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  • 1.
    Ekeroth, Ella
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Radiation Induced Oxidative Dissolution of UO22006Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This doctoral thesis is focused on oxidative dissolution of UO2 (as a model for spent nuclear fuel) induced by radiolysis of water and the effect of H2 on this process.

    Rate constants for oxidation of UO2 powder by different oxidants such as H2O2 in aqueous suspensions have been experimentally determined. The logarithm of the second order rate constant, ln k, for UO2 oxidation appears to be linearly dependent on the one-electron reduction potential, E0, of the oxidant. This indicates that the rate limiting step in oxidation of UO2 is one-electron transfer. The rate of UO2 oxidation by OH and CO3•- is estimated to be diffusion controlled, k = 10-3 m/min.

    The effect of carbonate on the oxidation kinetics using H2O2 in UO2 powder suspensions has been studied. The second order rate constant for the pure oxidation reaction between UO2 and H2O2 was determined to 4.4 x 10-6 m/min. The carbonate facilitated dissolution rate is found to be limited by diffusion and the rate constant for non-carbonate mediated UO22+ dissolution was determined to (7 ± 1) x 10-8 mol m-2 s-1.

    Furthermore, the relative efficiency (per electron) of one- and two-electron oxidants in causing dissolution of UO2 has been studied. The dissolution yields of one-electron oxidants are strongly dependent on the amount of oxidant (especially at low amounts).

    The reduction of UO22+ in carbonate solutions by H2 (p = 40 bar) at different temperatures has been studied as a function of time. The rate constant and the activation energy were determined, k298 =3.6 x 10 -9 M-1 s-1 and Ea = 130 ± 24 kJ/mol, respectively. The reduction of UO22+ to UO2 occurs in the absence of a catalyst.

    The relative impact of radiolysis products on oxidative dissolution of UO2 has been elucidated. The amount of dissolved UO22+ was measured as a function of time in γ -irradiated aqueous carbonate solutions saturated with various gases containing a UO2-pellet. The simulated dissolution rates were calculated from the surface area, oxidant concentrations and rate constants obtained from the linear relationship mentioned above. The simulated dissolution rates were compared with the experimental dissolution rates and the correlation was fairly good. Thus, the impact of each oxidant in the system studied can be estimated from the simulations. Only the molecular oxidants are of importance in systems of relevance for safety analysis of a deep repository.

  • 2.
    Ekeroth, Ella
    et al.
    KTH, Superseded Departments, Chemistry.
    Jonsson, Mats
    KTH, Superseded Departments, Chemistry.
    Oxidation of UO2 by radiolytic oxidants2003In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 322, no 03-feb, p. 242-248Article in journal (Refereed)
    Abstract [en]

    The kinetics of UO2 oxidation by H2O2 has been studied using aqueous suspensions Of UO2-powder. The second order,rate constant for the reaction between H2O2 and UO2 has been determined to 8 x 10(-7) m/min (based on the surface to solution volume ratio) in the absence of carbonate. By studying the reaction between UO2 and other oxidants, it is possible to draw conclusions concerning the mechanism. The logarithm of the second order rate constant, ln k, for UO2 oxidation appears to be linearly related to the one-electron reduction potential, E-0, of the oxidant. This indicates that the rate limiting step in the oxidation of UO2 is one-electron transfer. A Fenton like mechanism is plausible for the reaction between UO2 and H2O2. The diffusion controlled rate constant in this particular system is approximately 10(3) m/min, and therefore the reactions with OH- and CO3- are estimated to be diffusion controlled.

  • 3.
    Ekeroth, Ella
    et al.
    KTH, Superseded Departments, Chemistry.
    Jonsson, Mats.
    KTH, Superseded Departments, Chemistry.
    Eriksen, Trygve
    KTH, Superseded Departments, Chemistry.
    Ljungqvist, Kristina
    KTH, Superseded Departments, Chemistry.
    Kovacs, Sandor
    Puigdomenech, Ignasi
    Reduction of UO22+ by H-22004In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 334, no 1, p. 35-39Article in journal (Refereed)
    Abstract [en]

    The reactivity of H, towards UO22+ has been studied experimentally using a PEEK coated autoclave where the UO22+ concentration in aqueous solution containing 2 mM carbonate was measured as a function of time at p(H2) similar to 40 bar. The experiments were performed in the temperature interval 74-100 degreesC. In addition, the suggested catalytic activity of UO2 on the reduction of UO22+ by H-2 was investigated. The results clearly show that H-2 is capable of reducing UO22+ to UO2 without the presence of a catalyst. The reaction is of first order with respect to UO22+. The activation energy for the process is 130 +/- 24 U mol(-1) and the rate constant is k(298K) = 3.6 x 10(-9) l mol(-1) s(-1). The activation enthalpy and entropy for the process was determined to 126 kJ mol(-1) and 16.5 J mol(-1) K-1, respectively. Traces of oxygen were shown to inhibit the reduction process. Hence, the suggested catalytic activity of freshly precipitated U02 on the reduction of UO22+ by H-2 could not be confirmed.

  • 4.
    Ekeroth, Ella
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Roth, Olivia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    The relative impact of radiolysis products in radiation induced oxidative dissolution of UO22006In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 355, no 1-3, p. 38-46Article in journal (Refereed)
    Abstract [en]

    The relative impact of radiolysis products in radiation induced oxidative dissolution Of UO2 has been studied experimentally. The experiments were performed by y-irradiating an aqueous solution containing HCO3 and a UO2-pellet. The U(VI) concentration in the solution was measured as a function of irradiation time. The aqueous solution was saturated with Ar, N2O, N2O/O-2 (80/20), air and O-2 in order to vary the conditions and the initial oxidant yields. The measured rate of oxidation was significantly higher for the O-2- and air saturated systems compared to the other systems. Using oxidant concentrations derived from numerical simulations of the corresponding homogeneous systems and previously determined rate constants for oxidation Of UO2, the relative trend in rate of oxidation in the different systems was reproduced. The results from the simulations were also used to estimate the relative impact of the oxidative radiolysis products as a function of irradiation time, both for gamma- and alpha-irradiated systems. For 7-irradiated systems saturated with Ar, air or 02, the most important oxidant is H2O2 while for N2O- and N2O/O-2-saturated systems the most important oxidant is CO3.-. For a-irradiated systems the most important oxidant was found to be H2O2.

  • 5.
    Hossain, Mohammad Moshin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Ekeroth, Ella
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Effects of HCO3- on the kinetics of UO2 oxidation by H2O22006In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 358, no 2-3, p. 202-208Article in journal (Refereed)
    Abstract [en]

    The effect of HCO3- on the kinetics of UO2 oxidation by H2O2 in aqueous solution has been studied using powder suspensions where the concentration of H2O2 was monitored as a function of time. By varying the UO2 surface to solution volume ratio second order rate constants were obtained for HCO3- concentrations ranging from 0 to 100 mM. The second order rate constant increases linearly with HCO3- concentration from 0 to approximately 1 mM. Above 1 mM HCO3- the rate constant is 4.4 × 10-6 m min-1 independent of [HCO3-]. This indicates that the kinetics of the reaction depends on both oxidation and dissolution below 1 mM HCO3- while at higher concentrations it is solely governed by oxidation. Hence, the rate constant obtained at HCO3- concentrations above 1 mM is the true rate constant for oxidation of UO2 by H2O2. The results also imply that the reaction between HCO3- and oxidized UO2 on the UO2 surface (i.e. HCO3- facilitated dissolution) is limited by diffusion (ca 10-3 m min-1 in the present system). Furthermore, the experimental results were used to estimate the oxidation site density of the powder used (126 sites nm-1) and the rate constant for dissolution of UO22 + from the UO2 surface (7 × 10-8 mol m-2 s-1).

  • 6.
    Jonsson, Mats
    et al.
    KTH, Superseded Departments, Chemistry.
    Ekeroth, Ella
    KTH, Superseded Departments, Chemistry.
    Roth, Olivia
    KTH, Superseded Departments, Chemistry.
    Dissolution of UO2 by one- and two-electron oxidants2004In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 807, p. 77-82Article in journal (Refereed)
    Abstract [en]

    In this work, the efficiency of one- and two-electron oxidants in oxidative dissolution Of UO2 has been investigated. This was accomplished by measuring the U(VI)-concentration in solution after exposing UO2-powder to controlled amounts of oxidants in aqueous solution. The oxidants used in this study are H2O2, IrCl62-, CO3.- and OH..H2O2 acts as a two-electron oxidant while the remaining three oxidants are one-electron oxidants. CO3.- and OH. were generated using gamma-radiolysis. The results clearly show that the dissolution yields for one-electron oxidants (per electron pair) are lower than the yields for two-electron oxidants. Furthermore, the yields for one-electron oxidants increase with increasing amount of oxidant (especially at low amounts of oxidant). The rationale for this is that U(VI) is the main soluble species which is formed directly upon two-electron oxidation. For one-electron oxidants the primary oxidation product is U(V) which can form U(VI) upon reaction with a second oxidant. The probability for a second oxidation is however low at low concentrations of oxidant.

  • 7.
    Jonsson, Mats
    et al.
    KTH, Superseded Departments, Chemistry.
    Nielsen, Fredrik
    KTH, Superseded Departments, Chemistry.
    Ekeroth, Ella
    KTH, Superseded Departments, Chemistry.
    Eriksen, Trygve E.
    KTH, Superseded Departments, Chemistry.
    Modeling of the effects of radiolysis on UO2-dissolution employing recent experimental data2004In: / [ed] Oversby, VM; Werme, LO, 2004, Vol. 807, p. 385-390Conference paper (Refereed)
    Abstract [en]

    This study examines the effect of water radiolysis on the dissolution of uranium dioxide. A model is created to describe the system of uranium dioxide fragments in water, and the production and reactions of radiolysis products (using recent kinetic data). The system is evaluated under different conditions using MAKSIMA-CHEMIST. Conditions examined include presence of carbonate in the water and effects of hydrogen. The simulations are compared to experimental results on spent fuel dissolution. Surprisingly, the simulated U(VI)-release agrees within a factor of three with the experimentally found U(VI)-release. The inhibiting effect of hydrogen is clearly demonstrated by the simulations. From the results of the simulations we are also able to conclude that the main inhibiting effect of H-2 is the reaction with OH* and not the reduction of U(VI) to U(IV).

  • 8.
    Jonsson, Mats
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Nielsen, Fredrik
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Roth, Olivia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Ekeroth, Ella
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Nilsson, Sara
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Hossain, Mohammad Mohsin
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Radiation induced spent nuclear fuel dissolution under deep repository conditions2007In: Environmental Science and Technology, ISSN 0013-936X, Vol. 41, no 20, p. 7087-7093Article in journal (Refereed)
    Abstract [en]

    The dynamics of spent nuclear fuel dissolution in groundwater is an important part of the safety assessment of a deep geological repository for high level nuclear waste. In this paper we discuss the most important elementary processes and parameters involved in radiation induced oxidative dissolution of spent nuclear fuel. Based on these processes, we also present a new approach for simulation of spent nuclear fuel dissolution under deep repository conditions. This approach accounts for the effects of fuel age, burn up, noble metal nanoparticle contents, aqueous H-2 and HCO3- concentration, water chemistry, and combinations thereof. The results clearly indicate that solutes consuming H2O2 and combined effects of noble metal nanoparticles and H-2 have significant impact on the rate of spent nuclear fuel dissolution. Using data from the two possible repository sites in Sweden, we have employed the new approach to estimate the maximum rate of spent nuclear fuel dissolution. This estimate indicates that H-2 produced from radiolysis of groundwater alone will be sufficient to inhibit the dissolution, completely for spent nuclear fuel older than 100 years.

  • 9.
    Nielsen, Fredrik
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Ekeroth, Ella
    Studsvik Nuclear AB, Nyköping.
    Eriksen, Trygve E.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Simulation of radiation induced dissolution of spent nuclear fuel using the steady-state approach. A comparison to experimental data2008In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 374, no 1-2, p. 286-289Article in journal (Refereed)
    Abstract [en]

    Using the recently developed steady-state model for simulation of radiation induced dissolution of spent nuclear fuel in water we have estimated the rate of dissolution for relatively fresh fuel to 1.64 x 10(-9) Mol m(-2) s(-1). A series of experiments have been performed on fuel fragments in deoxygenated water containing 10 mM HCO3-. The dissolution rates obtained from these experiments range from 2.6 x 10(-10) to 1.6 x 10-9 Mol m(-2) s(-1). The leaching time in the experiments is 40 days or less and during this time the amount of released uranium increases linearly with time which indicates that the system has reached steady-state. The excellent agreement between the estimated dissolution rate and the dissolution rates obtained from the spent nuclear fuel leaching experiments indicates that the steady-state approach can indeed be used to predict the rate of spent nuclear fuel dissolution.

  • 10.
    Pehrman, Reijo
    et al.
    European Commission, Joint Research Centre, Institute for Transuranium Elements.
    Amme, Marcus
    European Commission, Joint Research Centre, Institute for Transuranium Elements.
    Roth, Olivia
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Ekeroth, Ella
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Nuclear Chemistry.
    Oxidative dissolution of actinide oxides in H2O2 containing aqueous solution: A preliminary study2010In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 397, no 1-3, p. 128-131Article in journal (Refereed)
    Abstract [en]

    Oxidative dissolution Of Spent nuclear fuel is an important issue in the safety assessment of a future geological repository for Spent nuclear fuel. Although UO2 Constitutes, in terms of mass, the majority of the spent fuel material, its main radiotoxicity is (after extended storage times) contained in actinides with half lives shorter than that of 238-uranium, such as isotopes of Np and Pu. Relatively little information is available on the dissolution behavior of Np and Pu in comparable environments. This work investigates the oxidative dissolution of NpO2 and PuO2 in non-complexing aqueous solutions containing H2O2 and compares their behavior with that of UO2. We have found that oxidative dissolution takes place for all three actinides in the presence of H2O2. Based on the obtained dissolution rates, we Would not expect the dissolution of the actinides to be congruent. Instead, in a System Without complexing agent, the release rates of Np and Pu are expected to be lower than the U release rate.

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