Low reactivity sodium-void benchmark studies in assemblies with civil and weapon-grade plutonium

Calculational analysis of the reactivity change produced by sodium removal from ZEBRA and BFS assemblies has been performed. The ZEBRA assembly simulated 1000 MW(e) heterogeneous fast power reactor design with decreased sodium-void reactivity effect. The BFS assembly was a full-scaled mock-up of the BN-800 sodium plenum fast reactor design. Both assemblies were charged with a large amount of plutonium of different isotopic composition. The ZEBRA assembly employed a reactor-grade plutonium with comparatively large amount of higher isotopes. BFS plutonium was weapon-grade plutonium with a low content of the higher isotopes. In experiments on both assemblies a large amount of sodium was removed. The experiments have been analysed by Russian both routine and recently developed computer codes. (author)