Control rod representation and heterogeneity effect in fast reactors

Diffusion theory based computer codes are generally used to design the core of a fast reactor. In these codes the control rods, like the fuel elements, are given an homogeneous representation. Over-estimation of the rod worth comes both from the diffusion approximation and from the homogeneous representation. The latter effect (heterogeneity effect) is much less known than the former. A critical analysis of the available calculation methods of the heterogeneity effect is performed in this work. A new simplified model is developed, based on the calculation of the self-shielding factor in a control rod in the center of a fuel element super-cell. The maximum flux at the cell boundary is used to average the cross sections. It is shown that, although the effective homogeneous cross sections are obtained from a central rod configuration, they can reproduce to within 4% the absorption of a central or a peripheral rod. It is shown too that in a small reactor the coarse-mesh CITATION-type diffusion calculation gives the same result as the transport calculation. In a large reactor an empirical correction factor is needed