FLUKA study of the actinides induced irradiation damage in Zircaloy-4 cladding during interim dry storage

Among the factors influencing the degradation of the spent nuclear fuel cladding in interim dry stor- age, the irradiation history and the average burnup at discharge must be considered. In fact, due to the pellet-cladding contact particularly affecting high burnup fuels, the inner surface of the cladding becomes increasingly exposed to the damage caused by the alpha decaying actinides present at the rim of the pellet. Moreover, due to the low temperature conditions characteristic of the interim dry storage, ther- mal recombination of the produced defects is not expected to occur. Here, we investigate the irradiation damage accumulated inside an irradiated Zircaloy-4 cladding 32, 55 and 100 years after the end of irra- diation and discharge from the reactor core. The considered cladded UO2 pellet belongs to a Pressurised Water Reactor (PWR) fuel rod consisting of five segments and having an average burnup at discharge of 50.4 GWdt−1 HM . The calculations performed with FLUKA 2021.2.0 Monte Carlo code show that the volume mostly affected by the irradiation damage corresponds to the ZrO2 layer formed between the pellet and the cladding. The actinides which are responsible for the alpha damage are mainly 242 Cm, 244Cm, 241Pu and 238Pu. The recoiling daughter nuclei during the alpha decays produce irradiation damage only within the first μm of oxide layer.