Fact Sheets

November 2013

Key Facts

  • Safety is the nuclear energy industry’s highest priority. That commitment includes protecting nuclear energy facilities from even the most extreme potential natural disasters, such as floods, hurricanes and earthquakes. Independent scientists calculate the most powerful natural disasters ever recorded or expected within the vicinity of a nuclear energy facility site. Based on that information, experts design and build the plants to endure those forces with an extra margin of safety. This approach doesn’t just meet federal standards—it exceeds them.
  • The flooding hazards that are considered in the design of nuclear energy facilities are local intense precipitation, river flooding, dam failure, hurricanes and storm surges, and tsunamis and seiches. Nuclear plant site selection, design features, structures and procedures combine to protect safety equipment even in the face of such events.
  • Companies that operate nuclear energy facilities are updating their evaluations of flooding hazards at the sites using state-of-the-art tools and the latest data available for each location. These reassessments will take into account the latest science on rising sea levels and how that could affect potential flood levels. This new information will supplement the original evaluations of external hazards, which were conducted when the facilities were built. The re-evaluation of flooding hazards is one of several new requirements the independent U.S. Nuclear Regulatory Commission has issued since the 2011 reactor accident in Japan.
  • The evaluations will allow companies to identify ways that flooding protection might be improved to enhance safety further. Flooding hazards large enough to require additional protection are extremely low-probability events and, in most cases, plant operators will have advance warning. For example, hurricanes and their associated storm surges, the largest flooding hazards for U.S. plants on the East and Gulf coasts, approach over several days.
  • Nuclear energy facilities in the paths of hurricanes, including Katrina and Superstorm Sandy, were protected during these events, and they sustained no damage to their safety systems. This demonstrated the effectiveness of the nuclear energy industry’s meticulous approach to design, its preparedness against severe storms, and its commitment to continuous safety improvement.

Flooding Protection Measures

Nuclear energy facilities must be able to safely withstand the maximum potential flood that could occur at the site, based on historical data, with extra margin to account for uncertainties in the data. In other words, experts calculate the most severe flooding that might be expected and then design the plant to withstand a flood that is even worse.

The calculated probable maximum flood must reflect all relevant flood sources such as local intense precipitation, rivers and streams, seiches, multiple dam failures, ocean storm surge, and tsunamis. The hazard evaluation also must take into account effects that might reasonably occur along with flooding and could affect the movement of water or the forces imposed by it, such as high winds and debris.

Nuclear plant safety systems and structures are designed to cope with the possibility of floods, hurricanes, tornadoes and earthquakes. A combination of design features, structures and procedures protects nuclear plant safety functions against flooding, including:

  • Placement of the entire facility or certain essential systems and structures at elevated levels.
  • Exterior barriers that protect safety equipment from flooding and the dynamic forces created by waves and current. Such barriers include levees, seawalls, bulkheads, revetments and breakwaters.
  • Plant designs that incorporate sealed engineered barriers, including reinforced concrete walls and watertight access openings for personnel and equipment to keep water from penetrating into areas containing safety-related equipment.
  • Site grading designed to cause water to flow away from buildings and equipment.
  • Site procedures and training to guide plant staff in their response to a flood.


Most flooding hazards give advance warning, such as hurricanes and other storms that bring heavy      precipitation. In these cases, operators will have time to safely shut down the reactor and initiate backup safety measures, such as sandbagging. Operators are trained in the use of emergency and flood response procedures so that in the unlikely event that a flood occurs, they will have practiced the actions that must be taken.


What Prompted the Flooding Re-Evaluations?

The nuclear energy industry and NRC regularly review events at facilities and identify lessons that can help make them even safer. The industry and the agency also examine new information as the state of knowledge evolves. New nuclear energy facilities being built in Georgia and South Carolina use new data related to flooding and state-of-the-art methodologies for evaluating flooding hazards. The NRC completed a new report related to flooding in 2010 (NUREG/CR-7046, published in 2012).

The 2011 tsunami that led to a reactor accident in Japan heightened attention to flood protection. After a major earthquake, a series of tsunami waves swept over the Fukushima Daiichi nuclear energy facility on Japan’s eastern coast. Flooding disabled the plant’s safety systems and led to a severe reactor accident. Within days of the events in Japan, the U.S. nuclear energy industry re-checked the capability of America’s reactors to respond to severe natural events. Actions included verifying that required materials and equipment are properly located to protect them from a flood, earthquake or other natural phenomena. The NRC subsequently required the owners of those facilities to perform flooding evaluations in detail.

Industry Evaluations Under Way

The companies that operate America’s nuclear energy facilities are performing flooding hazard re-evaluations that use state-of-the-art methods and conservative assumptions. These evaluations include a review of historical meteorological data and information on the most severe local storms on record. These complex evaluations can take several years.

Companies will compare the results of these re-evaluations with each facility’s flood protection capability. If the existing flood protection features at a site are sufficient for the hazard, no further action is needed.

These conservative re-evaluations may identify some sites where the maximum flooding hazard, however unlikely, is higher than reflected in the facility’s original design. If so, the company will take interim steps to increase flood protection or mitigate its effects. In addition, these sites also must perform an integrated assessment of the facility’s overall response capabilities to evaluate the consequences of such an event and identify equipment that can be used, if needed, to maintain safety during severe flooding. Because of the integrated assessment, the company may need to take additional steps to further enhance flood protection. If these steps are needed, plant owners must submit them within two years of submittal of the initial re-evaluation.