Ways to Solve the Safe Operation of VVER-1000 Reactors in Case of Loss of Original Rod Cluster Control Assemblies Supply

Abstract The article briefly presents the results and experience of lifetime extension of operating RCCAs manufactured in RF. It is shown that by reducing conservatism, until the criteria for maintaining the integrity of the fuel cladding are achieved, it is possible to extend the lifetime of almost all RCCAs currently operating at Ukrainian NPPs from 25500 to 38000 hours in the control group and from 75600 to 113500 hours in the shutdown groups. However, the substantiation should be carried out for each RCCA separately, since each one has its own individual history of operation in the reactor core. The main part of the article is devoted to the ways of increasing the efficiency and extension of RCCA lifetime, and to the development of control rods with improved characteristics. Among the proposed absorbers, the most acceptable neutron absorption efficiency is provided by metallic hafnium with an absorbing part diameter of 8.2 mm (without a cladding) and Dy2O3·HfO2 in the pellet version of filling the lower part of the control rod (up to 300 mm). In the upper part of the rod (from 300mm to 3500mm), Eu2O3·HfO2 composition can be used instead of B4C. Europium hafnate with a density of 8.0 g/cm3 and a tablet filling (6.85 mm) will exhibit an initial neutron absorption efficiency of 95.6% of B4C. The high neutron absorption efficiency will make it possible to use less efficient (n,y) absorbers with extended lifetime (Dy2O3·HfO2, Hf) with a length of more than 300 mm in the lower part without violating the neutron-physical criteria for the performance of the control rod.