Uncertainty and sensitivity analyses for the reduction factor of sheltering for radiation exposures

放射線被ばくに対する屋内退避の低減係数の不確実性解析と感度解析

廣内 淳 ; 高原 省五  ; 駒ヶ峯 弘志*

Hirouchi, Jun; Takahara, Shogo; Komagamine, Hiroshi*

屋内退避は原子力災害時の放射線被ばくに対する対策の一つである。吸入被ばくに対する屋内退避の効果は、屋内と屋外の累積放射能濃度または線量の比として定義される低減係数で表される。屋内濃度は、主に空気交換率,浸透率,屋内での沈着率に依存する。空気交換率は、風速,家屋の床面積で規格化した隙間面積,総建ぺい率などの周辺環境条件に依存する。本研究では、様々な環境条件下で粒子とIに対する低減係数の不確かさの範囲を検討し、低減係数の不確かさに最も影響を与えるパラメータを把握するために感度解析を行った。不確実性解析の結果から、算出された低減係数は環境条件や住宅の気密性によって大きく変動した。粒子とIの低減係数の不確かさの範囲はそれぞれ最大0.9および0.3であり、新しい家屋ほど小さかった。感度解析の結果、風速は低減係数に最も影響を与えるパラメータであった。また、風速は新しい家屋では低減係数に与える影響が小さかった。

Sheltering is one of the countermeasures against radiation exposure during nuclear accidents. The effectiveness of sheltering for inhalation exposure is often expressed by the reduction factor, which is defined as the ratio of the indoor to the outdoor cumulative radioactivity concentrations or doses. The indoor concentration is mainly controlled by the air exchange rate, penetration factor, and indoor deposition rate. Meanwhile, the air exchange rate depends on surrounding environmental conditions: the wind speed, leakage area normalized by the floor area of the house, and gross building coverage ratio. In this study, the ranges of the uncertainty of the reduction factors for particles and I were investigated under various environmental conditions, and sensitivity analyses were conducted to understand the parameter with the most influence on the uncertainty of the reduction factor. From the results of the uncertainty analyses, the calculated reduction factor was highly variable depending on the environmental condition and the airtightness of the houses. The uncertainty ranges of the reduction factor for particles and I were up to 0.9 and 0.3, respectively, and were smaller for newer houses. From the results of the sensitivity analyses, the wind speed was the most influential parameter on the reduction factor. Additionally, the wind speed was less influential for the reduction factor in newer houses.