Extension of double heterogeneity treatment method for coated TRISO fuel particles

This paper extends the methodology for treating the resonance self-shielding effect of doubly heterogeneous (DH) tristructural-isotropic fuel particles to take the coating layers of fuel grains into account. Due to the two-region assumption of fuel kernel and graphite matrix in the original derivation of Williams??? method, which was developed to replace the time consuming SCALE continuous energy spectrum calculations with more efficient equivalence theory and intermediate resonance approximation, the method can be applied only to the fuel kernels without coating layers. In order to overcome this limitation, Ji???s analytical expression of the Dancoff factor for multi-layer spheres has been incorporated into the DH method and the applicability has been successfully extended to the kernels with coating layers. It is demonstrated that the new method can predict the DH effects within 200 pcm order of errors against the reference Monte Carlo calculations for wide range of block type gas cooled reactor designs with coating layers.clos