Once-through Fuel Cycle

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

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

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.

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, 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.

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).
      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:

Nuclear Fuel Cycle