Some exploratory calculation have been performed, with ICARE/CATHARE V2 code (based on CATHARE2 V2.5 Mod7.1), to study the thermal behaviour of an irradiated fuel assembly, consequent to an instantaneous uncovery, during unload operations. Two situations have been analysed :
Partial and instantaneous draining of the pool, during the positioning of the irradiated fuel, in the spent fuel building (BK). The water level remains sufficient to assure the cooling of the spent fuel stored in the pool while the unloaded assembly, suspended above, will be exposed to the building atmosphere.
Instantaneous draining of the refuelling cavity in the reactor building (BR), during the handling of the irradiated fuel assembly inside the refuelling machine (MC).
The behaviour of the building atmosphere (BK or BR) is not simulated and, a constant composition (100% air) and temperature (50°C), has been assumed during the analysed transient. The buoyancy forces, leading to an up-ward air flow inside the fuel assembly and refuelling machine (chimney effect), have been considered imposing a constant pressure difference between the bottom and the top, simulating in this way, the progressive increase of the air flow rate as a function of the fuel assembly heating. The 3D model of the code has been activated to calculate the air behaviour inside and in the periphery of the fuel assembly.
The chemical interaction, between air and fuel claddings, has been taken into account with a preliminary model based on the existing literature data and assuming a parabolic behaviour for the reaction kinetics.
The performed exploratory calculations, demonstrate the flexibility of ICARE/CATHARE that can be used, without major difficulties, beyond its usual application field. However, to obtain a realistic description of this kind of accidents, a validation of the code models (heat transfer, hydraulic), against more specific codes and experimental data, is required. Moreover, the preliminary Zr-air oxidation model must be improved to take into account possible deviations from parabolic behaviour (break-away) and the role of nitrogen : formation of ZrN and consequent interaction with oxygen. Data for model validation are expected in the framework of MOZART experimental program carried-out by IRSN.
The application of ICARE/CATHARE to the simulation of a complete draining of spent fuel pools (fuel building or re-processing facilities), leading all stored assemblies to be uncovered, is under investigation.