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Transmutation of high-level radioactive waste
Partitioning and transmutation (P&T) promises to provide a technology to process high-level radioactive waste in such a way that the period of time during which it shows significant levels of radioactivity is reduced. Up to now, however, the P&T technologies have only been demonstrated on laboratory scale. Even with intensive research, it would probably take several decades for the technology to be ready for use. A final repository for high-level radioactive waste would still be necessary, as only a part of the high-level nuclear waste can be converted. For the foreseeable future, deep geological disposal will be the better alternative: such is the conclusion of a recent expert report commissioned by BASE.
Scientists around the world have been researching various ways to safely dispose of high-level radioactive waste for decades. One option, which only exists in theory so far, is the industrial application of transmutation to reduce the amount of radioactive waste. This process is designed to purposefully convert long-lived components of the waste into short-lived or stable substances.
How does this technology work? And is it an alternative to final disposal in deep geological rock strata?
What is transmutation?
Long-lived components such as uranium and plutonium are separated and converted into short-lived components, and energy is produced in the process.
Source: BASE
Transmutation is a physical process in which one element is converted into another. This already happens for some materials (especially uranium and plutonium) as a side effect of electricity generation in nuclear power plants. In the context of waste treatment, transmutation means that long-lived radioactive nuclei (radionuclides) are converted into short-lived or stable nuclei. A transmutation process for targeted industrial waste treatment does not yet exist. The hope is that, should this be technically feasible one day, the highly radioactive nuclear waste would no longer endanger people and the environment for hundreds of thousands of years, but only for a much shorter period. In return, however, the volume of low- and intermediate-level waste would probably increase significantly. A final repository for high-level radioactive waste would still be needed because only part of the high-level radioactive waste can be transmuted at all.
What does nuclear waste actually consist of?
High-level radioactive waste from nuclear power plants is a mixture of different substances or groups of substances, each of which have different properties. Since high-level radioactive waste has a high hazard potential over a very long period of time, it must be shielded from the environment in a repository for a million years. The main waste components are:
Uraniumshow / hide
Uranium is mined from the earth and processed into fuel. However, most of the uranium, a good 94%, is not fissioned during its use in the nuclear power plant, but is disposed of as part of the high-level radioactive waste. The uranium used in power plants only has a very low radiation level, and is not very mobile in the ground. Some P&T concepts therefore envision not converting it. Instead, it would be separated from the rest of the waste and sent directly to a repository.
Transuranic elementsshow / hide
Transuranic elements are substances that are formed when uranium captures neutrons instead of being split by them. This results in the formation of elements with a higher atomic number - i.e. with more protons - than uranium. The elements neptunium, plutonium, americium and curium are of particular importance. Transuranic elements account for about 1.5% of German high-level radioactive waste (in the form of fuel elements). They contribute to high radiation and large heat release of the irradiated fuel elements
Fission productsshow / hide
Fission products are formed when uranium or transuranic elements are split. Fission products account for about 4% of high-level radioactive waste (in the form of fuel assemblies). They contribute most to radiation in the first decades as well as to heat release from spent fuel. Fission products are often highly soluble in water and thus very mobile in the soil.
How does transmutation work in the context of waste treatment?
The envisaged technical process for waste treatment is called "partitioning and transmutation" (P&T) and consists of three steps: Separation (partitioning), fuel production, and conversion (transmutation).
During the partitioning process, transuranic elements are separated from the spent fuel. This process already exists for uranium and plutonium in reprocessing plants. Considerable technical development is needed to separate the other transuranic elements as well. So far, this has only been feasible in the laboratory.
The separated transuranic elements will then be processed into new fuel elements and bombarded with neutrons in special reactors. Some of the transuranic elements will be split and converted into short-lived or stable atomic nuclei. A small amount of long-lived fission products, such as iodine-129, will also be produced.
Yet, the P&T process would have to be repeated many times, since only a part of the transuranic elements can be converted each time.
Source: BASE
Is transmutation feasible in practice?
So far, there is no industrial-scale transmutation facility. According to an expert opinion commissioned by BASE, it could be many decades before this becomes possible. The research and development work would be associated with high costs.
According to model calculations, three to 23 of the nuclear power plants designed for transmutation would have to operate between 55 and 300 years to transmute a large proportion of Germany's transuranic elements. The P&T process would thus require the establishment of an extensive nuclear industry. This is not covered by the current legal situation in Germany. The reason: Following the reactor catastrophes at Chernobyl and Fukushima, there was a broad social consensus in this country that nuclear power plants should no longer be operated in the future. The last three nuclear power plants in Germany will be shut down by the end of 2022 at the latest.
Can transmutation replace a final repository?
Even with transmutation, a repository for high-level radioactive waste would be necessary. There are three particular reasons for this:
- Even with repeated transmutation, transuranic residues that would have to be transferred to a final repository will remain.
- Long-lived fission products (both existing and new) would have to be emplaced in a repository.
- Only a portion of the high-level waste is in the form of fuel assemblies. Approximately 40% of the waste was vitrified during reprocessing. In such cases, repartitioning would be much more challenging.
Furthermore, it should be noted that the amount of low and intermediate-level radioactive waste, for example from the dismantling of the plants, would increase considerably.
Conclusion
It is unclear when the transmutation technology for the industrial treatment of radioactive waste will be available, and how efficient it will be. Relying on this technology as a substitute for a final repository for high-level radioactive waste is thus incompatible with the principle of responsibility. This principle is enshrined in the Site Selection Act and stipulates that the best possible protection of humans and the environment from the effects of ionizing radiation, must be ensured, and unreasonable burdens for future generations must be avoided.
The future
Should partitioning & transmutation actually be developed to industrial maturity in the coming decades or centuries, and should it become possible to reduce the amount of high-level radioactive waste, the Site Selection Act provides for corrective options. According to the law, there must be a retrieval option for high-level waste until the repository is sealed.
Expert opinion on partitioning and transmutation
Concepts for partitioning and transmutation are being discussed and researched internationally. With the help of transmutation, high-level radioactive nuclear waste is to be processed in such a way that the radiation decreases more quickly. But, so far, this is only theory. BASE has commissioned an expert report to examine whether these concepts can be implemented in practice.
State of 2022.05.24