Power generation via nuclear fission results in the production of dangerous, long lived radioactive isotopes. Current protocol for handling this waste is typically long term storage in deep geologic repositories or shallow storage facilities, depending on the specific isotope. [1] However, accounting for the extreme half lives of some of these elements in the storage and disposal problem is not trivial task and requires strong consideration when discussing nuclear power as an option for the world's energy needs.
The application of particle physics and accelerator technologies to the process and utilization of the nuclear fuel cycle provides some additional considerations when addressing this problem. A couple of these considerations will be briefly addressed here, with links to detailed information in the references.
Nuclear transmutation of certain radioisotopes is possible using accelerator driven systems (ADS). A proton beam of high energy, on the GeV scale, is delivered to heavy metal target that drives the production of spallation neutrons. [2] The spallation neutrons can have sufficient energy to interact with nuclei of radioisotopes, leading to either fission or transmutation. Table 1 lists some of the isotopes that are produced and/or utilized in the nuclear fuel process of Uranium-235.
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| Table 1: Some important components of nuclear waste. |
An example transmutation scheme from a long lived radioisotope to a stable element: [3]
An alternative method for transmuting Iodine-129 has been demonstrated by photo-transmutation by using high power pulsed laser beams. [4] This non-conventional mode of acceleration, generates relativistic electrons within a high Z target (Au in this paper). The electrons produce a spectrum of bremsstrahlung x-ray and gamma rays that are absorbed by the Iodine-129 nucleus [5]:
I129 + γ → I128 + n
I128 → Xe128 + β- + ν (93.1%)
I128 → Te128 + β+ + ν → Xe128 + 2β- + 2ν (6.9%)
Transmutation of the most abundant isotope in spent nuclear fuel, Uranium-238, would lead the production Plutonium-239. Although, Plutonium-239 can be consumed as a fuel in breeder reactor it also is a concern for nuclear arms proliferation. Furthermore, when Plutonium-239 is fissioned it produces both short and long lived radioisotopes. For these reasons, transmutation of Uranium-238 purely for waste reduction on its own is not advisable.
To take accelerator transmutation of nuclear fuel one step further, research is being conducted on improving accelerator driven systems that will both produce nuclear power and reduce nuclear waste. [2] In these systems fission reactions are driven by the stream of neutrons delivered by the accelerator, which allows for fertile fuel to be used. A good candidate fuel for this type of system is Thorium-232, which is more abundant than current nuclear fuel sources. This isotope will absorb slow neutrons and transmute into a the fissile material Uranium-233. The process of converting Thorium-232 to Uranium-233 for fuel can be engineered to produce less waste by incinerating waste actinides and the long term fission products. ADS could also offer added security by processing the waste on site where Uranium-235 is burned.
© Matt Noll. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1]Y. A. Korovin, et al., "High energy activation data library (HEAD-2009)", Nuc. Instr. Meth. Phys. Res. A 624, 20 (2010).
[2] J. Lilley, Nuclear Physics: Principles and Applications (Wiley, 2001), p. 294.
[3] M. Kahn and J. Kleinberg, "Radiochemistry of Iodine," National Academy of Sciences, NAS-NS-3082, September 1977.
[4] K. W. D. Ledingham, et al., "Laser-Driven Photo-Transmutation of I129 - a Long-Lived Nuclear Waste Product," J. Phys. D 36, L79 (2003).
[5] E. W. Schneider et al., "Collective Excitations in 128Xe Observed Following the Decay of 128Cs and 128I" Phys.Rev. C, 19, 1025 (1979).