Technical basis for extended dry storage of aluminum-clad spent nuclear fuel

Abstract

This paper presents the technical basis for extended dry storage of aluminum-clad spent nuclear fuel (ASNF). This basis addresses and closes five knowledge gaps defined by the U.S. Department of Energy (DOE) Spent Nuclear Fuel Working Group (SNFWG) in DOE-ID/RPT-1575 “Aluminum-clad Spent Nuclear Fuel: Technical Considerations and Challenges for Extended (>50 Years) Dry Storage.” The SNFWG's primary concern pertaining to dry storage of ASNF is rooted in the presence of (oxy)hydroxide layers adherent to the aluminum cladding of the spent fuel that formed in service, indicating a risk of radiolytic molecular hydrogen (H2) generation in a strong gamma radiation field—a process that can be associated with disintegration of (oxy)hydroxide and the molecular decomposition of chemisorbed and physisorbed water bonded to it. Potential consequences are overpressurization of sealed ASNF dry storage canisters and creation of a flammable atmosphere in sealed and vented canisters. Further, there is the concern that the generation of other chemical species due to chemical reactions, such as nitric acid (HNO3), could lead to canister component corrosion. To address these issues, experimental work was conducted, including investigations of the performance of surrogate aluminum specimens and actual ASNF material featuring adherent (oxy)hydroxide layers during wet or dry storage. Further, a comprehensive characterization of the sample material was completed. Drying experiments were executed, providing better estimations of the amount of residual, chemisorbed, and physisorbed water after ASNF dry storage canister conditioning. These empirical activities allowed for the development of predictive numerical models to simulate the chemical species yield within sealed and vented dry storage canisters for 50 years. The findings of these activities lead to the main conclusion that extending current ASNF dry storage durations by 50 years is both safe and viable.