Probability Method of Fracture For Reactor Life Estimate

摘要

The state of the reactor vessel steel embrittlement as a result of neutron irradiation is reflected by its increase in ductile-brittle transition temperature (DBIR) in fracture toughness versus temperature curve. The increase in DBTTcan be measured experimentally. 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 can be characterized by its probability of fracture. To ensure a continued operation safety of the HFIR under appropriate vessel pressure and temperature, a periodic hydrostatic tests (hydrotest) is performed. If the fracture probability on performing the hydrotext exceeds a certain imposed value, then the vessel can not be tested hydro-statically and, therefore, it determines the end of the life of the reactor vessel. The conventional method of fracture probability calculations 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 preliminary version of a new method of fracture probability calculation has been developed by direct probability integration and more detail is shown in this paper. The present approach offers simple and expedient method to obtain numerical values of fracture probability and it retains all general features that a Monte Carlo simulation can possibly accomplish. This approach also provides a clear physical meaning on the probability of failure by fracture.