Tomofumi SAKURAGI Radioactive Waste Management Funding and Research Center (RWMC), Japan

1. Activation energy for parabolic corrosion kinetics of Zircaloy-4 by consecutive hydrogen measurement at 30 – 80℃ Tomofumi SAKURAGI Radioactive Waste Management Funding and Research Center (RWMC), Japan Osamu Kato Satoshi Yoshida Kobe Steel, Ltd, Japan Tsuyoshi TATEISHIKobelco Research Institute, Inc, Japan Acknowledgements Scientific Basis for Nuclear Waste Management XXXXI(2017), Sydney

2. Introduction Background • Disposal of spent fuel cladding • Spent fuel cladding (Zr alloys) is highly activated (e.g.14C) and thus is expected to be disposed in a deep underground repository • The corrosion phenomenon is regarded as a source of radionuclides leaching and hydrogen gas generation Top nozzle (stainless steel) Cladding (Zircaloy) Bottom nozzle (stainless steel) • Reprocessing facility • Shearing • UO2 dissolution • Metals compaction Canisters and package Geological disposal Hulls and endpieces Fuel assembly Disposal concept in Japan

3. Zircaloy corrosion Introduction • NPP operation temperature (260~400oC) 1) • Kinetics follows the Cubic rate law (pre-transition) • ΔW3 = kC×t • Rate constant, kC = k0 exp(-Ea/RT) • Activation energy, Ea = 113kJ/mol • Not only diffusion process in oxide film, being affected by complex phenomena (surface reaction, SPP, stress etc.) Oxide film Porous Pore Dense Zircaloy Mass transfer model in the oxide film Weight gain changes with corroded time under 260~400oC 1) E. Hillner, 1977, Zirconium in the Nuclear Industry, 3rd Int. Symp., ASTM STP 633, 211.

4. Zircaloy corrosion Introduction • Disposal temperature (~80oC) • Corrosion rate was estimated 1 nm/y by Shoesmith and Zagidulin 2) • At 30oC (hydrogen measurment), the corrosion rate decreased with time and was 5 nm/y (after 4 years) 3) • The corrosion kinetics followed the Parabolic rate law • However, due to not higher durable of the glass vessel, an experiment in rising temperatures has not yet been performed Agas=a×t0.55 t0.55 t0.60 Kinetics of hydrogen gas generation 3) Glass vessel for corrosion test at 30oC 3) 2) Shoesmith and Zagidulin 2011, J. Nucl. Mater. 418, 292. 3) Sakuragi et al. 2013, Mater. Res. Soc. Symp. Proc. 1518, 178.

5. Objective Introduction • Understanding the corrosion kinetic/process under the disposal temperature • Corrosion data up to 80℃ (measuring the hydrogen generation consecutively) • Improvement of the corrosion vessel • Evaluation of the rate law and the activation energy

6. Corrosion test Experimental Ar gas out (with H2) to API-MS • New test vessel • Steel vessel for durability • The inside coated with fluoro-resin • Monitoring the hydrogen gas generation (by API-MS) Double gaskets Cooling water out Cooling water in Water in Condenser Carrier Ar gas in Double gasket Water out Stainless Steel Vessel (Coated inside with fluororesin) Thermocouples Solution Zry Sample Heater Lagging

7. Corrosion test Experimental • Specimen • Zircaloy-4 (Foil sample) • Size:100×100×0.1 mm (100 foils/vessel) • Total surface area: 2.0 m2 • Polished • Initial hydrogen conc. 10 ppm • Method • Consecutive monitoring of H2 gas (Ar gas flow system) • Detector:Atmospheric pressure ionization mass spectrometry (API-MS, Hitachi Tokyo Electronics, UG-400) • Condition • Dilute NaOH solution (pH 12.5) • Argon atmosphere (Oxygen less than 0.1 ppb) • 30, 50, 80oC • 90 days Zircaloy-4 foil API-MS

8. Hydrogen generation Results • Rate law • Amount of hydrogen （mol/m2), Agas=a×tn • log Agas = log a + n log t • Exponent n is nearly 0.5 • Indicating the parabolic rate law

9. Activation energy Results • Arrhenius plot • Parabolic rate constant • ΔW2 = kP t • kP= k0 exp (-Ea/RT) • = 9.33×105exp (-10264/T) • Activation energy, • Ea = 85.3±9.7 (kJ/mol) • This energy is smaller than that in operating temp. of 113 kJ/mol1) (Cubic, 260 - 400oC) □ Zr4, Present work ○ Zr4, Previous3) ○ Zr2, Previous3) Present work (Parabolic), Ea=85.3±9.7 kJ/mol Hillner1977 (Cubic), Ea=113 kJ/mol1) 1) E. Hillner 1977, Zirconium in the Nuclear Industry, 3rd Int. Symp., ASTM STP 633, 211. 3) Sakuragi et al. 2013, Mater. Res. Soc. Symp. Proc. 1518, 178.

10. Diffusion data in ZrO2 Discussion • Activation energy of diffusion • The energy of diffusion and corrosion is generally corresponding Takagi et al., J. Nucl. Mater. 419, 339 (2011)

11. Oxide film characterization Results • Cs-TEM* with electrondiffraction • The surface oxide is well crystalized • Tetragonal-ZrO2 and Monoclinic-ZrO2 are confirmed • Crystal structure is similar to that in operation temp. Tetragonal Monoclinic * Corrector-Spherical Aberration TEM 80oC 80oC

12. Corrosion kinetics/process Discussion • Zr+ H2O -> ZrO2 + 2H2

13. Summary • By improving the experimental system of hydrogen measurement, the corrosion data in 30 - 80℃ hasbeenobtained • Corrosion kinetics follows the parabolic rate law and the activation energy is 85.3±9.7 kJ/mol • Simple diffusion-controlled process in oxide can be suggested for Zircaloy corrosion under repository temperature • Breakaway and post-transition corrosion behaviour, and the factors affecting the corrosion rate (irradiation, hydrogen embrittlement…) are future challenges for a safety case.

14. Thank you for your attention!