<p>The kinetics for radiation induced dissolution of spent nuclear fuel is a key issue in the safety assessment of a future deep repository. Spent nuclear fuel mainly consists of UO<sub>2</sub> and therefore the release of radionuclides (fission products and actinides) is assumed to be governed by the oxidation and subsequent dissolution of the UO<sub>2</sub> matrix. The process is influenced by the dose rate in the surrounding groundwater (a function of fuel age and burn up) and on the groundwater composition. In this licentiate thesis the effects of HCO<sub>3</sub>- (a strong complexing agent for UO2<sup>2+</sup>) and ionic strength on the kinetics of UO<sub>2</sub> oxidation and dissolution of oxidized UO<sub>2</sub> have been studied experimentally.</p><p>The experiments were performed using aqueous UO<sub>2 </sub>particle suspensions where the oxidant concentration was monitored as a function of reaction time. These reaction systems frequently display first order kinetics. Second order rate constants were obtained by varying the solid UO<sub>2 </sub>surface area to solution volume ratio and plotting the resulting pseudo first order rate constants against the surface area to solution volume ratio. The oxidants used were H<sub>2</sub>O<sub>2 </sub>(the most important oxidant under deep repository conditions), MnO<sub>4</sub>- and IrCl<sub>6</sub><sup>2-</sup>. The kinetics was studied as a function of HCO<sub>3</sub>- concentration and ionic strength (using NaCl and Na<sub>2</sub>SO<sub>4 </sub>as electrolytes).</p><p>The rate constant for the reaction between H<sub>2</sub>O<sub>2</sub> and UO<sub>2</sub> was found to increase linearly with the HCO3- concentration in the range 0-1 mM. Above 1 mM the rate constant is independent of the HCO3- concentration. The HCO<sub>3</sub>- concentration independent rate constant is interpreted as being the true rate constant for oxidation of UO<sub>2</sub> by H<sub>2</sub>O<sub>2</sub> [(4.4 ± 0.3) x 10-6 m min-1] while the HCO3- concentration dependent rate constant is used to estimate the rate constant for HCO<sub>3</sub>- facilitated dissolution of UO<sub>2</sub>2+ (oxidized UO<sub>2</sub>) [(8.8 ± 0.5) x 10-3 m min-1]. From experiments performed in suspensions free from HCO<sub>3</sub>- the rate constant for dissolution of UO<sub>2</sub>2+ was also determined [(7 ± 1) x 10<sup>-8 </sup>mol m<sup>-2</sup> s<sup>-1</sup>]. These rate constants are of significant importance for simulation of spent nuclear fuel dissolution.</p><p>The rate constant for the oxidation of UO<sub>2</sub> by H<sub>2</sub>O<sub>2</sub> (the HCO<sub>3</sub>- concentration independent rate constant) was found to be independent of ionic strength. However, the rate constant for dissolution of oxidized UO<sub>2</sub> displayed ionic strength dependence, namely it increases with increasing ionic strength.</p><p>The HCO<sub>3</sub>- concentration and ionic strength dependence for the anionic oxidants is more complex since also the electron transfer process is expected to be ionic strength dependent. Furthermore, the kinetics for the anionic oxidants is more pH sensitive. For both MnO<sub>4</sub>- and IrCl<sub>6</sub>2- the rate constant for the reaction with UO<sub>2 </sub>was found to be diffusion controlled at higher HCO3- concentrations (~0.2 M). Both oxidants also displayed ionic strength dependence even though the HCO<sub>3</sub>- independent reaction could not be studied exclusively.</p><p>Based on changes in reaction order from first to zeroth order kinetics (which occurs when the UO<sub>2</sub> surface is completely oxidized) in HCO<sub>3</sub>- deficient systems the oxidation site density of the UO<sub>2</sub> powder was determined. H<sub>2</sub>O<sub>2 </sub>and IrCl<sub>6</sub>2- were used in these experiments giving similar results [(2.1 ± 0.1) x 10-4 and (2.7 ± 0.5) x 10<sup>-4</sup> mol m<sup>-2</sup>, respectively].</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:kth-4172 |
| Date | January 2006 |
| Creators | Hossain, Mohammad Moshin |
| Publisher | KTH, Chemistry, Stockholm : Kemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Licentiate thesis, comprehensive summary, text |
| Relation | Trita-KKE, 0349-6465 ; 0603 |
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