Probability of failure of a reactor pressure-tube in case of loss of coolant accident

The reliability of a Zr-2 pressure-tube to be installed in a D2O-moderated BLWR is evaluated for the transient following LOCA. Reference is made to regions of pressure-tube wall where end effects and mechanical constraints have a negligible influence. Plastic instability is selected, as the criterion of failure. Overall reliability is the conditional probability (assuming the failure criterion and given a sample of experimental data) that applied stress does not exceed allowable strength in anyway. During LOCA pressure and temperature transients are represented in terms of stochastic processes non-stationary in time. The two processes are postulated statistically independent and their structure is defined according to a model of cumulated effects. On the other hand, since there is the need of considering all possible locations along the tube, mechanical properties and loads are described by means of stochastic processes supposedly stationary in space. The problem of stress vs. strength interference which ensues is simplified through the identification of a critical time instant. Then, overall reliability can be reduced to the calculation of space reliability. In an idealistic two-dimensional case, reliability is approximated by the probability of (zero-) level crossing of the sum of a number of stochastic processes. Finally, the concept of bayesian reliability may be established and a sample of new data (on material properties) can be worked out to calculate posterior distributions