Dry storage casks are used to store spent nuclear fuel after removal from the reactor spent fuel pool. Even prior to the Fukushima earthquake of March 2011, dry storage of spent fuel was receiving increased attention as many reactor spent fuel pools near their capacity. Many different types of cask designs are used, and one representative design is the TN-24P spent fuel cask, a non-ventilated steel cask with a shielded exterior shell and lid. The cask is typically filled with an inert gas such as helium, argon or nitrogen. In this paper, Computational Fluid Dynamics (CFD) calculation results for the thermal performance of the TN-24P cask using the commercial CFD software STAR-CCM+ are presented. Initial calculations employ a common approach of treating the fuel assemblies as conducting porous media with calibrated volume-averaged properties, and comparison to existing measured temperature data shows good agreement. One of the fuel assemblies is then replaced with a more accurate representation that includes the full geometric detail of the fuel rods, guide tubes, spacer grids and end fittings (flow nozzles), and the results shown are consistent with the initial analysis, but without the need for the assumptions inherent in the porous media approach. This hybrid modeling approach also permits the direct determination of important results, such as the precise location of peak fuel cladding temperatures (PCTs), which is not possible using the more traditional porous media approach.

In 2007, a severe transportation accident occurred near Oakland, California, at the interchange known as the “MacArthur Maze.” The accident involved a double tanker truck of gasoline overturning and bursting into flames. The subsequent fire reduced the strength of the supporting steel structure of an overhead interstate roadway causing the collapse of portions of that overpass onto the lower roadway in less than 20 minutes. The US Nuclear Regulatory Commission has analyzed what might have happened had a spent nuclear fuel transportation package been involved in this accident, to determine if there are any potential regulatory implications of this accident to the safe transport of spent nuclear fuel in the United States. This paper provides a summary of this effort, presents preliminary results and conclusions, and discusses future work related to the NRC’s analysis of the consequences of this type of severe accident.

On July 18, 2001, a freight train carrying hazardous (non-nuclear) materials derailed and caught fire while passing through the Howard Street railroad tunnel in downtown Baltimore, Maryland. The United States Nuclear Regulatory Commission (USNRC), one of the agencies responsible for ensuring the safe transportation of radioactive materials in the United States, undertook an investigation of the train derailment and fire to determine the possible regulatory implications of this particular event for the transportation of spent nuclear fuel by railroad.The USNRC met with the National Transportation Safety Board (NTSB) to discuss the details of the accident and the ensuing fire. Following these discussions, the USNRC assembled a team of experts from the National Institute of Standards and Technology (NIST), the Center for Nuclear Waste Regulatory Analyses (CNWRA), and Pacific Northwest National Laboratory (PNNL) to determine the thermal conditions that existed in the Howard Street tunnel fire and analyze the potential effects of those conditions on various spent nuclear fuel transportation package designs.The Fire Dynamics Simulator (FDS) code developed by NIST was used to determine the thermal environment in the Howard Street tunnel during the fire. The FDS results were used as boundary conditions for the COBRA-SFS and ANSYS ® computer models developed to evaluate the thermal performance of different package designs. The staff concluded that larger transportation packages resembling the TransNuclear Model No. TN-68 and HOLTEC Model No. HI-STAR 100 would withstand a fire with thermal conditions similar to those that existed in the Baltimore tunnel fire event with only minor damage to peripheral components. This is due to their sizable thermal inertia and design specifications in compliance with currently imposed regulatory requirements.For the TN-68 and the NAC International Model No. LWT (legal weight truck) transportation package, the maximum temperatures predicted in the regions of the lid and the vent and drain ports exceed the seals' rated service temperatures, making it possible for a small release to occur, due to CRUD that might spall off the surfaces of the fuel rods. While a release is not expected to occur for these conditions, any release that could occur would be very small due to a number of factors. These include (1) the tight clearances maintained between the lid and cask body by the closure bolts, (2) the low pressure differential between the package interior and exterior, (3) the tendency of such small clearances to plug, and (4) the tendency of CRUD particles to settle or plate out.USNRC staff evaluated the radiological con-sequences of the package responses to the Baltimore tunnel fire. The analysis indicates that the regulatory dose rate limits specified in 10 CFR 71.51 for accident conditions would not be exceeded by releases or direct radiation from any of these packages in this fire scenario. All three packages are designed to maintain regulatory dose rate limits even with a complete ...

Title 10 of the Code of Federal Regulations Part 71 section 73(c)(4), (10 CFR 71.73(c)(4)) requires that transportation packages used to ship radioactive material must be designed to resist an engulfing fire of a 30 minute duration and prevent release of radioactive material to the environment. In July, 2001, a derailed train carrying hazardous materials caught fire in a railroad tunnel in Baltimore, Maryland, and burned for several days. Although the occurrence of a fire of such duration during the shipment of spent nuclear fuel is unlikely, questions were raised about the performance of spent nuclear fuel casks under conditions similar to those experienced in the Baltimore tunnel fire incident. The U.S. Nuclear Regulatory Commission evaluates the performance of spent fuel transportation casks under accident conditions. The National Transportation Safety Board is responsible for investigating railroad accidents and identifying the probable cause(s) and offers recommendations for safety improvements. They are currently investigating the Baltimore tunnel fire accident. This paper assesses the performance of a spent fuel transportation cask with a welded canister under severe fire conditions. The paper describes the analytic model used for the assessment and presents a discussion of the preliminary results.

In 2007, a severe transportation accident occurred near Oakland, California, on a section of Interstate 880 known as the “MacArthur Maze,” involving a gasoline tanker truck which impacted an overpass support column and burst into flames. The fire caused the collapse of portions of the Interstate 580 overpass onto the remains of the tractor-trailer. The U.S. Nuclear Regulatory Commission, with assistance from Pacific Northwest National Laboratory, the Center for Nuclear Waste Regulatory Analyses, the Southwest Research Institute, and the National Institute of Standards and Technology, examined the accident conditions in order to characterize the fire and collapse that occurred, analyzed material samples from the collapsed I-580 overpass as well as the gasoline tanker truck, and developed a fire model of the accident. This was followed by development of a finite element analysis model to determine the impacts of this accident on the thermal and structural performance of a spent nuclear fuel (SNF) transportation package. The analysis results will be used to determine any potential regulatory implications related to the safe transport of SNF in the U.S. This paper provides a summary of this effort and presents some preliminary results and conclusions.