Christopher S. Bajwa

CFD Analyses of the TN-24P PWR Spent Fuel Storage Cask

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.Copyright

An Analysis of a Spent Fuel Transportation Cask Under Severe Fire Accident Conditions

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.Copyright

NRC's response to the National Academy of Science's transportation study: going the distance?

Abstract In February 2006, the National Academy of Sciences (NAS) published the results of a 3˙5-year study, titled Going the Distance, which examined the safety of transporting spent nuclear fuel (SNF) and high level waste (HLW) in the United States. NAS initiated this study to address what it perceived to be a national need for an independent, objective and authoritative analysis of SNF and HLW transport in the United States. The study was cosponsored by the US Nuclear Regulatory Commission, the US Department of Energy, the US Department of Transportation, the Electric Power Research Institute and the National Cooperative Highway Research Program. This paper addresses some of the recommendations made in the NAS study related to the performance of SNF transportation packages in long duration fires, the use of full scale package testing, and the need for an independent review of transportation security prior to the commencement of large scale shipping campaigns to an interim storage site or geologic repository.

Benchmarking of a Thermal Finite Element Approximation Scheme for Externally Cooled Spent Fuel Storage Casks

The US Nuclear Regulatory Commission (NRC) is responsible for licensing spent fuel storage casks under Title 10 of the Code of Federal Regulations Part 72 (10 CFR Part 72). Under these regulations, storage casks must be evaluated to verify that they meet various criteria, including acceptable thermal performance requirements. The purpose of the evaluation described in this paper is to establish the effectiveness of a medium-effort modeling approach and associated simplifying assumptions in closely approximating spent fuel cask component temperature distributions. This predictive evaluation is performed with the ANSYS® code, and is applicable to externally cooled cask designs. The results are compared against experimental measurements and predictions of the COBRA-SFS finite-difference code developed at Pacific Northwest National Laboratory.

Package Impact Models as a Precursor to Cladding Analysis

The evaluation of spent nuclear fuel storage casks and transportation packages under impact loading is an important safety topic that is reviewed as part of cask and package certification by the United States Nuclear Regulatory Commission. Explicit dynamic finite element models of full systems are increasingly common in industry for determining structural integrity during hypothetical drop accidents. Full cask and package model results are also used as the loading basis for single fuel pin impact models, which evaluate the response of fuel cladding under drop conditions. In this paper, a simplified package system is evaluated to illustrate several important structural dynamic phenomena, including the effect of gaps between components, the difference in local response at various points on a package during impact, and the effect of modeling various simplified representations of the basket and fuel assemblies. This paper focuses on the package impact analysis, and how loading conditions for a subsequent fuel assembly or fuel cladding analysis can be extracted.

Spent Fuel Storage in the Post-Yucca Mountain Paradigm

The current uncertainty surrounding the licensing and eventual opening of a long term geologic repository for the nation’s civilian and defense spent nuclear fuel (SNF) and high level radioactive waste (HLW) has shifted the window for the length of time spent fuel could be stored to periods of time significantly longer than the current licensing period of 40 years for dry storage. An alternative approach may be needed to the licensing of high-burnup fuel for storage and transportation based on the assumption that spent fuel cladding may not always remain intact. The approach would permit spent fuel to be retrieved on a canister basis and could lessen the need for repackaging of spent fuel. This approach is being presented as a possible engineering solution to address the uncertainties and lack of data availability for cladding properties for high burnup fuel and extended storage time frames. The proposed approach does not involve relaxing current safety standards for criticality safety, containment, or permissible external dose rates.© 2012 ASME

Spent Nuclear Fuel Transportation Package Seals in Beyond Design Basis Temperature Excursions

The US Nuclear Regulatory Commission (NRC) is studying the performance of seals in spent nuclear fuel (SNF) transportation packages exposed to fires that could exceed the hypothetical accident condition fire described in Title 10 of the Code of Federal Regulations, Part 71, such as the Baltimore Tunnel Fire that occurred in 2001, or the MacArthur Maze fire that occurred in 2007. The performance of package seals is important for determining the potential for release of radioactive material from a package during a beyond-design-basis accident. Seals generally have lower temperature limits than other package components and are the containment barrier between the environment and the radioactive package contents. The NRC Office of Nuclear Regulatory Research contracted the National Institute of Standards and Technology to conduct small-scale thermal testing to obtain experimental data of the performance of seals during extreme temperature exposures. The experimental testing consisted of several small-scale pressure vessels fabricated with a modified ASME flange design and tested metallic and polymeric seals, similar to those that might be used on an actual SNF transportation package. The vessels were heated in an electrical oven to temperatures as high as 800°C (1472°F), exceeding the rated temperatures of the seals in question. This paper will provide a summary of the testing conducted and present test results and conclusions. BACKGROUND

Thinking outside the box: options for transport of high burnup spent fuel

Abstract The current uncertainty surrounding the licensing and eventual opening of a long term geologic repository for the nation’s civilian and defense spent nuclear fuel and high level radioactive waste has shifted the window for the length of time spent fuel could be stored to periods of time significantly longer than the current licensing period of 40 years for dry storage. An alternative approach may be needed to the licensing of high burnup fuel for storage and transportation based on the assumption that spent fuel cladding may not always remain intact. The approach would permit spent fuel to be retrieved on a canister basis and could lessen the need for repackaging of spent fuel. This approach is being presented as a possible engineering solution to address the uncertainties and lack of data availability for cladding properties for high burnup fuel and extended storage time frames. The proposed approach does not involve relaxing current safety standards for criticality safety, containment, or permissible external dose rates.

Analyses of historical rail accidents to identify accident parameters impacting transport of spent nuclear fuel

Abstract As a regulatory authority for the transportation of spent nuclear fuel (SNF) in the USA, the Nuclear Regulatory Commission requires that SNF transportation packages be designed to endure a fully engulfing fire with an average temperature of 800°C (1475°F) for 30 min, as prescribed in Title 10 of the Code of Federal Regulations Part 71. The work described in this paper was performed to support the Nuclear Regulatory Commission in determining the types of accident parameters that could produce a severe fire with the potential to fully engulf an SNF transportation package. This paper describes the process that was used to characterise the important features of rail accidents that would potentially lead to an SNF transport package being involved in a severe fire. Historical rail accidents involving all hazardous material (i.e. all nine classes of hazardous material) and long duration fires in the USA have been analysed using data from the Federal Railroad Administration and the Pipeline and Hazardous Materials Safety Administration. Parameters that were evaluated from these data include, but were not limited to, class of track where the accident occurred, class of hazardous material that was being transported and number of railcars involved in the fire. The data analysis revealed that in the past 34 years of rail transport, roughly 1800 accidents have led to the release of hazardous materials, resulting in a frequency of roughly one accident per 10 million freight train miles (Because all of the data were obtained in the USA, which still uses distance measured in miles, and the primary source is an extensive database from the Federal Railroad Administration that is also in reported in miles, the data in this paper are reported in miles rather than kilometres. Conversion of miles to kilometres is by multiplication of 1·61.). In the last 12 years, there have only been 20 accidents involving multiple car hazardous material releases that led to a fire. This results in an accident rate of 0·003 accidents per million freight train miles that involved multiple car releases and a fire. Out of all the accidents analysed, only one involved a railcar carrying class 7 (i.e. radioactive) hazardous material.

Used Nuclear Fuel Transportation Package Seal Performance in Beyond Design Basis Thermal Conditions

The U.S. Nuclear Regulatory Commission (NRC) is evaluating the performance of seals in used fuel transportation packages during beyond-design-basis fires, similar to the Baltimore tunnel fire that occurred in 2001. The performance of package seals is important for determining the potential for a release of radioactive material from a package during a beyond-design-basis accident. Seals generally have lower temperature limits than other package components and are often part of the containment barrier between the environment and the cask contents. The NRC’s Office of Nuclear Regulatory Research (RES) funded the National Institute of Standards and Technology (NIST) to conduct small-scale thermal testing to obtain experimental data of the performance of seals during beyond-design basis temperature exposures. The experimental testing consisted of several small-scale pressure vessels fabricated with a modified ASME flange design, using commercial grade metallic seals, similar to those that might be used on an actual spent nuclear fuel transportation package. The vessels were heated in an electrical furnace for exposures up to 9 hours (hrs) at temperatures as high as 800°C (1472°F), which far exceeded the rated temperature of the seals in question. This paper will provide a summary of the testing completed as well as the preliminary results and conclusions of the experiments performed by NIST.