A study of the potential influence of frame coolant on HCLL-TBM nuclear response

Within the European Fusion Technology Programme, the Helium-Cooled Lithium Lead (HCLL) breeding blanket concept is one of the two EU lines to be developed for a long term fusion reactor, in particular with the aim of manufacturing a test blanket module (TBM) to be implemented in ITER. The HCLL-TBM is foreseen to be located in an ITER equatorial port, being housed inside a steel-supporting frame, actively cooled by pressurized water. This supporting frame has been designed to house two different TBMs providing two cavities separated by a dividing plate 20 cm thick. As the nuclear response of HCLL-TBM could vary with the supporting frame configuration and composition, a parametric study has been launched to investigate such an influence. Previous works dealt with the dependence of the nuclear response of HCLL-TBM on the configuration of an homogeneous frame while the present one has been focussed on the investigation of the influence of coolant distribution within the frame. A detailed parametric study of HCLL-TBM nuclear response has been performed by means of 3D-Monte Carlo neutronic and photonic analyses, simulating the frame in a semi-heterogeneous way. Three-dimensional heterogeneous models of HCLL-TBM and of the supporting frame have been set-up considering both the usual poloidal lay-out and a toroidal one and taking into account 9% Cr martensitic steel (EUROFER) as structural material. The models have been inserted into an existing 3D semi-heterogeneous ITER-FEAT one, simulating realistically the reactor lay-out up to the cryostat and providing for a proper D-T neutron source. The analyses have been performed by means of the MCNP-4C code, running a large number of histories (2·108) in such a way that results obtained are affected by statistical uncertainties lower than 1%. The results obtained are reported and critically discussed.