On-line monitoring for further corrosion reduction and better understanding of the activity build-up in LWR primary coolant systems

Int. J. Pres. Ves. & Piping 55 (1993) 163-177

On-Line Monitoring for Further Corrosion Reduction and Better Understanding of the Activity Build-Up in LWR Primary Coolant Systems K. M~ikel~i & P. A a l t o n e n Metals Laboratory, Technical Research Centre of Finland (VTT), Metallimiehenkuja 10, SF-02150 Espoo, Finland

ABSTRACT Uniform corrosion rates of materials in contact with the coolant in boiling water reactors (BWRs) and pressurized water reactors (PWRs) are low and this kind of material degradation is very unlikely to compromise the integrity of the primary pressure boundary. In a properly controlled environment the initial corrosion rates decrease reaching the low corrosion rate, typical for the steady state between the outer oxide surface and the environment. The water chemistry practice modifications can be used to decrease fuel corrosion, improve material integrity and slow down activity build-up in the primary circuit. Therefore, on-line water chemistry monitoring at high temperatures should be conducted to obtain local, reliable and thus useful information over long measurement periods at a specific loop site. In this paper, the effect of water chemistry on the corrosion product formation, activation, transport and redeposition on out-of-core surfaces is discussed.

1 INTRODUCTION Corrosion p h e n o m e n a in nuclear power plants (stress corrosion cracking, corrosion fatigue, general corrosion) can be prevented by right construction material selection, advanced plant design and tight water chemistry control. In a properly controlled environment the initial corrosion rates decrease reaching the low corrosion rate, typical for the steady state between the outer surface and the environment. Part of the 163 Int. J. Pres. Ves. & Piping 0308-0161/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland

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K. Mdkelii, P. Aaltonen

dissolved corrosion products will deposit on fuel elements when transported into the core. Activation of the corrosion products, the release from in-core surfaces, transport and redeposition on out-of-core surfaces lead to radiation build-up. Successful water chemistry control requires regular and continuous monitoring of such water chemistry parameters as dissolved oxygen content, pH, conductivity and impurity contents. Conventionally, the monitoring is carried out at low pressures and temperatures, which, however, has some shortcomings, because parameters such as pH, conductivity and redox potential are changing as a function of the temperature. In addition to this, the radiation effects can locally change the chemical composition of the water and therefore on-line monitoring should be carried out on-site in real operating environments. In power plants, extensive instrumentation and laboratory analysis programs are applied to provide rapid and reliable diagnosis of water chemistry. However, at present chemical on-line monitoring is used mainly in low temperature, low pressure conditions or by using grab samples. More relevant information concerning the chemical environment could be obtained by using high temperature, high pressure measurements at least for pH, conductivity and electrochemical corrosion potentials (ECP), which are all local, site-specific parameters of the environment.

2 P R E S S U R I Z E D W A T E R R E A C T O R (PWR) P R I M A R Y W A T E R CHEMISTRY AND C O R R O S I O N

The rates of general or uniform corrosion of materials in contact with the primary coolant in PWRs are quite low and do not compromise the integrity of the primary circuit. Corrosion product release rates are material-specific and vary as a function of pHv at high temperature aqueous environments and therefore the availability of corrosion products for deposition on fuel cladding surfaces is partly controlled by variations in pHv. Further, variations in pH-r can affect the transport and the deposition of corrosion products through solubility effects, zeta potential variations or as a result of a change in the form of the corrosion products released from corroding surfaces. In addition, the solubility is controlled by dissolution and precipitation rates, i.e. reaction kinetics. Therefore, improved analysis and narrow range control of primary circuit water chemistry, especially pHT, in PWRs can be successful in reducing general corrosion and thus the radiation build-up.

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2.1 Corrosion product solubility in P W R primary coolant

Tests at different plants have shown that at pHT6"9 there will be significantly less crud deposited on the fuel and a slower build-up of radiation fields on piping and steam generators than at lower pHT values. When the plant with the lower pHT changed to pHT of 6"9, there was no further increase in radiation fields during the subsequent fuel cycles. 1 Thus, the co-ordinated water chemistry regime for LiOH/H3BO3 buffer solution defines the minimum acceptable operational band. This co-ordinated chemistry regime was based on magnetite solubility. The studies of crud solubility and the data for nickel-iron spinel suggest that the pHT required to ensure the temperature coefficient of solubility to be positive should have an even higher pHT value than 6.9. 2 The solubility of oxides in high temperature aqueous solutions has been studied by,-for example, Thornton. 3 The solubilities of cobalt and manganese from crud at 300 °C versus pHT are plotted in Fig. 1. The solubilities increase rapidly if the pHT is lower than 7. Cobalt and

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Fig. 3. Deposition rate constant for cobalt and manganese depositing on Ringhals 2 crud at 300 °C.3 operating with constant, high pHT have lower radiation build-up than plants operated with low or varying pHT. 2 These few examples indicate the importance to measure continuously the real high temperature pHT values and to use the measured parameters to control the water chemistry within a vary narrow operating range during the whole fuel cycle. 2.2 Waterside corrosion of P W R fuel cladding U p to the today's target discharge burn-ups, the waterside fuel cladding corrosion has not restricted the operation strategies in PWRs. However, when the discharge burn-ups a n d / o r the temperature of the coolant are increased, the corrosion of the fuel cladding may become a life-limiting factor. If the elevated pHT chemistry is used to reduce the crud deposition on the fuel rods, it is necessary to confirm that the possible changes in water chemistry do not aggregate fuel cladding corrosion. Some reactor trials and loop experiments with increased lithium levels are in progress. Oxide thickness data from one U S A plant do not show any difference in oxide thickness after 12 months of > 2 . 2 p p m lithium

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exposure. 2 A good evaluation of the lithium increase effect is made possible by an extensive data base of plant-specific oxide thickness measurements prior to the elevated Li operation. The difference in oxide thickness before and after the elevated Li concentration operation is shown in Fig. 4. Similar fuel oxide evaluation results are obtained from other plants. It seems that any effects of Li are relatively minor, compared to the effects of temperature and zirconium composition, but as long term experiences from operation at elevated Li levels are still lacking, certain precautions should be carried out (i.e. monitoring of cladding oxidation). LiOH is used as an alkalizing agent in Western reactors, but in Soviet VVER-type PWRs K O H is being used to increase the pH value of the water. The effect of K + to zirconium corrosion is unknown even though the work done by Coriou et al. 4 shows that K O H is considerably less aggressive than LiOH. Unfortunately, their data only refer to the earlier stages of corrosion. The effect of different water chemistries on the materials' behavior requires extensive investigations in laboratories as well as in plants under well specified and accurately controlled environments, which can only be accomplished through high temperature p H r and ECP measurements.

3 E F F E C T S OF B O I L I N G W A T E R R E A C T O R (BWR) W A T E R C H E M I S T R Y ON C O R R O S I O N The uniform corrosion rates of materials in contact with the coolant in B W R are low, and this kind of material degradation is very unlikely to compromise the integrity of the primary pressure boundary. However for piping systems, sensitized microstructure coupled with residual

Monitoring of L WR primary coolant systems

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stresses produces a high degree of susceptibility to stress corrosion cracking in the presence of oxygenated coolant containing impurities? 3.1 Factors affecting stress corrosion cracking (SCC) of stainless steel The necessary condition for the IGSCC (intergranular stress corrosion cracking) is the presence of a certain amount of oxygen along with associated oxidizing species such as hydrogen peroxide in the coolant. Because sensitized microstructure and stresses are difficult to avoid, the obvious remedy to prevent IGSCC is the control of the environment. In general, by keeping the amount of oxygen low in the coolant SCC is inhibited, but the exact level is plant-specific. To prevent crack formation in the BWR stainless steel piping, the ECP in the cooling water has to be kept more negative than - 2 3 0 mV(SHE) in water that has a conductivity less than 0-3/~S/cm. 6 The dissolved oxygen content under normal BWR conditions is about 100-300 ppb (~g/kg), and the ECP of the stainless steel is between - 1 0 0 and + 1 0 0 m V ( S H E ) , thus supporting stress corrosion cracking. If the conductivity of the water is below 0.3 ~ S / c m , keeping the amount of oxygen below 20ppb is sufficient for maintaining the potential low enough to inhibit SCC. However, if the water contains powerful oxidative impurities such as chromates, not even 5 ppb O2 is low enough to inhibit SCC. Also, during transients the environment can cause high potentials, together with high conductivity, making fast crack growth rates possible (Fig. 5). One way to avoid IGSCC and IASCC (irradiation assisted stress 10-9 i

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Monitoring of LWR primary coolant systems

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Recent ECP measurements in Swedish plants show that the corrosion potential of the stainless steel can differ considerably between different piping locations even though the hydrogen dose rates were carried out according to BWR hydrogen water chemistry regulations. When the corrosion potentials in the autoclaves, which were connected to the primary loop piping, were decreased below the critical potential for IGSCC, the in-core measured potential was higher and only slightly affected by the hydrogen dosage, meaning that more hydrogen is needed to decrease in-core potentials below the critical value. If on-line potential measurements are carried out in a non-representative place, the corrosion potential can be in another place above the critical value for IGSCC and no protection is achieved. 7 The locations close to the core are more sensitive to transient chemistry conditions, because at more distant locations oxidants are consumed during the transport and corrosion potentials do not react to transients as no oxidants reaches the surfaces at these measurement points. Therefore, the adequate high temperature monitoring can be carried out only by measuring the water chemistry parameters continuously at several different locations.

3.2 Reduction of activity build-up Injection of zinc into the reactor system is the new method in BWR chemistry practice to reduce activity build-up. The radiation fields are reduced significantly by the presence of Zn in plants using normal water chemistry (NWC) (Fig. 8). The plants changing from NWC to H W C have in the beginning significantly increased shutdown radiation fields and zinc injections are hoped to mitigate them. 8 As part of the I

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corrosion inhibition effect of Zn, the soluble release of cations from construction materials is reduced, leading to lower release rates of materials containing cobalt. The same kind of effect is believed to happen to crud deposits on fuel cladding materials leading to suppressed release rates. 9 To learn more about the effects of zinc additions on the overall water chemistry, a proper high temperature on-line monitoring system is required, to give more information about the possible changes in ECP, pHv and high temperature conductivity values.

3.3 Waterside corrosion of BWR fuel cladding In the BWR core the dissolved oxygen in the coolant combined with irradiation enhances the corrision rate of zirconium. Impurities in the water continually deposit and leave the surface of the zirconium fuel cladding. On the hot surfaces of the fuel rods and in the typical pHT range of the coolant, the solubility of the corrosion products are easily exceeded. The crud deposited on the fuel cladding arises mainly from the general corrosion of the reactor structural materials such as stainless steel, Inconels, carbon steel, hard-facing alloys, brass, etc. The corrosion product layer on the fuel rods decreases heat transfer from the fuel to the coolant increasing the fuel temperature and accelerating the corrosion rate. Very little data have been reported of the HWC effects on zirconium corrosion or hydrogen pick-up. Generally it is believed that the hydrogen pick-up fraction of zirconium alloys increases with decreasing oxygen content of the water. In addition, the thermal diffusion towards colder parts may increase the hydrogen concentration locally, 1'' which could be verified by in-core ECP measurements. To minimize the deposition of insoluble materials on the fuel cladding as a result of nucleate boiling, the concentration of ionic impurities and the corrosion products in the coolant must be maintained at a very low level. Water chemistry control, in terms of the impurity levels and dissolved oxygen concentration as well as other oxidizing species produced by radiolysis, requires accurate experimental data particularly from in core ECP and high temperature conductivity measurements. 4 ON-LINE H I G H T E M P E R A T U R E W A T E R C H E M I S T R Y M O N I T O R I N G SYSTEM A computerized monitoring system, developed by VTT/Metals Laboratory and Imatran Voima Oy (IVO), for high temperature and pressure

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pHv, conductivity and ECP measurements has been in continuous use at the Loviisa power plant since 1988. The same kind of measurements have been performed also in the light water test loop of the O E C D Halden reactor in Norway since 1987. Special emphasis has been put on learning the effect of pHT and ECP control during the cooldown process in order to reduce further the background radiation build-up. The on-line monitoring system was used to measure the high temperature water chemistry parameters at the Loviisa power plant (VVER-440 reactor) during the shutdown for refueling outage in 1990. The system (Fig. 9), including the cell with electrodes, was connected to a sampling line coming from the core near the reactor water inlet point at Loviisa 1 PWR. During the shutdown the physical, operational and chemical changes produce large releases of corrosion products. The actions taken during the scheduled shutdown in Loviisa 1 include the following operations which are also indicated in Figs 10 and 11: (A) (B) (C) (D) (E)

The dosage of ammonia is stopped and removal of added chemicals with the primary water clean-up system is started. Starting and . . . . . . s t o p p i n g the degasification before the reactor reached subcritical state. The end of subcritical state and boronation of the primary water. Starting the degasification.

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Stopping the addition of ammonia and starting the total removal of added chemicals with the primary water cleaning system caused a decrease in the pHT and hydrogen concentration, which were further decreased after starting the degasification. The high temperature pHT decreased corresponding to the hydrogen overpressure and the electrochemical Nernstian response. The boronation decreased rapidly the high temperature pHT value to about 6.4 (Fig. 10). However after this, high temperature pHT started to fluctuate. Each minimum in the high temperature pH was followed by a peak in pHT until the auxiliary cooling decreased the temperature below 100 °C and the high temperature pH measurement was not reliable any more. The concentrations of dissolved Co in the primary coolant during the shutdown are presented in Fig. 11. Stopping the ammonia dosage and the following decrease in the hydrogen partial pressure did not enhance the dissolution of analyzed Co. At first the shift to lower pHT values immediately after the boronation increased the concentration of Co in the water. However, the subsequent increase in the high temperature pHT stopped the

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dissolution or decrease rate of corrosion products and therefore a distinct peak in the concentration of dissolved corrosion products was observed. The simultaneous measurements of pHT and concentrations of dissolved corrosion products in the primary coolant during the shutdown transient indicated the importance of primary coolant pHT in the dissolution and transport mechanism of corrosion products to out-ofcore surfaces (Figs 10 and 11). Fluctuations in measured parameters after the boronation in the primary loop revealed the importance of understanding the effects of all components in the loop like materials, flowrates, clean-up systems, etc., on the primary water chemistry. The mechanism of dissolution and transport of activated corrosion products can be studied by using theoretical models, but more effective control of the actual processes can only be carried out by on-line high temperature water chemistry measurements. Therefore, the improved pHT (measured on-line from reactor pressure vessel water inlet and outlet points) and temperature control of the primary coolant during normal operation and especially transients can be used to reduce further the activity build-up at plants.

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5 CONCLUSIONS The on-line water chemistry monitoring at high temperatures should be conducted to obtain local, reliable and thus useful information over long measurement periods at a specific loop site. The results obtained in PWR environments so far underline the differences in the water chemistry conditions during the steady-state operation and transients like shutdowns. The water chemistry practice modifications can be used to decrease fuel corrosion, improve material integrity and slow down activity build-up in the primary circuit. In BWR environments, the continuous monitoring of the ECP and the conductivity of the coolant is an efficient way to make sure that water quality is good enough to avoid the stress corrosion cracking of sensitized materials. Material testing in simulated reactor environments is necessary for the further improvement of construction materials. Simulation of reactor environments can be difficult, especially when tests are carried out in small-scale laboratory test loops where the coolant volume to the internal metal surface area ratio is different from that in real power reactors. In order to simulate the real conditions, representative to some parts of the reactor loop, the local water chemistry parameters of that site should be experimentally measured. Based on these parameters, materials testing can be carried out to obtain reliable estimations of the remaining material lifetime.

ACKNOWLEDGEMENTS The high temperature water chemistry study is a part of the Structural Integrity of Nuclear Components Research Programme funded by the Ministry of Trade and Industry (KTM) and the Technical Research Centre of Finland. The development work of the on-line monitoring equipment was additionally funded by Imatran Voima Oy (IVO).

REFERENCES 1. Wood, C. J., PWR Primary Water Chemistry Guidelines, Rev. 1, EPR1 NP-5960-SR, Palo Alto, CA, August 1988. 2. Wood, C. J., PWR Primary Water Chemistry Guidelines, Rev. 2, EPRI NP-7077, Palo Alto, CA, November 1990. 3. Thornton, E. W., Corrosion product solubility in PWR primary coolant,

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5. 6.

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8. 9. 10.

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12.

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Proc. JAIF Int. Conf. on Water Chemistry in Nuclear Power Plants, Tokyo, April, 1988, Japan Atomic Industrial Forum, pp. 750-4. Coriou, H., Grall, L., Meunier, J., Pelras, M. and Willernoz, J., Corrosion of Zircaloy in various alkaline media at high temperature, Proc. IAEA Conf. on Corrosion of Materials H, Europahaus, Salzburg, Austria, May, 1962, p. 193. Fox, M. J., A review of boiling water reactor water chemistry, NUREG]CR-5115, ANL-88-42, R5. US Nuclear Regulatory Commission, Washington, NRC FIN A2212. Indig, M. E. Recent advances in measuring ECPs in BWR systems, Proc. 4th Int. Symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors, Jekyll Island, USA, August, 1989, NACE, pp. 41111-41133. Molandar, A. and Jansson, C. In situ corrosion potential monitoring in Swedish BWR, Proc. 5th Int. Symposium on Environmental Degradation of Materials in Nuclear Power Systems--Water Reactors, Monterey, USA, August, 1991, American Nuclear Society, 1992, pp. 118-25. Wood, C. J., Zinc injection helps reduce radiation field build up in BWRs, Nuclear Engineering International, April 1991, pp. 39-40. Marble, W. J. & Cowen, R. L., Proc. Int. Conf. Water Chemistry in Nuclear Power Plants, Tokyo, 1991, pp. 55-65. Garzarolli, F., Bodmer, R. P., Stehle, H. and Trapp-Pritsching, S., Progress in understanding PWR fuel rod water side corrosion, Proc. ANS Top. Meeting on Light Water Reactor Fuel Performance, Orlando 1985, pp. 3]55-3]72. Shack, W. J., Kassner, T. F., Maiya, P. S., Park, J. Y. and Ruther, W. E., BWR pipe crack remedies evaluation, Proc. 14th Water Reactor Safety Information Meeting, Gaithersburg 1986, Vol. 2, Rep. NUREG/CP-0082, USNRC, Washington, DC, 1987, pp. 101-117. Aaltonen, P., J~irnstrtim, R., Kvanstr/Sm, R. and Chanfreau, E., On-line water chemistry monitoring for corrosion prevention in ageing nuclear power plants, Proc. JAIF Int. Conf. on Water Chemistry in Nuclear Power Plants, Fukui City, April 1991, Japan Atomic Industrial Forum, pp. 355-60.