Radioactive cesium atoms directly imaged for the first time

Thirteen years after the devastating nuclear disaster at the Fukushima Daiichi Nuclear Power Plant (FDNPP), a breakthrough analysis was completed by a team of researchers from Japan, Finland, America, and France. The study reveals the first-ever direct imaging of radioactive cesium (Cs) atoms in environmental samples emitted from the damaged FDNPP reactors.

Prof. Satoshi Utsunomiya from Kyushu University, Japan, led the research, which provides crucial insights into the lingering environmental and radioactive waste management challenges faced in Japan.

The findings highlight the formation of pollucite, a zeolite mineral, within Cs-rich microparticles (CsMPs) and the likely heterogeneity of cesium atoms distribution within the FDNPP reactors and the environment.

Analyzing radioactive cesium atoms

In 2011, the Great Tōhoku Earthquake and Tsunami caused three nuclear reactors at the FDNPP to undergo meltdowns due to a loss of back-up power and cooling.

Since then, extensive research efforts have focused on understanding the properties of fuel debris found within the damaged reactors, which must be carefully removed and disposed of.

However, many uncertainties remain concerning the physical and chemical state of the fuel debris, complicating retrieval efforts.

A significant amount of radioactive Cs was released from the damaged reactors in the form of poorly soluble, small (< 5 µm) particles with a glass-like composition, termed CsMPs.

Prof. Utsunomiya explained that the CsMPs “formed in the bottom of the damaged reactors during the meltdowns, when molten nuclear fuel impacted concrete.”

Despite abundant Cs in the microparticles, direct atomic-scale imaging of radioactive Cs has proven impossible due to the low concentration of Cs and the electron beam damage to the samples.

Prof. Gareth Law, a study collaborator from the University of Helsinki, explained that “this means we lack full information on the chemical form of Cs in the particles and fuel debris.”

Cesium: Understanding the basics

Cesium (Cs) is a soft, silvery-white metal that belongs to the alkali metal group in the periodic table. It has an atomic number of 55 and is the most electropositive and reactive of all the stable elements.

Cesium has a low melting point of 28.5°C (83.3°F) and a boiling point of 671°C (1,240°F). In nature, cesium occurs in the minerals pollucite and lepidolite, but it is relatively rare and is primarily obtained as a byproduct of lithium mining.

Radioactive isotopes of cesium

Cesium has several isotopes, both stable and radioactive. The most common stable isotope is cesium-133, while the most well-known radioactive isotopes are cesium-134 and cesium-137.

These radioactive isotopes are produced during nuclear fission reactions and have half-lives of 2.1 years and 30.2 years, respectively.

Cesium-137 is particularly significant because it is one of the main sources of radiation in nuclear fallout and can persist in the environment for decades.

Applications of cesium

Cesium has a wide range of applications in various fields:

• Atomic Clocks: Cesium-133 is used in the world’s most accurate atomic clocks, which define the international standard for time measurement.
• Photoelectric Cells: Cesium is used in photoelectric cells, which convert light energy into electrical energy, due to its high sensitivity to light.
• Drilling Fluids: Cesium formate is used as a high-density drilling fluid in oil and gas exploration, helping to stabilize high-temperature, high-pressure wells.
• Medical Applications: Cesium-131 is used in brachytherapy for cancer treatment, as it emits soft gamma rays that can effectively target tumors while minimizing damage to surrounding healthy tissue.
• Specialty Glass: Cesium is used in the production of specialty glass, such as night vision goggles and telescope lenses, due to its ability to increase the refractive index of glass.

Environmental and health concerns

While stable cesium is relatively harmless, radioactive cesium isotopes can pose significant environmental and health risks.

Cesium-137, in particular, can accumulate in the food chain and cause long-term contamination of soil, water, and vegetation.

Exposure to high levels of radioactive cesium can lead to acute radiation sickness, increased cancer risk, and other health problems.

Proper management and disposal of radioactive cesium waste are crucial to minimize these risks and protect public health and the environment.

Iron-rich pollucite connection to cesium atoms

In their previous work using a state-of-the-art high-resolution high-angle annular dark-field scanning transmission electron microscope (HR-HAADF-STEM), the team found inclusions of pollucite, a zeolite mineral, within CsMPs.

Prof. Law explained that “in past analysis we showed that the iron-rich pollucite inclusions in the CsMPs contained >20 wt.% Cs. In nature, pollucite is generally aluminum-rich. The pollucite in the CsMPs was clearly different to that in nature indicating it formed in the reactors.”

Informed by modeling and the knowledge that most of the Cs in CsMPs is fission-derived, the team set about painstaking analysis that finally led to the direct imaging of radioactive Cs atoms. Prof.

Utsunomiya expressed his excitement, stating, “It was incredibly exciting to see the beautiful pattern of Cs atoms in the pollucite structure, where about half of the atoms in the image correspond to radioactive Cs. This is the first time humans have directly imaged radioactive cesium atoms in an environmental sample.”

Reactor decommissioning and environmental remediation

The study’s findings go beyond the mere imaging of radioactive Cs atoms. The work sheds light on pollucite formation and the likely heterogeneity of Cs distribution within the FDNPP reactors and the environment.

Prof. Law emphasized, “Finding Cs containing pollucite in CsMPs likely means it also remains in the damaged reactors; as such, its properties can now be considered in reactor decommissioning and waste management strategies.”

“We should now also begin to consider the environmental behavior or Cs-pollucite and its possible impacts. It likely behaves differently to other forms of Cs fallout documented thus far,” added collaborator Emeritus Prof. Bernd Grambow from Subatech, IMT Atlantique Nantes University.

“Also, the effect on human health might have to be considered. The chemical reactivity of pollucite in the environment and in body fluids is certainly different than that of other forms of deposited radioactive Cs,” Grambow concluded.

Prof. Rod Ewing from Stanford University underscored the pressing need for continued research to inform debris removal strategies and environmental remediation, stating, “Yet again, we see that the pain-staking analytical efforts of international scientists really can unlock the mysteries of nuclear accidents, aiding long-term recovery efforts.”

Unlocking the mysteries of nuclear accidents

This important research led by Prof. Satoshi Utsunomiya and his brilliant international team of collaborators marks a significant milestone in understanding the aftermath of the Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster.

By directly imaging radioactive cesium atoms within pollucite inclusions in Cs-rich microparticles, the study provides crucial insights into the formation and distribution of radioactive materials within the damaged reactors and the environment.

These findings will undoubtedly inform future strategies for reactor decommissioning, waste management, and environmental remediation efforts, as scientists and policymakers work together to address the long-term consequences of nuclear accidents and ensure the safety and well-being of affected communities.

The full study was published in the Journal of Hazardous Materials.

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