APPLICATION OF THE STRUCTURAL RELIABILITY METHODS FOR JUSTIFICATION OF PRESSURE REDUCTION OF PERIODIC HYDROSTATIC TEST FOR PRIMARY CIRCUIT OF NPP WWER-1000

摘要

An analysis is formulated to determine how reductions in test pressures change the effectiveness of hydraulic strength tests (HT). The analysis is applied to Series 1000 Water-Water Energetic Reactor (WWER-1000) nuclear power plants (NPP) operating in countries of the former Soviet Union. During periodic HT for these reactors, the applied pressure exceeds the pressure for the normal operation mode (NOM) by a factor of more than 1.5. These HT pressures are among the highest in the nuclear industry. It is desirable to reduce the HT pressure in order to minimize potential damage to equipment due to pressurization during the tests. To justify a reduction in the HT pressure, a quantitative, risk-informed assessment of HT effectiveness for changes in HT pressures has been performed. This assessment follows the guidelines of INSAG-25 [1]. A deterministic analysis is used to calculate the HT minimum temperature, based on the estimation of the damage caused by HT and the evaluation of the warm prestressing effect. A probabilistic analysis is used to estimate the change in the probability of equipment fracture caused by a reduction in the HT pressure. The probabilistic analysis assumes that HT is performed as a destructive control method for replaceable components, so that HT combined with replacement or repair of defective components increases their reliability. A simple probabilistic analysis method, based on an exponent distribution law for defect depth and a lognormal law for the defect aspect ratio, taking into account laws for defect growth, has been proposed. The fracture probability is calculated as the proportion of defects that exceed critical sizes for NOM and HT. Limit load models are used for the determination of the critical size of defects. The variation in reliability is calculated as the difference between fracture probability during NOM after HT at routine and reduced pressures. The calculation results make it possible to analyze the effect of HT pressure reduction for WWER-1000 NPPs.