Nuclear Waste

Spent Fuel - Fuel Assembly
Typical fuel assembly

Nuclear waste, is primarily spent fuel removed from reactors after producing electricity. Nuclear waste is also a type of nuclear waste created by the reprocessing of spent nuclear fuel (e.g. waste formed by vitrification of liquid high-level waste). But in this case, the term high-level waste is preferred instead of nuclear waste. It must be noted, we have to distinguish between nuclear waste and radioactive waste. Nuclear waste contains nuclear material (fissile or fertile), while radioactive waste is any waste that contains radioactive material. Radioactive waste is a byproduct from nuclear reactors, fuel processing plants, hospitals, various industrial applications and research facilities. Radioactive waste is hazardous to most forms of life and the environment, and is regulated by government agencies in order to protect human health and the environment.

Spent Nuclear Fuel

Spent nuclear fuel, also called the used nuclear fuel, is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor) and that must be replaced by a fresh fuel due to its insufficient reactivity. The reduction of reactivity is a combinative effect of:

  • the net reduction of fissile nuclides,
  • the production of neutron-absorbing nuclides (non-fissile actinides and fission products)

Due to presence of high amount of radioactive fission fragments and transuranic elements, spent nuclear fuel is very hot and very radioactive. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor.  In nuclear power plants, spent nuclear fuel is usually stored underwater in the spent fuel pool on the plant.  Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged.

For once-through cycle, interim storage can be either at the power plant site or at a centralized location that stores the fuel from more than one power plant. After a minimum period of 50 to 100 years of interim storage, spent nuclear fuel must be transferred to a final disposal facility. Currently, the preferred option is a deep geological repository, which is an underground emplacement in stable geological formations. The once-through cycle considers the spent nuclear fuel to be high-level waste (HLW) and, consequently, it is directly disposed of in a storage facility without being put through to any chemical processes, where it will be safely stored for millions of years until its radiotoxicity reaches natural uranium levels or another safe reference level.

This strategy is favored by several countries: the United States, Canada, Sweden, Finland, Spain and South Africa. Some countries, notably Finland, Sweden and Canada, have designed repositories to permit future recovery of the material should the need arise, while others plan for permanent sequestration in a geological repository like the Yucca Mountain nuclear waste repository in the United States.

Typical Long-lived Radionuclides – Spent Fuel

Actinides

  • 239Pu. 239Pu belongs to the group of fissile isotopes. 239Pu decays via alpha decay to 235U with half-life of 24100 years. This isotope is the principal fissile isotope in use.
  • 240Pu. 240Pu belongs to the group of fertile isotopes. 240Pu decays via alpha decay to 236U with half-life of 6560 years. 240Pu has very high rate of spontaneous fission and has high radiative capture cross-section for thermal and also for resonance neutrons.
  • 236U. 236U is neither a fissile isotope, nor a fertile isotope. 236U is fissionable only by fast neutrons. Isotope 236U is formed in a nuclear reactor from fissile isotope 235U. 236U decays via alpha decay to 232Th with half-life of ~2.3×107 years. 236U occasionally decays by spontaneous fission with very low probability of 0.00000009%. Its specific activity is ~6.5×10-5 Ci/g.

Fission Products

  • 135Cs. 135Cs is a fission product (yield: 6.911%) with half-life of ~2.3×106 years.
  • 99Tc. 99Tc is a fission product (yield: 6.139%) with half-life of ~0.211×106 years.
  • 93Zr. 93Zr is a fission product (yield: 5.458%) with half-life of ~1.53×106 years.
  • 129I. 129I is a fission product (yield: 0.841%) with half-life of ~15.7×106 years.

Typical Medium-lived Radionuclides – Spent Fuel

Actinides

  • 241Pu. 241Pu belongs to the group of fissile isotopes. 241Pu decays via negative beta decay to 241Am with half-life of 14.3 years. This fissile isotope decays to non-fissile isotope with high radiative capture cross-section for thermal neutrons. An impact on reactivity of the nuclear fuel  is obvious.
  • 232U. 232U belongs to the group of fertile isotopes. 232U is a side product in the thorium fuel cycle and also this isotope is a decay product of 236Pu in the uranium fuel. 232U decays via alpha decay to 228Th with half-life of 68.9 years. 232U very rarely decays by spontaneous fission. Its specific activity is very high ~22 Ci/g and its decay chain produces very penetrating gamma rays.

Fission Products

  • 137Cs. 137Cs is a fission product (yield: 6.337%) with half-life of ~ 30.23 years.
  • 90Sr. 90Sr is a fission product (yield: 4.505%) with half-life of ~ 28.9 years.
  • 151Sm. 151Sm is a fission product (yield: 0.531%) with half-life of ~ 88.8 years.
  • 85Kr. 85Kr is a fission product (yield: 0.218%) with half-life of ~ 10.76 years.
References:
Nuclear and Reactor Physics:
  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Radioactive Waste