Study on reactivity effect of fission products for introducing burnup credit into the criticality safety evaluation of spent nuclear fuel

Okubo, Kiyoshi*; Suyama, Kenya; Uchiyama, Gunzo 

The amount of the spent nuclear fuel (SNF) stored at the nuclear reactor sites is increasing continuously in Japan. To correspond to such a situation, it is considered to take into account the decrease in the reactivity of SNF according to the burnup for the criticality safety control of SNF. This idea is called as burnup credit. If the negative reactivity effect of the fission product nuclides accumulated during the burnup is adopted into the burnup credit which considers only uranium and plutonium, the amount of fuel assembly that can be treated in the same facility is increased. This study reveals the reactivity effect of fission products has almost linear correlation with the increase of burnup SNF for both solution and heterogeneous systems. The negative reactivity effect of the selected fission product is equal to the increase of the burnup of approximately 20-25% for the solution system and 30-35% for the heterogeneous system respectively. It also implies that the estimation error of burnup value of 20% could be acceptable if we take the burnup credit adopting only uranium and plutonium isotopes, considering the fission products as the safety margin.