SNF Transportation Package Response to MacArthur Maze Fire Scenario: Leak Rate Determination After Seal Failure

Extensive and detailed modeling of the possible effect of the MacArthur Maze fire scenario on an over-the-road spent nuclear fuel transportation package has shown that the potential consequences could include release of radioactive material due to failure of the package seals. Structural and thermal modeling of the performance of the lid closure and closure bolts show that the lid closure bolts would maintain positive clamping force throughout the fire transient scenario, such that the total release possible, even with conservative and bounding modeling assumptions, is two to three orders of magnitude below the regulatory limit for accident conditions. Typical leak rate models, such as the ANSI standard ANSI N14.5, are based on the expectation of intact seals for the package. Very little analytical work has been done to investigate leak rates from failed seals, since seal failure is, by definition, unacceptable performance in real-world applications. In order to evaluate the potential release from the SNF package subjected to the conditions of this fire scenario, an analytical modeling approach was developed to determine bounding leak rate estimates through the interface of the package closure lid and body flange. This modeling approach postulates complete loss of the O-ring seal material, and assumes only metal-to-metal contact, maintained by the clamping force of the closure bolts, as it varies due to differential thermal expansion and changing internal package pressure during the transient. This paper describes the analytical approach used to perform the leak rate modeling for the SNF package, and presents results for the limiting design basis loading of the package.