I often return to the problem of radioactive waste disposal in this blog. In the U.S. alone, the cooling pools at nuclear reactor sites are rapidly filling up with spent nuclear fuel rods. The soonest that the U.S. will have a permanent geological repository for spent nuclear fuel is estimated to be 2050. In the meantime, it looks like the only realistic interim solution for spent nuclear fuel is storage in dry casks made of steel and concrete.
Scientists at the Tokyo Institute of Technology, Tohoku University, TokyoCity University and the Japan Atomic Energy Agency are collaborating on a novel approach to dealing with radioactive waste. An article describing the research was recently published in the journal Scientific Reports. This new approach could severely shorten the effective half-life of what are called long-lived fission products (LLFP) from hundreds of thousands of years to less than a hundred years. The specific type of radioactive waste being discussed is what is left over after uranium and plutonium have been recovered from spent nuclear fuel by application of reprocessing methods such as Plutonium Uranium Redox Extraction.
Ultimately, radioactive waste should be permanently stored deep underground in dedicated repositories. In the meantime, various methods are being considered to reduce the volume and toxicity of radioactive waste that has to be buried. One promising method is called the partitioning and transmutation method or P&T. In this approach, minor actinides (MAs) and LLFPs are separated from the spent nuclear fuel and then they are transmuted into nuclides which have a much shorter half-life. One of the limitations of the P&T method is the fact that the equipment necessary to separate the LLFPs from the rest of the waste is expensive. Another concern about P&T is that some LLFPs have a small neutron cross section which means that they have difficulty absorbing enough neutrons for transmutation to take place.
The Japanese researchers are working on a different route to transmutation for the LLFPs that does not require the LLFPs to be separated from the rest of the waste before transmutation. The waste to be transmuted is placed in the radial blanket and shield sections of a small fast breeder or fast spectrum reactor. The researchers found that placing yttrium deuteride (YD2), a moderator which slows down neutrons, in the radial blanket vastly improved the transmutation of the LLFPs. They say that the YD2 is able “to soften the neutron spectrum leaking from the core”.
The Japanese team chose to focus on six LLFPs: selenium-79, zirconium-93, technetium-99, palladium-107, iodine-129 and caesium-135. Computer modeling showed that the half-lives of these radioisotopes could be radically reduced by transmutation into other element so that the radioactive waste would “cool” off in a much shorter span of time that with other waste treatment methods.
If their method is utilized, the researchers say that the seventeen thousand tons of LLFPs current being stored in Japan could be disposed of in ten fast spectrum reactors. In addition, the new method can generate electricity and will aid in reducing the chance for the proliferation of nuclear weapons.
