Xps and Sem Studies on the Corrosion of UO₂Cointaining Plutonium in Demineralized and Carbonated Water.

Abstract

An oxidation and dissolution study has been performed on UO2 pellets containing ∼10 and ∼0.1 wt. % 238Pu, ∼10 wt. % 239Pu and on undoped UO2 to investigate the effects of radiolysis and composition on the corrosion behavior of spent fuel. The so-called alpha-doped UO2 is used to simulate the alpha-radiation field of different types of commercial LWR spent fuel after different storage times. Leaching experiments in demineralized and carbonated water at room temperature under oxidizing conditions showed that relatively high amounts of 238Pu were released. The leached surfaces were examined with X-ray Photoemission Spectroscopy (XPS), and the progressive surface oxidation was monitored. The oxidation of the U(IV) during the leaching experiments, in the materials doped with 238Pu resulted in precipitation of U(VI) phases: enhanced formation of studtite for the strongest radiation field and shoepite at low radiation field was observed on the surface of the pellet. Essentially no precipitation of Pu-rich phases was directly observed. Leaching in carbonated water and characterization of UO2 containing 239Pu under the same experimental conditions were performed and the results compared to those for alpha-doped UO2. The chemistry effects due to the presence of Pu in addition to alpha-radiolysis were investigated.

Spent fuel radionuclide source term model for assessing spent fuel performance in geological disposal. Part II: Matrix alteration model and global performance

“…Under repository conditions, experimental results have evidenced that the water radiolysis and the matrix oxidation are in most cases the limiting steps for the following reasons: (i) carbonates are significantly present within the environment and enhance the U(VI) release towards the solution, (ii) XPS measurements on un-irradiated UO 2 have shown the absence of a U(VI) layer [19,20] during UO 2 alteration in presence of carbonates. In addition, focusing on the first two steps is conservative in the sense that they will not lead to underestimation of the RN release rate.…”

Effect of temperature on studtite stability: Thermogravimetry and differential scanning calorimetry investigations

“…In recent years, an increasing effort has been directed to the study of uranyl peroxides (studtite and meta-studtite, UO 2 O 2 Á 4H 2 O and UO 2 O 2 Á 2H 2 O), due to the fact that these peroxides have been found to be formed under several dissolution experimental conditions in the absence of carbonates: UO 2 in presence of hydrogen peroxide [6], a-doped UO 2 [7,8], UO 2 under a [9], b [10], and c [11] radiation. However, the most relevant study to actual SNF alteration is the experiment performed by McNamara et al [12], who found these peroxides during the leaching of SNF.…”

Stability of uranium (VI) peroxide hydrates under ionizing radiation

“…In addition, the uranyl minerals may incorporate transuranium and fission product elements into their structure, and this process may retard the migration of these radionuclides from the repository to the near-surface environment (Burns et al 1997;Burns and Klingensmith 2006;Klingensmith et al 2007). There are only two uranyl peroxides known to occur in nature-studtite and metastudtite (UO 2 O 2 ·4H 2 O, Z = 4, C2/c; UO 2 O 2 ·2H 2 O, Z = 2, Immm)-but they have received much attention because they were produced in experimental alteration studies under several conditions: UO 2 dissolution in presence of hydrogen peroxide (Clarens et al2004), dissolution of α-doped UO 2 (Sattonnay et al 2001;Cobos et al 2003) and UO 2 under α- (Corbel et al 2006 McNamara et al (2003) reported that a mixture of both uranyl peroxides precipitated on the surface of SNF in contact with water after two years. In addition, the association of radionuclides with uranyl peroxides has been documented in several studies: (1) incorporation of Np into studtite (Douglas et al 2005); (2) incorporation of Np, Pu, Am, Cm, Sr, Cs, and Tc into metastudtite (McNamara et al 2004); and (3) sorption of Cs onto studtite (Giménez et al 2007).…”