Spent Fuel Transportation Package Response to the Caldecott Tunnel Fire Scenario (NUREG/CR-6894, PNNL-15346, Revision 1)

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Publication Information

Manuscript Completed: December 2006
Date Published: January 2007

Prepared by:
H.E. Adkins, Jr., B.J. Koeppel, J.M. Cuta, A.D. Guzman
Pacific Northwest National Laboratory
Richland, Washington 99352

C.S. Bajwa
Division of Spent Fuel Storage and Transportation
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

A. Hansen, NRC Project Manager

Prepared for:
Division of Spent Fuel Storage and Transportation
Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code J5167

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Abstract

On April 7, 1982, a tank truck and trailer carrying 8,800 gallons of gasoline was involved in an accident in the Caldecott Tunnel on State Route 24 near Oakland, California. The tank trailer overturned and subsequently caught fire. Because this event is one of the most severe of the five major highway tunnel fires involving shipments of hazardous material that have occurred world wide since 1949, the United States Nuclear Regulatory Commission (USNRC) selected it for analysis to determine the possible regulatory implications of such events for the transportation of spent nuclear fuel by truck.

The Fire Dynamics Simulator (FDS) code developed and maintained by the National Institute of Standards and Technology (NIST) was used to determine the thermal environment in the Caldecott Tunnel during the fire. The FDS results were used to define boundary conditions for a thermal transient model of a truck transport package containing spent nuclear fuel. The Nuclear Assurance Corporation (NAC) Legal Weight Truck (LWT) transportation package was selected for this evaluation, as it represents a typical truck (over-the-road) cask.

Detailed analysis of the response of the transport package to the fire was performed using the ANSYS^® computer code. The staff concluded that small transportation packages similar to the NAC LWT would probably experience degradation of some seals in this severe accident scenario. The maximum temperatures predicted in the regions of the lid and the vent and drain ports exceed the rated service temperature of the tetrafluoro-ethylene (TFE) or Viton^® seals, making it possible for a small release to occur due to CRUD that might spall off the surfaces of the fuel rods. However, any release is expected to be very small due to a number of factors. These include (1) the metallic lid seal does not exceed its rated service temperature and therefore can be assumed to remain intact, (2) the tight clearances maintained by the lid closure bolts, (3) the low pressure differential between the package interior and exterior, (4) the tendency for solid particles to plug small clearance gaps and narrow convoluted flow paths such as the vent and drain ports, and (5) the tendency of CRUD particles to settle or plate out and consequently not be available for release.

USNRC staff evaluated the radiological consequences of the package response to the Caldecott Tunnel fire. The results of this evaluation strongly indicate that neither spent nuclear fuel (SNF) particles nor fission products would be released from a spent fuel shipping package involved in a severe tunnel fire such as the Caldecott Tunnel fire. The NAC LWT design analyzed for the Caldecott Tunnel fire scenario does not reach internal temperatures that could result in rupture of the fuel cladding. Therefore, radioactive material (i.e., SNF particles or fission products) would be retained within the fuel rods. The potential release calculated for the NAC LWT package in this scenario indicates that any release of CRUD from the package would be very small – less than an A₂ quantity (see Section 8.2).