Monte-Carlo calculation of keff and neutron flux density in fast neutron critical assemblies with multi-group section data

The fast neutron spherical critical assemblies are analyzed by means of the Monte -Carlo method with multi-group section data. The calculated keff are consistent with the experimental results. The neutron flux density versus energy distribution presents a single maximum in the core made up of high enriched uranium and the distribution presents two maximums in the reflecting layer made up of natural uranium. The neutron spectrum hardens as the radius increases in the bare spherical assembly and softens as radius increases in the assembly with reflecting layer.