December 10, 2015 

The Honorable Edward J. Markey 

United States Senate 

Washington, DC 20510 

Dear Senator Markey: 

 On behalf of the U.S. Nuclear Regulatory Commission (NRC), I am responding to your 

letter of October 19, 2015, requesting information on the effects of extreme weather, higher 

water temperatures, and climate change on nuclear power plants over the past decade. 

Information responsive to your request is enclosed. 

In compiling a list of power reactor shutdowns and power reductions, NRC staff 

searched through events that were required to be reported to the NRC and also met the 

conditions described in your request. NRC’s reporting criteria generally require power reactor 

licensees to report reactor trips and the completion of technical specifications-required 

shutdowns. These criteria require licensees to report the state of the plant at the time the event 

becomes reportable, but do not require a discussion of the duration of a shutdown or power 

reduction. Therefore, durations provided in Enclosure 1 represent conservative NRC staff 

estimates for each listed shutdown or power reduction based on other information sources 

available to the NRC. It is likely that the Federal Energy Regulatory Commission, as the 

Federal agency responsible for monitoring reliability of the grid, also would have data regarding 

power plant changes induced by weather effects. 

Licensees’ capability to respond safely to extreme weather events is examined through 

the NRC’s baseline inspection program. Structures, systems, and components important to 

safety at nuclear power plants are required to be designed to withstand the effects of natural 

phenomena without loss of capability to perform their intended safety functions. The design 

bases for these structures, systems, and components reflect consideration of the most severe of 

the natural phenomena that have been historically reported for the site and surrounding area. 

The design bases also reflect margin to account for the limited accuracy, quantity, and period of 

time in which the historical data have been accumulated. Additional protection is provided 

through analysis, procedures, and maintenance practices. Together, these protections provide 

reasonable assurance of the continued safe operation of nuclear power plants under extreme 

environmental conditions. 

 In addition to a list of shutdowns and power reductions, you also requested a list of 

requests for permission to increase the water intake temperature, as well as the NRC’s 

justifications for approving or denying those requests. The NRC requires nuclear power plants 

to have an ultimate heat sink (UHS), a source of cooling water that is available under both 

normal and accident conditions, to remove heat from structures, systems, and components 

important to safety. It is the temperature of this UHS that limits the ability of the facility to 

remove heat from the plant. 
- 2 - 

 Enclosure 2 lists approved license amendments that changed the temperature limits of 

the UHS, as well as license amendment requests that were denied. License amendment 

requests that did not change the temperature limits, but are otherwise related to your request, 

as well as notices of enforcement discretion related to UHS temperatures, are also listed. Due 

to the volume of the correspondence associated with the listed actions, the accession numbers 

in the NRC’s Agencywide Documents Access and Management System for these materials are 

provided. 

If you need any additional information, please contact me or Eugene Dacus, Director of 

the Office of Congressional Affairs, at (301) 415-1776. 

Sincerely, 

/RA/ 

Stephen G. Burns 

Enclosures: 

As stated