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| Fig. 1: Neutrons strike U-235 nuclei and cause fission producing ~ 200 MeV of energy per fission. (Source: Wikimedia Commons) |
MITs Future of the Nuclear Fuel Cycle cites in today's once-through light-water reactors (LWRs), only about 5% of the energy value of the fuel is used in the reactor before the fuel is discarded as waste. [1] Standard once-through LWRs generate most of the world's nuclear electricity. [2] At first glance this sounds like a terrible efficiency number. We are used to thinking of efficiency as a single percentage that should be pushed as high as possible.
But this 5% figure does not mean that light-water reactors (LWRs) are inefficient in the same way a bad engine or a leaky insulation system is. LWRs operate near the practical limits for steam plants. Instead, the 5% statistic is about how much of the nuclear fuels potential energy we choose to exploit in the current fuel cycle.
Commercial LWRs use fuel pellets of uranium dioxide. Natural uranium is mostly the non-fissile isotope U-238, with only about 0.7% fissile U-235. [1] To sustain a chain reaction in ordinary water, the uranium must be enriched so that the fraction of U-235 is typically around 4- 5%. [2]
As depicted above in Fig. 1, inside the core, neutrons strike U-235 nuclei and cause fission, splitting them into lighter fragments, releasing more neutrons, and producing about 200 MeV of energy per fission event. [3] Some neutrons are also captured by U-238, gradually breeding plutonium isotopes that can themselves fission and contribute to power production. Meanwhile, fission products build up inside the fuel. Many of these fragments act as neutron poisons, soaking up neutrons and making it harder to keep the chain reaction going.
Fuel performance is therefore a compromise. On one hand, we would like to leave fuel in the reactor long enough to extract a lot of energy from it. On the other hand, as burnup increases these poisons build up and make the chain reaction harder to sustain, while also pushing the physical limits of fuel safety and integrity. As a result, fuel is discharged while much of its energy potential remains untapped raising the natural question...
A straightforward way to continue operating is simply to remove the spent assemblies and load fresh fuel. That is the U.S. once-through strategy. [2] From a thermodynamic standpoint, this choice looks wasteful. Chemical reprocessing of used fuel can raise the fraction of uranium energy that is ultimately fissioned. Reprocessing can separate residual uranium, newly created plutonium, and other fission products (i.e., true waste that cannot be burned further). Recovered uranium and plutonium can then be fabricated into new fuel and sent through reactors again.
Reprocessing is not just a physics question, however. For one thing, it is expensive. The MIT study concludes that, at historical uranium prices, once-through LWR fuel cycles were generally cheaper than full recycle, especially for countries without existing reprocessing infrastructure. [1] It is also politically sensitive, because pure separated plutonium is a proliferation risk. U.S. policy since the 1970s originally attempted to avoid commercial reprocessing for this reason and has more recently addressed reprocessing approaches often described as proliferation-resistant, even though all spent nuclear fuel contains plutonium. [4] And lastly, it moves, but does not eliminate, long-term waste liabilities. High-level waste volumes can be reduced, but new waste streams from reprocessing plants are created and repositories are still required. [1]
So while reprocessing is the obvious physics answer to 'What about the other 95%?', it has not yet been the obvious engineering, economic, or political answer in the United States. [1]
© Zac Maslia. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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] "The Future of the Nuclear Fuel Cycle," Massachusetts Institute of Technology, 2011.
[2] T. K. Kim and T. A. Taiwo, "Fuel Cycle Analysis of Once-Through Nuclear Systems," Argonne National Laboratory, ANL-FCRD-208, August 2010.
[3] "A Guide to the Nuclear Science Wall Chart," Contemporary Physics Education Project, 2019.
[4] Anthony Andrews, "Nuclear Fuel Reprocessing: U.S. Policy Development," Congressional Research Service Report, RS22542, March 2008.
