US panel: Fuel pool attack could trigger zirconium fire

13 April 2005

Although the panel submitted its classified report to congress and the US Nuclear Regulatory Commission (NRC) in July 2004, the public report only was released 6 April, about three weeks after NRC released public comments critical of the report – comments that the director of the academies’ study called “an incomplete and less-than-accurate characterisation” of the academies’ findings.

The unclassified academies’ report, Safety and Security of Commercial Spent Nuclear Fuel: Public Report, contains all the findings and recommendations of the classified report, but with all national security and safeguards information removed, said Louis Lanzerotti, who chaired the 15-member expert panel pulled together by the academies’ Board on Radioactive Waste Management in response to a mandate from congress. The panel spent six months gathering and analyzing data, and meeting with regulators, nuclear industry experts, and independent scientists.

Lanzerotti is a geophysics expert consulting for Bell Laboratories and Lucent Technologies and a distinguished professor for solar-terrestrial research at the New Jersey Institute of Technology. Other panel members included a former NRC division director in nuclear materials management, and experts in the behaviour of nuclear materials at high temperatures, penetration mechanics, ballistics and weapons technology, health physics, actinide chemistry, heat transfer, thermal hydraulics, structural engineering and terrorism.

The panel unanimously concluded that an attack that caused either partial or complete draining of a plant’s spent fuel pool might be capable of starting a high-temperature fuel cladding fire that could lead to the “release of large quantities of radioactive material into the environment.” The risk depends on a number of factors, including the type of attack, the design of the fuel pool, and the configuration of the fuel in the pool.

Because of the large differences among US nuclear plants, their spent fuel pools and dry spent fuel storage facilities, the academies’ report called on NRC to conduct a plant-by-plant vulnerability analysis to determine which plants are most at risk. The panel did not review individual plants, and does not have information on how many, or which, cooling ponds are at risk, said Lanzerotti.

The panel recommended two immediate measures that could reduce the potential for fuel cladding fires:

  • The reconfiguration of the position of fuel assemblies in the pools to more evenly distribute decay heat loads.
  • Making provisions to cool the fuel with water spray systems that could continue to operate even after a pool or the building housing it is damaged.
The first measure “could probably be implemented at all plants with minimal cost and time, and with little exposure of workers to radiation,” the panel found. The costs and benefits of the water spray option “should be examined to decide what requirements should be imposed.” The panel noted that water spray systems might not be needed at plants where the fuel pools are located below ground or otherwise protected.

As part of its study, congress asked the panel to examine the possible advantages of dry cask storage over pool storage at commercial power plants.

Pools are and will continue to be needed at all nuclear plants for the foreseeable future, the panel stressed, noting that fuel newly removed from the reactor needs about five years cooling time in a water pool before it can be loaded into casks.

For older fuel, however, dry storage has two advantages. It is a passive system that relies on air circulation for cooling, and it divides the spent fuel inventory into a number of individual, robust containers that contain only a small amount of the total inventory. Different dry cask systems available on the US market differed only slightly in robustness under different terrorist attack scenarios, the panel found.

While noting that congress did not specifically ask the panel to recommend whether to accelerate the transfer of fuel from pools to dry storage, the panel did say that after the NRC concludes its plant-specific analyses, the regulator “may conclude that earlier movements of spent fuel from pools into dry cask storage would be prudent at some plants.”

Congress had also requested the academies to examine the possibility that terrorists, particularly insiders or those aided by insiders, could steal spent fuel to use in a 'dirty bomb'. The panel concluded that the likelihood of such theft is small, however, the National Research Council (USNRC) should review security precautions for those plants where one or more fuel rods, or rod fragments, have been removed from a fuel assembly and are stored independently in a fuel pool.

Finally, the panel cautioned that some current classification and security precautions appear to be impeding the sharing of valuable information between USNRC and nuclear industry operators, and recommended that USNRC improve information sharing among the regulatory agency, plant operators and systems vendors.




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