Continuous leaking of radioactive strontium, cesium from Fukushima to the ocean

March 11 will be the 5th anniversary since the nuclear accident in Fukushima, Japan. The Tohoku earthquake and the series of tsunamis damaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) causing a massive release of radioactivity into the atmosphere and the Pacific Ocean. Since then, the Tokyo Electric Power Company (TEPCO) and the Japanese authorities have focused on controlling the water flowing in and out of the FDNPP and on decontaminating the highly radioactive water used as coolant for the damaged reactors (about 300 m3 a day, cubic meter = 1000 L). This cooling water is then stored in tanks and, to some extent, being decontaminated.

A new study recently published in Environmental Science and Technology, uses data on the concentrations of 90Sr and 134,137Cs in the coast off Japan from the moment of the accident until September 2013, and puts it into a longer-time perspective including published data and TEPCO's monitoring data available until June 2015. This study continues the work initiated after the accident in 2011 by some of the authors. These and other partners from Belgium and Japan are currently involved in the European FRAME project lead by Dr. Pere Masqué that aims at studying the impact of recent releases from the Fukushima nuclear accident on the marine environment. FRAME is encompassed within the European COMET project (https://wiki.ceh.ac.uk/display/COM/COMET-FRAME).

Seawater collected from the sea surface down to 500 m between 1 and 110 km off the FDNPP showed concentrations up to 9, 124 and 54 Bq·m−3 for 90Sr, 137Cs and 134Cs, respectively. The highest concentrations, found within 6 km off the FDNPP, were approximately 9, 100 and 50 times higher, respectively, than pre-Fukushima levels. Before the accident, the main source of these radionuclides was atmospheric deposition due to nuclear bomb testing performed in the 1950s and 1960s. The presence of 134Cs (undetectable before the accident) and the distinct relationship between 90Sr and 137Cs in the samples suggested that FDNPP was leaking 90Sr at a rate of 2,3 -- 8,5 GBq d-1 (giga-Becquerel per day) into the Pacific Ocean in September 2013. Such a leak would be 100-1000 times larger than the amount of 90Sr transported by rivers from land to ocean. Additional risk is related to the large amounts of water stored in tanks that have frequently leaked in the past. These results are in agreement with TEPCO's monitoring data which show levels of 90Sr and 137Cs up to 10 and 1000 times higher than pre-Fukushima near the discharge channels of the FDNPP until June 2015 (most recent data included in the study). The presence of 90Sr and 134,137Cs in significant amounts until 2015 suggests the need of a continuous monitoring of artificial radionuclides in the Pacific Ocean.