D-T neutron streaming experiment simulating narrow gaps in ITER equatorial port

There are gaps between the vacuum vessel port walls and the port plugs in ITER, providing possible radiation streaming paths. The dimensions of the gaps are ∝2 cm in width and ∝200 cm in depth and offset geometries are arranged on the middle of gaps to mitigate the streaming effect. The neutron streaming through gaps have been evaluated in the ITER machine design with analytic tools, such as Monte Carlo code in order to assure low enough radiation dose rates after shutdown for allowing personnel access. In the past, experimental verifications were made, simulating gaps among the blanket modules with about 50 cm depth, but not with full depth to the outside of the vacuum vessel. Therefore, further experimental verification with the full depth of the gaps is necessary. Neutronics experiments with a thin slit assembly simulating the gap between the port wall and the port plug have been conducted using the Fusion Neutronics Source (FNS) of the Japan Atomic Energy Agency under the ITER/ITA Task 73-10. The experimental assembly with a slit of 2 cm in width, 195 cm in depth and 3-cm offset at 56-cm depth from the surface was constructed with iron blocks. The distance from the D-T neutron source to the surface of the assembly was 20 cm. D-T neutrons were generated by 350-keV and 1-mA deuteron beam and tritiated titanium target (0.37 TBq). The intensity was about 10 to the power 11 neutrons/s. In order to evaluate distributions of neutron fluxes along the slit as a function of the depth from the assembly surface, fission rates were measured by U-238 and U-235 micro-fission chambers. The experimental errors of these fission rates were within 10%. This experimental result was analyzed by using the Monte Carlo code MCNP-4C and 3- dimensional Sn code ATTILA with the nuclear data library FENDL-2.1. JENDL-Dosimetry file 91 was adopted as U-238 and U-235 fission cross section data. From our experiment and analysis, the following facts were found: (1) Experimental data of U-238 and U-235 fission rates were obtained up to 116 cm in depth successfully; (2) The calculation results with MCNP-4C agreed well with the measured ones; (3) Preliminary calculation results with ATTILA showed considerable underestimation at the backward region through the offset structure. (orig.)