Abstract
Leaching tests of simulated fuel debris (CeO2, (Ce, Fe)O2, (U, Zr)O2 and (U, Zr, Ca)O2) were conducted by using the microchannel flow-through method. The pellet-shaped specimens were fixed with epoxy resin and polished. A Teflon sheet 0.16 mm thick with a slit 2 mm wide and 20 mm long was attached to the polished surface of the specimen, then various solutions, leachant, were passed through the slit at 6 μL/min as a microchannel. Samples of solutions, leachate, were collected every 3 hours, diluted with 2% nitric acid, and analyzed for Ce or U using ICP-MS to determine the dissolution rate.
The (U, Zr)O2 specimens separated into two phases when Zr was above 25%, while the (U,Zr,Ca)O2) specimen was a single-phase up to about 50% Zr, due to the presence of a few % of Ca. The porosity of the phase-separated specimens was more than 10%, whereas the porosity of the single-phase specimens was only a few percent.
Single-phase specimens show smaller dissolution rates than multi-phase specimens. This is due to the formation of cracks and voids in the specimen by density change during phase transformation, which increases the contact area. The dissolution rate of UO2 is larger than of CeO2 even for the same single-phase specimen and porosity. This could be because uranium is in a highly soluble oxidation state, while cerium remains in a less soluble tetravalent state. In the case of uranium-based single-phase specimens, the dissolution rate tended to be lower with higher Zr composition. When the leachant was 1M NaCl solution, the dissolution rate of CeO2 was slightly larger than that for pure water. Conversely, the normalized dissolution rate of uranium simulated debris was smaller for 1M NaCl solution than for pure water. This might be because uranium reacts with the NaCl component to form a protective film. SEM observation of the surface of the leached portion of the specimen before and after the leaching test showed elution of fine particles less than 1 μm in diameter and elution from the edge of the specimen. No significant dissolution of individual phases was observed in multi-phase specimens. Although the actual fuel debris initially contacted with seawater and then with freshwater, it is highly likely that particles of a few micrometers in diameter were dissolved. However, fuel debris larger than a few mm can be considered to dissolve only a part of the surface.
