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Comparison to the Influence of γ Radiation in the Presence of Dissolved

5.2 Experimental

7.3.4 Comparison to the Influence of γ Radiation in the Presence of Dissolved

This influence of electrochemical treatment can be compared to the observations of King et al [8, 11], who observed that γ-irradiation of a solution containing dissolved H2 also lead to a very negative value of ECORR in the range –0.6 V to –0.8 V and still decreasing after ~20 h. In the absence of dissolved H2, ECORR values were in the range –0.25 V to –0.35 V as observed here for the untreated electrodes. In experiments in which the radiation source was subsequently removed a similar relaxation in ECORR towards a value representing the oxidation threshold was similarly

-400 -300 -200 -100 0 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 U (V ) + U (V I) /U (t otal ) (A tomi c rati os fr om X P S ) Ar Ar-carbonate H2/Ar H2/Ar-carbonate (ECORR )ss(mV vs. SCE)

observed. The UO2 specimens used in the experiments of King et al. were undoped and not well characterized but likely to be closer in properties to UO2.002 than to Dy-UO2.

This similarity in the response of ECORR suggests a similar reduction of the UO2 matrix is induced by the combination of γ radiation and H2 to that caused electrochemically by the application of a potential sufficiently negative to reduce H2O. As illustrated schematically in Figure 7.5 in the presence of a potentiostatically applied potential reduction of UV states to UIV can occur both directly by electrochemical reduction and by reaction with absorbed H radicals. In the γ-radiation case a radiation-induced surface activation of H2 could produce the reactive H radicals leading to the reduction of UV states.

Figure 7.6: Schematic illustration comparing the proposed mechanisms for the electrochemical (A) and radiolytic (B) reduction of UV states within a doped or non-

stoichiometric UO2 matrix.

7.4

Summary and Conclusions

The influence of the electrochemical reduction of Dy-doped and non-stoichiometric UO2 has been compared. When the applied potential is sufficiently negative that H2O reduction occurs leading to the formation of reactive H radicals, the radicals are mobile within the matrix and lead to the

reduction of UV states within the oxide which are present due either to the DyIII doping or the non- stoichiometry. The extent of reduction is determined by the UV content of the oxide and the rate of production of H radicals. On subsequently switching to open circuit a relaxation of the corrosion potential suggests the reduction of UV is, at least partially, reversible. Comparison of the corrosion potential behavior observed in experiments in which H2-containing solutions are γ-irradiated suggests a similar mechanism is operative involving the radiolytic production of surface H radicals leading to matrix reduction.

7.5

References

[1] P. Carbol, J. Cobos-Sabate, J. Glatz, C. Ronchi, V. Rondinella, D.H. Wegen, T. Wiss, A. Loida, V. Metz, B. Kienzler, K. Spahiu, B. Grambow, J. Quinones, A. Martinez Esparza Valiente, The Effect of Dissolved Hydrogen on the Dissolution of 233U Doped UO2(s), High Burn-up Spent Fuel and MOX Fuel, Report TR-05-09, Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, 2005.

[2] S. Rollin, K. Spahiu, U.-B. Eklund, Determination of Dissolution Rates of Spent Fuel in Carbonate Solutions under Different Redox Conditions with a Flow-through Experiment, J. Nucl. Mater. 297 (2001) 231-243.

[3] D.W. Shoesmith, The Role of Dissolved Hydrogen on the Corrosion/dissolution of Spent Nuclear Fuel, Report NWMO TR-2008-19, Nuclear Waste Management Organization, Toronto, ON, 2008.

[4] M.E. Broczkowski, P.G. Keech, J.J. Noël, D.W. Shoesmith, Corrosion of Uranium Dioxide Containing Simulated Fission Products in Dilute Hydrogen Peroxide and Dissolved Hydrogen, J. Electrochem. Soc. 157 (2010) C275-C281.

[5] M.E. Broczkowski, J.J. Noël, D.W. Shoesmith, The Inhibiting Effects of Hydrogen on the Corrosion of Uranium Dioxide under Nuclear Waste Disposal Conditions, J. Nucl. Mater. 346 (2005) 16-23.

[6] M.E. Broczkowski, J.J. Noël, D.W. Shoesmith, The Influence of Dissolved Hydrogen on the Surface Composition of Doped Uranium Dioxide under Aqueous Corrosion Conditions, J. Electroanal. Chem. 602 (2007) 8-16.

[7] J.C. Wren, D.W. Shoesmith, S. Sunder, Corrosion Behavior of Uranium Dioxide in Alpha Radiolytically Decomposed Water, J. Electrochem. Soc. 152 (2005) B470.

[8] F. King, M.J. Quinn, N.H. Miller, The Effect of Hydrogen and Gamma Radiation on the Oxidation of UO2 in 0.1 M NaCl solution, Report TR-99-27, Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, 1999.

[9] A. Traboulsi, J. Vandenborre, G. Blain, B. Humbert, J. Barbet, M. Fattahi, Radiolytic Corrosion of Uranium Dioxide: Role of Molecular Species, J. Phys. Chem. C 118 (2014) 1071- 1080.

[10] M.E. Broczkowski, D. Zagidulin, D.W. Shoesmith, The Role of Dissolved Hydrogen on the Corrosion of Spent Nuclear Fuel, In “Nuclear Energy and the Environment”, American Chemical Society Symposium Proceedings, Vol 1046, Chapter 26, 349-380.

[11] F. King, D.W. Shoesmith, Electrochemical Studies of the Effect of H2 on UO2 Dissolution, Report TR-04-20,Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, 2004. [12] H. He, P.G. Keech, M.E. Broczkowski, J.J. Noël, D.W. Shoesmith, Characterization of the Influence of Fission Product Doping on the Anodic Reactivity of Uranium Dioxide, Can. J. Chem. 85 (2007) 702-713.

[13] M. Razdan, D.W. Shoesmith, Influence of Trivalent-Dopants on the Structural and

Electrochemical Properties of Uranium Dioxide (UO2), J. Electrochem. Soc. 161 (2014) H105- H113.

[14] H. He, R. Zhu, Z. Qin, P.G. Keech, Z. Ding, D.W. Shoesmith, Determination of Local Corrosion Kinetics on Hyper-Stoichiometric UO2+x by Scanning Electrochemical Microscopy, J. Electrochem. Soc. 156 (2009) C87-C94.

[15] M. Razdan, D.W Shoesmith, The Influence of Hydrogen Peroxide and Hydrogen on the Corrosion of Simulated Spent Nuclear Fuel, Faraday Discuss. 180 (2015) 283-299.

Chapter 8

8

Summary and Future Work

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