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

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

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.