In accordance with the IAEA guidelines INSAG-25 the general calculation basis for justification of pressure reduction of hydraulic pressure test (HT) for primary and second circuit of 1st and 2nd class equipment of NPP has been developed.

    On its basis in the framework of integrated risk-informed decision-making approach (IRIDM) the deterministic and probabilistic analyzes were carried out.

Deterministic approaches were used to: selection the defining elements; determining the minimum permissible temperature of the hydraulic tests; estimation of the change in the fatigue damage; assessment of warm prestressing effect.

    Probabilistic analysis are performed to quantify the effectiveness of hydraulic tests in general, as well as to estimate the probability of destruction of certain defining elements of the 1st and 2nd NPP circuits for reducing the pressure of hydraulic tests.

    Based on the analysis the technical solutions for reducing HT pressure of the primary and secondary circuits of WWER-1000 and WWER-440 Units have been developed and implemented for all of Ukrainian NPPs.