The chemistry of boiling: the CANDU steam generator.
Boiling water may appear to be a simple process when looking at a beaker over a Bunsen burner. Pure water escapes to the atmosphere and, depending upon the dissolved species present, a variety of reactions may occur as the solids concentrate and dry out. Within industrial boilers, boiling is a very complex process requiring control at the ppb level due to concentrating mechanisms and the need to minimize operating costs and maximize thermodynamic efficiency. Steam is produced at high pressure and recovered (as condensate) for reuse after performing work and releasing its heat.
Industrial boilers use carbon steel as the major structural material. Various alloys are used for individual components to meet specific needs. As all chemists know, mixing alloys may have advantages from the structural or heat-transfer points of view, but the combinations inevitably create galvanic cells. This means corrosion and corrosion product deposition. Carbon steel will be the anode and its surface must be converted to the black magnetic oxide, magnetite ([Fe.sub.3] [O.sub.4]), for protection. From a chemical point of view, the system is this thin layer (only a few [Micro] m thick) of magnetite supported by an extensive carbon steel framework and the chemical requirement is to maintain its integrity by keeping a high pH and eliminating oxygen.
The CANDU-PHW (CANadian Deuterium Uranium-Pressurized Heavy Water Reactor) is a high-pressure steam-generating system. Where fossil-fired plants use combustion heat to produce boiling, here heavy water cools the fuel and in turn transfers the heat to the boilers or steam generators as it flows through the tubes under pressure. Boiling occurs at the outside surface of the tubes to provide the steam to turn the turbine-generator system. Point Lepreau (PLGS) and Gentilly 2 (G2) each produce 1 Mg/s of steam at 258 [degrees] C and 4.55 MPa pressure. (The two CANDU plants are operated by the New Brunswick Electric Power Commission at Point Lepreau, NB and Hydro Quebec at Becancour, PQ respectively.) Steam production is a continuous cycle.
Condensate Extraction Pumps (CEP): The cycle may be considered as starting in the condenser hotwell. Chemically, the objective is to raise the pH in the feedtrain, steam generators and steam lines. A volatile amine such as ammonia or morpholine is used for this purpose in the CANDU system. Other amines such as cyclohexylamine are common in industrial plant utilities where there are long pipe runs.
Deaerator and Deaerator Storage Tank: After passing through three low-pressure (LP) feedwater heaters, the water temperature is above the atmospheric boiling point. Spraying the water into the vessel provides a large surface area from which a steam purge removes dissolved oxygen leaving less than 5 [Micro] g/kg. A scavenger such as hydrazine may be added to the storage tank to remove the last traces.
Boilers or Steam Generators (SG): After passing through an additional high-pressure (HP) feedwater heater, the water enters the steam generator. Boiling occurs at the tube surfaces. In nuclear steam generators, tubes tend to be high-nickel alloys such as Inconel-600 or Incoloy-800 within a carbon steel shell. The system is dynamic. Fresh makeup brings low-level impurities into the steam generator while steam leaves them behind. With time, species present at mg/kg levels in the makeup can reach several percent at the tube surface. When solubility limits are exceeded, deposition can occur. To put things into perspective, it does not take much corrosion to maintain a circulating iron concentration of only a few [Micro] g/kg. For most applications, this is an ultra-pure water. On the basis of a 90% capacity factor, 1.0 [Micro] g/kg would transport 300 kg of iron into the steam generator over a year.
Main Steam and Turbine: The turbine is constructed in two stages. Steam first enters the HP turbine. As energy is expended, the temperature drops enough to cause water droplets to form. At this stage, the wet steam passes through a moisture separator where the liquid water is removed and passed down through the system to recover its remaining energy by heating feedwater on the way up the chain. The dried steam goes to a reheater where it gains some superheating before proceeding to the LP turbine. A turbine is a major piece of rotating equipment (3,600 rpm) and very sensitive to deposition. Even a tiny amount of deposition on the blades could result in an imbalance situation that could cause major damage.
Condensers: When the steam leaves the LP turbine, it has given up essentially all its energy. As it is pure water, it can be condensed for reuse. Early CANDUs used Admiralty Brass condenser tubes; the newer units use stainless steel or titanium (seawater). With cooling water flowing through 40,000 tubes, it is vital that the integrity of these tubes be maintained. In a closed system, the cooling results in a vacuum and any leakage would bring cooling water into the steam generators.
The condensate is now ready for recycling through the system. To compensate for losses, fresh makeup is added to the system. Makeup is produced by ion-exchange usually with separate cation and anion beds followed by a mixed bed polisher.
Steam Generator Chemistry
Two chemistries are presently used for the CANDU steam-generating system: all-volatile treatment (AVT) and phosphate.
AVT: AVT uses only volatile products for corrosion control, eg. a volatile amine and oxygen scavenger. This choice is made to minimize the buildup of solids within the steam generator. AVT provides no buffering capacity and thus requires very tight control limits and strict adherence to them. Most nuclear stations in the world now use AVT.
Phosphate: Adding phosphate along with the amine and oxygen scavenger adds a buffering capacity to the pH control, but with a concentrating mechanism, phosphate concentrations could reach solubility limits. The phase diagram indicates several solid phases. With a high Na:[PO.sub.4] ratio, free caustic may exist with possible stress-corrosion cracking (SCC) of steam generator tubes. With a low ratio, free phosphoric acid may exist and lead to a phenomenon called phosphate wastage where tubes suffer OD thinning. Many nuclear stations have suffered from phosphate wastage and few now remain with phosphate control.
Only one CANDU, PLGS, now operates with phosphate chemistry. This provides a buffer against the effects of seawater ingress should a condenser tube leak. Control of the Na:[PO.sub.4] ratio has been tight from the start. An improved method was developed by AECL's Whiteshell Nuclear Research Laboratory (Dean et al.) to determine the ratio in the presence of other species. With the long reaction time, control has been performed manually rather than by computer. Plant experience (MacNeil and Silbert, 1988) indicated the system could not tolerate the high phosphate levels originally recommended within the industry. A reduced phosphate range has been established and mass balances performed to verify that the system could indeed tolerate the new range. This study was in agreement with fossil station work in Ontario Hydro (Stodola, 1986). Visual and NDT inspections showed no sign of the wastage that has plagued other nuclear steam generators.
Choice of Amine: The choice of amine is most critical in regions of two-phase (steam and water) flow. These exist within the steam generator, main steam and extraction steam lines and the moisture separator between the HP and LP turbines. A high pH is needed to maintain a protective magnetite surface. With copper alloys in the feedtrain, the pH range was limited to 9.2. For this morpholine was an excellent choice. As copper alloys were removed, pH control moved to the 9.5-9.8 range. Ammonia appeared more economical and became the choice for copper-free systems due to concerns about the presence of breakdown products from morpholine.
Extensive erosion/corrosion damage to several turbine systems in two-phase flow regions initiated a review of the chemistry. While ammonia may be the stronger base at room temperature, a comparison over the range from 0-300 [degrees] C shows the difficulty in relying upon room-temperature measurements. It is important that the distribution between the steam and liquid phases protects both. At the same time, the base strength should be high. In the 150-180 [degrees] C temperature range of the moisture separator region, ammonia tends to favour the steam phase while morpholine is almost equally distributed between the two. (Gilbert and Saheb, 1988). Both products have equal strength in this range with morpholine being the stronger base at higher temperatures. When PLGS substituted morpholine for ammonia, iron transport was reduced. NDT and visual observation confirmed a reduction in erosion/corrosion rates.
A major concern in the choice of amine is the possible formation of decomposition products that might be corrosive rather than protective. The reaction mechanism for the decomposition of morpholine in high pH, deoxygenated water (Gilbert and Lamarre, 1989), was determined initially on a laboratory scale using autoclaves and HPLC/HPIC analyses. Samples taken from both G2 and PLGS showed the presence of the various species, confirming the mechanism and also the distribution within the system. Glycolic and acetic acids are formed in the process. The role of weak organic acids in steam generator chemistry is somewhat controversial at the present time. While some think these species may lead to enhanced corrosion, there is no confirmatory evidence.
Only two mechanisms are available to remove impurities once they have entered the steam generator.
Blowdown continuously drains a small portion of the water from the steam generator and limits the extent to which impurities can concentrate. Economic, energy and environmental considerations limit blowdown to about 0.3% continuous flow with perhaps a few intermittent bursts above this. With this flow, it takes weeks to remove solids and deposit formation within the SG may be a competitive process. Mass balances from many systems indicate blowdown efficiencies of only 15-40%. In many industrial plant utility boilers, where there is much less emphasis on water quality, blowdown rates of 5-10% are common. (Unless there is some form of heat recovery, this constitutes a considerable portion of the annual $2 to 3-million fuel bill required to heat a university campus or industrial process.)
Condensate polishing passes the feedwater through a purification circuit, usually ion exchange. PLGS has installed a full-flow condensate polisher. They are expensive ($5-million per reactor unit), but could be justified at PLGS as a defence against the ingress of seawater. While operating polishers in the morpholinium mode was a new experience to CANDU-PHW operation, it caused no difficulties.
Dissolved Oxygen Control
Oxygen scavenging is accomplished primarily by the deaerator usually followed by addition of hydrazine as an oxygen scavenger.
[N.sub.2][H.sub.4] + [O.sub.2] [right arrow] [N.sub.2] + [2H.sub.2]O There are many arguments regarding the required concentration of oxygen scavenger to be maintained. Some set it high to ensure reducing conditions; others set it low on the basis that a trace of oxygen is required to form just enough oxide to ensure continuous on-line repairs to the magnetite surface. All CANDUs measure dissolved oxygen using in-line instrumentation and maintain concentrations below 5 [Mu]g/kg entering the steam generator.
The decomposition mechanism, for morpholine, suggests another alternative. Oxygen is required to form the acids. By relying upon the breakdown products of the morpholine, it should be possible to eliminate the need for hydrazine. Both G2 (Van Berlo and Dundar, 1986) and Tihange in Belgium use morpholine without adding hydrazine and are able to maintain a low-oxygen environment.
Monitoring and Control
Monitoring is accomplished by qualified laboratory personnel with a fully equipped laboratory operating on a 24-hr/day basis. The laboratory staff are supported by a number of in-line monitors. These include analyzers for dissolved oxygen, pH and sodium. The sodium analyzers continuously monitor the condensate for any trace of leakage and will alarm if only 1.0 [Mu]g/kg is detected.
Sampling presents the biggest difficulty. Steam and feedwater samples are taken from a high temperature, high pressure system while those from the condenser come from a system under vacuum. Before a sample can be taken, the sample must be brought to ambient temperature and pressure. It must also pass through hundreds of metres of stainless steel tubing to get to the sample point. In the process, a number of factors take their toll including: changes in equilibria and kinetics as the temperature changes and settling and adsorption in sample lines. The control becomes one of searching more for trends than absolute values. At the same time, these differences have to be considered to extrapolate back to system conditions.
To control a process, the first stage is to understand how it operates.
a. Phosphate control has been used for boilers throughout the world for many decades. While it is still used extensively with fossil boilers, nuclear steam generators were particularly sensitive to minor variations in the control range. Most nuclear stations overcame the problem by switching away from phosphate. By determining the limitations of the process, Point Lepreau was able to operate a successful program.
b. While morpholine chemistry is a new trend in worldwide PWR (pressurized water reactor) operation, it is an established procedure within CANDU operations. Studying the mechanism of the amine decomposition and the distribution of the decomposition products now provides better criteria for selecting the appropriate chemical environment to combat erosion/corrosion. The addition of an oxygen scavenger has been an established policy for decades; the reaction mechanism also indicates an indirect role of the amine as an oxygen scavenger. Essentially all stations in the world have relied on hydrazine to scavenge oxygen. G2 has operated since 1984 without problems, using morpholine only without any additional scavenger.
How do the CANDUs perform? Considering both the size of the system and its construction materials, a station such as PLGS is able to maintain circulating iron levels consistently below 2.0 [Mu]g/kg.
Comparing the 1988 capacity factors for the 322 stations in the world over 500 MW, shows five CANDUs in the top ten: Pickering-6 (#1), Pickering-5 (#5), Pickering-7 (#6), Point Lepreau (#7) and Gentilly-2 (#10). On the basis of lifetime capacity factors (i.e. since declared in service) Pickering-7 and Point Lepreau move up to #1 & #2.
Table 1 compares the integrity of CANDU steam generator tubes (Babcock & Wilcox Canada, Cambridge, Ont.) with those manufactured in other countries. The reliability can be assessed by the number of tubes plugged. This number includes both tubes that have developed leaks and those plugged during exploratory investigations. The CANDU steam-generator performance is orders of magnitude better than those of any other manufacturer in the world. [Tabular Data Omitted]
PHOTO : Brent Cummings, chemical technician at the Point Lepreau Generation Station of NB Power,
PHOTO : replaces the cover on a sodium analyzer after completing calibration.
PHOTO : Site of Gentilly Generating Station, Becancour, PQ
MARVIN D. SILBERT, FCIC Marvin Silbert and Associates, Willowdale, Ont. ROLAND GILBERT, MCIC Institut de recherche d'Hydro Quebec (IREQ), Varennes, PQ CYRIL K. MACNEIL, MCIC New Brunswick Electric Power Commission, Point Lepreau, NB
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|Title Annotation:||Canadian Deuterium Uranium-Pressurized Heavy Water Reactor|
|Author:||Silbert, Marvin D.; Gilbert, Roland; MacNeil, Cyril K.|
|Publication:||Canadian Chemical News|
|Date:||Sep 1, 1989|
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