According to news reports, five workers were accidentally exposed to high levels of radiation at the Oarai nuclear research and development center in Tokai-mura, Japan on June 6th. The Japan Atomic Energy Agency, the operator of the facility, reported that five workers inhaled plutonium and americium that was released from a storage container that the workers had opened. The radioactive materials were contained in two plastic bags, but they had apparently ripped.
This incident is a reminder of the extremely hazardous nature of these materials, especially when they are inhaled, and illustrates why they require such stringent procedures when they are stored and processed.
According to the earliest reports, it was estimated that one worker had inhaled 22,000 becquerels (Bq) of plutonium-239, and 220 Bq of americium-241. (One becquerel of a radioactive substance undergoes one radioactive decay per second.) The others inhaled between 2,200 and 14,000 Bq of plutonium-239 and quantities of americium-241 similar to that of the first worker.
More recent reports have stated that the amount of plutonium inhaled by the most highly exposed worker is now estimated to be 360,000 Bq, and that the 22,000 Bq measurement in the lungs was made 10 hours after the event occurred. Apparently, the plutonium that remains in the body decreases rapidly during the first hours after exposure, as a fraction of the quantity initially inhaled is expelled through respiration. But there are large uncertainties.
The mass equivalent of 360,000 Bq of Pu-239 is about 150 micrograms. It is commonly heard that plutonium is so radiotoxic that inhaling only one microgram will cause cancer with essentially one hundred percent certainty. This is not far off the mark for certain isotopes of plutonium, like Pu-238, but Pu-239 decays more slowly, so it is less toxic per gram. The actual level of harm also depends on a number of other factors. Estimating the health impacts of these exposures in the absence of more information is tricky, because those impacts depend on the exact composition of the radioactive materials, their chemical forms, and the sizes of the particles that were inhaled. Smaller particles become more deeply lodged in the lungs and are harder to clear by coughing. And more soluble compounds will dissolve more readily in the bloodstream and be transported from the lungs to other organs, resulting in exposure of more of the body to radiation. However, it is possible to make a rough estimate.
Using Department of Energy data, the inhalation of 360,000 Bq of Pu-239 would result in a whole-body radiation dose to an average adult over a 50-year period between 580 rem and nearly 4300 rem, depending on the solubility of the compounds inhaled. The material was most likely an oxide, which is relatively insoluble, corresponding to the lower bound of the estimate. But without further information on the material form, the best estimate would be around 1800 rem.
What is the health impact of such a dose? For isotopes such as plutonium-239 or americium-241, which emit relatively large, heavy charged particles known as alpha particles, there is a high likelihood that a dose of around 1000 rem will cause a fatal cancer. This is well below the radiation dose that the most highly exposed worker will receive over a 50-year period. This shows how costly a mistake can be when working with plutonium.
The workers are receiving chelation therapy to try to remove some plutonium from their bloodstream. However, the effectiveness of this therapy is limited at best, especially for insoluble forms, like oxides, that tend to be retained in the lungs.
The workers were exposed when they opened up an old storage can that held materials related to production of fuel from fast reactors. The plutonium facilities at Tokai-mura have been used to produce plutonium-uranium mixed-oxide (MOX) fuel for experimental test reactors, including the Joyo fast reactor, as well as the now-shutdown Monju fast reactor. Americium-241 was present as the result of the decay of the isotope plutonium-241.
I had the opportunity to tour some of these facilities about twenty years ago. MOX fuel fabrication at these facilities was primarily done in gloveboxes through manual means, and we were able to stand next to gloveboxes containing MOX pellets. The gloveboxes represented the only barrier between us and the plutonium they contained. In light of the incident this week, that is a sobering memory.