New-generation nuclear plants may be having trouble getting out of the gate, but that doesn’t mean that nuclear capacity additions are at a standstill. In fact, the 104 operating nuclear units in the U.S. have added substantial new capacity in the form of reactor and plant uprates over the past 20 years. Power uprates alone have added more than 5,600 MW since 1998 — the equivalent of five new nuclear plants.
The World Association of Nuclear Operators released the power industry’s 2008 report card in late March, proclaiming that safety and operating performance remained "Top Notch" with the year’s average capability factor of 91.1%. We’ve come to expect nothing less, as this is the ninth consecutive year that the U.S. fleet’s capability factor — a measure of a plant’s online time — has exceeded 90%, continuing to mark nuclear power as the most reliable source of electricity in the U.S.
Not content to merely improve and maintain these outstanding operating statistics, the industry also embarked years ago on a path of upgrading U.S. plants to produce additional power. The Nuclear Regulatory Commission (NRC), which is responsible for the regulation of all commercial nuclear power plants in the U.S., classifies power uprates into the following three categories: measurement uncertainty recapture, stretch power, and extended power uprates.
Measurement Uncertainty Recapture (MUR) Power Uprates. MURs entail improvements to feedwater mass flow measurement technology, through use of ultrasonic flow metering, to significantly reduce the uncertainty in core calorimetric computations. The NRC has updated regulations that now permit licensing using an uncertainty in the safety analysis allowance consistent with that determined in these improved calculations. Lowering the uncertainty can result in uprates up to 2%. MUR activity remains sporadic. Only Calvert Cliffs Units 1 and 2 are in the NRC’s pending approval queue for a 1.4% reactor thermal uprate.
Stretch Power Uprates (SPU). SPUs are typically up to around 7% of the original licensed power levels, as they typically take advantage of conservative measures built into the plant that previously were not included in design and licensing activities. These involve, at best, modest equipment replacement and little or no change to either the nuclear steam supply system (NSSS) or turbine by limiting increase in pressure (2% to 3%) to allow sufficient mass flow margin in the high-pressure (HP) turbine. Modifications for stretch uprates concentrate on procedures and equipment setpoints, making the uprate capability plant-dependent.
Extended Power Uprate (EPU). An EPU increases the original licensed thermal power output by up to 20% but requires significant modifications to the plant. The focus of this article is a review of the requirements for an EPU and estimates of the amount of power past, current, and future EPUs add to our nuclear inventory.
Early EPU Activity
Most of the early EPUs were performed on boiling water reactor (BWR) plants. Table 1 (p. 76) provides a list of NRC-approved EPUs. Fifteen BWRs have been approved to date, and nine were approved before the first pressurized water reactor (PWR) EPUs received approval from the NRC. A total of five PWRs received approval. Four of them requested rather modest increases. Constellation Energy’s Robert E. Ginna Nuclear Power Plant is the exception among the PWRs, having NRC approval for a 16.8 % power uprate increase (Figure 1). Table 2 identifies those plants expected to be approved this year for EPUs (Figure 2).
Table 1. Extended power uprates approved by the U.S. NRC. Source: NRC
1. New lease on life. Constellation Energy’s Robert E. Ginna Nuclear Power Plant, located along the south shore of Lake Ontario, in Ontario, N.Y., is one of the oldest nuclear power plants still in operation in the U.S., having begun operation in 1970. The plant is a single-unit Westinghouse two-loop PWR. The original steam generators were replaced in 1996, enabling an almost 17% EPU to be approved by the NRC two years later. Courtesy: NRC
Table 2. Extended power uprates pending NRC approval. Source: NRC
2. Triple play. Each of the three units at Tennessee Valley Authority’s Browns Ferry Nuclear Plant is expecting NRC approval of a 15% EPU in 2009. The NRC operating licenses for Units 1, 2, and 3 were renewed in May 2006, which will allow continued operation of the units until 2033, 2034, and 2036. Courtesy: TVA
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