Kizub P.A., Mitenkova E.F.
Nuclear Safety Institute of the Russian Academy of Sciences, Moscow, Russia
The stability and correctness of neutron source have a special significance in criticality calculations using Monte Carlo methods. In such calculations the correctness of local calculated characteristics, such as neutron reaction rates, directly depends from the steady-state neutron source.
The fission rate and neutron source distributions are considered for systems with dominance ratio close to one, such as loosely coupled systems. For Pin-cell Array with Irradiated Fuel (OECD/NEA benchmark) the stable asymmetric neutron source distributions are obtained. For this system the possibility of obtaining a symmetric distribution is demonstrated using weight windows in MCNP5 code.
For loosely coupled systems is shown that in calculations even with large statistics when providing sustainable neutron source it is still possible obtain incorrect effective neutron multiplication factor. The lowered multiplication factor is obtained for different initial neutron source distributions with 100000 neutrons per generation for non symmetric loosely coupled systems with unequal enrichment in symmetric zones. In these calculations the neutron source distribution remains persistent but is not correct.
The calculations have been performed using the TDMCC and MCNP5 codes.
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