Nuclear Power Plant Database FAQ

Background information on the most common safety-related issues at U.S. nuclear power plants.

View our nuclear power plant database for an interactive map of these and other safety issues at all commercial U.S. nuclear reactors.

What are elevated spent fuel pools?

Most nuclear reactors are powered by 12-foot long “fuel rods” that contain uranium, plutonium, and other metals and compounds. When the reactor has extracted most of the fuel’s energy, the rod is considered “spent” and removed to a cooling pool, where it remains until sufficiently cooled (usually for several years). Any loss of cooling from these pools can result in the water boiling off, the fuel rods igniting, and a subsequent—and significant—release of radiation.

Most reactors locate their cooling pools in reinforced subterranean pits, though certain designs use “elevated” spent fuel pools, in which the cooling pool is placed above-ground. These elevated spent fuel pools may be more likely to be drained of water or have their cooling systems disrupted during accidents or a major terrorist attack, and are more difficult to refill. Our database displays all reactors that store used fuel in these above-ground pools.

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What are fire protection problems?

Nuclear reactors are particularly vulnerable to fire, which can destroy key safety systems and lead to a reactor meltdown and the release of radiation. The arm of the government charged with nuclear safety, the Nuclear Regulatory Commission (NRC), estimates that the risk of meltdown from fire hazards is roughly equal to the meltdown risk from all other hazards combined—and that’s when the fire regulations are met. The fire hazard only increases when the regulations are violated.

In 1975, a fire at Alabama’s Browns Ferry nuclear power plant disabled a reactor’s primary and backup safety systems, allowing the reactor’s water levels to drop precipitously and nearly causing a meltdown. In response, the NRC developed new regulations in 1980 and again in 2004, requiring more robust fire prevention and mitigation strategies.

Today, decades after the Browns Ferry incident, many U.S. reactors are not in compliance with the NRC’s fire safety regulations and instead rely on unapproved systems and interim compensatory measures that weren’t intended for long-term use. Our database displays all reactors that remain out of compliance.

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What are groundwater leaks?

Powering nuclear power plants can produce large amounts of radioactively contaminated water. Typically, radioactive water is only present in secured, closed sections of the plant, and doesn’t pose a threat of environmental contamination. However, equipment failures, accidents, negligence, and human mistakes can lead to the release of this water via spills or leaks. In the past, this contaminated water has made its way onto public lands and into drinking water supplies.

Plants are legally permitted to release radioactively contaminated water to the environment, but only via controlled and monitored pathways that ensure the amounts released pose little harm to the public. Leaks and spills bypass this protection. Our database displays all reactors that have reported a past spill, leak, or other inadvertent release of radioactively contaminated water.

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What constitutes a near miss?

Near misses are events where combinations of broken or impaired safety equipment and/or poor worker performance spurred the Nuclear Regulatory Commission (NRC) to send a special inspection team, augmented inspection team, or incident investigation team to the site. Our database displays plants that experienced a near miss in the previous year.

The NRC dispatches a special inspection team when conditions may increase the chance of reactor core damage by a factor of 10. An augmented inspection team is sent when the risk is increased by a factor of 100. An incident investigation team is sent when the risk increases by a factor of 1,000 or more.

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What is earthquake risk?

In the 1990s, the Nuclear Regulatory Committee (NRC)—the federal agency charged with nuclear safety— determined that the hazard posed by earthquakes in the central and eastern parts of the United States was greater than previously believed. In 1996, the NRC enacted regulations that required new reactors built in these areas to be designed with protection against the greater hazard, but did nothing about improving seismic protection at existing reactors.

In 2005, the NRC began examining the potential risk from reactors operating with less protection than necessary against known seismic hazards, identifying several dozen reactors with levels of risk greater than what the reactors were designed to withstand. Those shortcomings have yet to be resolved. Our database displays all reactors facing this risk, as recognized by the NRC.

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What are flooding hazards?

Nuclear power plants are built next to rivers, lakes, and oceans because they require vast quantities of cooling water to carry away the large amounts of waste heat they produce. In the United States, nuclear reactors along rivers occasionally have one or more dams located upriver. If a dam fails, the ensuing flood waters could overwhelm the plant's protective barriers—an event not dissimilar to what happened at Fukushima, where a tsunami overcame the power plant’s seawall.

The Nuclear Regulatory Commission has identified several dozen reactors that face flooding hazards potentially greater than what they were designed to withstand. Though the extent of flooding risk is still unknown, our database displays all reactors facing this risk, as recognized by the NRC.

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What does "Heightened NRC Attention" mean?

Within the federal government’s reactor oversight process, safety levels at all operating reactors are evaluated every three months using performance indicators for approximately two dozen key parameters coupled with findings from NRC's inspections. A reactor is flagged as "heightened NRC attention" if its safety levels drop below specified thresholds. For these reactors, the arm of the government charged with reactor safety—the Nuclear Regulatory Commission—responds with additional inspection efforts to ascertain the causes of their declines and verify that steps are being taken to correct the problems.

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What's a year plus outage?

Nuclear reactors may be shut down for lengthy periods of time. The majority of such occurrences resulted from numerous violations of federal regulations that require plant owners to find and fix safety problems in a timely, effective manner, coupled with the government’s inability to detect those violations (allowing problems to multiply and worsen as a result). Our database displays all reactors that have experienced a year plus outage.

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What are “On-Site Dry Casks”?

When spent nuclear fuel is removed from a reactor, it cools off for a period of several years in a spent fuel pool. Once adequately cooled, it’s sometimes transferred to steel and concrete containers know as “dry casks.”

Unlike cooling pools, dry casks are passively cooled, meaning they don’t rely on electrical systems to keep spent fuel at safe temperatures. In the event of an accident or terrorist attack, this offers dry casks a significant safety advantage over cooling pools, as their cooling mechanisms aren’t affected by power outages. And because even the largest dry casks hold much less waste than cooling pools, far less radioactivity can be released from them.

An “on-site” dry cask means the nuclear power plant has one or more dry casks at the same location as the reactor, though the power plant’s cooling pools may still be overcrowded. Expediting the transfer of adequately cooled spent fuel from pools to temporary dry cask storage is a top security priority.

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