A deterministic model for corrosion and activity incorporation in nuclear power plants - ANTIOXI

1. A deterministic model for corrosion and activity incorporation in nuclear power plants - ANTIOXI WP1: REFINEMENT AND QUANTIFICATION OF THE MODEL FOR OXIDE FILMS Iva Betova, Martin Bojinov BG H2 Society

2. k k 2 4 V 2+ V y + 2+ H2O H+ O O O Fe k O aq CrIII Cr CrIII 3 6 + Cr m Cr Cr aq e - k 1 V V 3- V 3- Cr m Cr Ni 2 + aq k 3i k Fe y+ Fe Fe x+ x+ 1i aq i i kr Steel Water Inner layer Outer layer Deposit layer Ni Ni 2+ 2+ i i Fe Task WP1-1: Refinement and improvement of the principal scheme of interaction between the oxide on a structural material and the coolant originating species

3. Task WP1-2: Estimation of the rate constants of interfacial reactions and diffusion coefficients for individual constituents of the inner layer (Cr, Fe, Ni) • A quantitative procedure of determination of kinetic parameters for individual alloy constituents has been development • It is based on available in-depth compositional data for oxides obtained from ex-situ analyses • A fitting procedure for the calculation of the in-depth distribution of the individual alloy constituents in the compact layer is put forward • On the basis of the obtained database of kinetic constants and transport parameters, the rate of transport of minor species will be assessed

4. Task WP1-2 (continued) First approximation D D Ni Cr 3- 3- V V 3- 3- V V M M M M 2+ 6+ Cr Ni aq aq Alloy Inner layer Water k3Cr Cr(III) Cr(III) k1Cr Crm k3Fe D Fe e 2+ 2+ x+ Fe Fe Fe aq i i k1Fe k3Ni Fem Ni(II) Ni(II) k1Ni Nim

5. Task WP1-2 (continued) Transport equations Boundary conditions

6. Task WP1-2 (continued) Calculational results – AISI 316 in 0.1 M borate, 150-300 °C, 72 h oxidation, AES depth profiles 150 °C

7. Task WP1-2 (continued) Calculational results (continued) 250 °C

8. Task WP1-2 (continued) Calculational results (continued) 300 °C

9. Task WP1-2 (continued) Calculational results – temperature dependence Rate constants at the M/F interface

10. Calculational results – temperature dependence (continued) Task WP1-2 (continued) Rate constants at the F/E interface

11. Calculational results – temperature dependence (continued) Task WP1-2 (continued) Diffusion coefficients

12. Calculational results(continued) Task WP1-2 (continued) Apparent activation energies

13. Task WP1-2 (interim conclusions) • The kinetic and transport parameters in the inner layer of the oxide determined at first approximation using the proposed procedure • The obtained values of interfacial rate constants and activation energies in broad agreement with previous estimates based on electrochemical data • At 300 °C, also the elemental profiles in the outer layer(s) succesfully calculated based on a formal diffusion approach to the growth of outer layer crystallites • The correspondence between the calculated and experimental profiles can be improved by introducing the microstructure of the layer (Task WP1-3) Next step: Determination of the kinetic and transport parameters of minor species by applying the proposed procedure to data for the influence of Zn on the oxide growth on stainless steels

14. Task WP1-3:Introduction and quantification of the effect of the compact layer microstructure on the defect transport in that layer • Point defect transport - under the influence of diffusion and migration • Diffusion characterised by a diffusion coefficient and concentration • gradient of the diffusing species • Migration controlled by an effective field strength • Both the diffusion coefficient and the field strength assumed to be • functions of the oxide microstructure • The following effects will be taken into account: increased length of the jump, • higher jump frequency and lower activation energy at conductive grain boundaries • with respect to grain interior • In addition, the influence of double Schottky barriers at grain boundary on the • field strength will be introduced