A modified theta projection model for creep behavior of RPV steel 16MND5

Abstract

During a hypothetical severe accident of light water reactors, the reactor pressure vessel (RPV) could fail due to its creep under the influence of high-temperature corium. Hence, modelling of creep behavior of the RPV is paramount to reactor safety analysis since it predicts the transition point of accident progression from in-vessel to ex-vessel phase. In the present study we proposed a new creep model for the classical French RPV steel 16MND5, which is adapted from the “theta-projection model” and contains all three stages of a creep process. Creep curves are expressed as a function of time with five model parameters θi(i=1−4 and m). A model parameter dataset was constructed by fitting experimental creep curves into this function. To correlate the creep curves for different temperatures and stress loads, we directly interpolate the model’s parameters θi(i=1−4 and m) from this dataset, in contrast to the conventional “theta-projection model” which employs an extra single correlation for each θi(i=1−4 and m), to better accommodate all experimental curves over the wide ranges of temperature and stress loads. We also put a constraint on the trend of the creep strain that it would monotonically increase with temperature and stress load. A good agreement was achieved between each experimental creep curve and corresponding model’s prediction. The widely used time-hardening and strain-hardening models were performing reasonably well in the new method.