Estimation of temporal variation of tritium inventory discharged from the port of Fukushima Dai-ichi Nuclear Power Plant; Analysis of the temporal variation and comparison with released tritium inventories from Japan and world major nuclear facilities

福島第一原発港湾から流出したトリチウム量の経時変化の推定流出量変化の要因分析と福島事故前後の日本及びおよび世界の原子力施設との排出量比較

町田 昌彦  ; 岩田 亜矢子; 山田 進  ; 乙坂 重嘉*  ; 小林 卓也 ; 船坂 英之*; 森田 貴己*

Machida, Masahiko; Iwata, Ayako; Yamada, Susumu; Otosaka, Shigeyoshi*; Kobayashi, Takuya; Funasaka, Hideyuki*; Morita, Takami*

本論文では、福島第一原子力発電所の港湾口から沿岸へと流出するトリチウム量を、港湾内のトリチウムモニタリング結果から推定し、事故当初の2011年4月から2020年3月までのおよそ9年間に渡り、月間流出量を算出した。その結果、2015年の海側遮水壁閉合により、未知の流出はほとんど抑制されたことが分かった。また、この推定量を基に、日本全体の原子力施設からのトリチウム年間排出量を求め、事故前後の排出量の変遷を議論した。その結果、2015年以降、福島第一原子力発電所からの流出量は、事故前の約半分程度となっている一方、事故後の日本全体の排出量は大きく減少していることが分かった。

We estimate monthly discharge inventory of tritium from the port of Fukushima Dai-ichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2020 using the Voronoi tessellation scheme, following the tritium monitoring results inside the port started in Jun. 2013. As for the missing period from the first month of 1F accident, Apr. 2011 to May 2013, we calculate the tritium discharge inventory by utilizing the concentration ratio of tritium to Cs in stagnant contaminant water during the initial direct run-off period until Jun. 2011 and the discharge inventory correlation between tritium and Cs for the next-unknown continuous-discharge period from Jul. 2011 to May 2013. From all the estimated results over 9 years, we find that the monthly discharge inventory sharply dropped immediately after closing the seaside impermeable wall in Oct. 2015 and consequently coincided well with the sum of input inventories of drainage water and subdrain etc. purified water into the port. By comparing the above estimated results with those in the normal operation period before the accident, we point out that the discharge inventory from 1F port after the accident is not so large. Even the estimation value for the accident year 2011 is found to be comparable to the maximum of operating pressurized water reactors releasing relatively large inventories in the order. At the national level, the total domestic release inventory in Japan significantly decreased after the accident owing to the operational shutdown of most plants. Furthermore, the total Japanese discharge inventory including 1F are found to be minor compared with those of nuclear reprocessing plants and heavy-water reactors on a worldwide level. From the above results, we suggest that various scenarios can be openly discussed regarding the management of tritium stored inside 1F with the help of the present estimated data and its comparison with the past discharge inventory as well as those of other nuclear facilities.