by Jay T Cullen

The purpose of this post is to bring to the attention of readers here a review of the available measurements and science based investigations of the Fukushima Daiichi Nuclear Power Plant (FDNPP) and its impact on the Pacific Ocean ecosystem and public health. This post is part of an ongoing effort to summarize scientifically rigorous information about the disaster for interested readers. The new paper is a product of a working group on radioactivity in the ocean convened by the Scientific Committee on Ocean Research (SCOR) an international non-governmental non-profit organization. I highly recommend this paper for anyone who wishes to better understand what the international scientific community has found about the marine release, fate and impact of FDNPP-derived radionuclides in the marine environment.  The working group was made up of 10 experts from 9 different countries, including Japan, and published the open access paper in Annual Reviews.  The main findings of the review were as follows:

• The amount of ¹³⁷Cs released from the plant was ~50-fold less than the fall out from nuclear weapons testing in the 20^th century and ~5-fold lower than that released from Chernobyl in 1986. Total releases from Fukushima are similar to the discharges of ¹³⁷Cs from the nuclear fuel reprocessing plant Sellafield in the UK
• Initial releases in the weeks to months after the disaster which began on March 11, 2011 dwarf those from aggregated ongoing releases from the plant site
• The majority of radionuclide releases ended up in the Pacific Ocean with most deposition and input occurring close to the FDNPP
• Current range of estimates of the total ¹³⁷Cs ocean source term are 15-25 PBq (PBq = 10¹⁵ Becquerel where a Bq is one nuclear decay event per second). While many other radionuclides were released from FDNPP, the most likely isotopes to represent a health risk to the marine ecosystem and public are those of Cs given their longer half-lives for radioactive decay (¹³⁴Cs = ~2 yrs; ¹³⁷Cs = ~30 yrs) and higher relative abundance compared to other isotopes of concern in the FDNPP source term
• Because Cs is very soluble it rapidly dispersed in the ocean after the disaster given mixing, transport and dilution by ocean currents.  Peak levels of ¹³⁷Cs occurred close to the plant in 2011 where activity concentrations near FDNPP was tens of millions of times higher than before the accident. By 2014 ¹³⁷Cs concentrations in the central North Pacific was about six times the remaining weapons testing fallout and about 2-3 times higher than prior fallout levels in the northeast Pacific near to North America. Most of the fallout remains concentrated in the top few hundred meters of the ocean. Measurements being made by the Fukushima InFORM project indicate that maximum ¹³⁷Cs levels off the North American coast are likely to occur this year before declining to levels associated with background nuclear weapon testing before the accident by about the end of this decade
• There are unlikely to be measurable effects on marine life with the exception of coastal areas very close to FDNPP immediately after the disaster. Monitoring of fish species in Fukushima Prefecture show that about 50% of samples in coastal waters had radiocesium levels above the Japanese 100 Bq kg^-1 limit, but that by 2015 this had dropped to less than 1% measuring over the limit. High levels continue to be found in fish around and in the FDNPP port
• Given levels in seawater and marine organisms measurable impacts to human health through contact with the ocean and the consumption of seafoods are very unlikely

There are many informative graphics and moderately technical summaries of available studies found in the new paper.  The authors highlight the difficulty of monitoring radionuclides in the ocean  given the dynamic nature of the sea and logistical challenges presented by the temporal and spatial scales and low levels of FDNPP derived contamination going forward.  In addition to providing ongoing assessments of risk to the environment from the disaster it is likely that useful information about ocean circulation will be obtained through continued monitoring efforts.