The Effects of Fukushima on the Ocean

Mark Ibañez
May 14, 2018

Submitted as coursework for PH241, Stanford University, Winter 2018


Fig. 1: Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the accident. (Source: Wikimedia Commons)

The accident that occurred at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) took place on Friday March 11, 2011. This occurrence was triggered by The Great East Japan, or "Great Tohoku" Earthquake that had a magnitude of 9.0. [1] This earthquake then triggered a tsunami off of the coast of Fukushima. [1] The tsunami washed over the sea walls of the FDNPP, rendering the plant and its backup generators inoperative. Without power the cooling pumps could not be powered to remove the lingering heat from the fuel rods in the six reactors and fuel pools.

While the immediate cause of the outage could not be attributed to one just one thing, reactor units 1, 3 and 4 exploded int he following days. [2] An arial view of FDNPP post-explosion is displayed in figure one. This catastrophic accident was ranked as a level 7 on the International Nuclear and Radiological Event Scale. [2] Because of this, attention was immediately turned to inspecting and investigating possible health effects. More specifically, researchers employed special attention to the nuclear fallout's effect on the ocean and the species that inhabit it.

Impact on the Ocean

When the tsunami broke over the walls of the FDNPP, large quantities of contaminated fluids and other radioactive discharges leaked from the plant into the surrounding coastal and offshore waters. Some researchers and experts have assessed this incident as being the largest accidental release of radioactive materials into the oceans. [3] Consequently, some radionuclides, especially radioactive cesium (Cs-134 and Cs-137), have been detected in numerous marine products taken from the waters directly off the Tohoku area. [3] The radioactive Cs contamination of marine products in Fukushima Prefecture was especially serious.

From April 2011 to April 2012, more than 40% of fish species were found to have exceeded the Japanese regulatory limit of 100 Bq/kg-wet; Bq is the derived unit of radioactivity (one radioactive decay per second). This regulatory limit was enforced in April 2012 and applies to all wet foods, etc. produced after January 2012, other than water (regulatory limit: 10 Bq/L) and milk (50 Bq/L). [3]

In addition to this the waters off Fukushima Prefecture, where the Kuroshio current meets the Oyashio current, are fishing grounds where various fishing methods are used. In particular, the broad continental shelf off Fukushima Prefecture provides important fishing grounds for coastal fisheries, especially for trawling and gillnet fishing, which accounted for 31% of total landings in Fukushima Prefecture in 2010. [3] Again, this is extremely crucial to note because it is a central site for fishing, where resources are in abundance.

Impact Today

Researchers in the Tohoku area have assessed that the overall radioactive Cs concentrations in the marine products have decreased significantly. However, the different trends of radioactive Cs concentrations differ among different habitats in the ocean. It has been reported that higher concentrations of Cs have been observed in shallower waters south of the FDNPP. [3] One would expect that there would still have to be careful monitoring of marine products off Fukushima, especially around the FDNPP, in order to avoid any harmful health consequences.

© Mark Ibañez. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] H. Haba et al., "One-Year Monitoring of Airborne Radionuclides in Wako, Japan, After the Fukushima Dai-ichi Nuclear Power Plant Accident in 2011," Geochem. J. 46, 271 (2012).

[2] K. Kurokawa et al., "The Fukushima Nuclear Accident Independent Investigation Commission: Executive Summary," National Diet of Japan, 2012.

[3] T. Wada et al., "Effects of the Nuclear Disaster on Marine Products in Fukushima: An Update After Five Years," J. Environ. Radioactiv. 164, 312 (2016).