Steady- and transient-state analysis methodology of fully ceramic microencapsulated fuel via two-temperature homogenized model

Abstract

Recently, fully ceramic microencapsulated (FCM) fuel is proposed as a type of accident tolerant fuel (ATF). The FCM fuel is based on a proven safety philosophy that has been utilized operationally in VHTRs. In the FCM fuel, TRISO particles are randomly dispersed in a SiC matrix. Such a high heterogeneity in com-position gives advantages in terms of safety features of the fuel. However, the heterogeneous composition also leads to difficulty with explicit thermal analyses of such fuels. Therefore, an appropriate homogenization model becomes hn essential. For a fuel element of VHTRs, volumetric-average thermal conductivity model was used in the litera-ture. However, this volumetric-average thermal conductivity model is not conservative in that thus obtained temperature profiles are lower than the real values. Moreover, this model is unable to distinguish fuel-kernel and matrix temperatures. In this study, for a thermal analysis of the FCM fuel with such a high heterogeneity, a two-temperature homogenized model is investigated. The model is obtained by calculation of heterogeneous FCM fuel element using the particle transport Monte Carlo method devised for heat conduction problems. The temperature pro-files obtained by the model are more realistic than those from other models. Moreover, the two-temperature homogenized model can distinguish the fuel-kernel temperature from the SiC matrix temperature. Homogenized parameters used in the model are obtained by (i) matching steady-state analytic solu-tions of the model with the results from a Monte Carlo heat conduction calculation of a heterogeneous FCM fuel by HEATON, and (ii) by preserving total enthalpies in the fuel-kernels and the SiC matrix, respectively. The homogenized parameters have two desirable properties. First, they are insensitive to boundary conditions such as coolant bulk temperatures, thermal properties and thickness of the gap and cladding. Second, they are independent of power density. For validation, the two-temperature homogenized model is compared to the explicit modeling of the FCM fuel element via commercial FEM code package COMSOL at steady- and transient-states. In the model-ing, TRISO particles are randomly distributed in radial direction

however, there is only one axial plane due to memory requirement. By performing aforementioned procedures with temperature-dependent thermal properties of the con-stituent materials of the FCM fuel pellet, temperature-dependent homogenized parameters are obtained. With the parameters thus obtained, single-channel thermal analyses on the FCM fuel element were performed at steady- and transient-states. The results are compared to those of conventional UO2 fuel having the same ge-ometrical configurations but filled with homogeneous UO2 in the pellet in order to ascertain why the FCM fuel is considered as one of ATFs, in terms of operating temperatures. With the parameters thus obtained, coupled with the COREDAX code based on analytic function ex-pansion nodal (AFEN) method for the neutron diffusion model, an FCM fuel-loaded core is also analyzed via the two-temperature homogenized model at steady- and transient-states. The results are compared to those from the harmonic- and volumetric-average thermal conductivity models, i.e., the followings are compared: (i) keff eigenvalues, (ii) power distributions, and (iii) temperature profiles at the hottest single-channel at steady-state, (i) reactor power changes, (ii) reactivity changes, (iii) maximum temperature changes, (iv) average ener-gy deposition at transient-states, which is the key regulatory quantity for the reactivity initiated accident. The different thermal analysis models and the availability of fuel-kernel temperatures in the two-temperature ho-mogenized model for Doppler temperature feedback cause significant differences as revealed by comparison.