Radiolytic Gas Production from Aluminum Coupons (Alloy 1100 and 6061) in Helium Environments—Assessing the Extended Storage of Aluminum Clad Spent Nuclear Fuel

Abstract

Corrosion of aluminium alloy clad nuclear fuel, during reactor operation and under subsequent wet storage conditions, promotes the formation of aluminium hydroxide and oxyhydroxide layers. These hydrated mineral phases and the chemisorbed and physisorbed waters on their surfaces are susceptible to radiation-induced processes that yield molecular hydrogen gas (H2), which has the potential to complicate the long-term storage and disposal of aluminium clad nuclear fuel through flammable and explosive gas mixture formation, alloy embrittlement, and pressurization. Here, we present a systematic study of the radiolytic formation of H2 from aluminium alloy 1100 (AA1100) and 6061 (AA6061) coupons in “dry” (~0% relative humidity) and “wet” (50% relative humidity) helium environments. Cobalt-60 gamma irradiation of both aluminium alloy types promoted the formation of H2, which increased linearly up to ~2 MGy, and afforded G-values of 1.1 ± 0.1 and 2.9 ± 0.1 for “dry” and “wet” AA1100, and 2.7 ± 0.1 and 1.7 ± 0.1 for “dry” and “wet” AA6061. The negative correlation of H2 production with relative humidity for AA6061 is in stark contrast to AA1100 and is attributed to differences in the extent of corrosion and varying amounts of adsorbed water in the two alloys, as characterized using optical profilometry, scanning electron microscopy, Raman spectroscopy, and X-ray diffraction techniques.

Milestone 1.2.12: The Fate of Oxygen Radical Species in Corroded Aluminum Alloys under Irradiation

“…In this research program, radiolytic H2 generation from corroded surrogate aluminum alloy (AA) coupons, specifically AA1100 and AA6061, has been measured under a variety of temperatures, backfill gas compositions, and relative humidities. 2,[12][13] In these studies, the aluminum alloy coupons were corroded in high purity water (18.2 MΩ•cm) at ~95 o C for 30 days prior to cobalt-60 gamma irradiation. The corrosion procedure generated a heterogeneous corrosion layer, approximately 4-10 μm thick, consisting of a mixture of bayerite, nordstrandite, and gibbsite with small amounts of boehmite.…”

“…The corrosion procedure generated a heterogeneous corrosion layer, approximately 4-10 μm thick, consisting of a mixture of bayerite, nordstrandite, and gibbsite with small amounts of boehmite. [12][13] Additional studies were also completed using "non-native" aluminum corrosion plumes, created by corroding AA6061 plates for 350 days at ambient temperature to grow a thick bayerite layer representative of the deposits observed during inspection of Advanced Test Reactor (ATR) ASNF elements in the Idaho National Laboratory CPP-603 Fuel Handling Cave. 14 The generation of H2 from these corrosion layers was quantified as a function of the absorbed gamma dose up to tens of MGy.…”

Milestone 1.2.11: H₂ Production from Surrogate Non-Native Corrosion Plumes on Aluminum 6061-T6 Fuel Cladding Surrogates

“…To date, this program's irradiation studies have focused on the radiolytic formation of H2 from corroded surrogate aluminum alloy coupons, specifically 1100 and 6061, under a variety of conditions, including temperature, backfill gas composition, and relative humidity (RH) [16][17][18][19][20][21][22][23]. In these studies, corrosion was achieved by submerging the aluminum coupons in high purity water (18.2 MΩ•cm) at ~95 o C for 30 days, as benchmarked by Lister [24].…”

“…In these studies, corrosion was achieved by submerging the aluminum coupons in high purity water (18.2 MΩ•cm) at ~95 o C for 30 days, as benchmarked by Lister [24]. This procedure generated ~5 μm thick, heterogenous corrosion layers, with large contributions from gibbsite (Al(OH)3) and boehmite (γ-AlOOH) mineral phases [16,23]. The data gathered through the irradiation of these high-temperature-corroded aluminum coupons have been essential for the development of predictive computer models to support the technical basis for the extended dry storage of ASNF in the standard US-DOE canister [25][26][27][28].…”

“…Although samples of the non-native corrosion plumes were not collected for characterization [29], given the temperature dependence of aluminum corrosion processes [30], the mineral phase composition of these plumes is expected to be different from the initial passivated corrosion layer and the hightemperature-corrosion layers investigated by our team [16][17][18][19][20][21][22][23]. Differences in layer composition will impact the extent of H2 production from these plumes, as demonstrated by the differences in H2 generation found for irradiated gibbsite and boehmite powders [31][32][33][34][35].…”