The use of MNCP for neutronics calculations within large buildings of fusion facilities

The calculation of nuclear parameters within fusion facilities is complicated by the complex geometry and large size of the proposed buildings 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 operational dose rates in ITER building. The neutron and gamma fluxes during operation are calculated from the plasma region out to the cryostat of the machine using a two-dimensional discrete ordinates model (the subject of a companion paper) whereas a Monte Carlo MCNP model is applied in the rest of the building. In using this coupling approach, numerous joint Probability Mass Functions (PMFs) for the different phase space variables are used and constituted a specially-written source subroutine that is linked to MCNP. Along with the problem of proper source sampling and characterization, the physical size of the building, in comparison 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 (which is the NBI duct in this case). The fact that good results were easily 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 within large buildings such as in ITER.