夜次露天採掘場跡地の核種移行解析

The Radionuclide Transport Analysis in the former Yotsugi open pit site

長柄 収一; 松村 敏博; 佐藤 和彦 ; 時澤 孝之

Nagara, Shuichi; Matsumura, Toshihiro; Sato, Kazuhiko; Tokizawa, Takayuki

夜次露天採掘場跡地及びその周辺の核種移行解析を目的として、解析のためのパラメータの収集・整理を行い、核種解析コード「NESTOR」による一次元解析を実施した。解析に先立ち、解析用のデータセットは、露天採掘場跡地の特質及び廃棄物処分の情報から調査した。また、解析に用いるモデルは、夜次露天採掘場跡地の地形的特徴により7ブロックに分割した単純化したものを作成し、試錐孔を評価点と見なして核種移行解析に適用した。解析の結果、評価点で最も高くなる核種濃度は、措置後から約1万年経過したPo-210の8.5710Bq/mである。一方、U-238濃度の最大値は、約110Bq/mオーダーである。参考として、この数値は、人形峠保安規程の坑水・廃水の排出基準値よりも1桁低く、岡山県環境協定の河川水管理目標値と同オーダーである。現時点での解析結果と現在の地下水濃度を比較すると、前者の方がU-238濃度は約7桁低く、Ra-226濃度は約6桁低くなる。この原因は、露天跡地周辺に存在するバックグランドを考慮していないためと考えられる。

A one-dimensional numerical solute-transport analysis utilizing a simulation code named NESTOR, involving a literature survey on analytical parameters has been carried out for the purpose of an understanding radionuclide transport in groundwater in and around the former Yotsugi open pit site at the Ningyo-Toge Environmental Engineering Center, JNC. Prior to the analysis, data set was prepared referring to available documents on site characteristics and one on disposal of radwaste. A simplified model in which the site was divided into seven blocks according to its geographical-hydrological feature and has boreholes downward regarded as assessment points was applied on the analysis. As a result, The highest concentrations of radionuclide is 8.5710 Bq/m of Po-210 at assessment points. The highest concentration of U-238 is in the order of magnitude of 10 Bq/m. This figure is lower by 1 order than a standard value for mine- and waste-water on the safety rule of Ningyo-Toge mine and the same orders as surface-water on the agreement with Okayama prefecture. If the analysis result in this time and the present concentration in groundwater are measured, the former data became lower about 7 orders by U-238 concentrations and became lower about 6 orders by Ra-226 concentrations. It is thought that this cause is that U and Ra which exist around the former Yotsugi open pit sit before taking reclamation cannot be taking into consideration in analysis.