Spent Fuel Transportation Package Response to the Newhall Pass Tunnel Fire Scenario, Final Report (NUREG/CR-7207, PNNL-23708)

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Publication Information

Manuscript Completed: July 2016
Date Published: October 2016

Prepared by:
H. E. Adkins, Jr.¹, J. M. Cuta¹, N. A. Klymyshyn¹,
S. R. Suffield¹, K. B. McGrattan², and C. S. Bajwa³,
C. E. Beyer¹, and A. Sotomayor-Rivera⁴

¹Pacific Northwest National Laboratory
P. O. Box 999
Richland, WA 99352

²National Institute of Standards and Technology
Engineering Laboratory
100 Bureau Drive, Stop 8600
Gaithersburg, MD 20899-8600

³International Atomic Energy Agency
Vienna International Centre
P.O. Box 100
A-1400 Vienna, Austria

⁴U.S. Nuclear Regulatory Commission

J. Piotter, NRC Project Manager

NRC Job Code J5701

Office of Nuclear Material Safety and Safeguards
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

The U.S. Nuclear Regulatory Commission has established requirements for packaging and transportation of spent nuclear fuel assemblies under normal conditions of transport and for hypothetical accident conditions. Real-world accidents of greater severity are possible, but are of much lower probability, and the probability of such an accident involving a spent nuclear fuel (SNF) package is even lower. However, because of the potential consequences, the U.S. Nuclear Regulatory Commission has undertaken the examination of specific accidents to determine the potential consequences to an SNF package. The Newhall Pass tunnel accident of October 2007, which did not involve SNF, was selected for evaluation because of the length of the fire and the wide range of potential fire exposure scenarios.

The General Atomics GA-4 Legal Weight Truck transportation package was selected for this investigation. Based on fire modeling with the Fire Dynamics Simulator code, and physical examination of material samples obtained onsite, a bounding fire scenario was defined for this accident. Due to uncertainties in the overall fire timeline and incomplete information on the actual cargo of some of the trucks, five specific fire modeling cases were defined to bound the possible range of fire conditions.

Detailed thermal models of the GA-4 package were constructed for the ANSYS and COBRA-SFS codes, and transient evaluations were performed to determine the response of the package to bounding cases defining the fire scenario. The peak fire temperatures obtained in the Fire Dynamics Simulator modeling for vehicles at the hottest fire location and longest fire location were used to define the fully engulfing fire conditions for the GA-4 package. These evaluations also included the post-fire cooldown transient. The ANSYS model predicted higher fuel temperatures, due mainly to the simplified representation of the fuel region. In three of the five cases, this model predicts that the peak cladding temperature at the hottest fire location would exceed the short-term limit of 1058°F (570°C), shortly after the end of the vehicle fire at that location. The more realistic predictions from the COBRA-SFS model show that this limit would not be exceeded in any of these cases. At the location with the longest fire duration, both models show peak temperatures that are under this limit.

Fuel cladding performance was evaluated using the FRAPTRAN-1.4 code. For the fuel region temperature histories from the ANSYS modeling, burst rupture is predicted to occur at 1038°F (559°C). For the ANSYS model thermal analysis results, this indicates fuel failure for three of the five cases at the hottest fire location. For the fuel cladding temperatures predicted with COBRA-SFS, fuel rod cladding would not fail by burst rupture in this fire scenario in any of the cases evaluated. Both models predict that seal temperature limits will be exceeded for several hours of the post-fire cooldown transient.

The potential release from this fire scenario is bounded by that from the more severe MacArthur Maze fire scenario, where no more than approximately one-fourth of the mixture A2 quantity is predicted to be released. Since the regulatory limit is specified as an A2 quantity per week for accident conditions, the estimated release is below the prescribed limit for safety. This very conservative estimate indicates that the potential release from this package, were it to be involved in a fire accident as severe as the Newhall Pass Tunnel fire scenario, would not pose a risk to public health and safety.