Advancement of nuclear analysis method for ITER

Advancement of nuclear analysis, which means improvement and validation of radiation transport codes and nuclear data libraries, is a key issue for ITER licensing. We have developed an automatic conversion system from CAD data to MCNP input data in order to easily make precise MCNP input data even for complicated ITER structures. We also performed a narrow slit streaming experiment for analysis validation on streaming effects at JAEA/FNS. We developed an automatic conversion system from three-dimensional CAD drawing data to the Monte Carlo code MCNP input data under the ITER/ITA Task. This system consists of a void creation program (CrtVoid) and a conversion program (GEOMIT) from CAD drawing data to MCNP input data. First CrtVoid creates void region data. CrtVoid automatically divides the overall region to many small cubes, and the void region data can be created in each cube. Next GEOMIT generates surface data for MCNP from CAD data including the void data generated with CrtVoid. These surface data are connected, and cell data for MCNP input data are generated. By using this system, geometry data in MCNP inputs are automatically produced, which leads drastic time- and labor-saving. We applied this system to ITER 40 deg. benchmark model and Japanese test blanket module for ITER. The calculation with the generated input showed adequate neutron flux and nuclear heating distributions. There are slits between the vacuum vessel port walls and the port plugs in ITER, providing possible radiation streaming paths. In order to verify the nuclear analysis for such slit streaming, we have carried out a narrow slit streaming experiment at the Fusion Neutronics Source (FNS) facility in JAEA under the ITER/ITA Task. 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, which was located 20 cm from the D-T neutron source. Fission rates of U²³⁸ and U²³⁵ were measured along the slit with micro-fission chambers. Fission rates were obtained up to 116 cm in depth successfully. This experiment was analyzed by using the MCNP-4C with the nuclear data library FENDL-2.1 and other libraries. The calculation results with MCNP-4C agreed well with the measured ones. It is confirmed that the present ITER nuclear analysis method can also give accurate results for such slit streaming. (author)