CHEMICAL SCIENCE

Stabilize hexavalent americium, they let nuclear waste “turn waste into treasure”


In the eyes of scientists, nuclear waste is not “waste” in the full sense of the word, and there are many resources that can be used. For example, the nuclear waste in people’s eyes – plutonium-238 is a very good heat source and can be used for nuclear battery fuel, and the Curiosity rover in the United States uses plutonium thermonuclear batteries; Technetium 99M decay emits photons and iodine 131 decay emits electrons, which can be used for medical imaging examination and treatment; Carbon 14 can be used to determine the age of paleontological fossils.

In particular, “China is a country with depleted uranium, and the reprocessing and recycling of spent fuel is of great significance to the development of nuclear power in China.” Wang Yanwu, a professor at the State Key Laboratory of Radiation Medicine and Radiation Protection at Soochow University, said.

On April 20, Wang Yanwo’s team, together with researchers in the field of radiochemistry such as Tsinghua University, Colorado University of Mines in the United States, Jülich Research Center in Germany, and ShanghaiTech University, published the relevant results in Nature with the title of “Ultrafiltration separation between heteropolyacid compounds of hexavalent americium and lanthanides”. This achievement will provide a new way of thinking for the key nuclide separation process in spent nuclear fuel reprocessing and nuclear waste disposal.

Wang Yanwo (left) instructs students to do experiments. Photo courtesy of the research group

Precise extraction of americium from nuclear waste

“The uranium and plutonium in spent fuel can be extracted through a reprocessing process and put back into nuclear power plants as nuclear fuel, while the secondary actinides remaining after the separation of uranium and plutonium are difficult to reprocess.” Wang Yanwo introduced that these subactinides are highly radioactive, and the half-life of related isotopes varies from hundreds to millions of years, and can only be treated by deep burial after solidification. And these nuclear wastes are like “time bombs” underground, and once leaks occur, they will cause immeasurable harm to the natural environment.

Wang explained that americium, as one of the secondary actinides, is the main source of long-term radiotoxicity of nuclear waste after reprocessing, and if americium can be isolated for transmutation, it only takes 200 years for the radioactivity of these spent fuels to decay to the level of natural uranium ore. “The chemistry of americium is very similar to the trivalent lanthanides that coexist in nuclear waste, so its extraction and separation is one of the most challenging scientific problems in the field of radiochemistry.”

“If the trivalent americium can be oxidized to hexavalent, and the separation can be achieved by using the difference in coordination configuration between hexavalent americium and trivalent lanthanide, it can be expected to fundamentally solve the problem of lanthanum and actinium separation.” Wang Yaxing, co-corresponding author of the article and professor at the School of Radiation Medicine and Protection of Soochow University, told China Science News that hexavalent americium can only exist for a few seconds in the traditional extraction and separation process, and it is easy to be reduced to trivalent, resulting in difficulty in separation.

Through experiments, the research team designed an inorganic absent polyacid cluster that can accurately match the coordination configuration of hexavalent americium, which can not only undergo strong complexation with hexavalent americium, but also a protective group with “chemical inertness” and “steric hindrance effect” for hexavalent americium, which can stabilize hexavalent americium in acidic solution for more than 24 hours, thus providing a prerequisite for the subsequent separation process.

“With our separation method, americium can be precisely extracted from nuclear waste with a very complex chemical composition, with a single recovery rate of up to 91 %. Wang Yanwo said that mastering this technology can greatly reduce the long-term radiotoxicity of nuclear waste and free China from dependence on imported americium. As a rare element, americium has broad application prospects from thickness gauges and smoke alarms in daily life to space nuclear batteries for future development, and is an important strategic resource of the country.

“This experiment is extremely ingenious and has the potential to change the fundamental paradigm of global nuclear waste disposal.” This is what Nature reviewers said about the results.

After several “breakthroughs”, the best separation effect was finally obtained

This scientific research is far from as smooth as expected.

The team first had to face the oxidation of “americium”. Through repeated experiments, they found that the traditional method is only suitable for the oxidation of low concentrations of americium, and once the concentration is increased to the level of real nuclear waste liquid, the experimental data and results are very unsatisfactory. After the improvement, they used the “electrochemical” method for oxidation, and although the experiment was partially successful, they encountered new problems, which caused difficulties in separation due to incomplete oxidation of americium.

When everyone was at a loss, Wang contacted Xu Chao, an associate professor in the radiochemistry team of Tsinghua University. With his assistance, the team selected chemical oxidants produced by manufacturers recommended by Xu Chao to solve this long-troubled problem.

“Although we also used sodium bismuth oxide before, the same oxidant produced by different manufacturers can also cause huge deviations in experimental results.” Zhang Hailong, co-first author of the paper and associate researcher of the State Key Laboratory of Radiation Medicine and Radiation Protection of Soochow University, said.

Entering the extraction stage of americium, the team’s earliest strategy was to achieve separation by crystallizing high-valent actinides and low-valence lanthanides not crystallizing, and when this scheme was implemented, they found that the crystallization process would not only carry too many impurities, but also be affected by the crystallinization rate, and the separation effect was far from meeting expectations.

When they wanted to give up, the other work of the research group inspired them, why not try the ultrafiltration method widely used in the field of biology? Unexpectedly, this inspiration reaped “miraculous effects”. Using the significant size difference between americium polyacid nanocomplexes and hydrated lanthanide ions, combined with commercial ultrafiltration technology, the team finally obtained the best separation effect based on hexavalent americium.

“Wheel tactics” share the radiation dose

In the transuranic laboratory deep in Soochow University’s School of Radiation Medicine and Protection, three lab workers in bulky lead suits are nervously busy. The operator lowers his head to focus on the operation of radioactive material on the experimental platform, the auxiliary personnel cooperate according to the instructions of the main operator, and the third person holds a Geiger counting instrument for detecting radiation intensity to monitor whether there is a leak of radioactive material in real time. They also wear a radiation meter on their chest, and once the value of the meter on the main operator alarms, a rotation occurs between the three people.

“Radiation attenuates as it moves farther away from the source.” Li Yuan, co-first author of the article and scientific research assistant of the State Key Laboratory of Radiation Medicine and Radiation Protection of Soochow University, said that because the relevant isotopes of americium produce associated high-energy gamma rays during the decay process, when doing some experiments with larger radiation doses, researchers will adopt this “wheel tactic” to share the radiation dose while doing a good job of protection, and protect the health of experimenters to the greatest extent.

Continuous trial and error also brought anxiety to the research group, how to deal with the “mental internal friction” of scientific research, Wang Yanwo has his own “know-how”. No matter how busy he is, he always meets with stitches to lead the team to a game of basketball. “Through sports, it can not only relieve the heavy pressure of scientific research, but also form a good communication with the teachers and students in the group, and enhance the cohesion of the team.” Wang Yanwo said that the representative team formed by them has achieved good results in school and college competitions, and it has become a common practice to ask for a ball after scientific research.

“Every time everyone comes back to the lab after sweating on the court, they are in good spirits to meet the challenging work.” Li Yu said. (Source: China Science News, Wen Caifei, Yin Zhe)

Related paper information:https://doi.org/10.1038/s41586-023-05840-z



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