Combined BC/MD approach to the evaluation of damage from fast neutrons and its implementation for beryllium irradiation in a fusion reactor

摘要

The determination of primary damage production efficiency in metals irradiated with fast neutrons is a complex problem. Typically, the majority of atoms are displaced from their lattice positions not by neutrons themselves, but by energetic primary recoils, that can produce both single Frenkel pairs and dense localized cascades. Though a number of codes are available for the calculation of displacement damage from fast ions, they commonly use binary collision (BC) approximation, which is unreliable for dense cascades and thus tend to overestimate the number of created displacements. In order to amend the radiation damage predictions, this work suggests a combined approach, where the BC approximation is used for counting single Frenkel pairs only, whereas the secondary recoils able to produce localized dense cascades are stored for later processing, but not followed explicitly. The displacement production in dense cascades is then determined independently from molecular dynamics (MD) simulations. Combining contributions from different calculations, one gets the total number of displacements created by particular neutron spectrum. The approach is applied here to the case of beryllium irradiation in a fusion reactor. Using a relevant calculated energy spectrum of primary knocked-on atoms (PKAs), it is demonstrated that more than a half of the primary point defects (similar to 150/PKA) is produced by low-energy recoils in the form of single Frenkel pairs. The contribution to the damage from the dense cascades as predicted using the mixed BC/MD scheme, i.e. similar to 110/PKA, is remarkably lower than the value deduced from uncorrected SRIM calculations (similar to 145/PKA), so that in the studied case SRIM tends to over-predict the total primary damage level.