Nuclear Waste Disposal and Storage

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)

Storage and Final Disposal

Final disposal, or permanent disposal, is a final stage of the back end of the nuclear fuel cycle. Final disposal is unavoidable and common for all the strategies of nuclear fuel cycles, despite of the reduction in waste volume and radiotoxicity with current or future reprocessing techniques. This last step is the final disposal of the waste and whether it is untreated spent nuclear fuel or vitrified high-level waste arising from fuel reprocessing, it is still necessary to safely store them for the long-term until its radioactivity reaches safe levels. However, the period of time they require safe storage is highly dependent on the reprocessing technologies. For example, the once-through cycle comprises two main back end stages:

once-through fuel cycle - open fuel cycle
Nuclear Fuel Cycle. Source: Nuclear Regulatory Commission from US. License: CC BY 2.0

In this cases, the fuel assemblies are first after irradiation stored in spent fuel pools at the reactor site for an initial cooling period. 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 interim storaged. 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 (DGR), which is an underground emplacement in stable geological formations. Crystalline rock (granite, welded tuff and basalt), salt and clay are the most suitable formations for geological disposal. 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.

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