Fusion Programme - ENEA - Fusione

Fission TechnologyTemperature6004002000°2.61 m from thefuel bottom3.49 m from the fuel bottom2.15 m from the fuel bottom00 400 800 1200 1600Distance from system centre followingstraight line at 0° (mm)Fig. 8.44 - Temperatures of the system on the horizontalplane, at different heights from the fuel bottom (steadystate)positioned (fig. 8.43). The 3D geometry was transformed into 2D axialsymmetricgeometry, supported by the code. The modelling adoptedFig. 8.43 - Canister axonometric view allows simulation of partial and instantaneous uncovery of the spent fuelassembly, with water remaining at any level in the pool and cutting theassemblies in two zones: an upper portion, exposed to the building air,and a lower one immersed in water. The behaviour of the fluids (air and water) is not simulated andtherefore heat transfer by convection is imposed as a boundary condition by means of temperature andexchange coefficients. Radiation between the different sheaths is roughly simulated due to the constraintsimposed by the axial-symmetric geometry.A sensitivity analysis on fuel assembly power and water level was performed. The results allow a preliminaryquantification of the importance of the pool water level in reducing the maximum fuel assemblytemperatures and, consequently, to delay or avoid failure of the fuel rod cladding. Figure 8.44 shows thethermal behaviour of the system considering a water level 2.1 m from the fuel bottom, power of 4 kW forthe fuel assemblies in the central canister and 2 kW for the other assemblies.It is worth underlining that i) the current modelling, needing a change of the real geometry to a 2D axialsymmetric geometry, requires complex management of the heat transfer models of the ICARE/CATHAREcode [8.13], which can be done only by experienced users; ii) the results have to be validated against dataavailable and/or coming from dedicated experiments.2007 Progress Report138In accident management, an important measure is to re-flood a degraded core by waterAccident injection in order to mitigate the consequences of a severe accident. As observed in themanagement TMI-2 accident, debris beds may result from quenching of very hot rods during corere–flooding. The coolability of the rods is important if molten pool formation, expansion,and possible relocation in the lower head of the vessel are to be avoided. In addition, a significant hydrogensource may result from debris-bed oxidation at high temperatures.To support the preparation of an experimental programme promoted by IRSN to investigate debris–bedre–flooding phenomena, calculations were performedwith the ICARE/CATHARE V2.1 code under a bilateralagreement with IRSN to analyse debris–bed[8.13] S. Ederli, Dénoyage partiel et instantané d’unepiscine d’entreposage de l’usine de retraitement deLa Hague: Calculs exploratoires avec le code