Materials and Corrosion Product Behaviour Under CANDU-SCWR Conditions

1. Materials and Corrosion Product Behaviour Under CANDU-SCWR Conditions by William Cook University of New Brunswick Presentation at the Workshop of the Canadian National Committee of IAPWS Toronto, May 12, 2009

2. The supercritical water reactor is a logical evolution of nuclear reactor technology. • The benefits of operating a reactor with a supercritical cycle include: • Improved efficiency (up to ~ 48%) • Simpler cycle: • More compact core; • Smaller plant footprint; • Less fuel/used-fuel inventory; • More cost effective. • Potential for co-production of hydrogen.

3. CANDU-SCWR Design Parameters 25 MPa

4. Material selection will be the limiting factor in the design and operation of the SCWR. • To date, current reactor materials all pose problems for use in a SCWR. • Zr cannot be used due to excessive oxidation under supercritical conditions. May be used as the pressure tube, with a suitable insulator (ZrO2). • Alternate core materials (fuel cladding) will undoubtedly be required. • Austenitic stainless steels and nickel alloys are candidates but would suffer from SCC, IASCC and He-embrittlement (in the case of Ni-alloys). • F/M alloys are candidates but suffer from high oxidation rates.

5. The current knowledge about materials’ behaviour in SCW typically indicate the following generic statements: • “materials that do not corrode excessively, crack.” • “materials that do not crack, corrode excessively.” • New alloys or modified surfaces of currently available alloys are potential solutions: • F/M steels exhibit good SCC, creep and neutron activation characteristics. • Possibility of modifying F/M alloy surface to improve corrosion resistance: • Surface alloying (increasing Cr content) • Corrosion-resistant coatings (ZrO2 ?)

6. Oxide dispersion strengthened (ODS) alloys show particular promise for use in-core and elsewhere. • Based on a F/M steel substrate; • Yttria (Y2O3) particles dispersed through metal matrix through mechanical alloying. Increases high-temperature strength and lowers high-temperature creep. • MA956 chosen as base material for tests in SCW • Alloys with various chromium concentrations (9, 12, 14 & 25%) have been cast at CANMET-MTL; • Exposure in SCW in UNB’s test loop is under way.

7. Regardless of material chosen, corrosion products will inevitably be injected into the SCW coolant. • Poses a much greater risk for material or activity transport throughout the entire SCWR system due to: • No boiling in the core – corrosion products carried with bulk fluid; • Potential for large crud deposits in core and throughout the power cycle – drastic changes in fluid properties, particularly density and dielectric constant.

9. Estimation of the CP transport and deposits expected through the SCWR cycle require knowledge of many factors including: • Rate of corrosion product input to the coolant (essentially a function of the corrosion rate of the materials of construction); • Rate of dissolution or deposition of a surface deposit – a function of the bulk coolant corrosion product concentration, the CP’s solubility and a kinetic rate constant.

10. Mass balances for the corrosion release, transport and deposition can be formulated as follows: • Corrosion rate (dmsteel/dt): evaluated from experimental data of the alloy’s corrosion under given conditions. • Corrosion film growth: evaluated assuming oxide film replaces the metal corroded (q is the porosity of the corrosion film). • Corrosion release: evaluated from material not retained in the primary corrosion film.

11. A mass balance at the oxide-solution interface controls input to the bulk coolant and growth of the deposit (Kd= deposition kinetic constant; hm= mass transfer coefficient; Cbulk,Co/s,Csat= the bulk coolant and oxide-solution interfacial CP concentration and solubility): • Once the corrosion-product concentration in the oxide-solution interface is know, deposit growth (do) is calculated as:

12. Obviously, many of the parameters required are unknown or have very limited data. • The solubility of the primary corrosion product, (magnetite – Fe3O4) will be a key in predicting the overall deposition rates. • Estimations of its solubility in SCW can be made through limited experimental data or through thermodynamic calculations.

13. For example, consider the dissolution of magnetite to produce ferrous hydroxide: • Through the reaction’s equilibrium constant, we can estimate the activity (concentration) of the dissolved species: or

14. The Gibbs energy of reaction is simply calculated as the energy of formations of the reaction’s products minus reactants. • The Gf’s are evaluated at the desired temperature using the Helgeson, Kirkham and Flowers (HKF) extrapolation model. • As a starting point, the standard state thermodynamic data used was as presented in Beverskog’s iron Pourbaix work (1996). • The calculation was carried out for magnetite dissolution accounting for: • Ferrous and ferric hydrolysis species; • LiOH or NH3 used for pH adjustment – for the latter, no account has been taken for possible iron-ammonia complexes. • Ensuring electroneutrality of the solution is maintained.

19. It’s interesting to note that on ascent to and passing the critical temperature, all ionic species drop off to very low concentrations. • As expected, iron or magnetite solubility in the critical region will be controlled by the uncharged hydrolysis species, Fe(OH)2(aq) and Fe(OH)3(aq). Based on this preliminary thermodynamic assessment, Fe(OH)3(aq) is predicted to dominate above the critical point. • Once, all speciation has been accounted for, total solubility is calculated …

22. As seen in the speciation plots, the solubility drops precipitously at the pseudo-critical point and is dominated by the uncharged species. • A rough calculation, assuming the coolant is 1.5 x oversaturated in corrosion products as it enters the core yields fuel loadings of up to 19 mg/cm2 after one year of operation (CR = 1mm/yr; Kd= 0.1 cm/s).

23. It is important to note that the deposition kinetics will control bulk coolant concentration and the coolant will only “drop out” CP’s depending on the kinetics thus allowing oversaturation to develop around the circuit. • Significant deposition can also be expected in the steam lines and create problems in the turbines. • One possible solution is to use a two-loop configuration as in conventional CANDUs. • This has been modelled over 5 years of operation and shows significant decrease in efficiency as deposits build up in the steam generator.

24. SG deposit thickness and overall two-loop SCWR efficiency with Kd = 10-2 cm/s

25. Summary • The CANDU-SCWR will provide a significant improvement in plant efficiency and overall economics. • Materials must be developed that minimize corrosion rate and corrosion product input to the reactor coolant. • Corrosion product transport (and activity transport) and deposition will be significant issues in a SCWR cycle. • Chemistry dosing practices must be evaluated to minimize corrosion and corrosion product deposition.

26. Acknowledgements • R. Olive (PhD student); • W. Fatoux (MSc student); • NSERC for financial support; • NRCan and AECL for motivation to pursue this work.