Analysis of core physics test data and sodium void reactivity worth calculation for MONJU core with ARCADIAN-FBR computer code system

A computer code system ARCADIAN-FBR has been developed by utilizing existing analysis codes and the latest Japanese nuclear data library JENDL-3.3 for the analysis of core characteristics of fast reactors. In order to verify the applicability of ARCADIAN-FBR, the experimental data obtained in the MONJU core physics tests were analysed. In the analysis, the continuous-energy Monte Carlo code MVP in the ARCADIAN-FBR was used and the core characteristics, such as criticality, excess reactivity, isothermal temperature coefficient and control rod worth, were evaluated.The results of analyses indicated that the Monte Carlo code MVP using the nuclear data library JENDL-3.3 can predict the core parameters of MONJU with good accuracy. The validity of MVP code for fast reactor core analysis was confirmed through the verification using MONJU experimental data. As an application of ARCADIAN-FBR for the analysis of fast reactor core, the sodium void reactivity worth, which is an important parameter in the safety analysis of fast reactors was analysed for MONJU core. MONJU was shut down in 1995 and the operation was suspended over 13 years. During the long-term suspension, nearly half of 241Pu changed to 241Am with short decay constant.For the restart of MONJU, a number of initial MOX fuels were replaced by the another MOX fuels which contains higher plutonium enrichment. The sodium void reactivity worth is strongly affected due to the accumulation of 241Am.Therefore, the effect of 241Am accumulation on the sodium void reactivity worth was investigated for MONJU core. Using the Monte Carlo code MVP, the sodium void reactivity worths were calculated for the two MONJU cores, one is the initial designed core without accumulation of 241Amand the other is 241Am accumulated core for the restart. The sodium void reactivity worth was calculated when 100% of sodium within the wrapper tube in the active core was voided. The sodium void reactivity worths for the two MONJU cores is shown. As shown, the sodium void reactivity worth of the 241Am accumulated core is almost twice of the core without 241Am accumulation. The threshold fission cross section and neutron build-up factor of 241Am significantly increase over the 100 keV. Therefore, the spectral effect due to voiding is significantly larger in the 241Am accumulated core compared to the core without 241Am accumulation. As a result of calculation, it was confirmed that the accumulation of 241Am significantly influences on the sodium void reactivity worth and hence on the safety analysis of sodium-cooled fast reactors