10.38. The modeling in reload core design has normally been carried out at different levels of computational sophistication depending on the accuracy required as well as the dimensional representation and geometric detail that are appropriate. However, as computing capability advances, it is likely that levels can be combined with desirable sophistication applied earlier. In the first level, in which feed enrichment, new assembly number (batch size), older fuel reinsertion, and new cycle length are to be deter­mined, a multicycle point reactor model is appropriate. At the next level, a more sophisticated model is needed to develop, by iteration, the detailed loading pattern that is likely to meet licensing requirements. For this pur­pose, a three-dimensional nodal simulator using course-mesh representa­tion (§4.86) is normally used. A number of trial calculations may be needed to assess the power peaking effects of candidate loading patterns. There­fore, some compromise between accuracy and calculational efficiency is appropriate. Core symmetry may be assumed and only a one-fourth or one-eighth core need be considered.

10.39. Finally, to assure compliance with design limits, a detailed fine — mesh model that has been “qualified” by the U. S. Nuclear Regulatory

Commission is used to validate the final reload design. Two-dimensional diffusion theory codes have been used for this purpose for some years, but more recently, comparable results have been obtained by sophisticated nodal codes such as SIMULATE [8], which incorporate three-dimensional fine-mesh pin power reconstruction techniques.