Comparative stochastic - deterministic 2D calculations of WWER cores

Progress in computers power and development of complex Monte-Carlo codes make it possible to model neutron transport even in full-scale core models. This is especially useful in case of WWER reactor, since its core in contrast to RBMK has no periodic structure, which can be simulated by more simpler poly lattice. Full-scale Monte-Carlo core model allows to answer many interesting questions. But rises a new ones also. We have used MCU-REA code in 2D calculations of full cores, two of WWER-1000 and one of WWER-440. These problems were solved with modelling 640 millions of neutron histories in each case. Received assembly flux and power distributions can be used as benchmark solutions to test for instance pproximate deterministic coarse-mesh algorithms. Mesh cell one-group characteristics were calculated also with the help of MCU-REA by solving the problem of infinite lattice, consisted of given type assemblies. Apart from average cross-sections surface current-to-flux ratios were also obtained. The latter were used in so-called surface value approach, the plainest formulation of which was proposed in one's time by L.Majorov. Comparison of deterministic solutions with that of MCU-REA shows that coarse-mesh diffusion approach as expected gives quite poor agreement both in Kef and in flux distribution, especially in case of strongly non-uniform flux distributions. As for surface value approach, it produced solutions much closer to exact ones. A bit astonishing is that current-to-flux assembly characteristics, calculated in diffusion approximation with average crossections work nearly as good as those found in infinite lattice Monte-Carlo calculations (Authors)