THE USE OF MNCP FOR NEUTRONICS CALCULATIONS WITHIN LARGE BUILDINGS OFFUSION FACILITIES

The calculation of nuclear parameters within fusion facilities is comp licated by the complex geometry and large size of the proposed buildin gs housing the reactors. These complications make it impossible to use a single model, or code, to calculate the transport of neutrons from the plasma out into the rest of the building. In this paper, coupling two calculational models is demonstrated in calculating the operationa l dose rates in ITER building. The neutron and gamma fluxes during ope ration are calculated from the plasma region out to the cryostat of th e machine using a two-dimensional discrete ordinates model (the subjec t of a companion paper) whereas a Monte Carlo MCNP model is applied in the rest of the building. In using this coupling approach, numerous j oint Probability Mass Functions (PMFs) for the different phase space v ariables are used and constituted a specially-written source subroutin e that is linked to MCNP. Along with the problem of proper source samp ling and characterization, the physical size of the building, in compa rison to the tally region, drastically complicates the calculation. As a result of this, the use of non-analog techniques are needed to help in the transport of particles in regions far away from the source (wh ich is the NBI duct in this case). The fact that good results were eas ily achieved in the NBI room where there is a direct line of sight to the plasma, but as the detectors are placed further away, the results degenerate, exemplifying the need to use variance reduction techniques . Various techniques in the MCNP calculations are applied and compared and their usefulness is discussed. It is shown that MCNP can be used, in a limited fashion, to focus on specific high importance regions wi thin large buildings such as in ITER. (C) 1998 Elsevier Science S.A. A ll rights reserved.