Experimental determination and chemical modelling of radiolytic processes at the spent fuel/water interface

The Influence of Near Field Redox Conditions on Spent Fuel Leaching

In a repository, the spent fuel could come in contact with groundwater if the canister or container has breached. The system may be quite complex with oxygen-free water, uranium dioxide, a corroding metal, such as iron, and a radiation field present at the same time. In an anaerobic environment iron and mild steel will corrode and hydrogen will be evolved. The equilibrium hydrogen pressure for this reaction is very high. At some time after water intrusion, there will be large amounts of dissolved hydrogen in the near field, corresponding to a partial pressure at least equivalent to the hydrostatic pressure at the repository depth. For this reason, we investigated the leaching behavior of 0.25-0.5 mm sized fragments of PWR spent fuel (43 MWd / Kg U) in simulated groundwater solution (10 mM NaCl and 2 mM HCO3-) under 5 MPa hydrogen and argon pressure. In a leaching experiment under 5 MPa hydrogen at 25 °C, the total U concentration was found to be <10−8 M. After refilling of the autoclave with new solution at 70°C, the total U concentration first increased to 10−6.3M, and then quickly decreased to 10−8 M. The leaching behavior of uranium and other fuel components indicates that under pressurized hydrogen, the spent fuel dissolution is substantially hindered. Leaching results obtained after the substitution of hydrogen by argon at the same pressure and temperature are also presented. Finally, some results on spent fuel leaching under pressurized argon are presented and comparatively discussed.

The influence of near field hydrogen on actinide solubilities and spent fuel leaching

Large amounts of hydrogen are produced as a result of the anoxic corrosion of iron in the proposed container materials for some geologic repositories. Another hydrogen source, less important than the anoxic corrosion of iron, is the radiolysis of water by the spent fuel radiation. Gas phase formation occurs when the pressure of the hydrogen equals at least the hydrostatic pressure, around 5 MPa at 500 meters depth. The effects of 5 MPa hydrogen pressure on spent PWR fuel leaching and on uranium oxide solubility have been studied in carbonated solutions at 70 °C. The experiments were performed in a 1 liter autoclave, filled with 950 ml of a solution 10 mM NaCl, 2 mM NaHCO3 and with hydrogen at a pressure of 5 MPa in the remaining 50 ml free volume. The leaching behavior of 2 g PWR spent fuel powder of the 0.25-0.50 mm fraction, placed in a gold basket was studied during several months by analyzing 10 ml solution samples taken after regular time intervals. A few experiments were performed also with unirradiated U(IV) oxide. In both cases extremely low concentrations of uranium (less than 10-9 M were measured in the solution samples. Furthermore the uranium levels in solution remained practically constant during the whole leaching period (more than one year), indicating the absence of any oxidative dissolution of the spent fuel matrix. The same conclusion is confirmed by the constant (within analytical errors) levels of strontium, cesium, molybdenum, iodine and technetium during the whole leaching period. These results have been compared with the ones obtained during the leaching of a spent fuel pin in anoxic conditions, where the uranium and other radionuclides levels are several orders of magnitude higher.

The surface of spent fuel or U(IV) oxide is partially oxidized during storage, giving rise to relatively high levels of U(VI) in solution even during leaching in anoxic conditions. No such effect could be observed in the presence of 5 MPa hydrogen, indicating that this initial amount of U(VI) should have been reduced to U(IV). The experimental study of the influence of various parameters as temperature and pressure is still in progress.

Radiolytic formation of Tc(IV) oxide colloids

Technetium(IV) oxide colloids were radiolytically formed by γ irradiation of aqueous solutions of pertechnetate (TcO4−). Pertechnetate solutions (5.5×10−5-2.9×10−4M) were irradiated with bremsstrahlung from an electron linear accelerator at 40 and 17 °C. The color of irradiated solutions gradually changed to brownish black, suggesting the formation of Tc(IV) oxide colloids (TcO2·nH2O). A transmission electron microscopy (TEM) analysis showed that the size of colloids distributed around 30 to 130 nm in diameter. The characteristic X-rays from technetium and oxygen were simultaneously detected from colloid particles at the TEM measurements. Round-shaped colloids were produced by irradiation at 40 °C, whereas irregular-shaped colloid particles composed of tiny particles (2 nm in diameter) were produced at 17 °C. The concentration of TcO4−in the target solution gradually decreased with an increase of the absorbed dose, corresponding to an increase of the colloid yield. The yield of colloids sharply increased in the solution deaerated by Ar bubbling before irradiation, but strongly suppressed in the solution saturated with oxygen (O2) or nitrous oxide (N2O) gas. The fact suggests that hydrated electrons play an important role in the course of the reduction of TcO4−and that Tc(IV) oxide colloids were formedviasuccessive disproportionation reactions of Tc(VI) and Tc(V). The formation mechanisms of Tc(IV) oxide colloids are discussed.

The reduction of U(VI) by near field hydrogen in the presence of UO2(s)

In several recent works a large impact of dissolved hydrogen on spent fuel dissolution has been observed: the solution concentrations of all redox sensitive components of the spent fuel decrease with time, while gas phase analyses indicate levels of radiolytic oxygen below the detection limit. This indicates that at the relatively low temperatures of these studies (≤70 °C) hydrogen is activated. The aim of this study is to test one of the proposed mechanisms for hydrogen activation, namely by UO2(s) surfaces, which may act as hydrogen catalysts. Dissolved U(VI) carbonate species were used as oxidized species in order to test the reducing ability of hydrogen in the presence and absence of UO2(s). In most cases parallel experiments with the same conditions, but under Ar(g) atmosphere were carried out as blanks. Quartz lined autoclaves avoiding any contact of the solution with metallic parts were used. The experiments were carried out in two stages: first the stability of the (10-7-10-5) M U(VI) in 10 mM NaCl, 2 mM NaHCO3 solutions in the presence of dissolved hydrogen was tested. Then a gold net basket containing UO2(s) fragments was introduced in the autoclave and the concentration of U(VI) was followed at different time intervals by ICP-MS. Various dissolved hydrogen concentrations, temperatures and U(VI) concentrations were used, always spanning the range of these parameters in a proposed deep rock repository. Especially at the lowest U(VI) concentrations tested (∼100 ppb) the activation of dissolved hydrogen by UO2(s) surfaces is clearly visible. As a conclusion of this work it may be stated that in the temperature range (20-70 °C) investigated a) dissolved hydrogen does not reduce U(VI) carbonate species in the absence of a catalyst and b) in the presence of UO2(s) surfaces dissolved hydrogen reduces U(VI) in carbonate solutions, very probably to UO2(s).