Leaching Behavior and α-Decay Damage Accumulation of UO₂ Containing Short-Lived Actinides

Abstract

UO2 containing short-lived α-emitters, the so-called α-doped UO2, can simulate type (i.e. α- decay) and level of activity of spent fuel at the time when it might become exposed to groundwater in a geologic repository during storage. This allows studying α-radiolysis effects on the dissolution of the fuel matrix. Additionally, UO2 with high concentrations of α-emitters accumulate, during experimentally acceptable short times, the amount of decays, hence of property modifications, corresponding to long storage times for spent fuel. UO2 containing ∼10 and ∼0.1 wt% 238Pu was fabricated and tested. Leaching experiments in deionized water under unaerated conditions, with continuous monitoring of the evolution of the redox potential and pH in the leaching solutions, were performed. The Eh measurements showed a fast increase of the redox potential in the case of the material with the highest α-activity, while the UO2 containing ∼0.1 wt% 238Pu increased its potential more slowly. The redox potential for undoped UO2 decreased steadily during the experiment. As previously observed, higher fractions of U were released in the case of α-doped UO2 compared to undoped UO2. The fractions of U and Pu released during leaching from the α-doped materials were very similar, suggesting that congruent dissolution occurred. After leaching times longer than 10 h, only dissolved species were present in the solutions. Under these experimental conditions, characterized by relatively low values of the ratio sample surface/leachant volume, a dependence of the released amounts on the α-activity of the samples was observed. Periodical measurements of parameters like hardness, showed a rapid buildup of radiation damage in the material with the high α-activity. After more than two years, noticeable changes, namely an increase of the hardness, have begun to be observed also for the material with the low concentration of 238Pu.

Corrosion and dissolution studies of UO2 containing α-emitters

“…Also, it is not clear why the released fractions of Pu were smaller than the corresponding values for U for UO 2 -01. The pH did not show significant variations during the leaching tests and was in all cases between 6 and 6.5, as measured in previous experiments [17,18].…”

“…The evolution with time of the lattice parameter and of other properties of the α-doped materials under the effects of accumulating α-decay damage are reported elsewhere [17,18].…”

“…can be used [9,11,12,17,18]. Previous experiments performed by some of the authors investigated the leaching behaviour of UO 2 containing 238 Pu (simulating the α-activity of fresh and relatively old spent fuel) in deionized water under anoxic conditions and showed that enhanced U dissolution occurs in presence of enhanced α-radiolysis [11,12,17,18]. A dependence of the fuel dissolution on α-activity was measured under relatively low surface/volume conditions [18].…”

“…Previous experiments performed by some of the authors investigated the leaching behaviour of UO 2 containing 238 Pu (simulating the α-activity of fresh and relatively old spent fuel) in deionized water under anoxic conditions and showed that enhanced U dissolution occurs in presence of enhanced α-radiolysis [11,12,17,18]. A dependence of the fuel dissolution on α-activity was measured under relatively low surface/volume conditions [18]. This paper presents the results of leaching experiments of α-doped UO 2 containing 238 Pu in deionized water under anoxic conditions combined with surface analysis by XPS, performed for a better understanding of the oxidation-dissolution-reprecipitation behaviour.…”

The effects of alpha-radiolysis on UO2 dissolution determined from batch experiments with 238Pu-doped UO2

“…Cobos et al [24] and Rondinella et al [9,25] have published U dissolution data obtained with Pu-doped materials that had similar doping levels to the nominal 10 Ci/kg UO 2 material used in our study. They used doped materials with a variety of surface areas, from fine-grained (particle size <63 lm) to course-grained (63 lm <particle size <125 lm) material and a variety of discs and pellets.…”