Wet Storage of Spent Fuel

Spent Fuel - Fuel Assembly
Typical fuel assembly

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.

An irradiated 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. At first glance, it is difficult to recognize a fresh fuel from an used fuel. From mechanical point of view, the used fuel (irradiated) is identical as the fresh fuel. In most PWRs, used fuel assemblies stand between four and five metres high, are about 20 cm across and weighs about half a tonne. In contrast to the fresh fuel, which are simply shiny, the oxide layer forming on the surface of used fuel assemblies during the four-year fuel cycle makes them dark. Moreover,  Cherenkov radiation is typical only for spent nuclear fuel. The glow is visible also after the chain reaction stops (in the reactor). The cherenkov radiation can characterize the remaining radioactivity of spent nuclear fuel, therefore it can be used for measuring of fuel burnup.

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, where natural air circulation dissipates heat.
  • Wet Storage, where water is used as the heat conductor.

Wet Storage of Spent Fuel

Spent fuel pool
Spent fuel pool. Source: wikipedia.org License: Public Domain

Wet storage of spent nuclear fuel in wet storage pools is a common practice worldwide. It is very similar to at-reactor spent fuel storage, but in this case storage facilities are usually for many power plants. Irradiated 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. Many regulators have determined that dry storage of spent fuel at reactor sites is safer than wet storage. True AFR storage pools are independent of the reactor and all its services and can continue to operate after the reactor has been finally shut down and decommissioned. Centralized interim storage does not eliminate the need for developing a long-term disposal option, but it might provide other benefits.

A typical wet storage facility may have the following features:

  • Cask reception, decontamination, unloading, maintenance and dispatch
  • Underwater spent fuel storage (pool);
  • Auxiliary services (radiation monitoring, water cooling and purification, solid radioactive waste handling, ventilation, power supply etc.).

Spent fuel pools are fitted with stainless steel and aluminum racks that hold the fuel assemblies and are lined with stainless steel to prevent leaking. There are no drains that would allow the water level to drop or the pool to become empty. The plants have a variety of extra water sources and equipment to replenish water that evaporates over time, or in case there is a leak. Plant personnel are also trained and prepared to quickly respond to a problem. The water serves two purposes: it cools the fuel and shields workers at the plant from radioactivity. Although water is neither high density nor high Z material, it is commonly used as gamma shields. Water provides a radiation shielding of fuel assemblies in a spent fuel pool during storage or during transports from and into the reactor core. Although water is a low-density material and low Z material, it is commonly used in nuclear power plants, because these disadvantages can be compensated with increased thickness.

To conserve space, in all plants open storage racks were replaced with so called high-density racks that incorporate (boron-10) or other neutron-absorbing material to ensure subcriticality. Using such racks, fuel assemblies can be stored in about one half the volume required for storage in standard racks.

Safety of Spent Fuel Pool

Safety of AFR spent fuel pools stands on various criteria. These criteria are usually very similar to the criteria for AR pools and may be grouped according to following aspects:

  • Subcriticality. Fulfillment of this criterion is based on:
    • the design of the spent fuel pool,
    • requirements on boric acid diluted in water,
    • limiting of stored fuel (e.g. fuel enrichment, assembly burnup)
  • Cooling.  Fulfillment of this criterion is based on:
    • the design of the spent fuel pool (e.g. no drains below the top of the stored fuel elements),
    • requirements on water level in the pool,
    • requirements on active cooling elements (heat exchangers and heat sink).
  • Radiation Shielding.  Fulfillment of this criterion is based on:
    • the design of the spent fuel pool (water and concrete shielding),
    • the design of surrounding working area
    • requirements on water level in the pool,
  • Integrity. Fulfillment of this criterion is based on:
    • the design of the spent fuel pool,
    • the design of surrounding working area,
    • ensuring periodic inspections
References:
Nuclear and Reactor Physics:
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  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
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  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:

Spent Nuclear Fuel