Economy of BR2 fuel cycle with gadolinium as burnable absorber

A preliminary feasibility neutronics study has been performed for HEU (UAlx) and LEU (UMo-dispersed) fuels with various combinations of burnable poisons (within-meat and outside fuel meat). Reactivity and experimental performances, control rod motion, cycle length and fuel burn up (GWD/MTU) prior to discharge are compared in order to assess the fuel utilization. Former analyses have been focused on discrete absorbers located outside the fuel meat. Feasible designs using Cd-wires in the aluminum side plates of the BR2 standard fuel element were proposed for the HEU and for the LEU fuel systems. The necessity to sheath the Cd-wires in order to avoid cadmium solubility brings some technical issues, which increases the fabrication costs. This and also the limitations to manufacture very thin wires (if needed) is the reason to investigate within-meat absorbers, similarly to the ones used in the standard BR2 fuel element. The studies presented in this paper show that the economy of the fuel cycle can be significantly improved by using gadolinium poison in a form of homogeneous mixture with the fuel meat. At the same time, the experimental performances for gadolinium are similar as for the standard poisons (boron and samarium) used in the standard BR2 HEU fuel type. The neutronics calculations are performed by the MCNP6 code. (author)