More Than Replacing Parts
This isn’t to say that an EPU can be completed by merely making a few simple plant modifications. Increased thermal output requires greater thermal input into many of the plant systems and components, causing a potential reduction of required margins through lowered material properties and added burden on pumps, bearings, and seals. Increased flow accentuates flow accelerated corrosion (FAC) in pipes and other components. Increased mass flow has the potential of raising flow-induced vibration levels in systems and components to unacceptable levels, or changing the frequency of the exciting forces causing vibration where it previously did not exist.
The Electric Power Research Institute (EPRI) maintains a Lessons Learned database that identifies issues observed and resolved in previous power uprates and serves as an excellent information base for future uprates.
It is difficult to generalize about the perfect plan to complete an optimum EPU for a given plant. Differences between plants in initial regulatory approaches, past response to regulatory issues, and previous modifications and equipment change-outs to maintain plant operation all combine to make the EPU program for each plant unique. Even side-by-side "identical plants" frequently require separate plans to accomplish an equivalent EPU. For this reason, detailed studies are required for each plant. However, some general trends have been observed.
Design duty for overpressure protection and required relief capacity in the reactor coolant pressure boundary from normal operating and transient design conditions typically increase with increased power. This may require modification to primary and/or secondary side safety valves and safety relief valves. Otherwise, modifications to the reactor coolant pressure boundary have not been a major concern. Industry experience with power uprates to date has shown that the installed capacity of emergency core coolant systems is nearly always sufficient without modification. Auxiliary feedwater systems and emergency service water systems may require modification.
Major upgrades to the balance of plant (BOP) have been the focus of most EPUs. The turbine, main generator, main power transformer, and power train pumps often require replacement or modification. Components such as feedwater heaters, moisture separator reheaters, and heat exchangers are frequently replaced with larger units. Feedwater, condensate, and heater drain pumps, along with supporting components, typically require replacement or modification. This increases the demands of ISO phase bus duct cooling. Increased steam and feedwater mass flow often require replacement of piping to accommodate greater mass flow or to counteract the effect of FAC. The design must also consider any increased demand for demineralized water.
For each EPU, a high-pressure turbine retrofit (at least) is required, and because throttle margin can be achieved through the retrofit without an attendant increase in operating reactor pressure, the uprate can be analyzed and performed at constant pressure. Depending upon existing margins, the magnitude of the uprate and the condition of the turbines, it may be necessary to replace, repower, or modify the low-pressure and/or high-pressure portion of the turbine. In many cases, the condenser is either replaced or retubed. Plants with closed-loop cooling may also have to consider cooling tower upgrades, and plants with open cycles will need to evaluate thermal effects from the condenser outfall. Major modification to the generator and stator (rewinding) are expected. This may also require increased cooling for the generator. Transformers may also require replacement with larger units.
Replacement components are generally larger and heavier, which means that structures supporting these components are challenged and frequently require strengthening by way of modifications to the building structure and other foundations.
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