Drying of Spent Nuclear Fuel: Considerations and Examples

Abstract

A recently revised American Society for Testing and Materials consensus standard guide for drying of spent nuclear fuel (SNF) provides considerations and guidance for preparing SNF for its safe storage in a sealed dry storage system. The standard discusses (1) needs for drying, (2) techniques to dry, and (3) demonstration of adequate dryness. No specific approach is prescribed since the effective techniques and needs for drying depend on the specifics of the fuel and dry storage system. This paper discusses these topics using examples for both Zr-alloy-clad commercial SNF and for aluminum-alloy-clad research reactor SNF.

Residual water can include free water (liquid and/or vapor), physisorbed water bound to internal surfaces, and chemisorbed water incorporated into surface films, such as (oxy)hydroxides. The potential impacts of these residual waters are corrosion/oxidation, radiolytic breakdown into gaseous and/or reactive species, and canister pressurization.

For commercial SNF, inadvertent free water, even up to large amounts (e.g., 10+ mol), is not predicted to cause adverse corrosion degradation, except possible fuel oxidation for breached SNF. For aluminum-clad SNF, the production of radiolytic hydrogen with contribution from the chemisorbed water in its hydrated oxides is a primary consideration. For both SNF systems, canister pressurization is predicted to be well within the canister design, and flammability would not pose a safety concern using an oxygen limit of 5 vol % criterion. (Flammability control can be achieved by limiting either hydrogen or oxygen, and an oxygen limit is expected to be easier to meet in the presence of radiolytic H₂ generation.)

The two primary technologies for SNF drying, vacuum drying and forced-gas dehydration, are described herein, and drying tests and campaigns using these methods are cited. Dryness criteria and the methods used to detect and measure residual (free) water are also discussed.

Keywords:

• Spent nuclear fuel
• vacuum drying
• forced-gas dehydration
• dry storage

Acknowledgments

The authors acknowledge the many contributors to the development of the ASTM drying standard over its several revisions, most notably Bill Bracey, Andy Jung, Tae Ahn, Ricardo Torres, Brady Hanson, and Gary Smith.

This work was funded by the DOE Office of Nuclear Energy, Office of Spent Fuel and Waste Disposition, and by the Office of Environmental Management, Office of Technology Development.

Disclosure Statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was produced by Battelle Savannah River Alliance, LLC under contract no. 89303321CEM000080 with the DOE. This work was funded by the DOE Office of Nuclear Energy, Office of Spent Fuel and Waste Disposition, and by the Office of Environmental Management, Office of Technology Development. The publisher acknowledges the U.S. government license to provide public access under the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).