Oxidative dissolution of spent UO2 fuel in vapor and dripping groundwater at 90°C occurs via general corrosion at fragment surfaces. Dissolution along fuel-grain boundaries is also evident in samples contacted by the largest volumes of groundwater, and corroded grain boundaries extend at least 20 or 30 grains deep (> 200 μm), possibly throughout mm-sized fragments. Apparent dissolution of fuel along defects that intersect grain boundaries has produced 50 to 200 nm-diameter dissolution pits that penetrate 1–2 μm into each grain, giving rise to a “worm-like” texture along fuel-grain boundaries. Sub-micrometer-sized fuel shards are common between fuel grains and may contribute to the reactive surface area of fuel exposed to groundwater. Outer surfaces of reacted fuel fragments develop a fmne-grained layer of corrosion products adjacent to the fuel (5–15 μm thick). A more coarsely crystalline layer of corrosion products commonly covers the fine-grained layer, the thickness of which varies considerably among samples (from less than 5 μm to greater than 40 μm). The thickest and most porous corrosion layers develop on fuel fragments exposed to the largest volumes of groundwater. Corrosion-layer compositions depend strongly on water flux, with uranyl oxy-hydroxides predominating in vapor experiments, and alkali and alkaline earth uranyl silicates predominating in high drip-rate experiments. Low drip-rate experiments exhibit a complex assemblage of corrosion products, including phases identified in vapor and high drip-rate experiments.

Drip tests to measure radionuclide release from spent nuclear fuel are being performed at 90 °C at a drip rate of 0.75 ml every 3.5 days. The test conditions are designed to simulate the behavior of spent fuel under the unsaturated and oxidizing conditions expected in a potential repository at Yucca Mountain. The 238 U, 239 Pu, ,17 Cs, w Sr, "Tc, and l29 I content in the leachate after 581 days of testing is reported. These data provide valuable information for estimating the release of long-lived radionuclides. A comparison is made between our results and those of other researchers. The "Te release fractions are suggested to be representative of matrix dissolution.

No abstract

We have used X-ray photoelectron spectroscopy to determine the nitrogen 1s binding energies of azide, dinitrogen, and nitride complexes of several transition metals. Such data are of interest because the binding

Uranyl oxide hydrate phases are known to form during contact of oxide spent nuclear fuel with water under oxidizing conditions; however, less is known about the fate of fission and neutron capture products during this alteration. We describe, for the first time, evidence that neptunium can become incorporated into the uranyl secondary phase, dehydrated schoepite (U0,*0.8%0). Based on the long-term durability of natural schoepite, the retention of neptunium in this alteration phase may be significant during spent fuel corrosion in an unsaturated geologic repository.