Dose estimate in treatment and disposal of contaminated materials due to the accident at the Fukushima Nuclear Power Plant

福島第一原子力発電所事故に伴い発生した汚染物の処理と処分に関する線量評価

武田 聖司 ; 澤口 拓磨 ; 木村 英雄

Takeda, Seiji; Sawaguchi, Takuma; Kimura, Hideo

福島第一原子力発電所事故により放出された放射性セシウムが付着・混入した多様な汚染物(汚泥,災害廃棄物,土壌など)の保管,処理,埋設処分等にあたっては、放射線に対する作業者や公衆の安全性を確保する必要がある。本検討では、クリアランスレベルの評価手法を基本として、福島県における汚染物を取扱う施設,設備,作業の実態を反映させた汚染物の取扱に関わるシナリオおよびパラメータを設定し、作業者および公衆の受ける線量を計算した。各シナリオに対する線量計算から、特に高い線量を示した経路は処分場閉鎖後の跡地における建設業者および農業者ならびに居住者への外部被ばくであった。こうした処分後の跡地利用を制限した場合について、原子力安全委員会が示した線量の目安値に相当する放射性セシウム濃度をシナリオごとに算出した結果、全てのシナリオについて目安値を満足する濃度として8,900Bq/kgを得た。

Some kinds of material in the environment due to the accident at the Fukushima Nuclear Plant have been contaminated by radioactive cesium, which are represented by dehydrated sludge, surface soil, disaster wastes generated by the 2011 off the Pacific coast of Tohoku Earthquake and so on. Treatment (transportation, temporary storage or incineration) and disposal of the contaminated materials should be carried out while ensuring the safety of radiation for the workers and the public. In this study, in order to provide the technical information for making the guideline, the dose estimate for scenarios on the treatment and disposal is conducted, based on the method used for driving the clearance levels in Japan. Minimum activity concentration in contaminated material is calculated from the dose results, corresponding to the effective dose criterions indicated by the Nuclear Safety Commission of Japan (June 3, 2011). From the calculation result, it is suggested that it is necessary to forbid reusing the disposal site as construction, resident and agriculture in which the calculated doses for the public are higher than those in the other exposure pathways. Minimum activity concentration of radioactive cesium is derived to be 8.9 Bq/g for exposure pathway in landfill work under the condition of limited reuse of the site. In the case of the activity concentration below 8.9 Bq/g, the calculated dose of the resident due to direct and sky-shine radiation from the temporary storage place is less than 1 mSv/y, irrespective of the distance from the storage place.