Horizontally challenged1 January 2003
The steam generators used at VVER reactors have a number of advantages over other types of steam generator. By N Trunov, V Denisov and Ju Dragunov
The VVER's horizontal steam generator (SG) design is based on a different geometry from those used in the West and the tube material is stainless steel (08Kh18N10T grade). The benefits of this design are:
• A high water inventory and smooth behaviour in transients.
• Effective removal of non-condensable gases in accidents to provide natural circulation.
• The steam generation surface is under moderate load and has a simple separation scheme.
• Low velocities on the secondary side reduce damage from vibration and foreign object impacts.
• There is good access to the tubing from both primary and secondary side.
• There is space where sludge can accumulate.
• The horizontal tubing is resistant to degradation.
• The stepwise evaporation is effective in removing impurities.
• The tubes seldom rupture because they have a high load-carrying capacity.
So far the longest-operating SGs are at Novovoronezh 3, which started up in 1971.
Steam generators based on this philosophy are used at VVER-440s (PGV-440, sometimes referred to as PGV-4 or PGV-213) and VVER-1000s (PGV-1000). Several upgrades have been made, but the changes made were different at each unit and the upgrading process is still under way. The Table on page 16 gives the main technical details of SGs at each reactor.
The main design features were defined in the first commercial design. The horizontal bundle of U-shaped tubes is connected to the heating coolant collectors, made of stainless steel. The SG vessel is made of carbon steel. There is access to the collector from the top, through a flange joint in the vessel neck. The tube bundle supports are made of stainless steel. Steam passes from the vessel into the steam collector through nozzles on the upper vessel.
Maintenance access in the primary side collectors is via covered flanges in the upper part of the collector. The covers also have to be opened on the secondary side neck flanges. The covers were originally sealed with the help of two nickel gaskets but at some plants the joints now have improved gaskets of expanded graphite, making stud-tightening easier.
To prevent corrosive impurities from evaporating in the phase separation area the coolant collectors are provided with sealed protective enclosures that communicated with the secondary side steam space. After a series of problems with the tightness and inspection of the enclosures, they were replaced in some SGs with corrosion-resistant high-nickel cladding.
The separation devices are in the form of louvred stacks slightly inclined from the horizontal. There are no devices for steam load balancing in the original design. Later steam generators (at Rovno 1 and 2) were equipped with local perforated sheets in the area of the hot collector and the result has been less swelling in the area of maximum steam load and no flooding of the separator. The flange joint, which can be a point of corrosion from impurities, is also protected from moisture. Similar sheets were also used at Dukovany.
In the standard design, the feedwater is distributed in the hot side into the horizontal corridor between the upper and lower parts of tube bank. At some units where the distribution collectors were made of carbon steel, corrosion-erosion damage was detected and the feedwater distribution unit was modified. Replacement collectors of several different design versions were used.
• At Rovno 1 and 2 the modified feedwater collector is above the tube bundle and the feedwater is distributed, mainly, towards the vessel. Distributing nozzles are located non-uniformly, distributing the feedwater in compliance with the steam release profile.
• At Loviisa the feedwater collector has J-shaped distributing nozzles. Their purpose is to prevent the feedwater flow from stratification (resulting in water hammering) when water at 20ºC enters the main feedwater collector from the emergency standby water tanks that are a feature of the plant.
• In the design version, at Dukovany, feedwater is supplied downwards, into the horizontal corridor between the upper and lower parts of tube banks.
Along with the feedwater distribution system, the external blowdown lines were altered to improve the efficiency of dissolved impurity removal. The latter versions of the PGV-440 design have a device under the tube bundle used to wash sludge from the lower part of the SG vessel during planned maintenance.
The design's large margins has allowed power plant upgrades - at Loviisa, for example, a 9% upgrade has increased thermal load of each generator to 250MW.
The main design differences between PGV-440 and the PGV-1000 - and its upgrades, PGV-1000M and PGV-1000M+ - are due to the greater specific thermal load and the stringent overall dimensions requirements (due to transport restrictions). In PGV-1000 pearlitic steel was used for the collectors (instead of low-strength austenitic steel) and separation scheme with steam load balancing with the help of a submerged perforated plate (SPP).
The tube bundle in PGV-1000 steam generators is tightly arranged with staggered tubes. As in the PGV-440, the tube bundle is divided into four banks, between which downcoming corridors are arranged. The PGV-1000 originally had similar tubes spacers to the PGV-440 but later employed spacing strips with a perforation at the point of contact with the tubes.
The SG vessel has separate nozzles for main and emergency feedwater. The main flow of feedwater comes into the collectors under the SPP and distributed via nozzles oriented horizontally above the tube bundle towards each other. Corrosion-erosion wear detected in the course of operation made it necessary to replace all the carbon steel collectors with stainless steel versions.
At the louvers outlet there is a throttling perforated sheet. During separation tests at the first unit a steam-water mixture bypass phenomenon, was detected under the SPP rim from the hot side, making it difficult to ensure the moisture level was at the design value. To protect the lower edge of louvres from steam-water mixture flow a deflector baffle was placed above the gap between the vessel and the SPP rim from the hot side. Later the phenomenon was eliminated by closing the gap between the vessel and the tube bundle from the hot side using an additional perforated sheet. In new designs there is no rim. At some operating units and all new build units, louvres have been replaced by a steam perforated plate.
The SPP can help in steam separation with inertial bubbling-free heads. Each head is a perforated cylinder with a swirler inside. The moisture is separated inside the perforated nozzles. Steam goes directly to the steam space without bubbling through a water layer above the SPP so it is possible to decrease the swelling level above the SPP and to increase the inventory over available height of steam space. This improves the separation characteristics and allows an increase in the steam relieving area load or the nominal water level value.
Design updates were aimed at the improvement of coolant header operation reliability after detection of the cases of their damages.
During operation, the PGV-1000M version was modified to introduce stepwise evaporation. The feedwater was redistributed along the SG. Along with the relocation of the point of continuous blowdown, this decreased soluble impurity concentrations in the water space reducing corrosion in the collector and heat exchanger tubes.
Another design improvement has improved circulation within the secondary side space to reduce sludge accumulation in the individual sections of tube bundle, especially in the lower part, and related corrosion damage. The feedwater distribution was changed so part of it came into the downward channel, increasing the circulation ratio.
The most recent SGs have additional nozzles to connect passive heat removal systems, as well as a special nozzle and pipeline for reagent supply during chemical cleaning.
As well as the PGV-440 and PGV-1000 families, SG designs are at different stages of development. The PGV-640, for example, is designed for an intermediate power unit.
The next step in the horizontal SG is a modification of PGV-1000 with increased vessel diameter. PGV-1000 was sized so it could be transported by rail transport and this is why a staggered tube bundle was required. Increasing the vessel diameter to 4200mm will enable us to use a corridor tube bundle instead. Thermal and hydraulic characteristics are unchanged so the bigger SG can be used at a VVER-1000 without modifications to the plant. Optimising the tube arrangement will increase the bundle volume by 20% and have other advantages:
• Improved circulation in the tube bundle.
• Reduced sludge accumulation in the tubing space.
• Improved conditions for chemical and mechanical cleaning as well as for visual inspection.
This will increase tubing operational reliability, and provide an opportunity to justify a SG lifetime of 50 years and more. New transport arrangements would be required.
Horizontal SGs have been assumed in the design for the VVER-1500, which have a design life of 50 years. The main design features are similar to operating SGs, but the design and tubing is optimised on the basis of experimental and theoretical studies, taking into account available containment space. The advantages of the new design are:
• Increased reliability of tubing and better circulation.
• Simplified cleaning and sludge removal.
• Improved secondary-side access for tubing inspection.
• Better conditions for eddy-current inspection (increased tube bend radius).
PGV-1000 coolant header operation
At the end of 1986, cracking was detected on the cold outlet coolant headers of PGV-1000 SGs for the first time. The cracks - found in 25 SGs - arose in the ligaments between the tube holes at the edge of the zone of the so-called "cold" collector, where the residual and operational stresses are concentrated. Most of the failed SGs were replaced and two were repaired. The cracks propagated from the slot-hole area between the tube and the coolant header, from the secondary side, to through-wall cracks with damage to tubes welds.
Cracking was only detected on the cold collectors. A high level of conventionally elastic residual stress in collectors was revealed after heat exchanger tube expansion by explosion. It was also established that, at the operating temperature of the "cold" collector and total stresses close to a yield strength, the steel is subject to low-temperature creep. The initial stage of damage was therefore identified as corrosive cracking at slow rate of strain.
Studies of hydrodynamics, temperature distribution and pressure pulsations from the primary side under different conditions did not significantly influence the collector integrity. Solving the problem required a complex of measures involving a change of manufacturing technology, design modification, improved water chemistry and better inspection.
Since 1991 no SGs have been replaced because of collector cracking although they have operated at up to 120,000 hours (Kalinin 1).
The basic mechanism in tube damage is stress corrosion cracking. The cracking propagates when impurities (chlorides) collect on the outer surface of the tube. Sulphates are also activating agents. The cracks are mostly oriented along the tube axis and can be intercrystalline, transcrystalline or mixed.
Studies and operational experience testify that austenitic steel tubing has a high enough corrosive resistance, in the absence of pollution and conditions for local evaporation, but at a number of power units there are numerous damaged tubes. All have been in operation for long time without chemical cleaning and have suffered violations of water chemistry requirements and condenser leaks. The main cause of damage is sludge depositions in the space between the tubes. Chemical cleaning can prevent this.
At units where water chemistry quality is maintained and regular chemical cleaning is performed, tube damage is minimal. For example, at Khmelnitski 1 chemical cleaning is performed annually at one or more SGs during of cooldown. Only 10 tubes have been plugged due to corrosion since 1988.
VVER SG tubes are now plugged at a rate of 0.096% per year, compared to 0.6% repaired annually for vertical SGs.
PGV-440 collector cracking
Since 1975 the problems with cracking arose in the collector weld region, located under the protective enclosure made of steel 08Kh18N10T. At Novovoronezh 3, Greifswald and Kozloduy the cracks initiating from the secondary side were detected in the collector metal with a depth ranging from 28mm up to through-wall cracks. It was established that the damage resulted from chloride corrosive cracking from the secondary side. This was caused by inadequate checking of the tightness under the enclosure, allowing water to enter. The problem was solved after filling the sealed space of the enclosures with nitrogen, and pressure monitoring.
During operation of PGV-440s, corrosive cracking occurred in the studs and stud holes in the upper part of the primary side collectors. For different power units, the nature and places of crack forming were slightly different.
At a number of units the upper flange part of collectors was completely replaced. Numerous studies, performed by different organizations, on this phenomenon have not revealed the unequivocal cause of cracking, but all the investigations indicate the influence of four phenomena:
• High operational stresses.
• Corrosive influence of stud lubrication based on molybdenum disulphide.
• Influence of an alkaline condition, due to leaking from the primary side through both gaskets of the flange joint.
• Influence of secondary side water under non-design conditions (water ingress on flange joint from the SG water space).
The probability of water ingress can be reduced by placing the local perforated sheet in the region of the "hot" collector. Reliability of flange joints can be improved by monitoring the SG level, and not exceeding the operational design level. Other measures include:
• Improvement of the stud tightening procedure, without exceeding the design stresses.
• Change of stud lubricant type.
• Upgrading the studs and flange joint components.
• Use of the gaskets of expanded graphite instead of nickel gaskets in order to reduce the stud tightness force.
Since 1993, no cracking of studs and flange joint components has been recorded. Since 1999 the gaskets of expanded graphite have been used at several plants, allowing a reduction in the force of studs by a factor of 1.5.
Feedwater collector corrosion
During operation at a number of VVER-440s and VVER-1000s, the corrosion-erosion wear of the feedwater collectors, made of carbon steel, was observed. At a number of units through-wall wear on the collectors was detected.
A number of studies have shown that the damage to distributing nozzles has little effect on the SG hydrodynamics and its separation characteristics. Pipeline damage in the T-piece can cause water ingress onto the coolant collector, resulting in inadmissible thermocyclic loadings.
The problem can be solved by replacement with new collectors made of stainless steel. This has been performed at most VVER-1000s and at a number of VVER-440s.
Reliability and serviceability of commercial horizontal SGs were studied both at the development stage and, mainly, during operation. The following aspects were studied:
• Strength of the vessel components, coolant collectors, nozzles, and steam collector using strain and temperature measuring and mock-ups.
• Temperature fields both within the secondary side space, and in the primary side collectors.
• Vibrations of tube bundle and coolant collectors.
• Distribution of impurities in the water space and blowdown efficiency.
• Separation characteristics by steam moisture measurement in the SG steam space.
• Boundary of the actual level and its behaviour along the SG perimeter, including under transients.
• Circulation characteristics.
• Values of average and local steam qualities.
• Hydrodynamical conditions above and under submerged perforated sheet including the degree of perforation.
• Water chemistry conditions, including chemical cleaning, processes of corrosion and deposit formation.
The results of studies and operational experience are the rich material both for developing the next generation of horizontal SGs, and for all nuclear plant SGs.
Operational experience has demonstrated that horizontal SGs have several important advantages. In future, their operational characteristics will further improve as a result of the experimental and theoretical studies carried out. The main design solutions on the horizontal SGs can be used for new units of up to 1500MWe.