Managing Margins
An EPU is a major undertaking for an operating plant that requires the combined expertise of the plant staff, NSSS, turbine contractors, and, in most cases, nuclear EPC contractors. An initial but important step is to establish a margin management program (if the plant does not already have one) to ensure that adequate margins are available in systems, structures, and components (SSCs). Developing or updating the margin management program may be done in parallel with other EPU preparation steps.
Several "margins" are of interest in a margin management program. The Institute of Nuclear Power Operations (INPO) identifies three different ones for nuclear plant design: operating margin, design margin, and analytical margin (Figure 3).
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3. Managing margins. Relative nuclear plant margins as defined by the Institute of Nuclear Power Operations. Source: INPO
Operating margin is the difference between operating limits and the range of normal operation. The operating limit is analogous to design values in engineering terms. It accounts for, and envelops, all the potential operating conditions of the plant. Design codes and licensing criteria all include a certain margin, or safety factor, beyond the design limit, which address uncertainties in design, fabrication durability, reliability, and other issues. The difference between the analyzed design limit and the operating limit is this conservatism, which INPO calls the design margin.
Normal aging and plant operation can eat into each of these margins and requires constant attention by owners. Increased thermal output by an EPU imposes further demands on the operating limit. Even systems or components not directly affected by the power increases may not function as efficiently as intended following an EPU. For all of these reasons the margin management program becomes an important tool in performing an EPU.
The margin management program has two basic parts. One is analytical: ensuring that the design documents are current, correct, and consistent with the plant design features. The second part is more complex in that it requires a systemic assessment of the current condition of the physical plant thorough engineering walkdowns, review of condition reports, and other operational data. A through review of EPRI’s generic Lesson Learned databases is also important for identifying potential future issues.
Assuming that all the necessary studies have been performed and the decision has been made to consider an EPU, the next step is to conduct a feasibility study. Typically, an experienced architectural engineering firm, with support from an NSSS supplier and the turbine manufacturer, performs the study. Alternately, the NSSS supplier may take the lead with assistance by the turbine manufacturer and an architectural engineering firm.
Assuming that a decision has been made to consider an EPU, the next step is to conduct a feasibility study. An integrated team consisting of the owner’s plant staff, an experienced architectural/engineering firm, NSSS supplier, and the turbine/generator supplier should perform the feasibility study. This would minimize interface issues between the current operating experience at the nuclear plant, the NSSS, BOP, and turbine/generator equipment.
Potential modifications to the NSSS, the nuclear systems, the turbine and cooling system, and the BOP are studied. Initial evaluations are conducted to identify the potential power increases available through modifications of the NSSS, as discussed above. The turbine/generator is also evaluated to determine modifications required to meet the proposed uprated power needs. And finally, all the potentially affected nuclear and BOP systems and components are evaluated to determine the pinchpoints — those items that have suffered margin erosion due to the EPU modifications or other preexisting factors.
Included, or in parallel, with the feasibility study is a cost/benefit analysis. Typically, the greater the uprate, the greater the cost of the last kilowatt added. Most utilities are finding that, compared to other available alternatives, it is cost-effective to implement the greatest amount of added power possible from the EPU, provided that other outside factors demonstrate the need exists.
The next phase of the feasibility study is to identify modifications that are required to meet the EPU’s requirements and ensure that the modifications reestablish required margins. In some cases margin can be restored solely through more sophisticated analysis. In other cases hardware changes or plant modification are required. Preparation of equipment specifications and purchase orders are then placed for long-lead-time components. Typical long-lead elements include:
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High-pressure and low-pressure turbine replacement
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Main generator and auxiliary upgrades
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Transformer replacement
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Feedwater heater(s) replacement
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Pumps and motors (feedwater, condensate, heater drains, component cooling water)
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Spent fuel pool cooling heat exchangers
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Main steam reheaters
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Condenser and/or cooling tower upgrades
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Water treatment system upgrades
Based on the feasibility study, including the cost-benefit analysis, the owner will decide on the final upgrades/modifications required to meet the EPU goals. With this final list, a more detailed evaluation is performed that supports a Licensing Amendment Report (LAR) for NRC review and approval. The requirements of the LAR are provided in the NRC document RS-001, Review Standard for Extended Power Uprates. The LAR incorporates all the analytical results completed along with additional detailed evaluations of SSCs directly or indirectly affected.
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