Impact of Variation in Environmental Conditions on the Thermal Performance of Dry Storage Casks, Final Report (NUREG-2174)

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Publication Information

Manuscript Completed: December 2014
Date Published: March 2016

Prepared by:
Jorge Solis and Ghani Zigh

Prepared by:
Office of Nuclear Material Safety and Safeguards
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

During the certification review of the underground long-term spent fuel dry storage cask design, the Office of Nuclear Material Safety and Safeguards (NMSS) and the Office of Nuclear Regulatory Research of the U.S. Nuclear Regulatory Commission (NRC), identified low-speed wind as an environmental factor that may affect the thermal performance of this type of design. This led NMSS to investigate the impact of wind and other environmental variables on the thermal performance of different spent fuel dry storage cask designs.

During normal conditions of storage, environmental variables, such as ambient temperature, solar heating, relative humidity, elevation, and wind speed and direction, may affect the thermal performance of a ventilated dry storage cask. The thermal evaluation of a dry storage cask generally assumes a set of fixed environmental factors (e.g., average annual ambient temperature, quiescent conditions, sea level) that will bound all sites in the continental United States. However, for some sites, using average values may not be adequate, because more adverse ambient conditions could exist for prolonged periods of time, allowing a storage system to reach new steady-state conditions that could result in higher spent fuel cladding temperatures as compared to the steady-state conditions analyzed in the cask's safety analysis report (SAR) for normal conditions of storage. For cases with predicted small thermal margin, these adverse ambient conditions could result in peak cladding temperatures exceeding recommended limits for normal conditions of storage.

This report evaluates the thermal impact of varying environmental conditions on spent fuel dry storage casks. In addition, the report investigated the transient thermal behavior of a dry storage cask when it is subjected to a sudden boundary condition change, starting from the bounding conditions described in the SAR.

The results showed that, for the underground cask design, the peak temperature in the fuel package region, represented by a homogenous composite of the gas region, the fuel, and the cladding (hereafter referred to as the peak cladding temperature (PCT)) increases for low-speed wind, as compared to quiescent conditions. The analysis also showed that the PCT starts to decrease at higher wind speeds. For vertical aboveground casks with four vents, the PCT decreased as wind speed increased. For a postulated two-air-vent vertical dry storage cask, when wind direction is normal to the air vents, the PCT decreased as the wind speed increased. When wind direction is parallel to the air vents of the two-air-vent cask, the PCT increased as the wind speed increased. For horizontal aboveground casks with air vents located on the side, the wind speed and direction did not have any significant effect on the thermal performance of the cask, as the vents are not located normal to wind. For horizontal aboveground casks with inlet vents located on the front, when wind direction is facing the front of the cask, the thermal performance of the cask was improved, but when wind direction was parallel to the cask front, no significant effect was observed.

The NRC staff should consider the analysis results in this report when performing technical reviews, applicants should consider them when applying for cask certification, and the technical reviewer should consider them for applicability to a specific design. The results can also be used as additional guidance when considering the thermal impact of the environmental factors in the thermal performance of spent fuel dry storage systems.