Evaluation of the primary displacement damage in the neutron irradiated RBMK-1500 graphite

Numerical simulations of the neutron induced primary displacement damage in the RBMK-1500 reactor graphite were performed using the Monte Carlo technique based on the Norgett-Robinson-Torrens (NRT) model. The sequence of the displacement evaluation consists of the reactor core modelling using MCNP6 and GEANT4 codes and output parameters, such as recoil ion and neutron flux distribution used by SRIM2013 and NJOY2016 codes, respectively, to evaluate displacements per atom. The comparisons were made between these codes in order to evaluate the accuracy of each approach. The neutron flux in the spent RBMK-1500 graphite was sufficiently correctly represented by both MCNP6 and GEANT4 models. The comparison between SRIM and GEANT4 codes shows that GEANT4 predicts fewer vacancies by more than 16% than SRIM in the ion energy range from 1 keV to 1 MeV. The neutron energy required to produce recoils due to scattering is around 80 eV. Below this energy displacements occur mostly due to neutron capture reactions. The average displacement rate was found to be around 0.51 displacements per atom per full power year, which corresponds to the 1.36·1014 n/cm2s neutron flux.