Approaches to Neutronic Calculations to Justify the Lifetime Extension for VVER-1000 Control Rods

The uninterrupted operation of Ukrainian NPPs depends not only on timely delivery of fresh fuel, but also on the supply of a number of other critical core elements. One of these critical elements is control rods, which have their own lifetime and need to be replaced on a regular basis. If the diversification of nuclear fuel suppliers has been ongoing since the early 2000s [1], the diversification of control rods was at the initial stage at the beginning of the full-scale invasion and its completion requires some time to conduct the relevant tests and trial operation in the reactor core. For temporary meeting the NPP need for control rods, it was decided to justify lifetime extension for them by determining the residual lifetime. Control rod lifetime extension is based on the determination of the residual lifetime using an approach that takes into account control rod actual position during all fuel campaigns in which the control rods were operated. Excessive conservatism was laid due to the assumption that the immersion of the 10th working group in the core during all fuel campaigns is 70 %, but the actual position is 85–90 %. To reduce conservatism, it is necessary to take into account the actual position of the emergency control groups during all fuel campaigns in which the control rods were part of the control group that requires a large number of neutronic calculations to determine the neutron fluence and absorbing element burnup (boron-10). The obvious approach to determining the fluence of fast neutrons and boron burnup in absorbing elements is currently the use of the Monte Carlo method, which allows modeling neutron transport without any serious assumptions and approximations of the computational model, and, accordingly, present the most accurate results. The main disadvantage of Monte Carlo codes is that the simulation of complex systems is a long-term computational process, which can become a limiting factor. Considering the need to take into account the actual operation schedule of each control rod, the application of the Monte Carlo method to justify the operability in the lifetime extension for control rods is extremely problematic. The approach to neutronic calculations using the HELIOS code is proposed in the paper in terms of determining the fluence of fast neutrons and burnup of the boron-10 isotope in absorbing elements. The approach is based on the application of the neutron transport solution in two-dimensional geometry based on the method of the first collision probability and the subsequent synthesis of two- and one-dimensional neutron flux density distributions. The implemented approach is characterized by an accuracy comparable to the Monte Carlo method but is much more efficient in terms of computer time spent for calculations, which allows a large amount of calculations to be performed to justify the lifetime extension for each individual control rod, taking into account the real history of operation.