EVALUATION OF A CHEMICAL CLEANING FORMULATION FOR THE STEAM GENERATOR OF NUCLEAR POWER PLANTS A.L.Rufus, H.Subramanian, V.S.Sathyaseelan, Padma S.Kumar, S.Veena B.Anupkumar, M.P.Srinivasan, S.Velmurugan and S.V.Narasimhan Abstract Water and Steam Chemistry Division Bhabha Atomic Research Centre Facilities, Kalpakkam 603 102 (TN), INDIA A clean Steam Generator (SG) goes a long way in deciding the performance of any power plant. Continuous ingress of impurities from the feed water coupled with boiling in the SGs leads to sludge formation, which gets accumulated on the SG tubes, tube sheet and tube support plates. This not only decreases the heat transfer efficiency but also leads to many corrosion problems including through-hole pitting of SG tubes. In nuclear power plants such a situation is of much concern because of leak of active primary coolant into the SG leading to radiation exposure hazard. Periodic chemical cleaning of this deposited sludge is required not only to improve the performance but also to make available the plant for power generation. This paper describes the work carried out to evaluate the EDTA based chemical cleaning formulation to clean the nuclear steam generators having deposits containing copper and iron. It also discusses the benefit of adding corrosion inhibitor to control the corrosion of carbon steel during chemical cleaning. In addition, this paper gives the details and the results of a mock-up chemical cleaning carried out in an isolated failed heat exchanger from MAPS (Madras Atomic Power Station) using EDTA based formulations. Introduction Efficient feed water treatment and stringent water chemistry control have drastically reduced the level of contaminants in nuclear steam generators (SGs). However, over the years of operation and large volume of water which pass through results in the formation of deposits that gets accumulated on the tube sheet, tube support plates and tube surfaces affecting the performance of SGs. Besides, these deposits also lead to under deposit attack / crevice corrosion resulting in through-hole pitting of SG tubes. Such attacks are accelerated under the influence of boiling conditions prevailing in the SGs (1). SG fouling can be minimized by proper selection of structural materials, following stringent water chemistry control, employing on-line SG cleaning techniques such as use of polymeric acids. However, to avoid the risk of SG failure due already fouled SG, chemical cleaning is the only alternative. In SGs the possibility of added chemicals getting trapped in the flow restricted areas exists and hence nowadays organic chelating agents such as EDTA, citric acid etc., are being preferred as they are easily decomposable at the normal operating conditions of the SGs (2). As these are mild acids, the problems of corrosion are less. Hence, EDTA based formulations were evaluated for their efficiency for the chemical cleaning of the SGs. The steam generator deposits (foulants) are mainly the corrosion products along with the impurities ingressed from the condenser leak. The nature of the foulants is mainly dependent on the structural materials of the secondary system besides the water quality of the condenser cooling water (3). In MAPS (Madras Atomic Power Station), the shell side of the SG is carbon steel whereas the tube side is monel-400, which is an alloy of 70% Ni and 30% Cu and the condenser is the sea water cooled system. Table-1 gives the various
phases that constituted the SG sludge collected from one of the failed heat exchanger from MAPS. Table-1 Oxides and Silicates in the SG sludge Constituents Formula Magnetite Fe 3 O 4 Nickel ferrite NiFe 2 O 4 Nickel oxide NiO Metallic copper Cuprous oxide Cupric oxide Cu Cu 2 O CuO Magnesium calcium aluminosilicates -- Antigorite Mg 3 Si 2 O 5 (OH) 4 Clinochrysotile Mg 3 Si 2 O 5 (OH) 4 Talc 3MgO.4SiO 2.H 2 O Depending upon the composition of the sludge, the chemical cleaning process and the formulation varies. The sludge consists of three groups of constituents viz., iron based oxides, copper based constituents and the insolubles mainly consisting of silicates. Hence, it is required to have two different formulations, one for dissolving iron based oxides and the other to dissolve the copper containing constituents, which are accordingly termed as Iron formulation and Copper formulation. Iron formulation Iron formulation consists of de-aerated solution of 10% EDTA whose ph is adjusted with ammonia to 6. It is efficient in dissolving iron based oxides viz., magnetite and nickel ferrite. The efficiency increases in the presence of 1% N 2 H 4 which helps in the reductive dissolution of ferrites. Under identical experimental conditions, the percentage dissolution of SG sludge obtained in the absence of N 2 H 4 was 55 while it was 80 in the presence of N 2 H 4. Apart from dissolving iron based oxide, iron formulation also dissolves cupric oxide and cuprous oxide. However, in the case of cuprous oxide only 50% is dissolved due to disproportionation of Cu + ions to Cu 2+ and Cu 0. Corrosion inhibitor In any chemical cleaning process it is utmost important that only the foulants are dissolved efficiently without disturbing the underlying base metal. Hence, it is required to keep the corrosion of base metals at a minimum. Hence, studies were carried out to evaluate a suitable inhibitor for the process. The formulation used for the corrosion study was a de-aerated mixture of 5% of EDTA and 0.5% of N 2 H 4 (concentrations of both the constituents were halved) containing 0.1% inhibitor. The inhibitors evaluated were neutradine-135, rodine-92b and 1,2,3 benzotriazole. The ph was adjusted to 6 either with ammonia or NaOH and the temperature was maintained at 90 C. Corrosion rates for various structural materials such as carbon steel, monel-400, incoloy-800 and SS-316 were estimated from the weight loss. The corrosion rates for non-carbon steel surfaces were negligible in the absence of inhibitor and hence detailed study was carried out only with carbon steel. The corrosion rates obtained for carbon steel in NH 3 based formulation in presence of inhibitors neutradine /rhodine /benzotriazole were 0.07, 0 and 0.15µm/h respectively as against 1.9µm/h with the formulation containing no inhibitor. The corresponding inhibitor
efficiencies (I eff ) were estimated to be 96, 100 and 92%. In the case of rodine alone, visual observations showed a sticky coating on the surface of the coupon. From the efficiency values it is observed that all the inhibitors evaluated are very efficient in inhibiting the corrosion reaction of iron where hydrogen is evolved. Fe 3+ induced carbon steel corrosion Fe + + + 2+ 2H Fe H 2 The efficiency of the inhibitors on carbon steel corrosion was also evaluated in the presence of magnetite powder. Here, magnetite dissolves first to yield EDTA complexes of Fe 2+ and Fe 3+ in solution. The so formed Fe[III]EDTA complex then oxidizes iron (Fe 0 ) in the base metal and in the process gets reduced to Fe[II]EDTA complex. Thus, base metal is lost due to Fe 3+ induced corrosion. 2Fe [ III] EDTA + Fe 2Fe[ II] EDTA + Fe Studies were carried out to assess whether the inhibitors can inhibit the corrosion reaction due to Fe 3+ ions in solution. Figure-1 compares the corrosion rates in the presence of inhibitors, neutradine /rhodine /benzotriazole, with that obtained in the absence of inhibitor during the exposure of carbon steel surface to the formulation containing magnetite powder. The comparatively lower corrosion 0.6 rate observed in the case of rhodine is due to 0.4 the formation of sticky coating of the inhibitor on the surface thus providing a physical 0.2 barrier for the formulation to come in contact with the surface. Besides, the magnetite 0 particles also get stuck to the sticky coating and thereby their effective surface area decreases which results in lower Fe 3+ concentration and hence lower corrosion rate. In the case of neutradine and benzotriazole, the inhibition efficiencies are 38 and 15% respectively. Thus, from the observations it is seen that inhibitors are not that efficient in inhibiting the Fe 3+ induced base metal corrosion. Since the corrosion rates in the absence of inhibitor are only marginally (<2µm/h), the cleaning process can be applied even without inhibitor. In order to reduce corrosion, the ph of the iron formulation may be further increased. However, efficiency of the iron formulation at higher ph has to be investigated. Effect of ph adjusting agent on corrosion of carbon steel Experiments were also carried out with Na 2 EDTA based formulation in the presence and absence of the corrosion inhibitor, benzotriazole. The ph of the formulation was adjusted to 6 with NaOH. The corrosion rates in the presence of magnetite were found to be lower than the corresponding NH 3 based formulation. For Na based formulation, the corrosion rate in the absence of inhibitor was 0.9µm/h while it was 1.3 for NH 3 based formulation. The corresponding values in the presence of inhibitor were 0.8 and 1.1µm/h. Copper formulation Characterization of the sludge showed presence of copper metal to an appreciable extent. Iron formulation is incapable of dissolving metallic copper present in the sludge. Corrosion rate in µm/h 1.4 1.2 1 0.8 2+ Figure-1 Corrosion of CS in iron formulation during the simultaneious dissolution of magnetite No inhibitor Neutradine Rodine 92B Benzotriazole
Besides, there is plating of copper during the dissolution of sludge by iron formulation. Thus, there is a requirement for copper formulation which can dissolve both metallic and the plated copper. This formulation consists of 5% EDTA and an oxidizing agent. The ph of the formulation is adjusted with a mixture of NH3 and ethylene diamine (EDA) to 9.5. The temperature of application is room temperature. of a failed heat After evaluating chemical cleaning iron and copper formulations for their dissolution efficiency on synthetic oxides and sludge collected from SG and corrosion compatibility with specimens of SG structural material, a mock-up SG cleaning was done on a failed heat exchanger from MAPS#2 reactor. Figure-2 Effect of oxidant during the dissolultion of metallic copper by copper formulation 6 100 5 80 4 60 3 40 2 20 1 0 Time in h Chemical cleaning exchanger 120 % dissolution Figure-2 compares the percentage dissolution of metallic copper in copper formulation in the presence and absence of oxidant The oxidants evaluate were air and H2O2. From the graph it is clear that H2O2 is the best oxidant since 100% dissolution is obtained within half an hour of exposure. 0 Inert % Dissolution Air H2O2 Duration of expt SGs of MAPS reactors are of hair-pin type where, the feed water enters from the cold leg side. At the operating pressure and temperature, steam is generated in the SG which is fed to the steam drum and then to the turbine. The hot legs are almost flow restricted regions when compared to cold legs and hence prone to fouling followed by corrosion. In the failed heat exchanger, about two inches thick sludge was deposited on the tube sheet of the hot leg and a pin hole at about an inch from the tube sheet was observed. The cleaning was mainly aimed at the dissolution of sludge on the tube sheet of the hot leg and hence only a part-system cleaning was attempted by filling the hot leg with the formulation to a height of around two metres. The photograph and schematic of the engineering set-up for the chemical cleaning is shown in Figure-3. Figure-3 Photograph and schematic showing the SG chemical cleaning facility
The actual cleaning was done in three steps; first and third steps with copper formulation while the second step used iron formulation. In all the steps, a concentrated formulation was prepared and then injected into the SG cleaning facility. In the case of copper formulation, there was the risk of fenton s reaction, which is an exothermic decomposition of the concentrated formulation with H 2 O 2 and hence the cleaning was done in the temperature range 12 19 C. The duration of each copper step was around 4½ hours with intermittent addition of H 2 O 2. Care was taken to keep the iron in the system during the start-up of each copper step below 10ppm, since Fe catalyses fenton s reaction. In the case of iron step, 0.1% rodine-92b was used as corrosion inhibitor and the duration was 21 hours with intermittent addition of EDTA, N 2 H 4 and NH 3 as and when required. After each step, rinsing was also carried out. Table-2 gives the amount of various constituents of sludge expected to be present based on elemental analysis and the actual amounts dissolved in each step during the chemical cleaning. In the copper steps, almost all the copper present in the sludge dissolved while in the iron step the other constituents dissolved. At the end of the process, some insolubles were left out which were characterized by XRD as silicates. On calculation, ~ 80% of the sludge was dissolved. Subsequent to chemical cleaning, the shell portion near the tube sheet was cut open to examine the wetted region which showed clean surface. Table-2 Amount of various constituents dissolved during the mock-up chemical cleaning of the heat exchanger tube Metal ions % composition During the chemical cleaning, polished carbon steel and monel coupons were installed in the system and required number of coupons were removed at the end of each step to evaluate the corrosion rates. The estimated corrosion rates are given in Table-3. Table-3 Corrosion of structural materials during the mock-up chemical cleaning Values in g Copper step-1 Iron step Copper step-2 Total Expected Cu 26.8 651 3 616 1270 2443 Fe 30.0 3 2986 3 2992 3251 Ni 5.6 64 969 9 1042 600 Ca 7.3 17 98 8 123 577 Mg 0.3 0.4 9 0.2 9.6 30 Insolubles 5.0 -- -- -- 500 520 In terms of sludge 8267 10379 Values in µm/h Monel Carbon steel Copper step -1 0.18 0.004 Iron step 0.005 0.14 Copper step - 2 0.07 0.004 Over all process 0.33 0.04
The corrosion rate of monel in the first copper step is very high. This is because, immediately after injecting H 2 O 2 into the loop, the recirculation pump failed which was restored only after two days. The coupons were still in the system during these two days. However, in other steps and for the over all process, where the coupons were exposed to all the three steps, the corrosion rates were as expected. Conculsions EDTA based iron and copper formulations were evaluated for the chemical cleaning of SG. Besides, three inhibitors viz., neutradine-135/rodine-92b/1,2,3 benzotriazole were evaluated for their efficiency to inhibit carbon steel corrosion in iron formulation. A mock-up chemical cleaning was successfully carried out on a failed heat exchanger from MAPS#2 reactor wherein 8.3kg of sludge against an expected value of 10.4kg was dissolved with acceptable corrosion rates substantiating the efficiency of the EDTA based formulations. References 1. I.S.Hwang and I.G.Park, Control of Alkaline Stress Corrosion Cracking in Pressurized Water Reactor Steam Generator Tubing, Corrosion, 55 (6), 616-625, 1999. 2. R.Gilbert and L.Ouellet, Dissolution of Metal Oxides Accumulated in Nuclear Steam Generators: Study of Solutions Containing Organic Chelating Agents, Nucl. Technol., 68, 385-394, 1985. 3. F.Gonzalez, J.M.T.Raycheba and P.Spekkens, Examination of Corrosion Product Deposits from CANDU Steam Generators, Proc. Second Int. Conf. Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, California, 539-547, 1985.