This diary is the latest in an ongoing series that aims to provide information on the impacts of the Fukushima disaster on the health of the marine ecosystem and the public on the west coast of North America. The purpose of this diary is to address the relative releases of radioactive cesium (137-Cs) and plutonium isotopes to the environment from Fukushima. Both elements can cause short and long-term health problems. Plutonium is an alpha-emitting isotope that carries significant radiological health risks if internalized so understanding the amount released is key. Online and in some media there exists a misconception that "massive" amounts of Pu escaped from the reactors. Modeling studies and existing measurements demonstrate that the total activities of Pu isotopes released from Fukushima were 1 to 10 million times lower than the activity of 137-Cs reflecting Cs's much higher volatility. While there was ~3.5 times more Pu isotopes in the Fukushima reactors versus the Chernobyl reactor at the time of the respective disasters the percentage of core inventories released from Fukushima were about 100,000 times lower. Where possible I have linked to open-access scientific studies but some primary references, despite best efforts, are behind pay walls. More below the fold.
Those unfamiliar with the units scientists use to discuss radioactivity should consult a previous diary where a brief review is presented.
Model of Pu Release from Fukushima
In a study published in the peer-reviewed journal Environmental Science and Technology in 2012 Schwantes and co-workers (behind a pay wall unfortunately) considered radionuclide measurements made in the vicinity of the reactor sites to determine the extent of release of fission products and fuel. The study determined that the release of elements to the atmosphere was primarily controlled by how volatile and how easy it was to chemically reduce the oxide form of a radioisotope. The lower the boiling point of a radioisotope and the more easily its oxide could be reduced in the primary containment vessel at the site the more volatile was the element leading to more total release to the environment. Based on analyses of the fission product 137-Cs and the Plutonium isotopes 238, 239 and 240 Schwantes and colleagues determined that roughly 0.002 - 0.004% of the inventory of Pu of reactors 1 and 3 at the site were released to the environment. This study makes important predictions about the relative activities (Bq) of 137-Cs and Pu isotopes (Pu release about 1-10 million times less than 137-Cs) in the environment that can be tested by making measurements of air, soil and seawater. It is important to point out that this article did not consider the direct mobilization of elements to the ocean and that we will consider measurements of Pu in the ocean below.
Cesium and Plutonium Measured in the Air
The radioactive plume of elements released to the atmosphere from Fukushima was detected globally in the days following disaster and measurements of the activities of these elements in air were made. For example, The atmospheric signal from Fukushima was first detected over Europe on March 19 by a Comprehensive Test Ban Treaty Organization monitoring station in Reykjavik, Iceland. Maximum activities of 137-Cs of 0.05-1 milliBq/m3 air were measured over Europe which were 1,000 - 10,000 times lower than 137-Cs activities measured in the weeks following the Chernobyl disaster in 1986 (Povinec and others 2013). Pu from Fukushima was detected in air as well with activities of 239,240-Pu of ~45 nanoBq/m3 which is about 10,000 times lower than the activities detected over Europe resulting from the Chernobyl disaster. The relative activities of 137-Cs and Pu isotopes in the atmosphere are consistent with Pu release about 1 million times less than that of 137-Cs.
Cesium and Plutonium Measured in Soil in Japan
In an open-access, peer-review study Zheng and co-workers report on 137-Cs and Pu isotope activities in soils northwest and south within 20-30 km of the Fukushima reactor sites (see map).
Map showing soil sampling sites for Zheng et al. (2012)
Activity ratios of 137-Cs to the Pu isotopes were found to be 200,000 to 25,000,000 with 10-1,000 times less Pu relative to Cs released from the Chernobyl accident. A comparison Pu releases from Fukushima and Chernobyl disasters is presented in the following table from Zheng and others:
Comparison of Pu releases from Fukushima and Chernobyl nuclear disasters
The estimates of Pu releases from analyses of soils in Japan suggest that while there was >3.5 times the Pu present in the Fukushima reactors compared to Chernobyl at the time of the disaster about 100,000 times less was released from Fukushima to the environment. About 0.00002% of the core inventory of Pu was released from Fukushima according to their analyses.
Plutonium Released Directly to the Ocean
Less in known about the extent to which Pu was released to the marine environment. Measurements of coastal marine sediments and estuaries in Japan in 2011 and 2012 were carried out and reported by Bu and colleagues in the open-access, peer-reviewed journal Biogeosciences in 2013. By July 2012 they were unable to detect changes in the activity or isotopic ratios in the sediments owing to Pu originating from Fukushima. All the measurements showed that the Pu present in marine sediments reflected the presence of fallout from atmospheric weapons testing in the 20th century. There is a great need for sensitive measurements of Pu dissolved in seawater to determine how much Pu relative to 137-Cs was released directly to seawater. Ongoing monitoring of Pu in seawater is identified as a high priority task for the oceanographic community.
Taken together, modeling studies of the relative atmospheric releases of 137-Cs and Pu and measurements of the isotopes in air, soil and the marine environment suggest that a trace amount (0.002 to 0.00002%) of Pu from the core inventories at Fukushima were released to the environment. Release of Pu from Fukushima was about 100,000 times lower than the Pu release from Chernobyl in 1986 and 5,000,000 times lower than the Pu released from atmospheric weapons testing in the 20th century (Table 9).
Going forward more monitoring will needed to determine the distribution of this important alpha-emitter in the environment given the potential for significant radiological health risks. Given ongoing release from the disaster site monitoring of the activity and isotopic composition of Pu in the marine environment and its organisms will be a key component of determining the fate of Fukushima derived Pu.