Spent Fuel Transportation Package Response to the MacArthur Maze Fire Scenario, Final Report (NUREG/CR-7206, PNNL-23701)

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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, C. S. Bajwa², K. B. McGrattan³,
C. E. Beyer¹, and A. Sotomayor-Rivera⁴

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

²International Atomic Energy Agency
Vienna International Centre
P.O. Box 100
A-1400 Vienna, Austria

³National Institute of Standards and Technology
Engineering Laboratory
100 Bureau Drive, Stop 8600
Gaithersburg, MD 20899-8600

⁴U.S. Nuclear Regulatory Commission

J. Piotter, NRC Project Manager

NRC Job Code J5710

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 MacArthur Maze accident of April 2007, which did not involve SNF, was selected for evaluation because of the severity of the fire and the unusual structural consequences, in which the heat from the fire caused the verhead roadway segments to collapse onto the roadway where the fire was burning.

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. The complex and dynamic fire conditions are represented as a fully engulfing pool fire at 2012°F (1100°C) prior to the overhead roadway collapse, and as a smaller and less severe fully engulfing pool fire at 1652°F (900°C) afterward.

Thermal models of the GA-4 package were constructed for the ANSYS and COBRA-SFS codes, to determine the response of the package to the fire scenario, including the long post-fire cooldown transient. Additional detailed structural and thermal-structural models were developed using ANSYS and LS-DYNA for the roadway and package, which showed that the falling overhead segments could impose only relatively innocuous loads on the stainless steel body and DU gamma shield, compared to the hypothetical accident conditions structural loading that the package is designed to withstand.

Thermal evaluations of the package response to this fire scenario predict that the peak cladding temperature would exceed the short-term limit of 1058°F (570°C) long before the end of the fire. Maximum cladding temperatures on all rods in the package are predicted to exceed this temperature limit in the course of the transient, and remain above this limit for several hours. The maximum peak cladding temperature in the transient is predicted to be in the range of 1350-1400°F (732-760°C), and occurs approximately 3 hours after the end of the postulated fire accident. Temperatures in the regions of the package seals exceed the seal material limits for most of the fire duration.

The FRAPTRAN-1.4 code was used to estimate a fuel rod burst rupture temperature of 1097°F (592°C). Together with the temperature histories of the fuel, this suggests that there is the potential for all rods in the package to rupture in this fire scenario. The package seals are assumed to fail. However, a detailed thermo-structural model showed that the lid closure bolts maintain a positive clamping force throughout the transient, thus limiting the release. Using conservative and bounding modeling assumptions, the total possible release was estimated at approximately one-fourth of the mixture A2. 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. Therefore 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 MacArthur Maze fire scenario, would not pose a risk to public health and safety.