Safety of Spent Fuel Pool

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

Spent fuel pool (SFP) is storage pool for spent nuclear fuel from nuclear reactors. Spent fuel pool may be located inside the containment building or inside the fuel building (outside the containment building). When located outside the containment building, the two areas are connected by a fuel transfer system which carries the fuel through a normally closed opening in the reactor containment. In this case spent fuel is removed from the reactor vessel by a manipulator crane and placed in the fuel transfer system. In the spent fuel pool, the fuel is removed from the transfer system and placed into storage racks. After a suitable decay period, the fuel can be removed from storage and loaded into a shipping cask for removal from the site. Spent fuel pools are typically 12m or more deep, with the bottom equipped with storage racks designed to hold fuel assemblies removed from the reactor. A reactor’s pool is specially designed for the reactor in which the fuel was used and situated at the reactor site.

Safety of Spent Fuel Pool

Safety of spent fuel pools stands on various criteria. These criteria 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

Subcriticality of Spent Fuel Pool

As was written, subcriticality of the spent fuel pool is ensured by:

  • the design of the spent fuel pool,
  • requirements on boric acid diluted in water,
  • limiting of stored fuel (e.g. fuel enrichment, assembly burnup)

Today, spent fuel is usually stored in so called high-density racks or in the maximum-density rack (MDR).  Using such racks, fuel assemblies can be stored in about one half the volume required for storage in standard racks. Higher storage densities have been achieved without the risk of a nuclear chain reaction by adding neutron absorbing materials (typically boron) in storage racks and baskets, and dissolved in the water itself. These racks incorporate (boron-10) or other neutron-absorbing material to ensure subcriticality. Boron-10 is generally present in the chemical form of boron carbide (B4C) within a metal matrix (e.g., Boral and Metamic (trademark of Metamic, LLC)) or a polymer matrix (e.g., Boraflex (trademark of BISCO), Carborundum, and Tetrabor), although borated stainless steel incorporates the boron-10 atoms into the alloy composition.

In the high-density rack design, the spent fuel storage pool may divided into two separate and distinct regions which, for the purpose of criticality considerations, are considered as separate pools. In Region 2, maximum reactivity fuel is not allowed to load, while it can be loaded in the racks of Region 1 of the pool.

Most conservative approach requires that the multiplication factor, assuming flooding with pure water and infinite geometry, does not exceed 0.95 with a full loading of the maximum anticipated enrichment. To satisfy this design criterion, the assumptions in the criticality evaluation are as below:

  • The fuel assemblies have the maximum approved initial enrichment with the highest reactivity in fuel’s lifetime, and without the control rods (burnable poisons may be taken into account).
  • In the flooding condition, all soluble poison is assumed to have been lost, specify that the limiting keff of 0.95 (5% subcriticality) be evaluated in the absence of soluble boron.
  • The array of the fuel assemblies can be taken as infinite geometry, or with reflective boundary condition.
  • The effect of structure material and the fixed neutron absorber can be considered.
  • Unless the double contingency principle is taken, the presence of the boron in the moderation should not be considered. This principle shows at least two independent, unlikely and concurrent incidents have to happen to lead a criticality accident.

Cooling of Spent Fuel Pool

Decay HeatIt must be noted, 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.

However, even at these low levels (about 0.25% after ten days), the amount of heat generated requires the continued removal of heat for an appreciable time after shutdown. Decay heat is a long-term consideration and impacts spent fuel handling, reprocessing, waste management, and reactor safety.

The design of the reactor and the spent fuel pool must allow for the removal of this decay heat from the core by some means. If adequate heat removal is not available, decay heat will increase the temperatures in the fuel to the point that fuel melting and fuel assembly damage will occur as in the case of Fukushima. The degree of concern with decay heat will vary according to reactor type and design.

As was written, cooling of the spent fuel pool is ensured by:

  • 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).

The minimum water level in the fuel storage pool meets primarily the assumptions of iodine decontamination factors following a fuel handling accident. The specified water level shields and minimizes the general area dose when the storage racks are filled to their maximum capacity. The water also provides shielding during the movement of spent fuel. But it also provides with the coolant required for cooling spent fuel.

Cooling of the spent fuel pool is usually ensured by the spent fuel pool cooling system, which is designed to remove residual decay heat generated by spent fuel stored in the spent fuel pool. The system also maintains the purity of the spent fuel cooling water and the refueling water. The system is designed to provide with cooling of stored spent fuel and it may become necessary to cool all fuel assemblies from totally unloaded reactor. The design incorporates redundant active components (usually 3×100%) and it is one of safety systems. One of principal safety functions of the ultimate heat sink (UHS) is dissipation of maximum expected decay heat from the spent fuel pool to ensure the pool temperature remains within the design bounds for the structure.

Shielding of Spent Fuel in Spent Fuel Pool

It must be noted, 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 and radioactivity decreases under specified limits so that spent fuel may be reprocessed or interim storaged.

As was written, shielding of the spent fuel pool is ensured by:

  • the design of the spent fuel pool (water and concrete shielding),
  • the design of surrounding working area
  • requirements on water level in the pool,

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.

The minimum water level in the fuel storage pool meets primarily the assumptions of iodine decontamination factors following a fuel handling accident. The specified water level shields and minimizes the general area dose when the storage racks are filled to their maximum capacity. The water also provides shielding during the movement of spent fuel. But it also provides with the coolant required for cooling spent fuel.

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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  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 Fuel Pool