On-Board Experiment and Monte Carlo Analysis of Dose-Equivalent Rate Distribution in the LLW Shipping Vessel \"Seiei Maru\"

On-board experiments were carried out in a LL W ( low level radioactive wastes ) shipping vessel, the Seiei Maru, in which 2,880 LLW drums were loaded by 360 containers in six holds, and gamma-ray dose-equivalent rates were measured mainly on the hatch covers of the holds and also in the accommodation area. Furthermore, the gamma-ray doses were measured to estimate the contribution of the sky-shine on the quay when the Seiei Maru entered Mutsu Ogawara port. The continuous energy Monte Carlo code MCNP 4A (Briesmeister, 1993) was employed to analyze the measured gamma-ray dose-equivalent rates on the hatch covers of the holds. In order to give the gamma-ray source strength at every container in which 8 LLW drums were housed, 60 containers in a hold were modeled at every container in the present Monte Carlo analysis. This detailed model is called the \"heterogeneous model\" in this study. The homogeneous model in which 60 containers were homogenized through the hold of the LL W source region, was also taken up to compare the result with the heterogeneous model in the Monte Carlo analysis. The gamma-ray source strength was uniformly distributed in the homogenized region. The homogeneous model can be employed in the point kernel code QAD-CG (Hopkins, 1971}, the one-dimensional transport code ANISN (Engle, et al., 1967), and the two-dimensional code DOT 3.5 (Rhodes, et al., 1973). However, the heterogeneous model is difficult to calculate exactly except by the Monte Carlo method. In the QAD-CG code, the source geometry is able to model as precisely as the MCNP 4A, but the QADCG cannot give the source strength and energy spectrum at every container like the MCNP 4A