低レベル廃棄物の海上輸送における放射線安全性の評価

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On-board experiments were carried out with a LLW (low level wastes) shipping vessel, the Seiei Maru, in which 2,880 LLW drums were loaded by 360 containers in six holds. The gamma-ray dose-equivalent rates were measured mainly on the hatch covers of the holds and also in the accommodation area with a scintillation survey meter. 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 most of the gamma-ray sources of the LLW were ^<137>Cs and ^<60>Co. The continuous energy Monte Carlo code MCNP 4A with the next event surface crossing estimator was employed to analyze the measured gamma-ray dose-equivalent rates on the hatch covers of the holds. In order to give the the gamma-ray source strength at every container in which 8 LLW drums were contained, sixty 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 homogenized model in which sixty containers were homogenized through the hold of the LLW source region was also taken up to compare the result with the heterogeneous model. 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, the one-dimensional transport code ANISN, and the two-dimensional code DOT 3.5. 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 QAD-CG code is not able to give the source strength and energy spectrum at every container like the MCNP 4A. The Monte Carlo calculations with the heterogeneous model produced fairly good agreement as compared with the measured dose-equivalent rates on the hatch covers. On the other hand, the analysis with the homogenized model overestimated the measured values by a factor of 2.25 to 7.5. The maximum dose-equivalent rate of 0.05 μSv was observed in the navigation room, and the value is 1/36 of the controled value in Japan. The contribution of the sky-shaine could not be estimated by the measurements. Accordinglu, it was estimated by the Monte Carlo calculation. In consequence, the contribution of the sky-shain was approximately 11% of the gama-ray dose-equivalent rate at 50-m-distance from the ship on the quay.
独立行政法人海上技術安全研究所の論文
1996-12-11

低レベル廃棄物の海上輸送における放射線安全性の評価

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