Analysis of the releases produced by CANDU type irradiated fuel bundles accidentally remained in the air

In CANDU type reactors there are two situations for irradiated fuel bundles to remain accidentally in the air. One is represented by End Fitting Failure accident when one or more fuel bundles are ejected from the channels during the fuel handling process. The second one is determined by the uncovering of fuel bundles in the Spent Fuel Pool (SFP) due to a partial or total loss of cooling followed by a progressive evaporation of the water. In both cases there is a potential for the release of radio-nuclides that is dependent on the irradiation history and the duration between unloading of the bundle from the core and the entering into the atmosphere. The paper is devoted to investigate the release in these cases. The inventory at the moment of the accidentally entering into the atmosphere is calculated by ORIGEN code. The source term into the containment, and respectively in the SFP building, is obtained by treating the fuel degradation and the release from the fuel elements by ICARE module of ASTEC code. The most important influence factors on the source term formation (such as the velocity of the ventilation, the residence time) are identified and discussed. In a realistic CANDU SFP loss of cooling accident the potential of release is reduced due to relative fuel low burnup, long residence time, and specific configuration of the storage. The uncovering of the first plane of fuel bundles needs around 15 days for the heat-up and evaporation of water. In such case, a small convection (v ≥ 1.E−05 m/s) is enough to keep the temperature of the claddings under the failure thresholds. The paper identifies and investigates some other cases (with violation of the operational procedures) with a relevant release of fission products. The results are presented for the Caesium element.