A verification exercise for the AZNHEX code using the CEFR neutronics benchmark and the OpenMC and Instant codes

In 2018, the China Institute of Atomic Energy (Ciae) proposed a benchmark based on the start-up tests of the China Experimental Fast Reactor (CEFR), due to the large amount of experimental data that was obtained and made available for code validation. This benchmark represents an important opportunity for V & V of nuclear codes which, at first hand, could directly benefit the fast reactor community. For this benchmark several codes have been used worldwide; in Mexico for instance, the AZNHEX code was utilized, which is a neutron diffusion solver aimed for hexagonal geometry. Preliminary results showed large deviations and two main reasons were identified: a great influence on leakage phenomena (small core with very high energy neutrons) and scattering effects mainly related to large anisotropic effects. For the simplified core, vacuum boundary conditions (both radial and axial) were considered, and the reference solution was obtained with the Monte Carlo code OpenMC. The nodal multi-group macroscopic cross-sections were also obtained with the help of OpenMC. By means of the diffusion solver AZNHEX and the transport solver Instant, it is shown that scattering effects and a strong leakage dependence of the angular flux account for up to 8000 pcm s of this core configuration. (author)