Cooling 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.

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.

References:
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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.
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  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