The multipurpose reactor G.A. Siwabessy (RSG-GAS) initially used oxide fuel (U3O8-Al) with a heavy metal density of 2.96 gU/cm3. The initial RSG-GAS core configuration used a typical working core having a fuel loading pattern of 6/1 and 6/2. In this kind of working core, the number of new standard and control fuel elements added and removed from the core varied, including the fuel shuffling pattern in the core, making the core parameters unfixed.

The RSG-GAS core was converted from oxide fuel to silicide fuel (U3Si2-Al), with the same uranium density as an initial step to convert the RSG-GAS from oxide fuel to silicide fuel with a higher density to increase the operating cycle length. The fuel management strategy during the conversion from oxide to silicide fuel was carried out through several mixed oxide and silicide cores using a 5/1 fuel loading pattern to achieve an equilibrium core with all silicide fuel. The determination and calculation of the neutronics parameter of this mixed core were carried out using the inhouse Batan-FUEL and Batan-3DIFF codes.

The conversion process through the mixed oxide-silicide core was completed safely, and the equilibrium silicide core with a density of 2.96 gU/cm3 is still being used by the RSG-GAS to this day. The process of converting oxide fuel to silicide also showed that the neutronics and thermohydraulic parameters of the core did not experience significant changes, hence the RSG-GAS reactor operation and the support system could be maintained.