Southern Nuclear Company (SNC) is in the final stages of a 6-year programme to increase the generating capability at the Edwin I Hatch plant. Plant Hatch, as we call it, is a dual-unit station with BWR Mark 4 reactors originally rated at approximately 800 MWe. We have already been able to increase their combined net generation by about 230 MWe and expect to gain another 15 MWe in the year 2000 as all the upgrades become fully operational. The total cost of the increased capacity was less than $350 per kilowatt of installed capacity. The production cost associated with the 200+ megawatts increase is approximately 0.5 cents/kWh.

The MWe Improvement Programme was performed in two phases. The first phase consisted of a 5% increase in the licensed core thermal power (CTP) plus an upgrade to the cooling towers for the main condenser.† The second phase was another increase in the licensed CTP of 8% and an upgrade to the main turbine moisture-separator reheaters (MSRs). Phase 1 was completed in May of 1996 – in time for the 1996 Atlanta Olympic Games. Phase 2 has just been completed, and the additional power will be available for the peak summer period.


Five percent power uprate

In early 1994, SNC received funding from the owners* of Plant Hatch to begin two major projects. Both were designed to increase the electrical power output of the plant. The phase 1 power uprate raised the licensed CTP from 2436 MWt to 2558 MWt (5%) and raised the operating pressure by 30 psi (3%). The higher steamflow increased the electrical power output by 5% (40 MWe) per unit. The higher reactor pressure improved plant heat rate and minimised the modifications to the high pressure (HP) turbines. This is because with the direct-cycle BWR, an operating pressure increase of 30 psi results in a higher pressure not only in the vessel, but also at the main turbine control valves. Therefore, an increase in turbine throttle pressure causes a corresponding increase in the flow-passing capability of the HP turbine.

Both the NSSS and BOP components had adequate capability to handle the increases in steam flow, feedwater flow, pressure, and electrical power associated with the 5% uprate. Changes to the HP turbine were limited to the first stage nozzle. Selected instrumentation needed to be respanned or replaced.

Cooling tower upgrade

The cooling tower upgrade was accomplished by adding two twelve-cell counterflow helper cooling towers to reduce heat rate in the steam cycle. These towers were designed to increase cooling capacity and reduce condenser pressure with a gain of 9-10 MWe (net) during the summer. The helper towers were also sized to handle the additional duty from both the first and second phases of power uprating. Construction of the towers took place over a 9-month period while the units were operating and tie-ins to the existing cooling towers and circulating water flow system were made during the outages.

The cooling towers had been undersized for the plant since original construction. However, the increased duty from the power uprates (ie, more steam to the main condenser) would have resulted in power limitations and even higher heat rates during the summer months. Also, the existing towers had been in service for over 20 years. The helper towers allow one of the older towers to be removed from service for repair/refurbishment during the cooler months. These two factors, combined with the increase in plant efficiency, made the cooling tower upgrade cost-beneficial.

The phase 1 uprate and cooling tower projects were implemented following the Autumn 1995 Unit 2 and Spring 1996 Unit 1 outages. Performance testing for power uprate was completed during startup and power ascension testing following the outages. Cooling tower tests were completed in July 1996. The Phase 1 measured generation increases from this projects is shown in the table.


Extended power uprate

The phase 1 power uprate involved detailed analysis and testing of the capability and available margin of BOP and NSSS components. Therefore, this effort provided accurate data for estimating how far Plant Hatch could uprate a second time. Detailed feasibility studies were undertaken to estimate the costs of modifying limiting BOP components. Based on these studies, a target CTP level 8% above the first uprate was selected for the extended power uprate. This equated to a 13.4% increase overall in the licensed CTP. The second uprate was designed to increase the electrical output by 40-70 MWe per unit, depending on margins available in the BOP equipment.

As expected, the NSSS did not limit the extended power uprate. Studies by General Electric show that BWRs like Plant Hatch can safely increase core power and steamflow by 15%-20%. This available safety margin is due to (1) a conservative approach to design and analysis, and (2) improvements in nuclear fuel design. In addition, the safety systems in the plant were designed to be remarkably insensitive to increases in power level. For example, consider the impact of a higher power level on the calculated fuel peak cladding temperature (PCT) following a postulated loss-of-coolant accident (LOCA). Higher initial core power does not affect the size of the break in the piping nor the flow of water out the break. The power level also doesn’t affect the operation of the emergency pumps that add water to the vessel. The calculated PCT is mostly dependent on these factors, plus the initial power in the most limiting (ie, highest power) fuel bundles in the core. Uprating does not change the power of the most limiting fuel bundles. Therefore, for Hatch the impact of uprating on calculated PCT was insignificant.

However, many BOP components were affected by this second uprate. In particular, it was necessary to:

• Replace/modify several stages of the HP turbine to increase steam flow capacity.

• Modify the main generator stator water cooling and isophase bus cooling systems to accommodate higher power losses.

• Modify the condensate demineraliser system to decrease resin usage and system differential pressure.

• Modify the condensate/feedwater system to assure adequate suction and discharge pressure for the pumps.

• Increase main condenser tube bundle stacking at selected locations due to increased steamflow/duty.

• Improve plant instrumentation for monitoring heat rate parameters and temperatures on the main generator step-up transformers.

MSR refurbishment

The performance of the moisture-separator reheaters was an issue at Plant Hatch even before the uprates. Several tubes had already been plugged and heat rate testing over the years had confirmed that the existing tube bundles and chevron vanes were not as effective as desired. Studies had shown the existing MSRs were decreasing plant efficiency by 8-10 MWe per unit. The increased steam flow to the MSRs from the uprates would tend to make additional tube failures more likely, and Southern Company and industry experts doubted these heat exchangers would last for another 20-40 years. The extended uprate provided the final factor to allow MSR refurbishment to be cost-beneficial. Modifications to different stages of the HP turbines were needed to improve the flow-passing capability. Above some power, a completely new HP turbine would be required which could be cost-prohibitive. By carefully designing the main steam supply to the second stage of the MSR, it was possible to increase the available HP turbine flow margin while still improving plant heat rate. This is because the second stage of the MSR receives its reheat steam supply from the main steam lines upstream of the turbine control valves. (This high energy steam is used to superheat steam entering the low pressure sections of the turbine and then cascades down to the high pressure feedwater heaters.) By increasing steam flow to the second stage of the MSR, the higher steamflow from the reactor could be accommodated without replacing the HP turbine.

The phase 2 extended power uprate and MSR refurbishment projects were implemented following the Autumn 1998 Unit 2 and the Spring 1999 Unit 1 outages. Performance testing for power uprate and the MSRs was completed during the startup and power ascension testing following the outages.

The table summarises the measured generation increases from the projects.