First reliable estimates of highly radioactive cesium-rich microparticles released by Fukushima disaster

The flooding of the Fukushima Daiichi Nuclear Power Plant (FDNPP) after the disastrous earthquake on March 11 2011 caused the release of significant amounts of radioactive material, including cesium (Cs) isotopes 134Cs (half-life, 2 years) and 137Cs (half-life, 30 years). Initially scientists thought that all Cs was released in soluble form. Now however, they have realized that a part of the released Cs was in the form of glassy microparticles, formed at the time of the reactor meltdown; these particles were thrown over a wide area, but until now there has been no reliable estimate of how much radioactive cesium-rich microparticles was deposited in the surrounding area, and how this material was distributed.

Now a group of international scientists, led by Dr. Satoshi Utsunomiya (Associate Professor of Kyushu University, Fukuoka, Japan) has been able to give the first accurate estimates of the amount of the radioactive microparticles in the environment. This work describes the significance of the microparticles to current radiation levels, and provides fundamental data for a future re-evaluation of health risks from the highly radioactive microparticles which remain in the local environment.

"Most of the glassy microparticles are only a few microns in size, and were spread alongside the soluble cesium. The soluble cesium is generally bound to clay minerals after wet deposition, with the clay minerals also forming particles, so it was difficult to distinguish the cesium-rich microparticles from cesium absorbed on clay." said Dr Utsunomiya, "However, we realized that the cesium-rich microparticle has an extremely high radioactivity ~1011 Bq/g compared with the much lower radioactivity for cesium-sorbing clay particles, and this can be used to distinguish the two types. So we have established a novel procedure to quantify the cesium-rich microparticles by applying a quantitative autoradiography method."

Autoradiography exposes a photographic film or detector to a radioactive source, which causes the radiation to show up on the film (medical X-rays is the most common autoradiography technique). The team determined the threshold radioactivity for Cs-rich microparticles in the sieved fraction based on the relation between photostimulated luminescence signal and radioactivity. They applied this method to soil samples from 20 affected areas.

Dr Utsunomiya continued "In certain areas, these glassy particles are highly concentrated, so they are a major concern. We have found up to 318 of these particles in just 1 gram of soil, near the Fukushima Daiichi power plant. Most of these particles are still in the ambient environments, indicating the high stability.

Since the Fukushima accident we have been gradually coming to understand how the microparticles were distributed, and what this might mean to health and the environment. As you would expect, there are more radioactive particles nearer the reactor: we believe that there was a proportion of cesium released as soluble material, but we have found that the area south of the reactor contains a higher proportion of glassy particles. Our estimate is that around 78% of radioactive cesium was released as glassy particles. Many of the microparticles have been washed down from roofs and from plants, and have now gathered in radioactive hot spots.

Now that we have a better idea of the quantities involved and how the radiation has been distributed, it gives our team a better idea of how to approach the effect on health, which is obviously a major concern. This work does not imply that there is any additional radiation which has been missed -- the total amount of cesium released at Fukushima remains the same. However, the glassy particles have concentrated the radiation, which means that there is still much new work to be done to understand how this concentrated radiation might affect health"

Commenting on the work, Dr Ken Buesseler (Woods Hole Oceanographic Institution) said:

"The idea of microparticles has not been 'missed' in the assessment of total cesium levels in soil after Fukushima; it has been included, although this work highlights the fraction found in cesium microparticles. So we shouldn't think that there is additional radiation to worry about, but nevertheless in this highly concentrated form it may have different health impacts. These researchers have done a fine job of developing new tools to quantify these microparticles, and that is an important story to tell"