Critical mass and dimensions of bare 235U-H2O system

The benchmark experiment of bare 93.7 wt% enriched uranium metal sphere at GODIVA was analysed with a combination of continuous energy Monte Carlo code MCNP-4 and JENDL-3 library. The calculated effective multiplication factor keff was 1.003 ± 0.001. The most progressed combination of code and library was applied to 235U metal in basic geometries without reflector to find the sizes that give the same value of keff. The critical mass of bare 235U metal sphere was found to be 47.3 ± 0.4 kgU (radius 8.44 ± 0.02 cm), the critical cylinder diameter 11.92 ± 0.04 cm and the critical slab thickness 6.40 ± 0.02 cm. Disagreements from the corresponding data in the European and American handbooks were estimated to be originated in slightly different enrichments and reflector conditions. A series of experiments on high enriched uranium nitrate solution in cylindrical tanks conducted at the Rocky Flats Plant were also analysed, and the critical mass of water mixture of 235U was calculated and compared with the corresponding data found in the handbooks. The minimum critical mass was found to be 1.48 ± 0.02 kgU. (author)