Q&A with Professor Rodney C. Ewing, Frank Stanton Professor in Nuclear Security and co-director at the Center for International Security and Cooperation (CISAC) in the Freeman Spogli Institute for International Studies (FSI). Interview with Katy Gabel Chui.

Your previous research with this team helped identify the types of radioactive particles that can become airborne and were transported away from Fukushima during the 2011 nuclear disaster.

This most recent paper goes further to show how these Cesium (Cs)-rich silica particles behave in several types of fluids, including simulated human lung fluid, concluding that the particles are fully dissolved in the latter after more than 35 years. What might that mean for human health in the Fukushima area and beyond?

The first breakthrough was the recognition that such particles, a few microns in diameter, existed, a discovery by Japanese scientists at the Meteorological Research Institute, Tsukuba, in 2013. The particles are important because they were dispersed over distances of tens of kilometers and were “carriers” of highly radioactive Cs. Our team’s previous work, led by Professor Satoshi Utsunomiya, mainly focused on the characterization of the particles and their constituents at the atomic-scale and surveyed their distribution in the area away from the Fukushima Daiichi nuclear power plants. Our earliest work from 2016 can be found online. A good summary of the history of the work on these cesium-rich microparticles was recently published in Scientific American.

This latest paper published in Chemosphere is the 6th in a series of papers on the Cs-rich microparticles. We describe the behavior of these particles when exposed to different types of fluids: ultra-pure water, artificial sea water and simulated lung fluid. The microparticles dissolve in all three fluids, reaching a long-term but a continuing, slow rate of release after just three days. Essentially, the calculated release rate of cesium depends on the rate of dissolution of the silica glass matrix and the initial size of the particles. In the simulated lung fluid, the particles are modelled to fully dissolve after more than 35 years.

What is the simulated lung fluid made of, and how does it work in simulation? How do you estimate 35 years?

The constituents of typical lung fluid were simply mixed to simulate its composition based on a recipe reported by previous studies. The lung fluid is different from the other solutions because it contains organic compounds and has a different chemistry, i.e., higher sodium and chlorine content. The estimates of residence time in the body assumes that the particles are inhaled and find their way to the pulmonary system. The calculation of residence time is based on assumptions about the size and composition of the microparticles, and we used the long-term release rate from the experiments. We assumed a spherical shape and a constant decrease in size as the leaching process continued. The rate can vary depending on the actual shape, internal texture, composition (such as occurrence of intrinsic Cs-phase inclusions), and precipitation of secondary phases that may form a “protective” coating on the cesium-rich microparticles (CsMPs). The rate of release was fastest in the simulated lung fluid.

The important result is to realize that the Cs-rich silica particles dissolve slowly in the environment and in the body. Essentially, the release extends for several decades.

How can nuclear energy experts and policy makers use this research to avoid future risk?

Understanding the form and composition of materials that host and disperse radionuclides is the first step in completing a careful dose calculation. Based on these results, the fraction of Cs contained in the silica particles will not be rapidly “flushed” through the environment or the body, but rather will be released over several decades.