On the discrepancies between FIMA and specific burnup

Abstract

Nuclear fuel consumption is most commonly described by using two indicators: Fissions per Initial Metal Atom (FIMA) and energy generated per unit mass of initial heavy metal (GWd/tHM). FIMA indicates physical consumption of nuclides, while burnup is practical for industrial applications and comparisons. Both quantities are used as indicators in fuel cycle research and development studies.In this paper we demonstrate that the two analyzed indicators are not exactly proportional to each other. The exact ratio between these factors is derived to be dependent on the content of nuclear fuel. More precisely, it is a function of the ratio of the average energy generated in a system per fission to the average atomic mass of heavy metal. The predictions are proved by computation tests using the SERPENT code applied to common PWR assembly geometry. For the fuels comprising fissile isotopes of uranium and plutonium, the observed FIMA discrepancies are as big as 1.8% at burnup equal to 750 GW d/tHM. Concerning realistic content of industrial fuels such as UOX, MOX and 233U/Th loadings, the differences of FIMA reach 0.10% at about 48 GW d/tHM. The trends have been well predicted by the approximated theoretical model. The conclusions are meaningful for any comparisons of fuel cycle scenarios and are valid for any burnup calculations.