Justification of RCCAs lifetime extension at operating Ukrainian NPPs. Summary calculations

One of the ways to temporarily meet the needs of NPPs for rod cluster control assemblies (RCCAs) is to extend their lifetime. The justification for extending the lifetime of RCCAs should be based on the justification of mechanical reliability of control rods (CRs) and RCCAs in general, fulfillment of the criteria for physical efficiency of RCCAs, thermotechnical characteristics of RCCAs, taking into account changes in materials characteristics during the entire previous period of operation in the reactor core. In the course of the research, the operational data (RCCAs positions and power unit capacity) for each of the 61 RCCAs of all 13 VVER-1000 power units for the last almost 10 years were analyzed and systematized. The maximum fast neutron flux and 10 B burnup were calculated for each RCCA, and the margin before reaching the limit values was also calculated. The results of the performed research enabled to extend the lifetime of almost all RCCAs in VVER-1000 reactors of Ukrainian NPPs from 25,500 to 38,000 h in the automatic control group and from 75,600 to 113,500 h in the reactor scram group, except for twelve RCCAs from RNPP Unit 4 and six RCCAs from SUNPP Unit 1. An individual (separate) assessment of the lifetime was performed for these RCCAs, throughout which the operational integrity criteria for the maximum value of fast neutron flux in the lower part of CR cladding and 10 B burnup will be met.