Current Status of the Canadian Experimental Dry Storage Program for Irradiated Fuel

Abstract

Atomic Energy of Canada Limited and Ontario Hydro are jointly conducting three dry storage experiments in concrete canisters with irradiated fuel bundles from Ontario Hydro's Pickering and Bruce Nuclear Generating Stations. The objective of the work at WNRE near Pinawa, Manitoba, is to assess the behaviour of the bundles under storage conditions representative of those in an interim dry storage or an ultimate disposal facility. The bundles were irradiated at peak outer-element linear power ratings ranging from 22 kW/m to 53 kW/m, and outer-element average burnups of 181 MWh/kg U to 262 MWh/kg U. Prior to canister storage, the bundles were stored in reactor-site water bays for periods ranging between 1.8 and 8.8 years.

Undefected and intentionally defected bundles are being used in the experiments. Before the bundles were loaded into the canisters, they were characterized to establish their pre-storage condition. The first post-storage examinations were conducted on bundles stored for three sets of conditions: 44 months of storage in dry air at seasonally varying temperatures, 41 months in dry air at 150°C and ~30 months in air saturated with moisture at 150°C. No significant change was observed in the condition of any undefected fuel bundles. The most apparent change observed in the defected bundles was UO₂ oxidation. In a number of these elements, X-ray photoelectron spectroscopy indicated surface oxidation of the UO₂ grains to U₃O₈ or beyond. U₃O₈ was undetectable by X-ray diffraction, Fourier transform infrared spectrometry and by chemical analyses, which suggests that its concentration was very limited, However, α-U₃O₇ was detected by X-ray diffraction in several elements.