[en] SHEBA (Solution High Energy Burst Assembly) was built at the Los Alamos National Laboratory, New Mexico, in the 1980s. Both SHEBA-I and -II used uranyl fluoride solution with 4.95% enriched ²³⁵U. The fluoride solution is being replaced by 20% enriched ²³⁵U in nitrate solution. Two concentrations have been considered: 150 and 180 g/ell uranium. Temperature feedback, volumetric feedback, and its internal neutron source are calculated using transport codes and other computer programs