Application of the discontinuity factor theory to accelerate routine rod worth calculations for modular HTRs

A modular high temperature gas cooled reactor (HTR) is equipped with strong absorbers and many void areas in the control rod regions which are located in the graphitic side reflector. When using diffusion calculations, a special skill is required to homogenize these ex-core absorber regions. These difficulties arise from the fact that these absorber regions do not contain fission sources while the boundaries of the absorber regions are exposed to very strong neutron currents. This paper shows that a mere conventional homogenization technique is not enough to obtain acceptable results. It is shown that a discontinuity factor-corrected diffusion-solution can be found which yields very good, satisfactory results for fast, whole core diffusion calculations. The verification is done by comparing time-consuming transport calculations [SN-codes or multi-group MCNP-codes] with a fast running diffusion code, corrected by the newly proposed homogenization technique.