D-T neutron skyshine experiments and the MCNP analysis

The D-T neutron skyshine experiments were carried out at the Fusion Neutronics Source (FNS) of JAERI with a port at the roof in March 2003. The concrete thickness of the roof and the wall of a FNS target room are 1.15 and 2 m, respectively. The FNS skyshine port with a size of 0.9 x 0.9 m² was open during the experimental period. The source neutron intensity was ∼1.7 x 10¹¹ n/s in those skyshine experiments. The highest total dose rate measured was about 0.5 μSv/h at a distance of 30 m from the D-T target point and the dose rate was attenuated to about 0.1 μSv/h at a distance of 100 m and 0.02 μSv/h at a distance of 550 m. Those experimental results were analyzed by Monte Carlo code MCNP-4C with the nuclear data library JENDL-3.2, where the FNS building and the measurement field were modeled with a simplified cylindrical geometry, and the pine forest was modeled by homogenized cell with a height of 10m. The MCNP calculation agreed well both neutron and secondary gamma-ray dose rate distributions. (author)