3D analysis of the IFMIF Test Cell shielding performance using McEnea/MCNP code (Completion of the Analysis of the Test Cell Shielding performance. Comparison with results obtained with different methods)

In the frame of the International Fusion Materials Irradiation Facility (IFMIF) project the ENEA- Bologna group was committed to evaluate the shielding capability of IFMIF Test Cell and surrounding areas. Full 3D n-γ transport calculations have been performed by Monte Carlo MCNP codes in order to obtain the dose rates outside the frontal Test Cell shielding wall and in the Access/Maintenance room. The reference shielding material of the Test Cell walls was assumed to be heavy concrete. The ENEA-Bologna group has developed a new neutron source model, McEnea, based on the measurements of the neutron emission spectra in Li(d,n) reactions for 40 MeV deuterons performed at the “Cyclotron and Radioisotope Center (CYRIC)”, Tohoku University , Japan. The results of McEnea/MCNP calculations were compared with those obtained using other neutron sources, i.e. Oyama, McDeli and MCNPX-2.5b internal model. The results published by FZK group related to the coupled 3D Monte Carlo/discrete ordinates deterministic SN approach with McDelicious neutron source model were also analysed. The assessment shows how the definition of a reliable neutron source and a proper 3D computational approach to treat the radiation transport in concrete have a relevant impact on the response functions, as the dose rates. The following conclusions can be drawn: • High energy neutrons, over 40 MeV, have a significant importance for shielding behaviour (90% of the total dose). • The McEnea neutron source model, produced on the basis of experimental data should answer quite well on reproducing IFMIF experimental conditions. Furthermore, the appropriateness of the Monte Carlo MCNP code to treat physical phenomena of neutrons and gamma interactions in concrete gives enough confidence that the McEnea/MCNP approach is suitable to perform neutron shielding calculation for IFMIF plant. • The thickness of the heavy concrete shielding frontal wall of 400 cm is enough to keep dose rate in operating areas below the value of 10µSv/h. • Decay gamma sources do not significantly contribute to dose rates evaluated at the beam shutdown, outside the frontal test cell shielding wall and in the Access/Maintenance room behind a wall of 400 cm of heavy concrete (2µSv/h at 300 cm and 0.003µSv/h at 400 cm).