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Long-term research between the UK's National Nuclear Laboratory and the University of Manchester in the UK has explored the complex interactions between plutonium dioxide surface features and its surroundings. The new results provide more confidence in long-term storage strategies.
In many countries around the world, including the United Kingdom, the United States, Japan, and France, plutonium is separated from spent nuclear fuel and then stored in specially designed metal tanks in the form of oxide powder. In the UK, the final fate of this powder has not yet been determined; It can be discarded deep underground, or it can be used to make new fuel for future nuclear energy.
Some of the material in the UK has been in storage for almost 50 years, mainly at the Sellafield nuclear power plant in West Cumbria. Just like other radioactive elements, plutonium undergoes radioactive decay, so the conditions in the tank change gradually over time.
Dr. Robin Orr, Senior Technical Manager at the National Nuclear Laboratory, has extensive experience in working with plutonium: "The safe and reliable storage of plutonium is of great importance to the country. All available evidence confirms that our current solution is correct. The new study forms part of this evidence by gaining insight into the complex and dynamic interactions between plutonium and the environment inside the tank.
This insight has allowed us to gain a deeper understanding of the underlying science and engineering requirements so that we can inform Sellafield Limited's storage and conditioning solutions. ”
Plutonium dioxide has a strong affinity for water and can easily be "sucked" from the atmosphere. Historically, laboratory experiments have shown that ionizing radiation is energetic enough to break down this water to form hydrogen. This rarely happens in storage tanks, suggesting that some additional process inhibits hydrogen production.
The new research, led by the UK's National Nuclear Laboratory on behalf of the Nuclear Decommissioning Authority and Sellafield***, focuses on historic storage tanks. Over a period of six years, dozens of samples of plutonium oxide powder with different properties were tested at the National Nuclear Laboratory, the world's leading specialized laboratory. Samples of different ** – and therefore different mixtures of plutonium isotopes and trace elements – were analyzed in an atmosphere where relative humidity and composition both vary in a controlled manner.
Plutonium produced in the UK is unique in that it is produced from two different types of reactors. The Magnox reactor is a product of the UK's historic civilian nuclear program. The plutonium isotopes they produced were less radioactive than those produced in alternative advanced gas reactors (AGRs), which were later used in the UK. Different processes are used to separate the isotopes from the spent fuel.
AGR fuel is processed at the Thermal Oxide Retreatment Plant or THORP Reprocessing Plant in Sellafield, Cumbria, UK. Once plutonium is separated from other elements in spent nuclear fuel, it is processed into oxide powder, and the exact processing steps in the UK are different from those used in other countries.
The dynamic interaction within the tank is also unique. According to Dr. Orr, "You can't turn off radiation from nuclear material, so even if you measure it, the plutonium and the environment inside the tank are changing." In the past, we have conducted controlled experiments on materials similar to plutonium oxide, which have provided some clues, but this is not the same as studying plutonium itself. You really need to look at the real material.
Because the environment inside the tank is changing so quickly, researchers look for trends or patterns in the data to help narrow down what might be happening to inhibit hydrogen production.
As the relative humidity decreases, so does the production of hydrogen. Intuitively, this makes sense, as the amount of water limits the amount of hydrogen that can be produced. At very low relative humidity, water is strongly absorbed by the powder, which binds to the powder and prevents it from escaping as a gas.
Samples in a normal air atmosphere tend to produce more hydrogen than samples in an argon or nitrogen atmosphere. This suggests that a mixture of oxygen, nitrogen, carbon dioxide, and trace gases in the air alters the chemical reactions caused by radiation.
Radiation, like particles released by plutonium, causes a cascade of reactions; Even a simple molecule like water can go through dozens of them. The energy produced by radiation can be absorbed by matter and can even be transferred from one material to another.
The interface between plutonium oxide powder and its surface water is very important. Powders have very small pores, so small that water molecules are more likely to come out of the pore walls** than to collide with other water molecules. These interactions also affect radiation chemistry.
Higher porosity means more surface area of the powder. Surprisingly, there is no clear correlation between surface area and hydrogen production, probably because small differences in powder composition affect surface chemistry.
Dr Luke Jones previously worked on the radiolysis of water during his research at the University of Manchester and is now a Senior Research Technologist at NNNL. He explains the challenge of understanding the interface between water and powder, "There are many different variables to consider. Even if you are using a non-radioactive substitute for plutonium dioxide, with an external radiation field that can be controlled, you still need to consider the purity of the sample as well as the microstructure of the powder.
Small differences can have a significant impact on the myriad of chemical reactions that occur when water is irradiated. When water is confined to pores, it affects how water molecules and debris produced by radiation diffuse. If the diffusion is restricted, then it affects the rate of the reaction. Different chemicals located on the surface of the powder can also interact with water fragments.
There are some discrepancies in the data collected from the tank. Samples from the Magnox reactor (via Sellafield's Magnox posttreatment plant) tend to exhibit less variability than samples from the Thorp posttreatment route. Since Magnox-derived plutonium contains isotopes that are less radioactive than those of the THORP processing route, the radiation dose rate appears to affect the chemical reaction.
A lower dose means that the concentration of certain reaction products will be lower, which in turn will affect further reactions. This variability in the data suggests that the mechanisms influencing hydrogen production are very sensitive to small changes in the powder surface.
These trends also suggest that the type of atmosphere inside the tank also plays a role in inhibiting hydrogen production; Previously, it was thought that only the interaction of the powder surface was important. Dr. Orr explains the importance of these results, "Radiation chemistry depends on many factors. Our results show that even small changes in trace elements on the surface of the powder, the porosity of the powder, and the atmosphere in the tank can affect a myriad of chemical reactions. "
The results show that temporary storage inside these specially designed tanks is the right choice; Although they have a dynamic, changeable environment inside, we can see that there is little change.
The UK has a proud nuclear heritage dating back to the 1950s, when the world's first industrial-scale nuclear power plant was connected to the national grid and began providing carbon-neutral electricity to the country. Radiation science has long been part of this legacy. The current research program at the US National Nuclear Laboratory began in 2011 with the aim of revealing the dynamic processes that occur in stored plutonium. The initial work focused solely on natural gas production.
In collaboration with researchers in the United States, a large amount of information was gathered in the following years that showed that the powder was stored safely. Since then, there has been a major shift in research to reveal the details of radiation chemistry. This data will be used to form theories and create specific experiments to test them.
The findings were published in the journal Frontiers in Nuclear Engineering.
More information: Kevin Webb et al., Effects of relative humidity, surface area, and production routes on plutonium dioxide surface water hydrogen production, Frontiers in Nuclear Engineering (2023). doi： 10.3389/fnuen.2023.1127504