The production of H 2 in the radiolysis of water adsorbed on micron-sized particles of CeO 2 and ZrO 2 has been examined. Radiation chemical yields of H 2 increase substantially with decreasing number of adsorbed water layers when the yield is determined with respect to the energy deposited directly by γ-rays to the water. These yields reached values of 20 and 150 molecules of H 2 per 100 eV for one to two water layers on CeO 2 and ZrO 2 , respectively, compared to 0.45 molecule/100 eV in bulk liquid water. The yields of H 2 determined with respect to the total energy deposited in both the oxide and water were found to have a smaller, but observable, dependence on the amount of water adsorbed. Radiolysis of ZrO 2 with γ-rays produced about 5 times more H 2 than CeO 2 for the equivalent amount of water adsorbed. The results suggest that the increase in H 2 production is due to the transfer of energy, possibly by an exciton, from the oxide to the water. O 2 production was at least an order of magnitude less than H 2 . The yield of H 2 in the 5 MeV helium ion radiolysis of water on CeO 2 is the same as with γ-rays, but the results with ZrO 2 are substantially lower. The H 2 yields with helium ion radiolysis may be nearly independent of the type of oxide.
The production of H 2 in the γ-ray and 5 MeV helium ion radiolyses of water adsorbed on 0.4 and 1 µm particles of UO 2 has been examined. The yields of H 2 determined with respect to the total energy deposited in the oxide-water system is 0.016 molecule/100 eV, which is about the same as similarly sized CeO 2 particles. Radiation chemical yields of H 2 increase substantially with decreasing number of adsorbed water layers when the yield is determined with respect to the energy deposited directly by γ-rays in the water. A yield of about 40 molecules of H 2 per 100 eV energy directly absorbed by four layers of water on UO 2 compared to 0.45 molecule/100 eV in bulk liquid water strongly suggests that energy is being transported from the oxide through the water interface. The results with UO 2 have been combined with data on ZrO 2 and CeO 2 to give H 2 yields for a wide range of water loading. H 2 is mainly produced from chemisorbed water in the γ-radiolysis of ZrO 2 , and its yield decreases in the presence of O 2 to values comparable to those for UO 2 and CeO 2 . The yields of H 2 in the 5 MeV helium ion radiolysis of water on UO 2 , CeO 2 , and ZrO 2 are similar, indicating that these materials may be suitable for estimating H 2 production in transuranic waste.
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