The state of the reactor vessel steel embrittlement as a result of neutron irradiation is reflected by its increase in ductile-brittle transition temperature (DBTT) in fracture toughness versus temperature curve. Higher DBTT implies a decrease in fracture toughness and an increase in the chance of vessel fracture in brittle fracture mode. The extent of degradation that the High Flux Isotope Reactor (HFIR) vessel has experienced is characterized by its probability of fracture that is defined as a probability count of the number of critical cracks in the reactor vessel based on a distribution of possible cracks. In this paper, the fracture probabilities under the accident pressure conditions against possible HFIR operating life are calculated for the safety analysis of the reactor vessel. Conventional methods of fracture probability calculation such as that adopted by the NRC-sponsored PRAISE CODE and the FAVOR CODE developed in this Laboratory are based on Monte Carlo simulation. Heavy computations are required. The present calculations are based on a new method of fracture probability calculation that was developed by applying direct probability integration [1]. This method offers simple and expedient procedure to obtain numerical values of fracture probability yet retains all possible features that a Monte Carlo simulation can possibly accomplish.

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