U.S. NRC Blog

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An Explanation of Capacity Factor

Neil Sheehan
Public Affairs Officer
Region I

Earlier this year, the nuclear power industry announced it had set a record for reliability in 2015. The measuring stick for this achievement is what is known as “capacity factor.” But what exactly is that?

Put simply, capacity factor compares how much energy was generated against the maximum that could have been produced at continuous full-power operation during a specific period of time. It’s similar to baseball’s on-base percentage, which counts how many times a hitter reaches base versus the number of opportunities in the batter’s box.

Nuclear is one part of the energy generation mix.

Nuclear is one part of the energy generation mix in the U.S.

The Nuclear Energy Institute, the industry’s trade organization, preliminarily pegged the capacity factor average for all of the nation’s reactors at 91.9 percent last year. It added that this was a new record, edging out the previous one set in 2007.

An update issued by the U.S. Energy Information Administration on June 24 upped the 2015 total to 92.2 percent. EIA also lists nuclear power’s capacity factor in 2014 as 91.7 percent and 2013 as 89.9 percent.

For comparison purposes, other segments of the energy production sector had the following reliability ratings in 2015 (according to the EIA): Coal – 54.6 percent; natural gas-fired combined cycle – 56.3 percent; conventional hydropower – 35.9 percent; wind – 32.5 percent; solar photovoltaic – 28.6 percent; solar thermal – 22.7 percent; landfill gas and municipal solid waste – 67.6 percent; other biomass, including wood – 52.9 percent; and geothermal – 71.7 percent.

EIA allows visitors to its website to check capacity factors dating back to 1973. A review of this data shows that reactor reliability rates started out in the upper 40s/low-to-mid-50s percent range during the U.S. commercial nuclear power fleet’s early days.

By 1991, the level had climbed to 70.2 percent and in 1998 to 85.3 percent. Since the start of the new millennium, the capacity factor average has been in the upper 80s/lower 90s range.

Then-NRC Chairman Nils Diaz, in congressional correspondence issued in March 2001, wrote that increases in capacity factor could be attributed to decreases in the amount of time that plants were shut down for repairs, refueling and maintenance.

For its part, the NRC focuses not on the number of operational hours for plants but rather that they remain safe whether or not they are operating. The agency does, however, track the number of unplanned shutdowns as a measure of plant performance.

5 responses to “An Explanation of Capacity Factor

  1. steamshovel2002 November 17, 2016 at 9:06 am

    There goes the NRC again with giving us selective information. Capacity factor is a leading indicator on the health of the organization. If you have poor capacity factor, violations of rules and increase errors are sure to follow. It’s a predictor of future performance. Is there a relationship at Watts Bar with unit two’s capacity factor problems at first startup and widespread employee intimidation? Pilgrim had increasing issues with capacity factor and the worst LOOP rate in the nation, leading up to their SRV failures and decision to shut down. Grand Gulf had horrendous capacity factor issues this past year recently ending with a prolonged safety shutdown to regain control of their operations department. There still shutdown.
    The nuclear industry has always protected their weak plants to the detriment of the rest of the industry. It is sickening to think about the cost and excess regulation that has been hoisted upon the well operating plants by the weak and declining capacity factor plants.
    If you wanted a safer industry you would better portray capacity factor to the public. Give us capacity factor much like google finance gives us stock prices and rolling averages. Give us individual, regional, utility plant and total capacity factor on simple graph. One click away to daily, weekly, monthly, yearly and whatever capacity factor on one page. Let us pick the bones off you weak actors. Hiding capacity factor is how the nuclear industry establishment thinks they protect the industry. I totally disagree with it. I think it would make you stronger.
    I could make the case that 20% to 30% of national capacity factor is the result of dangerous and secret nuclear industry political contributions deregulation and the NRC just not following their rules. And special compensations if a plant maliciously gets into trouble need a shutdown. It is a very slippery slope the NRC is playing with.

    And a high and prolong good capacity factor plant can be particularly dangerous. It leads to complacency. Everyone writes off or can’t see the decline of a good capacity factor plant, some unseen organization disruption…then a big event erupts out of seemingly nowhere, shocking everyone. Davis Besse is the example of this. Usually in hindsight, these plant’s organizations were terribly dysfunctional, unseen by outsiders and NRC. At least that is their story.

    Mike Mulligan
    Hinsdale, NH

  2. fermi2Mike November 9, 2016 at 11:56 am

    Being a nuke and having worked as a fossil plant training manager I can say the following.

    Nukes are designed to follow load, Fact is they don’t. So Pitbull is more correct.
    Fossil plants don’t measure capacity factor. It’s a purely nuclear term.

  3. kitemansa November 7, 2016 at 1:07 pm

    The real point is that Caacity Factor, among dispatchable sources, is more a description of fuel costs (or availability) than anything else. High capacity factor, low fuel costs. It is only when talking about the unREliables that it takes of the implication of lack of reliablilty.

    (Public Pit Bull) You are not quite accurate as to the load variability of nuclear power. It can be changed quite a bit, it is just rather stupid to do so.

    What we should be using to compare various sources is the “Unplanned Loss of Capability Factor” (ULCF) but defined such that it takes into account a random long term time factor so that solar PV cannot claim a low value because they KNEW the sun would go down.

  4. Public Pit Bull November 6, 2016 at 5:14 am

    I think you may have hit the ball hard NRC but I think you hit a foul ball by not explaining another reason nuke plants have a high capacity factor. Nuke plants are designed to be operated at full power 24/7 i.e. base loaded. They are dispatched by energy centers to be operated at full power. Power in nuke plants cannot be easily changed, they simply cannot maneuver like all other forms of energy production. Nuke plant operate at full power & everyone else changes power output to cover changes in electricity demand. Other sources of power are flexible; nuke plants are anything but.

  5. roger6t6 November 3, 2016 at 1:31 pm

    Thanks Neil. One of the best explanations of capacity factors I’ve seen. The day after the World SEries, your baseball analogy is particularly apt.

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