IMPROVEMENT OF RADIATION EMBRITTLEMENT DEPENDENCES FOR WER-1000 PRESSURE VESSEL MATERIALS ON SERVICE-LIFE EXTENSION

摘要

The results received by now while investigating the sets of surveillance specimens for WER-1000 reactor pressure vessels (RPV) are the most representative for studying and evaluation of radiation embrittlement of materials of this type of RPVs. It is primarily due to the fact that neutron irradiation of surveillance specimens (SS) of WER-1000 RPV materials is carried out with a flux density corresponding to the inner surface of a reactor pressure vessel opposite to the core with the average lead factor for sets of surveillance specimens intended for evaluation of radiation embrittlement varying from 0.5 to 3. On the base of SS research results, standard dependences have been developed and are currently in use for radiation embrittlement of WER-1000 RPV materials with the operation range limited by the value of fast neutron fluence of 6.4 × 10~(23) neutron/m~2 with the energy above 0.5 MeV where the general transition temperature is modelled as a sum of radiation and thermal contributions and the thermal component has a form of a function with the maximum which is hypothetically caused by carbide precipitation and coagulation [1,2]. Microstructure investigations of SS materials performed at the National Research Centre "Kurchatov Institute" after different time exposures failed to prove the hypothesis of carbide hardening under temperature aging and. at the same time, unambiguously showed the existence of significant radiation-and temperature-stimulated phosphoms segregation on grain boundaries and in interphase areas [3-5]. Thereby, there arose a necessity to develop a dependence that would account for the physical processes proceeding in the material under irradiation at the reactor operation temperature. Forecasting of the RPV material condition on service life extension up to 60 years and more requires an expansion of the validity range of the developed dependences to the maximum fast neutron fluence of - 8.0×10~(23) neutron/m~2. With this aim in view, the SS material was subjected to accelerated irradiation to gain higher fast neutron fluences and conservatism of the suggested dependences was verified on their extrapolation. On using the accelerated irradiation results for verification of the forecast conservatism for the developed dependences, account was taken of the effects related to the difference in time and rate of accelerated irradiation as compared to the conditions of RPV wall irradiation.