Modeling of the SB-CVD Process Used for TRISO Coated Fuels Fabrication

For a few years, AREVA and the Commissariat a` l’Energie Atomique (CEA) have been conducting an extensive R&D program on V/HTR fuels with the objective is to optimize the TRISO fuel coatings produced in a Spouted Bed Chemical Vapor Deposition (SB-CVD) reactor. Numerical simulation models of this SB-CVD process have been developed in this work, describing physical and chemical phenomena occurring in high temperature spouted bed reactors. These models have been used to link external operating conditions (gas flow rate, precursor concentration, temperature, etc.) to local deposition conditions (concentration and temperature fields, deposition rate profiles, etc...). The adopted strategy has been to develop simplified models based on a process engineering approach, which require low computational efforts but can handle complex chemical systems and provide relatively accurate predictions. A model based on a hydrodynamic stream tube formulation (pseudo 2D model) and including a complete description of heat and mass transfer has thus been developed. Bed hydrodynamic has been described using high temperature correlations developed in the frame of this work. Radiation and heat transfer at reactor walls, which are of key importance for an accurate description of coupled transfer phenomena, have been implemented in the model formulation. The heterogeneous and homogeneous chemical mechanisms involved in the SB-CVD process have been first selected from the literature, then developed and reduced according to the main reaction paths. The pseudo 2D model developed and the specific high temperature correlations used have been validated by in-situ pressure and temperature profile measurement in 2^″ and 3^″ diameter SB-CVD reactors and finally by measurements of deposition rates on coated ZrO₂ and UO₂ kernels. The results presented in this paper show that the model is capable of handling rather large chemical schemes and combines simplicity and relatively good accuracy; hence it can be used for preliminary design and optimization of HTR fuel coating fabrication. Calculations have shown that particular attention must be paid to the heat transfer description in high temperature spouted bed reactors. A forthcoming work will focus on further model validation by varying the experimental conditions and using different SB-CVD furnace sizes and configurations. In addition further analyses and optimization studies of the chemical mechanisms involved are planned, which aim to increase the model accuracy and reliability. A better understanding of the SB-CVD process through accurate modeling will be very helpful for the optimization of coating deposition parameters on an industrial scale and for the design and scale up of large SB-CVD reactors.