a)permanent address: The Institute of Applied Energy, Shinbashi SY Bldg. 8F , Nishi-Shinbashi, Minato-ku, Tokyo, , Japan

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1 P R E P R I N T ICPWS XV Berlin, September 8 11, 8 Evaluation Method on Flow Accelerated Corrosion of Components in Cooling Systems of Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics Hidetoshi Okada a), Shunsuke Uchida a), Masanori Naitoh a), Yasushi Uehara a), and Seiichi Koshizuka b) a)nuclear Power Engineering Corporation, Nishi-Shinbashi Yasuda Union Bldg. F -- Nishi-Shinbashi, Minato-ku, Tokyo, 15-, Japan b)the University of Tokyo, 7--1, Hongo, Bunkyo-ku, Tokyo, , Japan a)permanent address: The Institute of Applied Energy, Shinbashi SY Bldg. 8F 1-1-, Nishi-Shinbashi, Minato-ku, Tokyo, 15-, Japan hokada@iae.or.jp The evaluation procedure based on the coupled analysis of flow dynamics and corrosion is proposed in order to predict the wall thinning rate of piping in the secondary system at PWRs. As a first step in the procedure, the flow in the secondary system is evaluated by the 1D CFD code, RELAP5. In the second step, the distribution of oxide concentration in the secondary system is calculated by the oxygen-hydrazine reaction code, RADIOLYSIS-NH. In the third step, the distribution of mass transfer coefficients are calculated by the D CFD code, PLASHY. Based on information gathered from these steps, zones highly susceptible to FAC are identified. For these zones, the wall thinning rate is calculated by means of a corrosion model that is composed of static electrochemical analysis coupled with dynamic double oxide layer analysis. The wall thinning rates calculated are in good agreements with those obtained from actual plant data and experiments. Countermeasures were investigated in order to reduce the wall thinning rate based on the proposed model. By changing the hydrazine injection point, the oxygen concentration is held at 5 ppb in zones that are highly susceptible to FAC and less than.5 ppb at the inlet to the steam generator. Introduction Recently, power uprate, extended cycle length, and plant lifetime extension have been considered as the important measures for improving the economical competitiveness of nuclear power plants (NPPs) in Japan. In order to apply these measures to existing NPPs, it is necessary to maintain and monitor piping integrity over longer operating periods and extended lifetimes. The unexpected piping rupture in the secondary cooling system at the Mihama- pressurized water reactor(pwr) power plant in was caused by flow accelerated corrosion (FAC) [1]. A similar event occurred at the Surry- PWR in 1986 []. Therefore, evaluation methods to determine thinning rate of piping due to FAC is needed to ensure the safe operation of NPPs. This paper s focus is on FAC within the secondary system of PWRs. An evaluation procedure is proposed that couples the analysis of flow dynamics and corrosion in order to more precisely predict the thinning rate of piping. The corrosion model selected is composed of static electrochemical analysis coupled with dynamic double oxide layer analysis. Various countermeasures are investigated in order to reduce the wall thinning rate. Evaluation Procedures to Determine FAC Rate Major Parameters to Determine FAC As shown in Fig.1, FAC is composed of two major processes: one is the corrosion process which is a chemical process, and the other is the flow dynamics process which is a physical process []. The former is an essential process to cause FAC and the latter is an accelerating process to enhance FAC occurrence. Metallic ions, mainly ferrous ions (Fe + ), are released into the water at the boundary layer where their concentrations become supersaturated; some of the Fe + will form oxide particles that are deposited on the metal surface to produce a magnetite oxide layer. The oxide layer acts as a barrier that prevents the additional release of Fe + (corrosion reaction). The thickness of the boundary layer is much affected by flow dynamics. For these processes, oxygen concentration ([O ]) in the boundary layer also plays an important role in the oxidation of magnetite to hematite which

2 contributes to much higher corrosion resistance. The key factor affecting FAC is the thickness of the oxide film on the pipe surface, which is the result of the corrosion process. However, at the same time, this oxide film serves as a protective barrier that inhibits the corrosion rate [], []. Determination of oxide film oxide film mass transfer coefficients by flow dynamics analysis (inner layer) (outer layer) Fe + release to bulk base metal oxide film growth ionic concentrations precipitation of oxide release of oxide erosion by water droplets shear stress by flow main flow boundary layer cooling water diffusion of oxidants corrosive conditions Major elemental models Figure 1: Elemental models to evaluate FAC. : corrosion (chemical term) : flow dynamics (physical term) The major parameters that influence FAC can be divided into three groups: material parameters, flow dynamics parameters and environmental parameters, as shown in Fig. []. A danger zone can be identified where the critical conditions for each parameter overlap. These critical conditions are necessary for FAC to occur; we refer to this as the FAC occurrence zone in Fig.. Therefore, FAC can be prevented by controlling each parameter such that this danger zone is avoided. As a minimum, wall thinning should be evaluated for those locations where these three critical conditions are present, as determined by this first evaluation step. flow pattern mass transfer coeff. >threshold oxidant [O ]<5 ppb <-.V ph ph<9. FAC occurrence zone temperature 1<T<18 Cr content [Cr]<.% [Fe + ] [Fe]<1/[Fe] sat wall thinning (Double oxide layer model) : coupled analysis : individual analysis Figure : FAC occurrence zone indicated by major parameters. Evaluation Procedures FAC is divided into five steps as shown in Fig.. In the first step, distributions of flow velocity and temperature along the flow paths in the cooling system are calculated with the 1D computational fluid dynamics (CFD) code, RELAP5 [5]. Corrosion conditions, e.g., [O ] and electrochemical potential (), along the flow path are also calculated with a 1D O -hydrazine reaction code, RADIOLYSIS-NH []. The detailed flow pattern along the flow path is calculated with the D or D CFD code, PLASHY [6], to determine the distribution of mass transfer coefficients at key locations in the cooling system. Step 1: 1D CFD code Step : 1D O -N H reaction code Step : D CFD code Step : Chart evaluation Step 5: Wall thinning calculation code Figure : Evaluation steps for FAC. Distributions of flow velocity and temperature in the cooling system Corrosion conditions, e.g., [O ] and, in the cooling system Distributions of mass transfer coefficients Danger zone evaluation Distributions of wall thinning rate and : flow dynamics : corrosion The danger zones, that is, regions that are highly susceptible to FAC, are evaluated by using the major parameters determined from steps 1 to. Finally, wall thinning rates at these locations of interest are calculated with a newly developed wall thinning calculation code. The code system is described in Table 1. Evaluation Procedure to Determine Environmental Factors of FAC Distributions of flow velocity and temperature along the flow path in the cooling system are calculated with the 1D plant system code, RELAP5 [5]. Oxygen concentration distribution along the flow path is calculated based on oxygen hydrazine reaction in the feed water. The details of the calculation procedures have been presented in a previous paper []. Calculated results of residual [O ] along the flow path are shown in Fig.. The mixing rate, χ, is calculated by the CFD model and then applied in the hydrazine-oxygen reaction calculation. is one of the candidates for high temperature water chemistry sensors for corrosive conditions caused by O and N H, where is expressed as a function of [O ] and [N H ] [7]. Evaluation Procedure to Determine Mass Transfer Coefficients Two key parameters that influence FAC is the thickness of the boundary layer and the mass transfer coefficients between the bulk water and the

3 Table1: Evaluation code system for FAC. Calculation targets Flow pattern [O ],[Fe + ] and current density Input Computer programs Output Reactor parameters Geometries heat flux 1D CFD(RELAP5) D, D k-ε CFD code (PLASHY, α-flow) D LES code Flow rate Reactor parameters Flow velocity S/V rate Mixing rate N H -O reaction code RADIOLYSIS-NH [O ] and [Fe + ] distribution along flow path ph [O ] Oxide film parameters Mass transfer coefficients Static electrochemistry mode MIXED-POT and current density at location of interest Coupling calculation Electrochemistry model(static) Oxide film growth model(dynamic) Major points Anodic/cathodic current density Oxide film formation Input Oxidant concentration Corrosion current density ph Mass Transfer Coefficient Oxide film thickness Oxide properties(fe O /Fe O ratio) Output Corrosion current density Wall thinning rate Oxide properties(fe O /Fe O ratio) boundary layer. The flow pattern along the flow path is very complicated and should be analyzed by a combination of 1D, D and D CFD codes. Calculated precise flow velocity distributions are obtained, which are applied to evaluate friction velocity distribution at the pipe surface, distributions of turbulent energy, K, and the mass transfer coefficient, h m [8]. [O ] (ppb) LPH1-: 1st-th low pressure heaters HPH6: 6th high pressure heaters condenser outlet deaerator 6 5 temperature [O ] 5 HPH6 A deaerator 15 B LPH LPH without mixing 1 LPH C LPH1 5 1 suitable interaction perfect mixture time from condenser outlet (s) Figure : Corrosive condition calculation. temperature (C) Turbulence energy and mass transfer coefficient calculated with the D k-ε code, PLASHY, for piping with an orifice just after a vent section are shown in Fig. 5. Downstream from the orifice a backward current results in high turbulent energy and a large mass transfer coefficient, which enhance FAC in the flow dynamic process. mass transfer coefficient (mm/s) 1 Pipe inner diameter: 5cm Thickness of boundary layer: ~1μm orifice reattachment point distance from inlet point (m) Figure 5: Mass transfer coefficients calculated with D CFD code, PLASHY.

4 Evaluation Procedure to Determine Wall Thinning Rate Wall thinning rate is calculated by coupling the static electrochemistry model and dynamic oxide film growth model (Table 1) [], [9]. Anodic and cathodic current densities and s are calculated with the static electrochemistry model and the ferrous ion release rate determined by the anodic current density is used as input for the dynamic oxide film growth model. The thickness of oxide film and its characteristics are used in the electrochemistry model to determine the resistance of cathodic current from the bulk water to the surface and anodic current from the surface to bulk water. The static electrochemistry model consists of an Evans diagram, as shown in Fig.6. The cathodic current is determined by the reduction reaction of oxidant at the surface, where the oxidant concentration is calculated by its diffusion through surface boundary and oxide layers. The anodic current is determined by the sum of currents due to ferrous ion release from the metal surface and the hydrazine oxidation reaction. Both currents are also determined by their diffusion through surface boundary and oxide layers. As a balance of the anodic and cathodic current densities, electrochemical corrosion potential,, is determined. The minimum potential is determined by the hydrogen generation potential, which is determined by ph and temperature of the water. anodic and cathodic current densities I a, I c (A/m ) hydrogen generation I a : total anodic current I c * of hydrazine oxidation I a* : anodic current of carbon steel* corrosion current I c :cathodic current *: anodic current for potential (V-SHE) a given oxide film Figure 6: Wall thinning calculation (Electrochemistry mode). The oxide film growth model is shown in Fig. 7. In the model, ferrous ions are released from the base metal through the oxide layers, which consist of magnetite inner and hematite outer layers []. Some of the dissolved ferrous ions are removed to bulk water and other precipitants on the surface as magnetite particles or are adsorbed on the particle surface. Some of magnetite particles are oxidized and become hematite. The conversion factor from magnetite to hematite is expressed as a function of. Calculated wall thinning rates are shown in Fig. 8. The calculated and measured results [1]-[1] for mass transfer coefficient dependence of wall thinning rates are shown in Fig. 8(a) and those for [O ] dependence are shown in Fig. 8(b), while calculated and measured results for ph dependence of wall thinning rates are shown in Fig. 8(c). The calculated results were in good agreement with the measured ones. flow mass transfer bulk water dissolution release adsorption δ boundary layer outer oxide layer (hematite particles) oxide layer oxidation oxide particle oxide particle (magnetite) (hematite) base metal inner oxide layer (magnetite particles) Figure 7: Modified double oxide layer model.

5 corrosion rate (mm/y) corrosion rate (mm/y) corrosion rate (mm/y) (a)mass transfer coefficient dependence measured : Satoh, et al. ([O ]<1ppb) [1] corrosion rate mass transfer coefficient, k (m/s) (b)[o ] dependence 1 1 (k:.5 m/s) 1 (k:.1 m/s) corrosion rate (k:.5 m/s) -. (k:.1 m/s) measured -. : Brush and Pearl (ph:7, k:.5m/s) [11] [O ] (ppb) (c)ph dependence measured : Heitmann and Schub [1] corrosion rate ([O ]:.1 ppb). k:.5 m/s [O ]: -. 1 ppb ([O ]:.1 ppb) [O ]: 1 ppb ph (-) Figure 8: Results of wall thinning calculation. Discussion (V-SHE) (V-SHE) (V-SHE) plants reported by EPRI) have been shown in Reference [1]. One countermeasure for copperfree plants is to increase the ph to more than 9.5. However, the low pressure feedwater heater tubes of some PWR plants still use copper alloy, which precludes increased ph values. Hence, it is important to control [O ] in the feed water for FAC mitigation. Another candidate for water chemistry control that can mitigate FAC is oxygen injection, which has been successfully applied in BWRs and FPPs. A slight increase in feed water [O ] without increasing O input to the steam generator is also considered. For this, it is essential to determine [O ] concentration precisely at the location of interest. Oxygen concentration is kept at a sufficiently low level at the condenser hotwell and hydrazine is injected at the outlet of the condensate demineralizer to reduce [O ] as low as possible and to mitigate the effects of oxygen on steam generator tubing. In order to increase [O ] in the FAC danger zone and to keep it lower than the target value, [O ] is calculated by changing the hydrazine injection point. [O ] (ppb) condenser outlet deaerator [O ] LPH LPH LPH1 deaerator LPH LPH inlet temperature HPH6 LPH inlet time from condenser outlet (s) Figure 9: countermeasures ([O control]). temperature (C) Two cases are compared in Fig. 9. One case injected oxygen at the usual point, the condensate demineralizer outlet, and the other case injected oxygen at the LPH inlet. The first case has 5 ppb O at the condenser outlet, which means about five ppb O remained in the danger zone and [O ] at the steam generator inlet should be lower than.5 ppb. The oxygen concentration control should be monitored using the sensor with a suitable reference electrode. Major countermeasures for FAC of carbon steel at fossil power plants (FPPs) (examples at US 5

6 Conclusion The evaluation procedure based on the coupled analysis of flow dynamics and corrosion was proposed in order to predict the wall thinning rate of piping in the secondary system at PWRs. As the first step in the procedure, the flow in the secondary system is evaluated by the 1D CFD code, RELAP5. In the second step, the distribution of oxide concentration in the secondary system is calculated by the oxygen-hydrazine reaction code, RADIOLYSIS-NH adopting the flow calculated in the first step. In the third step, the distribution of mass transfer coefficients are calculated by the D CFD code, PLASHY. Based on the information obtained from these steps, regions that are highly susceptible to FAC, or danger zones, are identified. For these danger zones, the wall thinning rate is calculated by means of the corrosion model which is composed of static electrochemical analysis coupled with dynamic double oxide layer analysis. The wall thinning rates calculated by the proposed model were compared with measurements taken from actual power plants and from experiments and found to be in good agreement. Countermeasures were investigated in order to reduce the wall thinning rate based on the proposed model. By changing the hydrazine injection point, the oxygen concentration is held at 5ppb in these FAC susceptible danger zones while oxygen concentration is less than.5 ppb at the inlet to the steam generator. Acknowledgements This research was partially performed under the Developmental Resource for Innovative and Viable Nuclear Energy Technology and is based upon work supported by a grant from the Japanese Ministry of Economy, Trade and Industry. Literature [1] Nuclear and Industrial Safety Agency: Final report about Mihama- secondary system piping failure, Tokyo (5)(in Japanese). [] C.Z. Czajkowski: Metallurgical an 18 Inch Feedwater Line Failure at the Surry Unit Power Station, NUREG/CR-868 BNL-NUREG-557 (1987). [] S.Uchida, M.Naitoh, Y.Uehara, H.Okada and S.Koshizuka: Evaluation Method of Corrosive Conditions in Cooling Systems of Nuclear Power Plants by Combined Analyses of Flow Dynamics and Corrosion. PowerPlant Chemistry, 9, (7). [] S.Uchida, M.Naitoh, Y.Uehara, H.Okada, S.Koshizuka and D.H.Lister: Flow Accelerated Corrosion of PWR Secondary Components by Corrosion analysis Coupled Flow Dynamics Analysis. Proc. 1 th Int. Conf. Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Whistler (7). [5] The RELAP5 Code Development Team: RELAP5/MOD Code Manual. NUREG/CR- 555, INEL-95/17, Idaho National Engineering Laboratory, Idaho Falls(1995). [6] R.Takahashi, K.Matsubara and H. Koike: Implementation of a Consultative Expert System for Advance Fluid Dynamics Analysis Code α-flow. Proc. nd Int. Forum on Expert System and Computer Simulation in Energy Engineering, Erlangen(199). [7] M.Ullberg, P.Andersson, K.Fruzzetti, et al.: Modeling the oxygen-hydrazine reaction using electrochemical kinetics. Proc. 1 th Int. Conf. Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Whistler(7). [8] S.Uchida:Latest Experience with Water Chemistry in Nuclear Power Plants in Japan. PowerPlant Chemistry, 8, 8-9 (6). [9] S.Uchida, M.Naitoh, Y.Uehara, H.Okada, S.Koshizuka and D.H.Lister: Evaluation Methods for Flow Accelerated Corrosion in Nuclear Power Plants by Coupling Analysis of Corrosion and Flow Dynamics. Proc. Int. Conf. on Flow Accelerated Corrosion 8, Lyon (8). [1] T.Satoh, Y.Shao, W.G.Cook, D.H.Lister and S.Uchida:Flow-Assisted Corrosion of Carbon Steel Under Neutral Water Conditions. Corrosion, 6, (7). [11] E.G.Brush and W.L.Pearl: Corrosion and Corrosion product Release in Neutral Feedwater. Corrosion, 8, 19 (197). [1] H.G.Heitmann and P.Schub: Initial experience gained with a high ph value in the secondary system of PWRs. Proc. Third Meeting on Water Chemistry of Nuclear Reactors, London (198). [1] B.Dooley and K.Shields:Cycle Chemistry for Conventional Fossil Plants and Combined Cycle/HRSGs. Proc. EPRI Cycle Chemistry Conf., Houston (). 6