Replacement steam generators

The first of the Babcock & Wilcox Canada (B&W) replacement steam generators (RSGs) began with the Millstone 2 steam generator replacement programme for Northeast Utilities in 1988.

Manufacture is presently underway on replacement recirculating steam generators for the Calvert Cliffs 1 & 2 units of Constellation Nuclear and replacement once-through steam generators for the Oconee 1, 2 & 3 units of Duke Energy. These sites are the first to have applied for and received approval for life extension of 20 years beyond their original operating licence.

Beyond the RSGs for PWR plants, B&W has produced recirculating steam generators for all but one of the commercial Candu reactors for a total of 223 Candu steam generators. In addition, B&W is building replacement closure heads for PWR reactors, has built many heat exchangers, pressurisers and other reactor components for Candu systems and has been continuously involved in manufacturing nuclear power plant equipment since the late 1950s.

Recirculating steam generators

The steam generators for Calvert Cliffs 1&2, which are currently in pre-installation preparation and manufacture respectively, are the most recent of the replacement recirculating steam generators (RRSGs) as listed in the Table. The picture on this page shows the unit 1 RRSGs being off loaded from the barge by a multi-wheeled, self propelled transporter. The two units shown are the lower end, heat transfer sections to which the existing steam drums complete with new drum internals will be added. This method, which involves a drum to lower shell pressure vessel weld at the middle of the transition cone, is used where handling the RSG complete with drum is not feasible due to building opening size or internal constraints. These steam generators replace the CE OEM SGs. These RSGs have a number of significant features as shown in Figure 1 (which depicts an RRSG of similar concept but for a different OEM SG replacement application).

These RSGs feature Alloy 690TT tubing - the selection and quality of the tubing is the most important single factor in SG design since almost all SG degradation mechanisms eventually affect the tubing.

The pressure vessel envelope was made to match the OEM SGs as is necessary to ensure licensability of the replacements. A fully welded primary divider plate has been used to replace the tubesheet to primary head stay cylinder function thus avoiding the obstruction to access in the head and the divider plate leakage experienced with the prior floating type of divider plate.

The tube supports are lattice grids made up of intersecting flat bars. The U-bend supports are also flat bar type supports and all are of Type 410S stainless steel material. These flat bar configurations provide an open arrangement and line contact tube support so as to ensure free flowing recirculation conditions and the avoidance of tube conforming crevices or trapped spaces which may accumulate deposits and encourage corrosion. The tube supports are positioned to provide effective support against flow induced vibration due to the high two phase flow loadings and also for seismic, shipping and handling loads. The drum internals which replace the OEM equipment incorporate B&W CAP primary and cyclone type secondary separators as used on all RRSGs, all of which have performed very well, with exceedingly low moisture carry over.

Current B&W Candu steam generators typically incorporate integral preheaters and use Alloy 800 tubing but otherwise incorporate tube supports, drum internals and other features similar to the RRSGs. These features were in fact initially developed for the Candu SGs and subsequently adapted for use in RRSGs.

Once through steam generators

Replacement once through steam generators (OTSGs), as shown in Figure 2, are also currently under construction for Oconee 1, 2 and 3. These reactor units were the original lead units of the B&W reactor programme and their SGs have performed very well over the years. The OEM OTSGs, however, have had a number of problems, including most recently free span axial cracking in the upper spans and, previously, tube failures adjacent to the tube free lane.

The replacement OTSGs have been designed to retain the thermal and performance characteristics of the original units. This was done because the design of SGs is very tightly integrated. Thermal characteristics of heatup and cooldown and the thermal hydraulics of the downcomer flow, feedwater heating, level control, and so on are highly interdependent with each other and with the structural and hydraulic configuration of the existing design. The tubing is again Alloy 690TT. The pressure vessel retains the envelope of the OEM SGs but is of high strength material to minimise weight. The support skirt is replaced with a conical base in order to improve access for pipe work and manway opening. The tube supports of the OEM were the introduction to the industry of the broached plate design. The replacement SGs retain the broached plates but in Type 410S material. They also feature hour glassed flow passages and surface finish enhancement using the electrohone process. The plate positioning has also been optimised to enhance support against flow induced vibration in the upper tube span.

Many other design enhancements have been added as well including feedwater header configuration improvements, deletion of the tube-free-lane, addition of more inspection ports and seal welding of infrequently used penetrations.

Reliability record

Presently there are 134 Candu SGs and 30 PWR RSGs in service. An additional eight Candu SGs await startup at Qinshan 1 & 2, and 10 RSGs are at various stages of manufacture for Calvert Cliffs 1 & 2 and Oconee 1, 2 & 3. The only installed B&W SGs which have been indefinitely shutdown are 16 Candu SGs at Bruce 1 & 2. An additional 48 Candu SGs at Pickering 1-4 and 16 at Bruce 3 & 4 are expected to return to service after extended outages.

As of December 1998 there were 240 SGs in service with over 800,000 tubes. Of these, the total number of tubes removed from service to December 1998 was 4528 for a repair rate of 6 x 10-4 repairs per tube year of operation (1 x 10-3 would be good; 1 x 10-4 would be exceptional). This includes leaks, non-leaking defects, tubes sacrificed to gain access to other tubes and tubes removed as samples for monitoring purposes. Note that 84% of these tubes (3774) are in only two of the 38 reactor units.

Comparing the B&W SG tube performance history through 1998 against that of other major SG vendors indicates that B&W recirculating SGs, along with Framatome, are second to Westinghouse in tube-years of operating history; and, along with Siemens, have a tubing repair rate of less than one tube plugged per 1000 tube-years of operation.

Thermal hydraulic performance

Moisture carryover performance has been particularly good for the 30 replacement SGs now in operation at nine plants in the USA. All have had performance and moisture carryover tests performed during startup and all had test results in the 0.02 to 0.05% range, a fraction of the required values. Also, controllability of all of the RSGs has been excellent, with the water level being stable and responding well to operating transients.

The Millstone Unit 2 RSGs were the first steam generators designed by B&W with Alloy 690 tubes and they were the first Alloy 690 RSGs that met their heat transfer expectations. This includes the effects of the reduced conductivity of Alloy 690 tubing versus that of Alloy 600.

The thermal performance of Millstone 2, R E Ginna, Catawba 1, W B McGuire 1 and 2, St Lucie 1, Byron 1 and Braidwood 1 RSGs have all accurately matched the predicted best estimate startup performance. Although the warranted startup performance of Donald C Cook 1 RSGs was satisfied, the startup nozzle pressure was less than the best estimate prediction which was made using the same methodology as that for the other RSGs. A number of factors have been identified which together could account for the difference including steam nozzle pressure loss and dynamic head, tube wall thickness, subsequent enhancement of boiling nucleation by slight fouling and measurement uncertainties.

Secondary side visual inspections during refuelling outages as well as tubesheet water flushing maintenance activities have confirmed minimal deposition within well operated SGs. With good control of feedwater, iron ingress (concentrations of 1-2ppb are achievable) less than 20lbs of tubesheet deposition is expected in a typical RSG. Exceptional water level controllability and light tubesheet deposition within the tube bundles are evidence of effective liquid recirculation through the SG.

Tube reliability

Of the 30 RSGs now in operation, 26 have received 100% eddy current inspection during in service inspection. Of these, 12 have experienced limited fretting wear. The other 14 RSGs have no evidence of any wear. ECT indications have resulted in 23 plugged tubes out of a total population of 176,282 in the 26 inspected SGs.

The wear indications can be characterised into "typical" fretting wear and non-typical "pit-like" indications. The small number of observed non-typical indications have the appearance of pitting due to corrosion. However, corrosion is not suspected and these have been attributed to highly localised wear at support bar locations due to some type of small raised asperity affixed to the bar. This condition is thought to have been present since start of operation and is therefore not an ongoing mechanism or cause for ongoing concern. While the largest non-typical indication observed is less than 10% through wall and well below normal plugging criteria, due to the unavailability of calibration for these non-typical indications, most have been plugged.

Fretting wear characterised as "typical" has either rectangular or tapered indications consistent with the contacting surfaces between flatbars and tubes. A rectangular fret is a fret that has an axial length equal to the length of the tube to support contact area and of more or less equal depth along its length. A tapered fret is caused by contact between the tube and support bar which creates a tapered indication with a contract length less than the bar width.

A number of tubes with small typical wear indications have been left in service to allow ongoing monitoring. The typical wear indications have been found in the upper central core region of the U-bend. That is, in the larger radius region of the bundle (but not in tubes right at the periphery) and toward the axis of the steam generator - in the in-plane direction, at support bars near the tube apex mostly on the cold leg side and in the out-of-plane direction in tube columns near the vessel centreline.

The typical fretting indications have been attributed to isolated large tube to bar clearances at one or several support locations of affected U-bends. These U-bend support designs have been optimised for the SG flow and operating conditions provided that the support points are effective. Effectiveness of flat bar supports is considered to be primarily a function of uniformly tight clearances. Maintaining uniformly tight clearances is a function of selecting a sufficiently low value for the nominal clearance and also of avoiding design, manufacturing, shipping and handling practices which may result in isolated oversize clearances. B&W is implementing a reduction of nominal design clearance from a diametrical clearance of approximately 0.004-0.005 inch on B&W RSGs in operation to a value in the range of 0.0015 inch or less.

Recognising the importance of tube to flatbar clearances, B&W initiated a programme to measure gaps within the U-bend assembly during the pre-service eddy current inspection or eventually during in-service inspection in operating units. The goal of the measurements is to use the best technology available to better understand the variability in actual gaps and to reconfirm that isolated large clearances really are the cause of the fretting.

Design and operational enhancements

Future technologies emerging to further increase SG reliability and minimise maintenance activities include:

i) Pre-passivating the inner surfaces of Alloy 690TT tubing to reduce primary circuit activation following replacement.

ii) Using dispersant feedwater additives to maximise blow down efficiency and minimise contaminant buildup.

iii) Adding loose parts trapping systems within RSGs to avoid possible tube damage due to foreign material ingress with the feedwater.

iv) Addition of a "mud drum" sludge collection system to trap recirculating sludge and reduce its deposition.

v) Further improvements to the already highly uniform, low ECT noise, highly corrosion resistant tubing and eliminating the few remaining tube imperfections.

The latter is a tall order considering that a determined effort to reduce tube manufacturing irregularities during a current replacement project has resulted in a very high ECT signal to noise ratio of well over 30 and a total of three and four MBMs respectively for the first and second RSGs completed.

Regarding the need for tube ID surface passivation, general corrosion of Inconel Ni-Cr-Fe SG tubing in PWR plants contributes greatly to radiation fields in primary side circuits due to the release, activation and subsequent deposition on RCS surfaces of corrosion products. This phenomenon is more pronounced in new RSGs which have not undergone hot conditioning in which case the pre-existing tube inner surface oxide layer is thin and is able to release some of its nickel-rich alloy constituents. With the continued movement towards improved ALARA radiation levels, efforts within the industry have focused on developing pre-oxidation (passivation) processes for Alloy 690 tubing in RSGs.

B&W has developed its own passivation technology which takes advantage of the high chromium content of Alloy 690TT tubing to produce a thin, high chromium, low nickel oxidised barrier on the inside surface of the steam generator tubes. This is done during normal tube processing thereby avoiding the risks characteristic of alternative processes such as damage to the steam generator during manufacture or penalty due to late delivery or delayed commissioning.

Regarding dispersants, work is underway to evaluate the effectiveness of dispersants as feedwater additives for use in B&W steam generators. The desire is to maximise the fraction of particulate that is removable by blowdown while ensuring minimal risks and uncertainty with respect to the integrity of the tubing and other materials. This in turn would reduce the net fraction of contaminant that remains and accumulates as deposits on tubes, tubesheets and supports. With less deposits, the frequency of waterlancing and chemical cleaning could be greatly reduced.
Related Articles
Byron implements fuel assembly mapping system
MHI delivers RSGs to SONGS
Tables

B&W steam generator experience