A method for deriving reactor operating limits using probabilistic fracture mechanics

A light water reactor is controlled so that it remains within permitted zones on a pressure-temperature operating diagram. The line bounding the prohibited area of possible brittle fracture is related to the material nilductility transition temperature which increases during reactor life because of the effect of irradiation. There is an increasing incentive to quantify the implied margins of safety and to establish a rational basis for the brittle fracture boundary. A method is proposed to derive a new brittle fracture boundary, based on fracture mechanics, which relates reactor operation to a defined level of vessel failure probability. The objective of the method is to derive lines of constant failure probability on the pressure-temperature operating diagram. Central to the derivation of the new boundary is the calculation of vessel failure probability which is, in our case, based on statistical distributions of defect depth and fracture toughness. Failure probability is calculated as a function of time during an operating transient, and families of pressure-temperature curves for given values of failure probability constructed for particular times during the vessel life. Before a probabilistic brittle fracture boundary can be used to define plant operation, agreement must be reached on the level of risk which is generally acceptable. The method proposed is general, and improvements and modifications to the data and theoretical p procedures will strengthen the case for the introduction of operating limits which are related to a defined level of risk. (author)