1. Zinc Addition: Theory and Experience Presented To: KEPRI & KHNP Presented By: Dr. Robert Litman

2. Terminal Objective Successful addition of zinc to the RCS to reduce dose rates and reduce primary water stress corrosion cracking (PWSCC) initiation and propagation.

3. Historical Perspective Studies of corrosion films formed on stainless steel and nickel alloys began in the 1950’s, because of the significant commercial importance of ferromagnetic spinels. The first use of zinc injection in BWRs in the US began in the early 1980s. A reduction in crack initiation was realized, and other programs such as noble metal injection and hydrogen water chemistry were also initiated.

4. Historical Perspective The first use of zinc injection in a PWR was at Farley in 1994. The purpose was to mitigate crack initiation and growth in SG tubes. This was 2 cycles before Farley was scheduled to replace their SGs due to extensive plugging from cracking. Zinc injection in PWRs in Germany began in 1996. The purpose was to reduce plant dose rates. There are about 10 US plants that are currently using zinc injection

5. What Properties Does Zinc Affect? The three principal effects that zinc addition hopes to achieve are: Reduction of dose rates on system components by reducing the amount of certain radionuclides Mitigate stress corrosion crack propagation in Alloy 600 components Mitigate stress corrosion cracking initiation Zinc injection has been shown to reduce the thickness of Alloy 600/800 oxide films. Zinc has been shown to have a significant effect on reducing the general corrosion rates of SS and Inconel (Alloy 600) alloys.

6. In Which Chemical Forms is Zinc added? Zinc Oxide Formula ZnO Formula weight 81.4 Solubility: Dilute acetic acid Zinc Acetate Formula Zn(C 2 H 3 O 2 ) 2 2H 2 O [note that it is in the dihydrate form] Formula weight 219.5 Solubility: Very soluble in water

7. Naturally Occurring or Enriched? Zinc has five stable isotopes: Abundance Cross-Section Longest half life (barns) activation product 64 Zn48.6%0.76244 d 66 Zn27.9%0.9Stable 67 Zn4.1%7Stable 68 Zn18.80.0713.8 h 70 Zn0.60.0084 h

8. Concentration of Zinc in RCS The range of concentration that has been used is from 5 to 100 ppb. 5 to 10 ppb range is used for control of radiation fields measured on plant components during shutdowns 20 to 45 ppb range is for minimizing crack growth initiation, and may inhibit crack growth rate.

9. Metal Grains Oxide Film CRUD Layer Metal surface grains

10. Metal Grains The structure of a metal grain is not regular. No two grains are exactly the same. The grain represents the basic unit of the metal. The grain boundary at the water interface oxidizes to form a corrosion film through a series of solid state and diffusion transport reactions. The primary corrosion film formed will reflect the composition of the grain. After that, the corrosion film will be affected by the environment it experiences.

11. Mechanism of Zinc Incorporation Nickel ferrites and nickel chromites, which are formed in the CRUD layer, have the configuration of a spinel. Spinels have a combination of structures that have both tetrahedral and octahedral coordination complex fields. The composition of these spinels is chromium rich, with basic composition of Ni x Cr y Fe 3-x-y O 4 ; the corrosion particle is non-stoichiometric. It was discovered that zinc has the strongest affinity for tetrahedral position in spinels of all the transition metals. Thus, if zinc is added to the RCS liquid, an equilibrium will be reached with those tetrahedral coordination sites that are accessible to liquid ↔solid interfaces.

12. Mechanism of Zinc Incorporation (continued) Zn incorporation occurs in layer 1 1 2 3 4 5 RCS Flow Zn 2+ Co Ni Fe

13. What are the Requirements for Zinc Addition? Unit above 30% power CVCS mixed bed demineralizer in service Zinc injection flow piping aligned to VCT RCS silica <1.5 ppm A plant specific target and control band concentration for zinc has been established Initial (conservatively low) injection rate has been established Maximum injection rate has been established Sample and analysis schedule for RCS provides enough data to properly assess zinc concentrations and changes over short periods of time.

14. Potential Effects on the RCS Mixed bed resin Solubility of crud and RCS filter doses Effect on liquid radwaste handling Chemical effects of zinc on valves or pump seals

15. Initial Zinc Injection Considerations Zinc will be incorporated into the mobile CRUD layer of all RCS system surfaces It will take about 2 weeks to see residual zinc (>1 ppb) Initial rate of surface exchange will exceed the injection rate 58 Co activity will increase as Ni and Co are displaced from the CRUD layer Zinc will be removed by the ion exchange resins, returning some lithium to the RCS

16. Initial Zinc Injection Considerations (continued) If zinc feed is stopped, zinc return will occur over a longer period of time than it took to establish it as part of the CRUD layer. 65 Zn will become measurable (up to ~5x10 -3 μCi/ml, at about 30 ppb)

17. What is the Resin Capacity for Zinc? The letdown demineralizer bed is ~75 ft 3, with about 35 ft 3 as cation resin in the lithium form. How much zinc can be exchanged on the resin bed? Bed Capacity= [35 ft 3 ][2.87x10 4 cc/ft 3 ][1.6 meq/cc][3.25x10 -2 g/meq] = 5.02x10 4 grams If the zinc injection rate is at 5 ppb, with letdown at 120 gpm, how many days will it take to exhaust the resin? [5.02x10 4 g] [120 gal/min][3.78 L/gal][5x10 -6 g/L][1.44x10 3 min/day] = 1.6x10 4 days If the resin is only used for one cycle, it will never reach exhaustion.

18. How Much Zinc Needs to be Added? Once the RCS surfaces have become saturated with zinc, the major removal mechanism is through letdown ion-exchange. At 120 gpm and a concentration of 5 ppb, each day the demineralizer removes: (120 gpm)(3.78 L/Gal)(5x10 -6 g/L)(1440 min/da) = 3.27 g per day must be replaced There are other removal mechanisms which are small, and the surface never becomes completely saturated.

19. Solubility and Equilibrium Effects Zinc does not complex effectively with boric acid and is strongly retained by the cation exchange resin. This means that it will most likely displace twice its concentration as Li. However at 40 ppb, an increase in RCS Li on the demineralizer outlet of 0.08 ppm may not distinguishable by some lab techniques. Concentration of 58 Co can be expected to increase since zinc displaces both cobalt and nickel in crud films. The 65 Zn produced is 100% soluble, whereas the other radionuclides will be both soluble and insoluble. The increase in circulating 58 Co leads to higher dose rates on filters but not a significant increase in the number of filters.

20. Effects on Liquid Radwaste Since there is additional, non-borated liquid being injected into the RCS, operators must remove that same volume of liquid daily to maintain water balance. This amounts to 1 to 25 gallons per day. Because zinc is uncomplexed and divalent it will have a small effect on resin capacity. However due to its small concentration this is negligible.

21. Effects on Other Plant Components RCP Seals Valves

22. Experience with Zinc at US PWRs Farley (Southern Nuclear Company) Diablo Canyon (Pacific Gas and Electric) Callaway (AmerenUE) D.C. Cook (American Electric Power) Sequoyah (Tennessee Valley Authority) Palisades (Nuclear Management Corporation)

23. Callaway Zinc addition is through the sample return line back to the VCT: Letdown Sample Line Primary Sample Sink Flush line to VCT VCT Zinc acetate addition bottles (plastic, each about 20 liters) Small positive displacement pump (same type as used for Ion chromatography analysis). To charging pump suction From Letdown Demineralizers

24. Diablo Canyon Principal reason for using zinc was to Mitigate PWSCC crack growth rate in their Alloy 600 MA tubes Secondary reason was to reduce SG channel head dose rates

25. Sequoyah (TVA) Principal reason for zinc injection was to reduce plant dose rates Continuous injection through sample return to VCT (similar to Callaway set up) Operational verification Shiftly - Skid operation, valve alignments, RCS sample flush, letdown flow, CVCS mixed bed DP and reactor coolant filter DP Daily - RCS total zinc 72 Hours - RCS total Co-58 and Co-60 Weekly - RCS total nickel and transition metal radionuclides (total, insoluble and soluble) Monthly - Zinc feed solution concentration

26. Sequoyah (TVA) Requirements for Zinc injection Unit at or above 30% power CVCS mixed bed demineralizer in service RCS hot leg sample flow aligned to VCT RCS not being sampled (so that the flush of the zinc into the VCT is not disturbed) RCS silica <1.5 ppm RCS zinc <10 ppb RCS nickel < 3 ppb

27. Sequoyah Injection Module

28. Palisades Principal reason for use of zinc was to reduce dose rates at the SG manway areas. This would reduce worker exposure during outages. Depleted zinc was used

29. Palisades

30. Farley Farley was about to replace steam generators due to tube plugging from IGSCC. Injected zinc at about 40 ppb. Ni concentration never exceeded detectable 58 Co concentration increased by a factor of 2-5 over non-zinc cycles. 60 Co concentration increased 1.5-3 times non-zinc cycles. Realized significant dose rate reductions during the outage.

31. Farley Zn injection started

32. Summary of Zinc Injection Parameters (At steady state conditions) Plant (Depleted or Natural, D or N) Zn 2+ Concentration Feed Solution g/L Injection Rate mL/hr (g/Da) Target RCS Zn Concentration ppm Frequency of Analyses Zn, Ni, 58 Co Callaway (D) D.C. Cook 18.122.8 (9.9) 5 D, W, D Diablo (N) Farley (N) Palisades (D) 0.6 3785(24) 1900-2271 (24- 28) 40 40 D, W Sequoyah (N)10.3 g/L60 (14) 5S, B, D S = Shiftly, B = twice per week, D = Daily, M= monthly

33. Recirculating Autoclave Tests for Zinc Effect on Oxide Thickness Reference: Corrosion, Vol. 56(6), p.623 (2000) Summary of Results: 1. The initial thickness of the oxide film (no zinc) is about 1 micron. 2. Zinc is incorporated into the outermost 0.1 μ of nickel ferrite on Alloy 600 tubes at about 1-6 atom%. 3. This effect occurs whether the zinc is injected simultaneously with the film formation, or if it is injected after film formation is established. 4. The thickness of the oxide film decreases as the exposure to zinc (ppb-days) increases. 5. It will take about 50 days to achieve equilibrium.

34. Recirculating Autoclave Tests for Zinc Effect on Oxide Thickness Reference: Water Chemistry of Nuclear Reactor Systems, Vol. 7, p.573 (1996) Results: 1. The oxide layer on stainless is 2-3 microns thick. 2. Corrosion films on stainless steel have up to 30 atom% of zinc in the outermost corrosion layer. 3. The stainless corrosion films can be translocated to Alloy 600 surfaces. This raises the apparent atom% zinc on the alloy 600 surface compared to the experimental tests.

35. CRUD Thickness and Chemistry Farley SG tube crud was ~20 atom% zinc after 1 cycle at 40- 45 ppb, however this decreased to <0.6 atom% after a 100nm thickness of the crud Fuel crud ‘black’, relatively thin, Coated the entire fuel assembly, and and contained 2-4% Zn Fuel oxide thickness was 10-20 μm for first burned fuel and 30-40 μm for twice burned fuel. Both higher than anticipated. Detailed evaluation indicated that zinc did not cause the increase in oxide thickness, nor did it have other deleterious effects on fuel.