel beltline to a temperature well above the operating temperature of the reactor for an extended period of time sufficient to remove the microstructural changes caused by radiation, is the only known method for restoring toughness properties to materials degraded by neutron radiation. The re quirements for conducting thermal annealing and re starting the plant after annealing are set forth in 10 CFR 50.66. Although thermal annealing has not yet been applied to a U.S. commercial power reactor, it has been successfully applied to other reactors. Two reac tor vessels that have been successfully annealed are the Army's SM-1A in 1967 (Ref. 1), and the BR-3 in Mol, Belgium, in 1984 (Ref. 2). Both of these reactors operated at temperatures low enough to permit "wet annealing" at a temperature of 650°F using the reactor coolant pumps as the heat source.