The new theoretical method, implemented in APOLLO-2 code as the TIBERE procedure, allows to take into account, the influence of assembly heterogeneity on neutron leakage This method lies on the heterogeneous B1 procedure; however this formalism, in its exact form, should lead to very complex calculations; but, assuming a small number of approximations, it is possible to reduce it to a formalism using only calculation tools very close to those existing in APOLLO-2 It is the purpose of the TIBERE model, which allows to define the leakage cross-sections as additional absorption cross-sections having a space and energy dependence, thus cell-consistent leakage rates can be defined in order to perform the whole core calculation The study of this refined heterogeneous leakage treatment was undertaken because of the insufficiency of the homogeneous leakage model, especially in cases when an assembly contains voided or almost voided zones, so that the streaming effect may become important The fission rate comparisons between the experimental results obtained from the series of experiments EPICURE where the water was substituted, in the central part of reactor, by aluminium in order to simulate void and the results of the whole reactor calculations were accomplished, where leakages were calculated by the classical (homogeneous) procedure and by the TIBERE (heterogeneous) procedure of APOLLO-2 code They prove that the heterogeneous treatment of leakage give results which lie on the whole closer to the experimental values In other words, the classical leakage procedure underestimates the depression in the zone with aluminium, due to streaming This new model could be used, of course, for PWR's with other types of fuel, and also for other types of reactors. (Author
