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HIGH ACCURACY LARGE SCALE MONTE CARLO AND DETERMINISTIC TRANSPORT CALCULATIONS FOR CRITICAL SYSTEMS

The Monte Carlo method with nuclear point data currently offers the possibility for the most accurate description of the neutronics of critical systems, since practically no approximations are required. However, for many problems deterministic methods are applied for performance reasons and the advantage of the absence of statistical uncertainties. In the present paper, the results of high accuracy Monte Carlo and deterministic SN transport calculations with pin cell resolution for two recent benchmark problems, namely the VENUS-7 series of critical experiments with LWRtype lattice and the stationary part of the PWR MOX/UO2 core transient benchmark, are presented. Very good agreement between the results for multiplication constants, reactivity values, and pin-wise fission rate distributions is obtained, thereby demonstrating the adequacy of the SN method including the necessary approximations, like energy group condensation and spatial homogenization over pin cells, to perform large-scale neutronics calculations. Currently, most 3-D neutronics full-core LWR calculations are performed with codes based on the diffusion approximation to the Boltzmann transport equation, using nuclear data homogenized over large spatial zones (typically fuel assembly sections) and collapsed to very few energy groups (typically two), generated in advance by cell codes. With the capabilities of high performance computers available today, it has already become possible to perform such large scale calculations within the SN formulation of the transport theory or similar methods, without the necessity of the diffusion approximation, and a much finer resolution concerning geometry and energy. Nevertheless, to be able to perform these calculations in reasonable computer time, especially for transient problems where a large series of calculations equivalent to stationary problems have to be performed, certain approximations are advantageous. These are the homogenization of cross sections over small spatial zones (typically pin cells) and the condensation to an appropriate number of energy groups. The adequacy of these solution methods should be assessed by comparing the calculation results with measurements and with results obtained with reference calculation methods. Such a reference is the Monte Carlo method with nuclear point data, within which it is possible to model the problem geometry at an arbitrary level of accuracy without a substantial influence on the computing time needed. With modern high performance computers, it is possible to reduce the statistical uncertainties inherent in the Monte Carlo