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Selection criteria for the best secondary water chemistry
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Nuclear Engineeringand Design 160 (1996) 193-201
Selection criteria for the best secondary water chemistry Francis Nordmann,
Jean-Marie
Fiquet
EDF-Groupe des Laborawire, 21 all~e Priv~e, F-93206 Saint-Den[s, France
Abstract This paper describes, for PWR plants, the approach for selecting the best chemistry--pH, amine, corrosion inhibitors--according to the secondary system characteristics, .,ucb as presence or not of copper alloys, steam generator tubing alloy, tube support plate design, sludge pile importance. The impact of condensate polisher, sludge lancing, chemical cleaning, as well as other ways of eliminating undesirable compounds or mitigating them are also discussed. For plants with simultaneous presence of carbon steel and copper alloys, alternate amines like morpholine, or new reagents such as ethanolamine (ETA), can be selected to manage erosion-corrosion of carbon steel and decrease corrosion transport, at a pH acceptable for copper alloys (9.2 at 25 °C). In more recent units, with an all ferrous system, a high pH operation, with only hydrazine addition, the easiest way, or with combined hydrazine and morphollne or ETA will be of some benefit against steam generator corrosion. When Alloy 600 has been selected, inhibitors such as boric acid, or maybe titanium oxide or cerium in the future. needs to be added in steam generators in order to decrease intergranular corrosion progression. In addition, caustic and lead contaminations must be strictly avoided, while sludge and deposits will be eliminated by lancing and chemical cleaning, if necessary.
1. Introduction The new challenge in selecting the best secondary water chemistry for the secondary system of PWR plants includes: (i) the adequacy to existing materials, such as copper alloys, carbon steel, steam generator materials; (ii) minimizing various types of corrosion such as flow-assisted corrosion of carbon steel, intergranular corrosion of steam generator tubing, denting; (iii) the limitation of operating costs and wastes, as well as their effect on the environment, through acceptable reagents and concentrations. This paper describes the most recent approaches considered all
over the world to fulfil the above objectives in connection with the secondary system design and materials.
2. Basis for pH reagent selection The main objective of the pH reagent selection is to obtain the conditions for the lowest possible corrosion rate of the different materials present in the whole steam-water system. Thus, a volatile reagent must be selected in order to obtain the alkaline pH in the entire system. Stainless steels, sometimes used for condenser and some heaters,
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F. Nordmann, J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
and always for the steam generator tubing, have a lowest generalized corrosion rate in slightly alkaline condition but can accept a rather wide range of pH. In contrast, carbon steel (often used at least for high pressure heaters) or copper alloys (mainly used in oldest condensers and some low pressure heaters or moisture separator reheater tubing) are compatible within a limited range of pH. In addition to corrosion of materials, other issues must be considered when selecting the water treatment control agent and the pH to be used. These include: • corrosion transport to the steam generator, giving rise to deposits where corrosion might occur; • impact on condensate polisher operation or on blowdown demineralizers, if any of these; • low toxicity of reagent and acceptable release to the environment; • commercial availability and cost; • thermal stability and type of decomposition products; • required concentration for the desired pH. Thus, existing materials as well as the presence of condensate polishers have a large impact on pH and reagent selection. 2.1. A m m o n i a
Up to the mid-1980s, ammonia was mainly used since it is easy to implement, with a long experience, known characteristics, absence of decomposition products, low cost and no adverse effect except ammonia corrosion and formation of soluble copper-ammonia complexes if pH is above 9.2 at room temperature. In addition, its use is even easier since it is the main decomposition product of hydrazine, generally added as a reducing agent. This will allow the use of a higher hydrazine content, which is favourable for obtaining a reducing environment as described in Section 2.4. However, ammonia has one main disadvantage. The maximum room temperature pH compatible with either waste release to the environment, resin exhaustion or copF,:r alloy corrosion, the high temperature operating pH is insufficient to minimize corrosion product transport along the feed
water train, originating from carbon steel generalized corrosion. Moreover, it is also insufficient to protect carbon steel components, in liquid phase, from flow-a3sisted corrosion (erosion-corrosion). In effect, in the presence of copper alloy, the maximur,, pH of 9.2 at 25 °C leads, at 175 °C (the most sensitive temperature) to an unacceptable rate of erosion-corrosion, as demonstrated by the severe accident which occurred on a pipe in the Surry US plant in 1986. In the absence of copper alloys pH has to be increased, at least up to 9.7 at 25 °C, to avoid erosion-corrosion (Berge, 1985). However, in the presence of condensate polishers operating in normal operation and in hydrogen cycle only, regeneration frequency will be too high. However, even in the absence of condensate polishers, a room temperature pH of 10, as applied or considered for some German units (Odar, 1994), is appropriate for minimizing corrosion products transport but is hardly acceptable in tern is of ammonia concentration required which will necessitate either: (i) the release of blown down water from the steam generator to the environment, or (ii) a frequent regeneration of blowdown demineralizer resins, or (iii) operation of resins after exhaustion by ammonia that will decrease their efficiency for sodium elimination at such a high ammonia molar concentration. Finally, ammonia treatment with a 25 °C pH of about 9.7 seems acceptable only for plants without copper alloys and without condensate polisher continuously in operation. 2.2. M o r p h o l i n e In the presence of copper alloys in the absence of condensate polishing plants, morphoiine has been satisfactorily used in all French PWRs since 1984 and in some other countries, mainly the USA. The main objective of the reagent selection is to protect carbon steel from erosion-corrosion and to decrease corrosion products transport. With morpholine, a 175 °C pH of 6.8 in two phase flow can be obtained with a 25 °C pH of 9.3; the same high temperature pH would require a room temperature ot' l0 with ammonia (Berge,
F. Nordmann, J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
1986). This can be explained by two advantages of morpholine over ammonia. (1) The evolution of morpholine dissociation constant vs. temperature which is less decreasing than that of ammonia. (2) The morpholine distribution coefficient (which is close to relative volatility) between steam and water is close to 1, giving a constant concentration all over the steam-water system; with ammonia liquid drains have a lower concentration. The main disadvantages of morpholine are the relatively high molar concentration required, which is comparable with that of ammonia at a pH of 9.7, and the possibility of increasing organic compound~ content. The consequence of the molar concentration is, as with ammonia at a high pH, observed on condensate or blowdown polishers, This is why only a few plants with condensate polisher have selected morpholine. French PWRs operating with morpholine do not have such a purification system. In addition, blowdown demineralizer cation resins are generally kept in operation after morpholine exhaustion so as to increase their life .~~:ration. This is because the French decide not to regenerate the ,'esins to avoid risk of secondary system contamination by either regeneration reagents or resin fines. In addition, nuclear grade resins are used. The second main consequence of morp_holine is the increase of organic acids, acetate and formate concentration which can be observed in some cases. French laboratories (Dauvois, 1986) have largely studied the decomposition steps of morpholine, hydrolysis followed by oxidation, and the organic ions generated in various parts of the secondary system of operating PWRs. It has been shown that the highest organic acid concentrations have been observed in the presence of oil or grease pollution, particularly with significant air ingress at the condenser level, thus confirming the oxidation step in morpholine decomposition. Even though laboratory studies never revealed any corrosion of steam generator tubing associated with the c,rganic acid content observed in operating units, their concentration should be ~.s low as possible if cation conductivity is the key
on-line monitor for secondary system overall contamination. Depending on the units, cation conductivity blank with morpholine treatment, compared with ammonia, is typically increased by 0 to 0.15 I~S cm - t . A higher impact has to be considered as an indicator of a significant organic contamination and/or air ingress. Finally, morpholine treatment appears to be one of the best solutions for plants still having copper alloys, where the pH cannot be raised with ammonia to adequately protect carbon steel from erosion-corrosion and generalized corrosion, which give rise to corrosion products transported to the steam generator where they accumulate on the tubesheet in a sludge pile in which eoncentratable contaminants will corrode the tubing. In France, over 300 reacter-years of morpholine trealment data has largely proved its feasibility.
2.3. Advanced amines
In the 1980s, several alternative amines have been thoroughly investigated by various countries. mainly in the UK, USA and France. Cyclohexylamine is of lesser interest than morpholine because of the higher molar concentration and higher room temperature pH necessary to obtain the same protection of carbon steel at high temperature as with m0rpholine. 2-Amino-2-methylpropanol (AMP) is suitable for the pH but is not stable enough for use in PWRs with recireulating steam generators, particularly in the absence of condensate polisher. Owing to the length of the resident time at high temperature in the system, AMP will be thermally decomposed before its removal by the air ejectors. The proportion of AMP eliminated and replaced by fresh AMP is consequently low. Once-through steam generators have a much shorter residence time and condensate polishers, thus AMP will be less thermally d ~ o m l ~ s e d but similarly eliminated by air ejector; the proportion of fresh, non-decomposed AMP is consequently higher. This is why it has been successfully used with OTSGs (Donaldson, 1992) at Wylfa (Wales, UK) and Vandellos (Spain).
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F. Nordmann. J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
Quinuelidine and N-methylpyrrolidine have not been proved to be adequate for use in power plants. Electric Power Research Institute (EPRI) (Burns, 1993) has recently stimulated many US utilities to the use of ethanolamine (ETA), after a theoretical investigation followed by plant tests. The main advantage of ETA is the lower molar concentration needed, compared with moq~holine, to get the target pH at operating t.eml~:rature. Consequently, the load on condensate polisher or blowdown demineralizer resin will be lower with a beneficial effect on one or several of the following parameters: (i) operating cost; (ii) run length of cation resin in hydrogen form; (iii) reduced frequency of resin regeneration; (iv) better possibility of increasing hydrazine concentration for the time of operation with ammonia elimination on cation resin in H form compared with resin exhausted with morphoiine. According to EPRI tests, corrosion products transport with ETA is lower than with ammonia and even lower than with morpholine. However, Corrosion of copper alloys, still existing in a number of plants, is ten times more important with ETA than with morpholine under oxygen saturated conditions at room temperature with 200 ppm of each amine (Burns, 1993). This can be a significant influence on steam generator tubing corrosion (where copper transport has been shown to be detrimental for denting in the late 1970s) or, more recently, for intergranular corrosion of Alloy 600 (Stutzmann, 1994). Furthermore, the relative volatility of about 0.6 for ETA, instead of 1 for morpholine, will not ~ve a constant concentration along the different parts of the steam-water system. Consequently, some portion of copper alloy components with too high an ETA concentration may have an increased corrosion. The organic acids produced by thermal decomposition of ETA are reported to be present at lower concentration than with morpholine in some cases (Donaldson, 1992). However, it has been expl~ined that organic acid should not be a large problem in clean plants operating with morpholine and without organic contamination (oil or grease) and without important air ingress.
Finally, the first resultt~ from US plants with ETA seem promising, particularly when condensate polisher is used continuously. However, a strong attention should be focused, if copper alloys are present, on copper transport to the steam generator, specifically if oxygen content is not very low in the condensate. 2.4. Reducing reagent evaluation
A reducing environment in the secondary side of steam generators is of utmost concern (i) for mitigating denting of tubes al intersections with carbon steel tube support plates, particularly for sea-water cooled plants, and (ii) for trying to minimize occurrence and progr,:ssion of intergranular cracking of Alloy 600 lubing, which is the main secondary side degradation of steam generator tubing in most Western countries. Several studies in Japan (Kistfida, i987) and Sweden (Bjornkwist, 1991) have demonstrated the influence of the electrochemical potential (ECP) on intergranular stress corrosion cracking and the relation between hydrazine content and ECP. One largely used remedy is the addition to the feed water of high hydrazine concentration above 100 ppb, as recommended by at least American, Japanese, German and Swedish organisations. However, hydrazine addition is not the way to solve air ingress into the condenser, which has to be strictly searched for by plant staff, since it has been shown that hydrazine added in the condensate water does not react significantly with oxygen. It can partially reduce hematite into magnetite and must be added at a sufficient concentration. Furthermore, for plants with copper alloys, hydrazine must not be increased in too large a proportion if ammonia, resulting from thermal decomposition of hydrazine, is not eliminated by the condensate polisher operating continuously in H form. In addition, as described above, the drawbacks of high hydrazine concentration with ammonia elimination by condensate polisher are the operating costs, the release of wastes to the environment associated with resin regeneration and the potential detrimental influence on flow-assisted corrosion of carbon steels.
F. Nordmann, J.-tl4. ~r'iquet/ Nuclear Engineering and Design 160 (1996) 193-201
Table I Reagent selection synopsis SG
Copper alloy
Colldensate polisher Yes
No -Morpholine pH 9.2; efficient against erosion corrosion; large experience. -ETA pH 9.2 also to be considered -Morpholine-NaH4 pH 9.6 several advantages.
RSG
Yes
-ETA pH 9.2; _cent but interesting approach; check Cu corrosion.
RSG
No
-ETA pH 9.6; Advantage of low molar concentratiGn -N,Ha--NH 3 pH 9.7; easy, high N~Ha. Above selection or AMP
OTSG
Moreover, there is a recent trend in the USA to consider the carcinogenic effect of hydrazine release to the environment; this is why other reducing chemicals are under evaluation for their reducing power, thermal stability and toxicity. 2.5. Reagent selection synopsis
Table l gathers together the respective advantages of the various reagents and 25 °C pH selections discussed in Section 2, ,'~ccording to secondary system design.
3. Steam generator chemistry 3. I. Phosphate chemistry
Before 1974, most PWRs with recirculating steam generators were operating with coordinated phosphate treatment, mainly for its buffering effect in the presence of cooling water in-leakage. However, the Beznau (Switzerland) experience has seriously demonstrated the limitatiot~s of such a treatment on Alloy 600. At the time, when the N a - P O 4 molar ratio was too high, caustic intergranular corrosion was predominant. Ther,, with a lower ratio acid wastage occurred, and finally, even with strict control of the ratio, it appeared that both degradations were able to take place in
-N,H4-NIt~ pH 9.7 to 10; easy implementation.
different loca,,ions of the tubing due to hide-out of chemicals and to the following global reaction, which is, in fact, the result of several reactions: 3Fe304 + 8Na3PO4 + 12H20 ~ 6FePO~ -~- Fe3(PO4) 2 -t- 24NaOH Consequently, the reaction between trisodium phosphate and magnetite produces free caustic, thus making possible the inte¢granular attack of the tubing. In the K W U plants, with Alloy 800, which is much less susceptible to intergranular corrosion than Alloy 600, phosphate treatment has been recommended for a long time. Despite decreasing the phosphate concentration, wastage was still occurring and phosphate treatment has bo~:n progressively given-up for all volatile treatment (AVT) as recommended by other manufacturers in Western countries. However, AVT, withe, at buffering power, cannot accept significant sea water leakages, which generate acidic compounds. Thus several remedial actions need to be considered, with at least a few implemented: • selection of tight condenser, such as titanium tubing; • development of helium tests for leak detection; • use of condensate polisher;
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•
stringent guide lines for condensate water and steam generator blowdown chemistry. At the present time, a few old plants without previous problems in phosphate treatment are still applying it successfully. However, it must be recognized that all these units are of an older design, with lower power, lower temperature and lower thermal heat flux, and thus cannot guarantee the same results on more powerful units. Only Doel 4, where severe attack of Alloy 600 tubing occurred, is a modem unit which switched recently to phosphate treatment as a response to degradation? in AVT (Roofthooft, 1992) to try to mitigate intergranular corrosion, even though some wastage is anticipated to develop. The main objective is to add a few years of safe operation by the time of a scheduled steam generator replacement, because of a lower concern to safety issues from wastage than from cracking. The last inspections, however, showed that crack progression had not stopped. 3.2. A V T c h e m i s t r y - - a c i d chemistry
For high rated power units of recent generation, only AVT should be considered as a viable option, even if the selection of inadequate materials like Alloy 600, very sensitive to stress corrosion cracking, imposes the application of a strictly controlled contamination level with the complete absence of caustic pollutions. Sophis+icated methods to avoid local concentration of alkaline compound in areas with restricted flow are carried out increasingly frequently; they include means to decrease the sodium to a strong acid anions molar ratio in order to compensate for the slight volatility of chloride, allowing sodium, almost non-volatile and insoluble in steam, to more efficiently concentrate in crevices, according to hide-out return studies. However, only some of the hide-out results in plant operating conditions have demonstrated a higher hideout rate for sodium than for chloride, while sulphate is more hidden-out and adsorbed than sodium. The most recent ways to decrease the Na/C! ratio in operating steam generators are:
•
decrease of sodium contamination, but practical limits have been reached; • injection of ammonium chloride which will, after hydrolysis, generate hydrochloric acid to neutralize the sodium hydroxide; • increase of cation to anion resin ratio in such a way that anions leach while sodium does not; • modification of regeneration process with a different acid and leaving a small fraction of anion resin in the cation resin before regeneration by hydrochloric acid that will increase subsequent release of chloride. It is not yet clear if these last three methods of direct or indirect acid addition might be superior to the simple approach of the first method, contaminant limitation. 3.3. Corrosion inhibitors
Although many efforts have been made to avoid stress corrosion cracking on the secondary side of steam generators, most of the plants over 10 years old with Alloy 600 are more or less severely affected, especially when the tube support plates have drilled circular holes; even despite a high purity chemistry. Before it was possible to mitigate against such a type of corrosion, remedies like those applied for denting mitigation in the late seventies have been implemented. Boric acid, which in addition to its passivation effect on the film at tube surface, is able to neutralize alkaline chemistry, was the first inhibitor to be injected in the secondary system. It had already been qualified tor its compatibility with the steam-water circuit materials. Unfortunately, Alloy 600 cracking is not as easy to avoid as carbon steel fast linear corrosion, and boric acid only has a limited efficiency on field application. Of course, in the laboratory, where the test had been accelerated by increasing the caustic concentration, boric acid is very efficient since it neutralzes the caustic excess, moving back to a standard chemistry which will not induce cracking in the short duration of laboratory tests. However, in operating plants, cracking of Alloy 600 is the result of three well-known parameters: material
F. Nordmann. J.-M. Fiquet / Nuclear Engineerhtg and Design 160 (1996) 193-201
sensitivity, stress level and chemistry severity. Thus, boric acid is only acting on the last one, if it is a caustic environment, and this explains why its remedial effectiveness varies in a range between no effect and a slope progression decrease of about two-fold. Nevertheless, to increase the steam generator life up to an adequately scheduled replacement, boric acid can still largely be used in many of the already cracked older steam generators. Obviously, bode acid is added only when there is no other option, since it has two main inconveniences: ® the increase of cation conductivity blank which is usually employed for contamination detection, but this can be obviated by monitoring chloride and sulphate more frequently; • it complicates the use of demineralizer purification resin either at steam generator blowdown or in condensate polisher; this can also be solved. Recently, EPRI pushed toward development of another corrosion inhibitor, titanium oxide (Paine, lO93), which might be more systematically efficient than boric acid. The first step was qualification in model boilers, with the same restrictions as for boric acid effectiveness; the second step was plant testing in the USA. There are difficulties in obtaining the required concentration of the desired oxide (anatase rather than rutile) going through the steam generator from feedwater addition of either sparingly soluble compounds or more easily soluble compounds containing contaminating anions. Thus, a non-negligible number of sensitive steam generators will likely have been replaced before a finn evaluation of titanium oxide, or other inhibitors such as cerium, in operating plants.
4. Contaminants elimination--design Several ways of eliminating contaminants are implemented in different plants. 4.1. Condensate polishers
Condensate purification by filters and resins are
systematically used with once-through steam generators which do not have blowdown. On recirculating steam generators, the option of condensate polisher depends on the utility. The main advantages are: (i) to be able to continue operation in the presence of limited pollutant ingress in the condenser, usually cooling water; (it) to decrease corrosion product transport to the steam generator; (iii) to eliminate pH reagent, permitting a higher hydrazine concentration injection. However, there are a number of disadvantages, partly discussed in Section 2: • high investment co~t; • operating costs; • large production of wastes, potentially radioactive; • risk of contamination by regenerant chemicals or resin fines. Laboratory studies and plant experience have shown that contamination with decomposition products of resin fines, giving reduced sulphur compounds, or with sodium hydroxide throw, may constitute a very detrimental hazard for steam generator integrity (Daret, 1993). Since reliable condenser exists and low leak detection with helium has been carried out, it looks more desirable to avoid continuous ¢,peration of condensate polisher which can constitute a higher risk of contamination than a purification method. This does not exclude the interesting, but expensive concept, of having such a condensate polisher in by-pass, ready for use in case of contamination. A cheaper option is to only have a lower flow rate system specifically aimed at eliminating contaminants before and during start-up, even though all the efforts have been made to implement the best lay-up and cleanliness conditions, as done in French PWRs. The other claimed advantage of condensate po!ishcr, particularly used in Japan, is the elimim , lon of ammonia on the cation resin, permitting an increase of hydrazine concentration, with the advantages described in Section 2.4, particularly for plant with copper alloys where pH cannot be raised above 9.3.
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It still looks difficult to compare the benefits of (i) replacing all copper alloy components from the secondary system with ferrous alloys, (ii) adding condensate polishers and (iii) waiting for Alloy 600 steam generator replacement with a more resistant tubing alloy sud: as 690 or 800. 4.2. Copper alloy elimination
As described above, copper alloy elimination from the steam water system has many advantages: • possibility of increasing the pH and thus decreasing: (i) corrosion transport of oxides at the origin of the sludge pile where corrosion occurs and deposits on heat exchange surfaces; (ii) generalized and flow-assisted corrosion of carbon steels; • possibility of increasing hydrazine and reducing environment; • positive influence on denting and stress corrosion cracking, sensitive to the presence of copper in the steam generator.
4.3. Lancing and chemical cleaning
The need for clean steam generators has been fully explained (Berge, 1992). Even when resistant alloys, such as 690 or 800, will be the only ones in use in steam generators, the need for clean steam generators will still apply so as to avoid the risk of corrosive contaminant concentrations in deposits and to keep high heat flux and performance of the steam generator. Of course, when Alloy 600 is in operation, there is a more acute necessity to keep the generators almost free of deposits, especially if they contain oxidizing oxides or lead or other detrimental dements, or in presence of species such as aluminosilicate gels where impurities can concentrate (Sal~, 1993). This has been recently evidenced by the large leak which occurred under deposit in the free span of a Palo Verde tube, in the USA, seriously affecting .~afety issues.
Lancing efficiency of the steam generators with high pressure water has been improved with robots like CECIL, and the method can be considered free of adverse potential effects since no chemical is used. The only point to be considered is the cost of the operation to define its frequency, It can, for example, be implemented at each shut-down for refuelling. Chemical cleaning has been used for many years in PWR units, mainly as a remedial action for specific cases, but is preventively applied to fossil fired steam generators. It is more efficient, particularly if the steam generator ha'.'. ',c he completely cleaned, either for elimination of a specific deposit usually on the tube sheet or for restoring the heat flux on the tube surface. Nevertheless, utilities are reluctant to perform steam generator cleaning for several reasons: • associated unavailability of the plant for some methods; • use of chemicals giving undesirable wastes; • cost of operation; • risk of steam generator corrosion if the cleaning solution is not correctly inhibited; • insufficient effectiveness for corrosion limitation. The discovery of the right solution can still be considered as a real challenge for the second part of the 1990s. Even when such a solution, hopefully, exists it will remain necessary to avoid the transport to the steam generator of contaminants such as lead which have a specific action on corrosion of any of the alloys in use or in consideration: 600, 800, 690. E D F has worked on actions during maintenance works to avoid the use of lead containing products and to keep them out of the secondary system when no other product can ,be used. Another option to avoid the formation of sludge piles on tubesheet, where chemicals will concentrate and almost inexorably corrode the tube, is to use horizontal steam generators, as in W E R design of eastenl Europe, provided that other improvements in the design and operation are carried out to insure a safe and long life of the component.
F. Nordmann, J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
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5. C o n c i s i o n
References
Chemistry. always b_ad the ambi:.i.ot+s objectives to optimize o p e r a t i o n o f the s e c o n d a r y system a n d to c o m p e n s a t e for i n a p p r o p r i a t e design a n d materials selections. In t h e fecdwater train, the s i m u l t a n e o u s presence o f c a r b o n steel a n d c o p p e r alloys will necessitate alternative a m i n e s like m o r p h o l i n e , able to m a n a g e e r o s i o n - c o r r o s i o n o f c a r b o n steel at a p H acceptable for c o p p e r alloys (9.2 at 25 °C). If c o n d e n s a t e polishers are in operation, new reagents c a n be selected s u c h as E T A , for which a lower m o l a r c o n c e n t r a t i o n is necessary. However, c o n d e n s a t e polishers will have the a d v a n t a g e o f eliminating a m m o n i a a n d m a k i n g possible a n increase o f h y d r a z i n e c o n c e n t r a t i o n , t h a t will be o f s o m e benefit for stress c o n ' o s i o n cracking. In m o r e recent u n i t s with a n all ferrous system, a h i g h p H operation, with only hydrazine addition (the easiest way), or with c o m b i n e d h y d r a z i n e a n d m o r p h o l i n e o r E T A will h a v e a large beneficial i m p a c t o n s u b s e q u e n t s t e a m g e n e r a t o r corrosien. W h e n Alloy 600 h a s been selected, this will n o t be sufficient to g u a r a n t e e a safe o p e r a t i o n o n a long t e r m basis for s t e a m g e n e r a t o r tubing; in this case, inhibitors s u c h as boric acid, or m a y b e t i t a n i u m oxide or cerium in the future, m a y help to delay the s t e a m g e n e r a t o r replacement if it h a s n o t already been required by stress corrosion cracking o n the p r i m a r y side. Finally, a strict limitation o f caustic a n d lead c o n t a m i n a t i o n s seems o f u t m o s t necessity a n d is a m o r e p r u d e n t a p p r o a c h t h a n trying to eliminate c o n t a m i n a n t by c o n d e n s a t e polishers, o r neutralizing caustic by chloride addition. Nevertheless, ~ludge a n d deposits elimination by lancing a n d chemical cleaning will help to m a i n t a i n clean s t e a m generators. A s far as chemical cleaning is considered for systematic use, the d e v e l o p m e n t o f a mild solution w i t h o u t a n y d i s a d v a n t a g e s a n d with a sufficient efficiency, r e m a i n s a real challenge for the second part o f the 1990s.
Ph. Berge et al., Corrosion-~rosion clans ies circuits secondaires. Investigations et ren~es apport~. SFEN !nt. Symp.. FontevramJ. France, Septembre 2-6. 1985. Ph. Berge, The use. need and desirable properties of amine for use in steam-water circuits. CEOn-Amine Workshop Pruc., Bristol, UK, April 29-May I, 1986, Paper AMI. Ph. Berge and J.M. Fiquet. The need for clean steam genera. tors+ Water Chemistry of Nuclear Reactors Systems. Vol. 2, BNES, London, October, 1992, Paper 48. L+ Bjornkwist and A. Molander, Potential measurelnents in side-stream autoclaves on PWR feedwater, EPRI Workshop, Washington, DC, 1991. G.D. ~'ttrns and P.L Millen, Advanced amine application guide lines, Final Rep. TR-102952 Rev. O. September, 1993. (Electric Research Powe~ institute). J. Daret and D. Frron, Model boiler tests--phosphates. 1993 EPRI Workshop on Secondary-Side Initiated IGA/SCC, Minneapolis, USA, October 14-15, 1993. v. Dauvois et al., Laboratory and plant investigations on decomposition products of morpholine in the secondary system of French PWRs, Conf. on Water Chemistry, BNES, Bournemouth, UK, October 1986, Paper 101. R.M. Donaldson et al.. Selection of amines for use as alkalising agents in the steam/water circuits of nuclear power plants, Water Chemistry of Nuclear Reactor Systems, VoL 2, BNES, London. October. 1992, Paper 61. A. Kishida et al., The causes and remedial measures of sleam generator tube intergranular attack in Japanese PWRs, 3rd Int. Syrup. on Environmental Degradation of Materials in Nuclear Power Systems, Traverse City, USA, 1987. S. Odar and K. Kuhnke, 10 Years field experience with high AVT chemistry, int. ConL on Chemistry in Water Reactors-Operating Experience and New Developments, Nice, 24-27 April, 1994. P. Paine, IGA/SCC inhibitors, 1993 EPRI Crevice Chemistry Workshop, Boston, USA, 28-30 June, 1993. R. Roofthooft et al.. Steam generator secondary side corrosion problems in Dnel 4, Water Chemistry of Nuclear Reactor Systems, Vol. 2, BNES, London, October 1992, Paper 66. g. Sala el al., Local chemistry and formation of deposits on the secondary side of steam generators. A laboratory study, 6th Int. Syrup. on Environmental Degradation of Materials in Nu"lear Power Systems, San Diego, CA, USA, August. 1993. A. Stutzmann and F. Nordmann, Studies of IGA related parameters and remedi~, int. Conf. on Chemistry in Water Reactors--Operating Experience and New Developments, Nice, 24-27 April, 1994.
,. i',
iai,. ELSEVIER
Nuclear Engineeringand Design 160 (1996) 193-201
Selection criteria for the best secondary water chemistry Francis Nordmann,
Jean-Marie
Fiquet
EDF-Groupe des Laborawire, 21 all~e Priv~e, F-93206 Saint-Den[s, France
Abstract This paper describes, for PWR plants, the approach for selecting the best chemistry--pH, amine, corrosion inhibitors--according to the secondary system characteristics, .,ucb as presence or not of copper alloys, steam generator tubing alloy, tube support plate design, sludge pile importance. The impact of condensate polisher, sludge lancing, chemical cleaning, as well as other ways of eliminating undesirable compounds or mitigating them are also discussed. For plants with simultaneous presence of carbon steel and copper alloys, alternate amines like morpholine, or new reagents such as ethanolamine (ETA), can be selected to manage erosion-corrosion of carbon steel and decrease corrosion transport, at a pH acceptable for copper alloys (9.2 at 25 °C). In more recent units, with an all ferrous system, a high pH operation, with only hydrazine addition, the easiest way, or with combined hydrazine and morphollne or ETA will be of some benefit against steam generator corrosion. When Alloy 600 has been selected, inhibitors such as boric acid, or maybe titanium oxide or cerium in the future. needs to be added in steam generators in order to decrease intergranular corrosion progression. In addition, caustic and lead contaminations must be strictly avoided, while sludge and deposits will be eliminated by lancing and chemical cleaning, if necessary.
1. Introduction The new challenge in selecting the best secondary water chemistry for the secondary system of PWR plants includes: (i) the adequacy to existing materials, such as copper alloys, carbon steel, steam generator materials; (ii) minimizing various types of corrosion such as flow-assisted corrosion of carbon steel, intergranular corrosion of steam generator tubing, denting; (iii) the limitation of operating costs and wastes, as well as their effect on the environment, through acceptable reagents and concentrations. This paper describes the most recent approaches considered all
over the world to fulfil the above objectives in connection with the secondary system design and materials.
2. Basis for pH reagent selection The main objective of the pH reagent selection is to obtain the conditions for the lowest possible corrosion rate of the different materials present in the whole steam-water system. Thus, a volatile reagent must be selected in order to obtain the alkaline pH in the entire system. Stainless steels, sometimes used for condenser and some heaters,
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F. Nordmann, J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
and always for the steam generator tubing, have a lowest generalized corrosion rate in slightly alkaline condition but can accept a rather wide range of pH. In contrast, carbon steel (often used at least for high pressure heaters) or copper alloys (mainly used in oldest condensers and some low pressure heaters or moisture separator reheater tubing) are compatible within a limited range of pH. In addition to corrosion of materials, other issues must be considered when selecting the water treatment control agent and the pH to be used. These include: • corrosion transport to the steam generator, giving rise to deposits where corrosion might occur; • impact on condensate polisher operation or on blowdown demineralizers, if any of these; • low toxicity of reagent and acceptable release to the environment; • commercial availability and cost; • thermal stability and type of decomposition products; • required concentration for the desired pH. Thus, existing materials as well as the presence of condensate polishers have a large impact on pH and reagent selection. 2.1. A m m o n i a
Up to the mid-1980s, ammonia was mainly used since it is easy to implement, with a long experience, known characteristics, absence of decomposition products, low cost and no adverse effect except ammonia corrosion and formation of soluble copper-ammonia complexes if pH is above 9.2 at room temperature. In addition, its use is even easier since it is the main decomposition product of hydrazine, generally added as a reducing agent. This will allow the use of a higher hydrazine content, which is favourable for obtaining a reducing environment as described in Section 2.4. However, ammonia has one main disadvantage. The maximum room temperature pH compatible with either waste release to the environment, resin exhaustion or copF,:r alloy corrosion, the high temperature operating pH is insufficient to minimize corrosion product transport along the feed
water train, originating from carbon steel generalized corrosion. Moreover, it is also insufficient to protect carbon steel components, in liquid phase, from flow-a3sisted corrosion (erosion-corrosion). In effect, in the presence of copper alloy, the maximur,, pH of 9.2 at 25 °C leads, at 175 °C (the most sensitive temperature) to an unacceptable rate of erosion-corrosion, as demonstrated by the severe accident which occurred on a pipe in the Surry US plant in 1986. In the absence of copper alloys pH has to be increased, at least up to 9.7 at 25 °C, to avoid erosion-corrosion (Berge, 1985). However, in the presence of condensate polishers operating in normal operation and in hydrogen cycle only, regeneration frequency will be too high. However, even in the absence of condensate polishers, a room temperature pH of 10, as applied or considered for some German units (Odar, 1994), is appropriate for minimizing corrosion products transport but is hardly acceptable in tern is of ammonia concentration required which will necessitate either: (i) the release of blown down water from the steam generator to the environment, or (ii) a frequent regeneration of blowdown demineralizer resins, or (iii) operation of resins after exhaustion by ammonia that will decrease their efficiency for sodium elimination at such a high ammonia molar concentration. Finally, ammonia treatment with a 25 °C pH of about 9.7 seems acceptable only for plants without copper alloys and without condensate polisher continuously in operation. 2.2. M o r p h o l i n e In the presence of copper alloys in the absence of condensate polishing plants, morphoiine has been satisfactorily used in all French PWRs since 1984 and in some other countries, mainly the USA. The main objective of the reagent selection is to protect carbon steel from erosion-corrosion and to decrease corrosion products transport. With morpholine, a 175 °C pH of 6.8 in two phase flow can be obtained with a 25 °C pH of 9.3; the same high temperature pH would require a room temperature ot' l0 with ammonia (Berge,
F. Nordmann, J.-M. Fiquet / Nuclear Engineering and Design 160 (1996) 193-201
1986). This can be explained by two advantages of morpholine over ammonia. (1) The evolution of morpholine dissociation constant vs. temperature which is less decreasing than that of ammonia. (2) The morpholine distribution coefficient (which is close to relative volatility) between steam and water is close to 1, giving a constant concentration all over the steam-water system; with ammonia liquid drains have a lower concentration. The main disadvantages of morpholine are the relatively high molar concentration required, which is comparable with that of ammonia at a pH of 9.7, and the possibility of increasing organic compound~ content. The consequence of the molar concentration is, as with ammonia at a high pH, observed on condensate or blowdown polishers, This is why only a few plants with condensate polisher have selected morpholine. French PWRs operating with morpholine do not have such a purification system. In addition, blowdown demineralizer cation resins are generally kept in operation after morpholine exhaustion so as to increase their life .~~:ration. This is because the French decide not to regenerate the ,'esins to avoid risk of secondary system contamination by either regeneration reagents or resin fines. In addition, nuclear grade resins are used. The second main consequence of morp_holine is the increase of organic acids, acetate and formate concentration which can be observed in some cases. French laboratories (Dauvois, 1986) have largely studied the decomposition steps of morpholine, hydrolysis followed by oxidation, and the organic ions generated in various parts of the secondary system of operating PWRs. It has been shown that the highest organic acid concentrations have been observed in the presence of oil or grease pollution, particularly with significant air ingress at the condenser level, thus confirming the oxidation step in morpholine decomposition. Even though laboratory studies never revealed any corrosion of steam generator tubing associated with the c,rganic acid content observed in operating units, their concentration should be ~.s low as possible if cation conductivity is the key
on-line monitor for secondary system overall contamination. Depending on the units, cation conductivity blank with morpholine treatment, compared with ammonia, is typically increased by 0 to 0.15 I~S cm - t . A higher impact has to be considered as an indicator of a significant organic contamination and/or air ingress. Finally, morpholine treatment appears to be one of the best solutions for plants still having copper alloys, where the pH cannot be raised with ammonia to adequately protect carbon steel from erosion-corrosion and generalized corrosion, which give rise to corrosion products transported to the steam generator where they accumulate on the tubesheet in a sludge pile in which eoncentratable contaminants will corrode the tubing. In France, over 300 reacter-years of morpholine trealment data has largely proved its feasibility.
2.3. Advanced amines
In the 1980s, several alternative amines have been thoroughly investigated by various countries. mainly in the UK, USA and France. Cyclohexylamine is of lesser interest than morpholine because of the higher molar concentration and higher room temperature pH necessary to obtain the same protection of carbon steel at high temperature as with m0rpholine. 2-Amino-2-methylpropanol (AMP) is suitable for the pH but is not stable enough for use in PWRs with recireulating steam generators, particularly in the absence of condensate polisher. Owing to the length of the resident time at high temperature in the system, AMP will be thermally decomposed before its removal by the air ejectors. The proportion of AMP eliminated and replaced by fresh AMP is consequently low. Once-through steam generators have a much shorter residence time and condensate polishers, thus AMP will be less thermally d ~ o m l ~ s e d but similarly eliminated by air ejector; the proportion of fresh, non-decomposed AMP is consequently higher. This is why it has been successfully used with OTSGs (Donaldson, 1992) at Wylfa (Wales, UK) and Vandellos (Spain).
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Quinuelidine and N-methylpyrrolidine have not been proved to be adequate for use in power plants. Electric Power Research Institute (EPRI) (Burns, 1993) has recently stimulated many US utilities to the use of ethanolamine (ETA), after a theoretical investigation followed by plant tests. The main advantage of ETA is the lower molar concentration needed, compared with moq~holine, to get the target pH at operating t.eml~:rature. Consequently, the load on condensate polisher or blowdown demineralizer resin will be lower with a beneficial effect on one or several of the following parameters: (i) operating cost; (ii) run length of cation resin in hydrogen form; (iii) reduced frequency of resin regeneration; (iv) better possibility of increasing hydrazine concentration for the time of operation with ammonia elimination on cation resin in H form compared with resin exhausted with morphoiine. According to EPRI tests, corrosion products transport with ETA is lower than with ammonia and even lower than with morpholine. However, Corrosion of copper alloys, still existing in a number of plants, is ten times more important with ETA than with morpholine under oxygen saturated conditions at room temperature with 200 ppm of each amine (Burns, 1993). This can be a significant influence on steam generator tubing corrosion (where copper transport has been shown to be detrimental for denting in the late 1970s) or, more recently, for intergranular corrosion of Alloy 600 (Stutzmann, 1994). Furthermore, the relative volatility of about 0.6 for ETA, instead of 1 for morpholine, will not ~ve a constant concentration along the different parts of the steam-water system. Consequently, some portion of copper alloy components with too high an ETA concentration may have an increased corrosion. The organic acids produced by thermal decomposition of ETA are reported to be present at lower concentration than with morpholine in some cases (Donaldson, 1992). However, it has been expl~ined that organic acid should not be a large problem in clean plants operating with morpholine and without organic contamination (oil or grease) and without important air ingress.
Finally, the first resultt~ from US plants with ETA seem promising, particularly when condensate polisher is used continuously. However, a strong attention should be focused, if copper alloys are present, on copper transport to the steam generator, specifically if oxygen content is not very low in the condensate. 2.4. Reducing reagent evaluation
A reducing environment in the secondary side of steam generators is of utmost concern (i) for mitigating denting of tubes al intersections with carbon steel tube support plates, particularly for sea-water cooled plants, and (ii) for trying to minimize occurrence and progr,:ssion of intergranular cracking of Alloy 600 lubing, which is the main secondary side degradation of steam generator tubing in most Western countries. Several studies in Japan (Kistfida, i987) and Sweden (Bjornkwist, 1991) have demonstrated the influence of the electrochemical potential (ECP) on intergranular stress corrosion cracking and the relation between hydrazine content and ECP. One largely used remedy is the addition to the feed water of high hydrazine concentration above 100 ppb, as recommended by at least American, Japanese, German and Swedish organisations. However, hydrazine addition is not the way to solve air ingress into the condenser, which has to be strictly searched for by plant staff, since it has been shown that hydrazine added in the condensate water does not react significantly with oxygen. It can partially reduce hematite into magnetite and must be added at a sufficient concentration. Furthermore, for plants with copper alloys, hydrazine must not be increased in too large a proportion if ammonia, resulting from thermal decomposition of hydrazine, is not eliminated by the condensate polisher operating continuously in H form. In addition, as described above, the drawbacks of high hydrazine concentration with ammonia elimination by condensate polisher are the operating costs, the release of wastes to the environment associated with resin regeneration and the potential detrimental influence on flow-assisted corrosion of carbon steels.
F. Nordmann, J.-tl4. ~r'iquet/ Nuclear Engineering and Design 160 (1996) 193-201
Table I Reagent selection synopsis SG
Copper alloy
Colldensate polisher Yes
No -Morpholine pH 9.2; efficient against erosion corrosion; large experience. -ETA pH 9.2 also to be considered -Morpholine-NaH4 pH 9.6 several advantages.
RSG
Yes
-ETA pH 9.2; _cent but interesting approach; check Cu corrosion.
RSG
No
-ETA pH 9.6; Advantage of low molar concentratiGn -N,Ha--NH 3 pH 9.7; easy, high N~Ha. Above selection or AMP
OTSG
Moreover, there is a recent trend in the USA to consider the carcinogenic effect of hydrazine release to the environment; this is why other reducing chemicals are under evaluation for their reducing power, thermal stability and toxicity. 2.5. Reagent selection synopsis
Table l gathers together the respective advantages of the various reagents and 25 °C pH selections discussed in Section 2, ,'~ccording to secondary system design.
3. Steam generator chemistry 3. I. Phosphate chemistry
Before 1974, most PWRs with recirculating steam generators were operating with coordinated phosphate treatment, mainly for its buffering effect in the presence of cooling water in-leakage. However, the Beznau (Switzerland) experience has seriously demonstrated the limitatiot~s of such a treatment on Alloy 600. At the time, when the N a - P O 4 molar ratio was too high, caustic intergranular corrosion was predominant. Ther,, with a lower ratio acid wastage occurred, and finally, even with strict control of the ratio, it appeared that both degradations were able to take place in
-N,H4-NIt~ pH 9.7 to 10; easy implementation.
different loca,,ions of the tubing due to hide-out of chemicals and to the following global reaction, which is, in fact, the result of several reactions: 3Fe304 + 8Na3PO4 + 12H20 ~ 6FePO~ -~- Fe3(PO4) 2 -t- 24NaOH Consequently, the reaction between trisodium phosphate and magnetite produces free caustic, thus making possible the inte¢granular attack of the tubing. In the K W U plants, with Alloy 800, which is much less susceptible to intergranular corrosion than Alloy 600, phosphate treatment has been recommended for a long time. Despite decreasing the phosphate concentration, wastage was still occurring and phosphate treatment has bo~:n progressively given-up for all volatile treatment (AVT) as recommended by other manufacturers in Western countries. However, AVT, withe, at buffering power, cannot accept significant sea water leakages, which generate acidic compounds. Thus several remedial actions need to be considered, with at least a few implemented: • selection of tight condenser, such as titanium tubing; • development of helium tests for leak detection; • use of condensate polisher;
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•
stringent guide lines for condensate water and steam generator blowdown chemistry. At the present time, a few old plants without previous problems in phosphate treatment are still applying it successfully. However, it must be recognized that all these units are of an older design, with lower power, lower temperature and lower thermal heat flux, and thus cannot guarantee the same results on more powerful units. Only Doel 4, where severe attack of Alloy 600 tubing occurred, is a modem unit which switched recently to phosphate treatment as a response to degradation? in AVT (Roofthooft, 1992) to try to mitigate intergranular corrosion, even though some wastage is anticipated to develop. The main objective is to add a few years of safe operation by the time of a scheduled steam generator replacement, because of a lower concern to safety issues from wastage than from cracking. The last inspections, however, showed that crack progression had not stopped. 3.2. A V T c h e m i s t r y - - a c i d chemistry
For high rated power units of recent generation, only AVT should be considered as a viable option, even if the selection of inadequate materials like Alloy 600, very sensitive to stress corrosion cracking, imposes the application of a strictly controlled contamination level with the complete absence of caustic pollutions. Sophis+icated methods to avoid local concentration of alkaline compound in areas with restricted flow are carried out increasingly frequently; they include means to decrease the sodium to a strong acid anions molar ratio in order to compensate for the slight volatility of chloride, allowing sodium, almost non-volatile and insoluble in steam, to more efficiently concentrate in crevices, according to hide-out return studies. However, only some of the hide-out results in plant operating conditions have demonstrated a higher hideout rate for sodium than for chloride, while sulphate is more hidden-out and adsorbed than sodium. The most recent ways to decrease the Na/C! ratio in operating steam generators are:
•
decrease of sodium contamination, but practical limits have been reached; • injection of ammonium chloride which will, after hydrolysis, generate hydrochloric acid to neutralize the sodium hydroxide; • increase of cation to anion resin ratio in such a way that anions leach while sodium does not; • modification of regeneration process with a different acid and leaving a small fraction of anion resin in the cation resin before regeneration by hydrochloric acid that will increase subsequent release of chloride. It is not yet clear if these last three methods of direct or indirect acid addition might be superior to the simple approach of the first method, contaminant limitation. 3.3. Corrosion inhibitors
Although many efforts have been made to avoid stress corrosion cracking on the secondary side of steam generators, most of the plants over 10 years old with Alloy 600 are more or less severely affected, especially when the tube support plates have drilled circular holes; even despite a high purity chemistry. Before it was possible to mitigate against such a type of corrosion, remedies like those applied for denting mitigation in the late seventies have been implemented. Boric acid, which in addition to its passivation effect on the film at tube surface, is able to neutralize alkaline chemistry, was the first inhibitor to be injected in the secondary system. It had already been qualified tor its compatibility with the steam-water circuit materials. Unfortunately, Alloy 600 cracking is not as easy to avoid as carbon steel fast linear corrosion, and boric acid only has a limited efficiency on field application. Of course, in the laboratory, where the test had been accelerated by increasing the caustic concentration, boric acid is very efficient since it neutralzes the caustic excess, moving back to a standard chemistry which will not induce cracking in the short duration of laboratory tests. However, in operating plants, cracking of Alloy 600 is the result of three well-known parameters: material
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sensitivity, stress level and chemistry severity. Thus, boric acid is only acting on the last one, if it is a caustic environment, and this explains why its remedial effectiveness varies in a range between no effect and a slope progression decrease of about two-fold. Nevertheless, to increase the steam generator life up to an adequately scheduled replacement, boric acid can still largely be used in many of the already cracked older steam generators. Obviously, bode acid is added only when there is no other option, since it has two main inconveniences: ® the increase of cation conductivity blank which is usually employed for contamination detection, but this can be obviated by monitoring chloride and sulphate more frequently; • it complicates the use of demineralizer purification resin either at steam generator blowdown or in condensate polisher; this can also be solved. Recently, EPRI pushed toward development of another corrosion inhibitor, titanium oxide (Paine, lO93), which might be more systematically efficient than boric acid. The first step was qualification in model boilers, with the same restrictions as for boric acid effectiveness; the second step was plant testing in the USA. There are difficulties in obtaining the required concentration of the desired oxide (anatase rather than rutile) going through the steam generator from feedwater addition of either sparingly soluble compounds or more easily soluble compounds containing contaminating anions. Thus, a non-negligible number of sensitive steam generators will likely have been replaced before a finn evaluation of titanium oxide, or other inhibitors such as cerium, in operating plants.
4. Contaminants elimination--design Several ways of eliminating contaminants are implemented in different plants. 4.1. Condensate polishers
Condensate purification by filters and resins are
systematically used with once-through steam generators which do not have blowdown. On recirculating steam generators, the option of condensate polisher depends on the utility. The main advantages are: (i) to be able to continue operation in the presence of limited pollutant ingress in the condenser, usually cooling water; (it) to decrease corrosion product transport to the steam generator; (iii) to eliminate pH reagent, permitting a higher hydrazine concentration injection. However, there are a number of disadvantages, partly discussed in Section 2: • high investment co~t; • operating costs; • large production of wastes, potentially radioactive; • risk of contamination by regenerant chemicals or resin fines. Laboratory studies and plant experience have shown that contamination with decomposition products of resin fines, giving reduced sulphur compounds, or with sodium hydroxide throw, may constitute a very detrimental hazard for steam generator integrity (Daret, 1993). Since reliable condenser exists and low leak detection with helium has been carried out, it looks more desirable to avoid continuous ¢,peration of condensate polisher which can constitute a higher risk of contamination than a purification method. This does not exclude the interesting, but expensive concept, of having such a condensate polisher in by-pass, ready for use in case of contamination. A cheaper option is to only have a lower flow rate system specifically aimed at eliminating contaminants before and during start-up, even though all the efforts have been made to implement the best lay-up and cleanliness conditions, as done in French PWRs. The other claimed advantage of condensate po!ishcr, particularly used in Japan, is the elimim , lon of ammonia on the cation resin, permitting an increase of hydrazine concentration, with the advantages described in Section 2.4, particularly for plant with copper alloys where pH cannot be raised above 9.3.
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It still looks difficult to compare the benefits of (i) replacing all copper alloy components from the secondary system with ferrous alloys, (ii) adding condensate polishers and (iii) waiting for Alloy 600 steam generator replacement with a more resistant tubing alloy sud: as 690 or 800. 4.2. Copper alloy elimination
As described above, copper alloy elimination from the steam water system has many advantages: • possibility of increasing the pH and thus decreasing: (i) corrosion transport of oxides at the origin of the sludge pile where corrosion occurs and deposits on heat exchange surfaces; (ii) generalized and flow-assisted corrosion of carbon steels; • possibility of increasing hydrazine and reducing environment; • positive influence on denting and stress corrosion cracking, sensitive to the presence of copper in the steam generator.
4.3. Lancing and chemical cleaning
The need for clean steam generators has been fully explained (Berge, 1992). Even when resistant alloys, such as 690 or 800, will be the only ones in use in steam generators, the need for clean steam generators will still apply so as to avoid the risk of corrosive contaminant concentrations in deposits and to keep high heat flux and performance of the steam generator. Of course, when Alloy 600 is in operation, there is a more acute necessity to keep the generators almost free of deposits, especially if they contain oxidizing oxides or lead or other detrimental dements, or in presence of species such as aluminosilicate gels where impurities can concentrate (Sal~, 1993). This has been recently evidenced by the large leak which occurred under deposit in the free span of a Palo Verde tube, in the USA, seriously affecting .~afety issues.
Lancing efficiency of the steam generators with high pressure water has been improved with robots like CECIL, and the method can be considered free of adverse potential effects since no chemical is used. The only point to be considered is the cost of the operation to define its frequency, It can, for example, be implemented at each shut-down for refuelling. Chemical cleaning has been used for many years in PWR units, mainly as a remedial action for specific cases, but is preventively applied to fossil fired steam generators. It is more efficient, particularly if the steam generator ha'.'. ',c he completely cleaned, either for elimination of a specific deposit usually on the tube sheet or for restoring the heat flux on the tube surface. Nevertheless, utilities are reluctant to perform steam generator cleaning for several reasons: • associated unavailability of the plant for some methods; • use of chemicals giving undesirable wastes; • cost of operation; • risk of steam generator corrosion if the cleaning solution is not correctly inhibited; • insufficient effectiveness for corrosion limitation. The discovery of the right solution can still be considered as a real challenge for the second part of the 1990s. Even when such a solution, hopefully, exists it will remain necessary to avoid the transport to the steam generator of contaminants such as lead which have a specific action on corrosion of any of the alloys in use or in consideration: 600, 800, 690. E D F has worked on actions during maintenance works to avoid the use of lead containing products and to keep them out of the secondary system when no other product can ,be used. Another option to avoid the formation of sludge piles on tubesheet, where chemicals will concentrate and almost inexorably corrode the tube, is to use horizontal steam generators, as in W E R design of eastenl Europe, provided that other improvements in the design and operation are carried out to insure a safe and long life of the component.
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5. C o n c i s i o n
References
Chemistry. always b_ad the ambi:.i.ot+s objectives to optimize o p e r a t i o n o f the s e c o n d a r y system a n d to c o m p e n s a t e for i n a p p r o p r i a t e design a n d materials selections. In t h e fecdwater train, the s i m u l t a n e o u s presence o f c a r b o n steel a n d c o p p e r alloys will necessitate alternative a m i n e s like m o r p h o l i n e , able to m a n a g e e r o s i o n - c o r r o s i o n o f c a r b o n steel at a p H acceptable for c o p p e r alloys (9.2 at 25 °C). If c o n d e n s a t e polishers are in operation, new reagents c a n be selected s u c h as E T A , for which a lower m o l a r c o n c e n t r a t i o n is necessary. However, c o n d e n s a t e polishers will have the a d v a n t a g e o f eliminating a m m o n i a a n d m a k i n g possible a n increase o f h y d r a z i n e c o n c e n t r a t i o n , t h a t will be o f s o m e benefit for stress c o n ' o s i o n cracking. In m o r e recent u n i t s with a n all ferrous system, a h i g h p H operation, with only hydrazine addition (the easiest way), or with c o m b i n e d h y d r a z i n e a n d m o r p h o l i n e o r E T A will h a v e a large beneficial i m p a c t o n s u b s e q u e n t s t e a m g e n e r a t o r corrosien. W h e n Alloy 600 h a s been selected, this will n o t be sufficient to g u a r a n t e e a safe o p e r a t i o n o n a long t e r m basis for s t e a m g e n e r a t o r tubing; in this case, inhibitors s u c h as boric acid, or m a y b e t i t a n i u m oxide or cerium in the future, m a y help to delay the s t e a m g e n e r a t o r replacement if it h a s n o t already been required by stress corrosion cracking o n the p r i m a r y side. Finally, a strict limitation o f caustic a n d lead c o n t a m i n a t i o n s seems o f u t m o s t necessity a n d is a m o r e p r u d e n t a p p r o a c h t h a n trying to eliminate c o n t a m i n a n t by c o n d e n s a t e polishers, o r neutralizing caustic by chloride addition. Nevertheless, ~ludge a n d deposits elimination by lancing a n d chemical cleaning will help to m a i n t a i n clean s t e a m generators. A s far as chemical cleaning is considered for systematic use, the d e v e l o p m e n t o f a mild solution w i t h o u t a n y d i s a d v a n t a g e s a n d with a sufficient efficiency, r e m a i n s a real challenge for the second part o f the 1990s.
Ph. Berge et al., Corrosion-~rosion clans ies circuits secondaires. Investigations et ren~es apport~. SFEN !nt. Symp.. FontevramJ. France, Septembre 2-6. 1985. Ph. Berge, The use. need and desirable properties of amine for use in steam-water circuits. CEOn-Amine Workshop Pruc., Bristol, UK, April 29-May I, 1986, Paper AMI. Ph. Berge and J.M. Fiquet. The need for clean steam genera. tors+ Water Chemistry of Nuclear Reactors Systems. Vol. 2, BNES, London, October, 1992, Paper 48. L+ Bjornkwist and A. Molander, Potential measurelnents in side-stream autoclaves on PWR feedwater, EPRI Workshop, Washington, DC, 1991. G.D. ~'ttrns and P.L Millen, Advanced amine application guide lines, Final Rep. TR-102952 Rev. O. September, 1993. (Electric Research Powe~ institute). J. Daret and D. Frron, Model boiler tests--phosphates. 1993 EPRI Workshop on Secondary-Side Initiated IGA/SCC, Minneapolis, USA, October 14-15, 1993. v. Dauvois et al., Laboratory and plant investigations on decomposition products of morpholine in the secondary system of French PWRs, Conf. on Water Chemistry, BNES, Bournemouth, UK, October 1986, Paper 101. R.M. Donaldson et al.. Selection of amines for use as alkalising agents in the steam/water circuits of nuclear power plants, Water Chemistry of Nuclear Reactor Systems, VoL 2, BNES, London. October. 1992, Paper 61. A. Kishida et al., The causes and remedial measures of sleam generator tube intergranular attack in Japanese PWRs, 3rd Int. Syrup. on Environmental Degradation of Materials in Nuclear Power Systems, Traverse City, USA, 1987. S. Odar and K. Kuhnke, 10 Years field experience with high AVT chemistry, int. ConL on Chemistry in Water Reactors-Operating Experience and New Developments, Nice, 24-27 April, 1994. P. Paine, IGA/SCC inhibitors, 1993 EPRI Crevice Chemistry Workshop, Boston, USA, 28-30 June, 1993. R. Roofthooft et al.. Steam generator secondary side corrosion problems in Dnel 4, Water Chemistry of Nuclear Reactor Systems, Vol. 2, BNES, London, October 1992, Paper 66. g. Sala el al., Local chemistry and formation of deposits on the secondary side of steam generators. A laboratory study, 6th Int. Syrup. on Environmental Degradation of Materials in Nu"lear Power Systems, San Diego, CA, USA, August. 1993. A. Stutzmann and F. Nordmann, Studies of IGA related parameters and remedi~, int. Conf. on Chemistry in Water Reactors--Operating Experience and New Developments, Nice, 24-27 April, 1994.