Influence of beryllium block repositioning on poison level and reactivity

The poisoning of beryllium matrix in the Maria reactor by Li6 and He3 has been analysed for a model case comprising a core fragment containing one row of beryllium blocks. The purpose was to check the idea of reducing the beryllium block poisoning by its cyclic repositioning between the core center and the core periphery. A three dimensional distribution of the isotope has been obtained. calculations have been carried out by the REBUS-3 code with microscopic cross sections taken from a library created by the WIMS-ANL code. The first period of reactor operation, 1975-1985, has been simulated. Afterwards the blocks were repositioned and burnup of the core fragment was simulated to evaluate the influence of beryllium blocks shuffling on the accumulated poison and reactivity. (author)