Reduction of pressure vessel neutron fluence - Industry response to 10CFR50. 61

Certain overcooling transients with overpressurization can result in increased pressure vessel stress fields, which increase the probability of failure. The conditions that contribute to vessel damage in these pressurized thermal shock (PTS) events are vessel embrittlement due to fast neutron irradiation and the existence of flaws on the inside surface of the pressure vessel. On July 23, 1985, the US Nuclear Regulatory Commission (NRC) added the PTS rule 50.61 to the Code of Federal Regulations (CFR) in a section entitled Fracture Toughness Requirements for Protection Against Thermal Shock Events. While most plants have sufficient pressure vessel toughness, several pressurized water reactor plants required a significant reduction of the vessel irradiation rate to meet the requirements of 10CFR50.61. The collective industry response to meet the 10CFR50.61 requirements via flux reduction is discussed.