VALIDATION OF A PIN-BY-PIN HETEROGENEOUS METHOD AGAINST LWR MOX BENCHMARKS

The 3-D pin-by-pin model based on the advanced heterogeneous reactor theory was developed in RRC “Kurchatov Institute” for analysis of consequences of reactivity accidents in LWRs. This model implemented in the BARS neutronic code and coupled with the RELAP5/MOD3.2 thermal-hydraulic code allows simulation of a wide range of transients. Previously, the BARS code package (including the UNK code used for neutron database generation) was validated against LWR UO2 numerical and experimental benchmarks. This paper is focused on the validation for MOX-fuelled LWR systems. The UNK validation includes a number of various MOX fuel unit cells available in open literature. The multiplication factor, neutron balance, and different reactivity effects have been investigated. Comparison with calculations performed by Monte Carlo or transport codes shows a good agreement in calculation of the major neutronic parameters of MOX fuel. The spatial (pin-by-pin) power effects were analyzed for the LWR MOX experimental and numerical benchmarks. The BARS calculations were performed using 5-group neutron databases generated by UNK on the basis of ENDF/B-VI. The effective multiplication factor and pin-power distributions were compared against the measurements and other calculations. Pin-power distributions calculated by BARS agree with rather high accuracy with the experimental data or Monte Carlo or SN calculations. The root-mean square deviations do not exceed 1.5%. The validation results show that the BARS code package can be used in calculations of LWR MOX-fuelled systems with a precision adequate for most design purposes.