Wait and See

nuclear fuel cycle
Nuclear Fuel Cycle. Source: Nuclear Regulatory Commission from US. License: CC BY 2.0

The nuclear fuel cycle is a process chain consisting of a series of differing stages. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of used fuel as an option for nuclear energy, the stages form a true cycle. In general, nuclear fuel cycle consists of steps in the front end (the preparation of the fuel), steps in the service period (fuel burnup), and steps in the back end (reprocessing or disposal of spent nuclear fuel).

  • Front end of nuclear fuel cycle. The front end of the nuclear fuel cycle starts with the mining of uranium in the mines and ends with the delivery of the enriched uranium to the nuclear fuel assembly producer. Therefore, the front end of fuel cycle consist of:
    • Uranium mining, milling and mill tailings,
    • Conversion
    • Fuel enrichment
    • Fabrication of fuel assemblies
  • Service Period. Service period includes transport of fuel assemblies within a power plant, in-core fuel management, fuel utilization and storage in the spent fuel pool.
  • Back end of nuclear fuel cycle. The back end of the nuclear fuel cycle involves managing the spent fuel after irradiation. Therefore, the back end of fuel cycle consist of:
    • spent fuel interim storage
    • fuel reprocessing,
    • final disposal of radioactive waste or spent fuel.

Open Fuel Cycle – Once-through Fuel Cycle

Wait and See approach - fuel cycle
Nuclear Fuel Cycle. Source: Nuclear Regulatory Commission from US. License: CC BY 2.0

Most countries with nuclear programs use some type of interim storage as their back end strategy. They have explicitly decided to take a “wait and see” approach to spent fuel management, leaving their spent fuel in interim storage, which leaves both the reprocessing and direct disposal options open for the future. The wait and see option, as the name implies, proposes interim storage until some solution for permanent storage and disposal will be developed in the future. For many operators, the spent fuel represents also a strategic material, since the spent nuclear fuel still contains about 96% of reusable material. Storing spent fuel and waste for several years allows heat release and radioactivity to subside. Despite the continued debate over the future of the fuel cycle, a quiet consensus has developed that for the near term, simply storing spent fuel while continuing to develop more permanent solutions is an attractive approach. Currently, spent fuel is being stored in at-reactor (AR) or away-from-reactor (APR) storage facilities and we can identify two basic solutions of interim storage:

  • Dry Storage
  • Wet Storage

Interim Storage

Interim storage is a temporary solution that plays a central role in the management of the most highly radioactive materials: spent nuclear fuel and vitrified waste resulting from reprocessing such fuel. Since spent nuclear fuel is compact, plant operators are able to store fuel assemblies for a long time. It must be noted, the spent nuclear fuel is due to presence of high amount of radioactive fission fragments and transuranic elements 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.

Dry storage cask
Behältermodell CASTOR V/19. Source: GNS Gesellschaft für Nuklear-Service mbH

Dry storage is most often based on the use of spent fuel casks. As its name implies, dry storage of spent fuel assemblies differs from wet storage by making use of gas or air instead of water as the coolant and metal or concrete instead of water as the radiation barrier. In dry storage systems, sufficiently cooled spent fuel is not stored underwater, but it is loaded in these casks (or vaults or silos). If on-site pool storage capacity is exceeded, it may be desirable to store the spent fuel in modular dry storage facilities, which may be at the reactor site (AR) or at a facility away from the site (AFR). Spent fuel is transferred from spent fuel pools to thick metal casks or thinner canisters. These casks are then drained, filled with inert gas, and sealed. The thick casks can be placed directly on a concrete pad, while the thinner canisters are placed in concrete casks or vaults to provide radiation shielding. Many regulators have determined that dry storage of spent fuel at reactor sites is safe for at least 100 years, and generally considers dry storage safer than pool storage.

Types of Nuclear Fuel Cycles

As was written, the back end of the nuclear fuel cycle involves managing the spent fuel after irradiation. Therefore, the back end of the fuel cycle consist of:

  • spent fuel interim storage
  • fuel reprocessing,
  • final disposal of radioactive waste or spent fuel.

There are three main types of nuclear fuel cycle:

  • Once-through fuel cycle. In fact, an open fuel cycle is not a real cycle. This strategy assumes that the fuel is used once and then sent to long-term storage without further reprocessing. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle or a once-through fuel cycle as the uranium components go through the reactor just one single time. The once-through cycle comprises two main back end stages:
    • interim storage
    • final disposal.
  • Twice-through fuel cycle. The twice-through cycle strategy assumes, that the spent nuclear fuel will be reprocessed in order to extract the uranium and plutonium, which can either be recycled as fresh nuclear fuel for its use in a nuclear reactor that is adapted to this type of fuel.
  • Closed fuel cycle. The closed fuel cycle is an advanced fuel cycle, which purpose is to achieve nuclear power sustainability by further reducing radiotoxicity of the final waste, as well as improving resource utilization, while maintaining its economic viability. There are currently different types of advanced fuel cycles under research, but most of them are based on the use of:
    • Advanced Nuclear Reactors
    • Fuel Reprocessing

These types of fuel strategie are based on specific processes in entire fuel cycle (the back end, the front end, as well the service period). In fact, all these scenarios are theoretical and the practical solution will be in any case a combination of these options. In all cases, the fuel assemblies are first after irradiation stored in spent fuel pools at the reactor site for an initial cooling period. It must be added, any strategy for managing spent nuclear fuel will be built around combinations of many options, and all strategies must ultimately include permanent geological disposal.

The choices of nuclear fuel cycle (open, closed, or partially closed through limited spent fuel recycle) depend upon the technologies we develop and societal weighting of goals (safety, economics, waste management, and nonproliferation). Once choices are made, they will have major and very long term impacts on nuclear power development. Today we do not have sufficient knowledge to make informed choices for the best cycles and associated technologies.

See more: The Future of the Nuclear Fuel Cycle. An interdisciplinary MIT study. MIT, 2011. ISBN 978-0-9828008-4-3.

References:
Nuclear and Reactor Physics:
      1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
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      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:

Nuclear Fuel Cycle