Accurate resonance absorption calculations including 2-D effects and their representation in multigroup lattice physics codes

Abstract

Recent extensions of accurate transport theory calculations of resonance absorption in reactor unit cells by finite-element techniques to a 2-D description of the cell are described. Although very time consuming, they are now a feasible alternative for benchmark calculations, in particular when applied to limited energy ranges. They have provided independent evidence for the validity of transport treatments based on a 1-D model with suitable boundary conditions at the outer unit cell boundary. The latter remain as the principal technique against which the procedures for handling resonance absorption in multigroup lattice physics codes can be checked. An improved equivalence relation between heterogeneous and homogeneous assemblies, due to Y. A. Chao, has been applied to evaluate the resonance shielding in a multigroup code in the MUFT group structure. Examples are given of the extent to which the accuracy of the calculations is improved, by comparison with benchmark calculations. Inevitable shortcomings in the treatment of resonance absorption in multigroup lattice physics codes are discussed. These are similar in nature to the problems which arise in conventional methods of fuel assembly homogenization as a preliminary to multidimensional few-group flux and power-distribution calculations in full reactor cores. In the same way, in which discontinuity factors can be defined rigorously and applied with good accuracy in practice, to overcome these difficulties, group correction factors can be used for the resonance absorption problem. Their definition involves the flux calculated by the multigroup code in the preservation of resonance reaction rates resulting from benchmark calculations, and not the flux which is obtained in the benchmark transport theory code. A description is given of a practical way for evaluating the group correction factors to be applied to the multigroup code resonance treatment, so that they can be used subsequently in rapid applications of the code. The corrections account implicitly for the approximations introduced by intermediate resonance theory and equivalence theory and, in particular, also for the neglect of interference effects between the resonances belonging to different nuclides.