An approach is proposed for predicting the change in the reliability of reactor pressure vessels of nuclear power plants. A computer program was used to perform a series of calculations for finding the invariant classification as a function of the degree of stochastic action on a time interval. A stable solution was obtained by modeling the stochastic action with a root mean square deviation of up to 15% of the instantaneous normalized value of the parameter. The proposed approach is not universal, but has the advantages of easily correcting forecast data, avoiding extreme conservatism, and usability at all stages of design, fabrication, assembly, and operation. 7 refs., 1 tab.