Removal of 90% of radioactive cesium in soil with "salt," "vacuum," and "800 degrees" — Japan Atomic Energy Agency

The Japan Atomic Energy Agency (JAEA) discovered that heating soil contaminated with radioactive cesium to 800 degrees in a vacuum after adding sodium chloride (salt) can remove 90% of the cesium in a short time. A new phenomenon called high-speed ion exchange appears to be involved. Over the next two years or so, JAEA plans to conduct demonstration experiments to verify whether cesium can be removed at low cost from about 10 kilograms of soil.

Radioactive cesium released from Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant, which was damaged in the Great East Japan Earthquake of 2011, fell to the ground with rain and other precipitation. Cesium is difficult to remove because it penetrates as ions into the layered structure of clay minerals in soil. Meanwhile, among radioactive cesium isotopes, cesium-137 has a physical half-life of 30 years, causing long-term environmental contamination. Therefore, the development of low-cost and efficient decontamination methods is needed.

According to Principal Researcher Iwao Shimoyama who studies material science at the Sustainable Functional Materials Science Group, Advanced Science Research Center, Nuclear Science Research Institute of the JAEA, previous decontamination methods involved melting soil containing radioactive cesium together with salt at 1000-1300 degrees to remove vaporized cesium. However, the higher the heating temperature, the greater the energy costs, so reducing the processing temperature was a challenge.

Shimoyama and his colleagues thought that cesium might vaporize more easily even at low temperatures in a vacuum, potentially improving decontamination efficiency. They collected soil samples from Tomioka Town, Fukushima Prefecture, and confirmed a radioactive concentration of 10,000 becquerels, exceeding the 8,000 becquerels per kilogram threshold that the Ministry of the Environment has established as a standard for safely processing waste.

They heated soil and an equal amount of salt in a near-vacuum state at 10-20 pascals pressure, about one ten-thousandth of atmospheric pressure. As a result, the decontamination rate for radioactive cesium increased at 600-700 degrees, reaching about 90% at 800 degrees after approximately 60 minutes. In aqueous solution reactions, even after about 5000 minutes, the decontamination rate remained below 30%.

Even when heated to 800 degrees in atmosphere without creating vacuum conditions, the decontamination rate was only about one-sixth of that achieved in a vacuum. Additionally, when salt was not mixed in, creating vacuum conditions did not improve the decontamination rate as much as expected.

To understand why removal was possible at a low temperature when salt was added in a vacuum, Shimoyama and his colleagues focused on the layered structure of the clay minerals that are penetrated by cesium ions in soil. X-ray diffraction experiments revealed that clay minerals increase the distance between layers up to 500 degrees upon heating, but when further heated to 700 degrees, the interlayer distance rapidly contracts.

Combined with the fact that salt vaporizes directly from solid to gas at 700 degrees in a vacuum and cesium removal began at temperatures lower than the salt's transition temperature, it became clear that ion exchange occurred where cesium ions exited through gaps created by the expansion of layered structures in clay minerals and sodium ions—also cations but slightly smaller in size—entered instead, and the interlayer distance contracted by sandwiching the sodium ions.

"Ion exchange is familiar in aqueous solutions. I think high-speed ion exchange occurring in a vacuum is a new phenomenon in decontamination," said Shimoyama. Moving forward, the researchers will test the effects of additives other than salt as basic research. For demonstration tests, the first stage will verify over two years whether 10 kilograms of soil can be efficiently decontaminated, and the second stage will scale up to 100 kilograms to verify efficiency.

The research was published in the electronic version of Journal of Environmental Management, an international academic journal, on June 19.