Physics design of 2Â MW upgraded Apsara research reactor

This paper describes the physics design calculations for equilibrium core of a proposed 2 MW upgraded Apsara reactor. As a part of upgrading, the existing highly enriched uranium (HEU) fuelled core is planned to be replaced with low enriched uranium (LEU) fuelled core in order to enhance the design safety features and the maximum available thermal neutron flux of the reactor. In existing Apsara, the maximum thermal neutron flux level is about 1 × 1013 n/cm2/s at designed power (1 MW) whereas in upgraded version, flux will be 6.1 × 1013 n/cm2/s. The design of the proposed LEU fuel assembly is similar to existing HEU fuel assembly except the fact that U3Si2-Al dispersion type fuel with low enrichment (17 wt.%) is adopted in the fuel design. Existing Apsara core of 7 × 7 array of fuel assemblies with 77 mm lattice pitch will be modified to 8 × 8 array with 79.7 mm lattice pitch, in which active core will be placed at the centre in a 4 × 4 array and remaining positions will be filled up by BeO reflector elements, irradiation positions, etc. Calculation strategy is first validated against benchmark problems describing similar kind of core conversion and then applied to present core.