Coal, Gas, Nuclear

Southern Nuclear Integrates MSR Refurbishment with Power Uprate Program

Issue 4 and Volume 104.

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Southern Nuclear’s Plant Hatch Nuclear Generating Station in Baxley, Ga., Includes Two BWR (boiling water reactor) units-an 810 MW gross-power unit commissioned in 1975 and an 820 MW gross-power unit commissioned in 1978. In the course of component evaluation in preparation for an 8 percent power uprate program, Southern Nuclear determined that refurbishing the existing moisture separator reheaters (MSRs) would add capacity and improve reliability, while offering substantial savings in high-pressure (HP) turbine modifications.

Performance of the existing MSRs (four vessels per unit, for a total of eight vessels) had significantly deteriorated over the years in terms of their contribution to unit output and had experienced continuous high maintenance costs. While the rate of deterioration was high in the early operating years, it had later stabilized, and its adverse effects on unit operation became nearly constant. Some of the factors that had inhibited MSR redesign and reconstruction in the past included their relative inaccessibility beneath the turbine deck, high costs inherent in a BWR unit modification, such as radioactive-waste disposal, contaminated work-zone issues, and the labor-intensive undertaking of removing and reworking internal components.

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It was obvious to Southern Nuclear, however, that the adverse impacts of the 8 percent power uprate program on the MSRs could increase the rate of deterioration to a totally unacceptable level and limit their ability to function properly.

As a further incentive, Plant Hatch is actively pursuing a renewal of its operating license, and it was apparent that the existing MSR reheater bundles would require refurbishment at some time, even at the current reduced deterioration rate. Additionally, refurbishment would improve low-pressure (LP) extraction steam quality, benefiting extraction piping and last-stage turbine bucket degradation.

Southern Nuclear decided, therefore, to coordinate the refurbishment of the MSRs with the 8 percent power uprate program, and to do it in a novel way to minimize costs. This approach involved two significant items:

1. Refurbish-but not totally redesign and reconstruct-the MSRs by using the existing internal structure with minimal alterations. While this would not take full advantage of modern, high-performance MSR technology, it would likely restore the original unit gross output, halt further MSR deterioration and eliminate the continuing high maintenance costs.

2. Avoid the expensive HP turbine modification that would have been required to accommodate the increased HP steam flow associated with the 8 percent power uprate. This involved re-sizing the HP reheaters to consume the maximum steam possible within the available MSR size constraints, and permitted retaining a safe HP turbine control-valve flow margin.

MSR Refurbishment

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Figure 1 shows sectional elevations of the original and refurbished MSRs. In the original MSR designs, there were minor differences between Units 1 and 2 due to MSR technology advances in the years between unit commissioning. These minor differences were relatively easily circumvented, however, and the new LP and HP reheaters for both plants are essentially duplicates.

The refurbished moisture separator section of the MSRs employs modern high performance, Type 430 stainless steel, double pocket, chevron moisture separators to achieve near zero moisture carryover to the LP reheaters. The existing structural arrangement, of course, precluded altering the relatively tortuous cycle-steam path between the moisture separator outlets and the LP reheaters. This contributed to a high cycle-steam pressure drop, which could have been significantly reduced by using a modern MSR structural redesign.

The original circular impingement plates at the cycle-steam inlets to the MSRs also increased the pressure drop across the MSRs. Thermal Engineering International decided these plates were unnecessary in conjunction with the new stainless-steel bundle rear bulkheads and U-bend caps. To confirm the anticipated effects of the removal of these circular impingement plates, a computational fluid dynamics study was performed, and corresponding full-scale model tests were conducted. These tests confirmed that the effect would be positive, as anticipated.

Since major structural changes could not be made, the locations and physical envelopes of the LP and HP reheaters could not be radically altered. Nevertheless, the required two-pass LP and HP reheaters were “custom designed” to produce a low terminal temperature difference (TTD) in the HP tube bundle and an even lower TTD in the LP tube bundle. In addition, in both LP and HP tube bundles, selected individual tubes were orificed to provide optimum steam distribution through each bundle. Note that instead of the original, essentially rectangular LP tube bundle, a slightly octagonal bundle could fit into the existing structural envelope. This improved the aspect ratio (tube rows vs. tube columns) and increased the number of tubes in each LP reheater to more than 1,200, thereby enhancing heat transfer from the increased steam flow resulting from bypassing the HP turbine.

In the original design, both 5/8-inch and 3/4-inch finned, carbon-steel tubing had been used in some of the MSR reheater tube bundles. In the refurbished MSRs, only 3/4-inch Type 439 stainless-steel, finned (27 fins per inch) tubing is used throughout all the MSRs.

Although tube failures due to thermal stress had not been a major contributor to operational deterioration in the original reheater tube bundles, the new reheaters incorporate a flexible tube-support system that allows for controlled, intermittent relief of mismatched thermal expansions between the hotter tubes and the progressively cooler shrouding plates. This concept employs a “slide-plate” design, so that the slide plates can automatically adjust, in plane, to any tube or structural thermal expansion without restraining the tubes or damaging the plates themselves.

Instead of using orifice restrictors in the condensate discharge lines to provide a fixed excess-steam/condensate flow from the LP and HP reheaters, manually operated throttling valves are now used. These valves, in conjunction with integral thermocouples installed in selected reheater tubes, permit occasional manual flow trimming to optimize MSR operation to maximize output gain.

Reheater Installation

Each of the MSR reheater tube bundles was shipped in a special container via tractor/trailer. To clear a path for the bundles and rigging-support structures, a large number of interferences had to be removed, including conduit cable trays, piping, valves and pipe supports. The piping ranged in size from small-bore vents and drains to a 72-inch diameter feedpump turbine exhaust duct. Additionally, a platform capable of supporting the weight of the MSR reheater bundles had to be constructed on one side of the condenser to locate the bundles at the MSR shell elevation. After new bundle insertion, the interferences were reinstalled and functionally tested.

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Figure 2 shows three steps in the complicated and challenging process of installing a new HP reheater bundle (the LP reheater bundle is already in place). The process involved “angling” the new reheater 30 degrees from vertical in its shipping container and through the equipment hatches in the turbine deck, while supported by the turbine crane. Radio communication was necessary for maneuvering the bundles since the crane operator could not see them directly. The bundles were then positioned horizontally at the appropriate MSR shell level, their shipping containers removed, several chain lifts hung from the special, axially mobile trolley rails, and the reheaters aligned at their ultimate elevation within the MSR. A block-and-tackle rig then dragged each HP reheater bundle into the MSR shell. The accompanying photo shows a partially inserted HP tube bundle and the many lifts that support it from the trolley beams. A similar process was used to remove the old contaminated tube bundles. These were covered with “shrink wrap” and transported to special oversized trailers for eventual shipment, decontamination and disposal.

This process, in one direction or the other, had to be repeated 16 times per unit with the special trolley rails relocated eight times. The lessons learned from certain difficulties experienced during the Unit 1 MSR reheater installation process were incorporated into the succeeding Unit 2 procedure, improving the latter’s installation schedule.

After completing bundle insertion, Southern Nuclear pressure-washed the MSR shells to prevent a massive chemistry excursion or the intrusion of foreign material into the reactor vessel before sealing. This was necessary because of the proximity of scrap from the extensive amount of cutting, grinding and welding resulting from MSR internal modifications. The wash was successfully completed and resulted in minimum impact on the unit’s normal startup chemistry.

Performance Results

Using accepted performance parameters-pressure drop, TTD, etc.-Southern Nuclear demonstrated that the MSR refurbishment project resulted in a capacity benefit of approximately 10 MWe in Unit 1 and 7 MWe in Unit 2. This is roughly equivalent to the loss experienced over the years attributed to MSR performance deterioration. Further, if the MSRs had not been refurbished in concert with the 8 percent uprate program, the actual uprate achievable would have only been about 7 percent.

The MW production benefits, combined with cost savings from the HP turbine modifications, enhanced long-term MSR reliability and reduced maintenance costs, confirm the cost-effectiveness of the project. This unique concurrent refurbishment and uprate project should also demonstrate to other nuclear plant operators the advantages of intimately tying together MSR redesign and reconstruction issues with power uprate considerations.

Authors-

Marion R. Price, principal engineer, has been associated with the Southern Co. for 24 years in thermal cycle design and major equipment bid evaluation for a variety of their power plants. He is a registered Professional Engineer and holds a BS degree in mechanical engineering from the University of Alabama.

Tim G. Wells, senior engineer, has been associated with the Southern Co. for more than 15 years in project, system and program engineering plants. He is a registered Professional Engineer and holds a BS degree in mechanical engineering from Clemson University.

Clement W. Tam, director of MSR operations, has been associated with Thermal Engineering International for 26 years in all phases of moisture separator reheater (MSR) design, development, component and model testing, fabrication support, installation and field testing. He is a registered Professional Engineer and holds both BS and MS degrees from California State University.

Abraham L. Yarden, senior vice president, has been associated with Thermal Engineeering International for more than 32 years in heat-transfer systems. For the last 22 years, he has specialized in all phases of moisture separator reheater MSR technology. He holds both BS and MS degrees from the University of California.