WWER-1000 reactors specimens

Abstract This chapter describes the embrittlement processes in WWER reactor pressure vessel (RPV) materials during operation - radiation embrittlement and thermal ageing. Current trend curves for both types of WWER RPV materials are given and explained. Surveillance specimen programmes are shown, as their results are used for RPV integrity and lifetime evaluation. Finally, anneahng of the RPV is proposed as the most efficient mitigation measure. 

Within FP-5 project RETROSPEC work will be done to improve the evaluation of the neutron doses induced in reactor structural materials in those cases where no or unreliable data from surveillance specimens are available (for example the older generation of WWER-440 type reactors). The objectives of this project are to develop procedures and guidelines for retrospective dosimetry which are independent of the type of steel and of the time elapsed after removing the material from the reactor. 

Scheme of a surveillance specimen assembly in WWER-1000/V-320 reactor. 

Research work has been undertaken to determine the dependence of irradiation embrittlement on material chemistry, heat treatment and service factors (irradiation conditions, temperature, coolant chemistry, etc.) in Russian research reactors and WWER-440 reactors. The irradiations were carried out mainly on surveillance specimens irradiated in nuclear power plants in locations where the inlet temperature was 270 °C (for WWER-440). The intent was to provide carefully controlled irradiation conditions in terms of temperature and neutron spectrum. The resulting major conclusion was that a substantial body of data established an irradiation-induced increase of the brittle fracture temperature, (similar to Tnj), of the general form of (CF) (FF) ... 

For plants under construction, a modification of the surveillance programme is considered. Specimen containers will be located in positions representative of vessel wall conditions at Temelin NPP. Based on the fracture mechanics analysis, it was recommended to heat up the accumulator water to 5S°C and to prevent injection of ECCS water with temperatures below 20°C for all the plants. The use of low leakage core loading patterns in WWER-1000 reactors would reduce RPV wall fluence by approximately 30%. It was planned to introduce partial low leakage loading patterns at some plants during 1994 (fuel assemblies with high bumup to be placed at the core periphery). 

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