Analyses of heat load in ITER NBI duct and neutron streaming through pressure relief line

Detailed heat load distribution in NBI installed near plasma is the important data for the mechanical and thermo-hydraulic design of the ITER (International Thermonuclear Experimental Reactor) NBI ducts. Three-dimensional Monte Carlo shielding analyses was conducted by MCNP-4C code with FENDL-2 nuclear data library to obtain the distribution. The nuclear heating rate distribution in the NBI duct wall was evaluated by neutron and gamma-ray transport calculations with a neutron source distribution estimated for a fusion power of 500 MW. The surface heat flux of the NBI duct wall was evaluated by photon transport calculations with a photon source distribution due to bremsstrahlung and line radiation for a radiation power of 136 MW. The results exhibit that the nuclear heating rate in the NBI duct wall facing the plasma is 5 - 7 MW/m3 at the first wall surface location. The heating rate in the duct wall hidden from the plasma is nearly equal to the value in the facing wall. However, at the location about 1 m from the first wall surface, the nuclear heating rate in the facing wall is larger by a factor of 2 - 3 than those in the duct wall hidden from the plasma, since the attenuation in the hidden wall is steeper than that in the wall facing the plasma. The surface heat flux at the first wall surface location on the duct wall facing the plasma is 0.17 - 0.18 MW/m2, which is about 7 times larger than the heat flux 0.02 - 0.03 MW/m2 on the wall hidden from the plasma. In the meantime, the surface heat flux about 1 m from the first wall surface location in the duct wall facing the plasma is about 0.02 MW/m2, and that hidden from the plasma is 0.006 - 0.007 MW/m2. The pressure relief lines should be designed so as to suppress the dose rate due to the activation of the rupture disc in the pressure suppression system below 10 μSv/h that is allowed for hands-on access. The neutron streaming effect has been analyzed using a simple code DUCT-III for several design options of the pressure relief lines. The radioactivity of the rupture disc induced by neutrons has been calculated with ACT4 code. The neutron flux distributions in several energy ranges and dose rate distributions after the machine shut-down are estimated along the pressure relief lines. These results clarify that the duct size larger than 1.2 m x 1.2 m in cross section, more than one bends and the first leg longer than 3 m are required to suppress the streaming effect below the allowable level. (author)