Performance evaluation for rapid-dose estimation of radioactive plume dispersion based on pre-simulation database of wind conditions by large-eddy simulation

Large-eddy simulationによる風況の事前計算データベースに基づく放射性物質拡散の迅速推定手法の性能評価

佐藤 拓人  ; 中山 浩成   ; 佐藤 大樹   

Sato, Takuto; Nakayama, Hiromasa; Satoh, Daiki

本研究では、Nakayama et al. (2021)の手法に基づき、Large-eddy simulationモデルによる事前計算データベース(pre-sim DB)と現地観測を組み合わせ、原子力施設周辺における放射性プルームの拡散を迅速にモニタする実用的なフレームワークを開発した。提案手法の風況の再現性と大気拡散解析の性能を調査するため、pre-sim DBを用いてスタック周辺のモニタリングポストにおける線量率を評価し、観測値と比較した。pre-sim DBを用いる手法は、モニタリングポストにおける空間線量率の時間変化をよく再現できたものの、いくつかのピークを過大評価した。特に風の鉛直シアが顕著な場合、pre-sim DBを用いる手法の結果には顕著な誤差が見られた。これは、pre-sim DBを用いる手法が単一の観測点における値を用いて流れ場の復元を行うために、風の鉛直シアを取り込めないことが原因と考えられる。複雑な風況の再現性に関して手法的な限界は見つかったものの、Large-eddy simulationモデルを用いた非定常計算と比べて低い計算コストで再現が可能なことから、pre-sim DBを用いる手法は、線量率の迅速なシミュレーションに対して有用なツールとなりうるだろう。

We developed a framework for rapid monitoring of radioactive plumes in the vicinity of nuclear facilities based on a quick and practical high-resolution atmospheric dispersion simulation method that combines a large-eddy simulation (LES) model pre-simulation database (pre-sim DB) of wind conditions and onsite meteorological observation results, as proposed by the previous study. However, this framework was not quantitatively demonstrated using measurement data. In this study, we evaluated the performance of the wind condition reproduction and plume dispersion analysis methods. Air dose rates observed at monitoring posts around the stack were compared with the values reproduced by the method using the pre-sim DB, and the reproducibility of both air dose rate and flow field was discussed. The pre-sim DB-based method successfully captured the temporal variation of air dose rates at the monitoring posts, though it tended to overestimate the peak values. Particularly when the vertical wind shear was pronounced, the method using the pre-sim DB could cause significant errors. This is likely because the method relies on wind conditions from a single observation point, which inherently limits its ability to represent vertical wind shear within the pre-sim DB. Despite these limitations, particularly in reproducing complex wind fields, the method utilizing the pre-sim DB offers a valuable and practical tool for rapid dose rate simulation due to its lower computational cost compared to unsteady simulations using an LES model.