US nuclear plants must report on electric protections: NRC

Washington (Platts)--20Dec2013/529 pm EST/2229 GMT


US nuclear plant operators must provide information to the Nuclear Regulatory Commission by February about an electrical vulnerability that could affect safety systems and how each plant plans to mitigate it, the agency said in a letter made public Friday.

The vulnerability affects almost all US nuclear stations and has received regulatory attention since January 2012, when Exelon's 1,210-MW Byron-2 reactor near Rockford, Illinois, lost power to its safety systems after a partial loss of off-site power known as an open phase condition.

The event revealed what the NRC called a "design vulnerability" in nuclear plant power systems related to power provided to them from the grid to feed internal plant loads.

Electricity is distributed on the grid using three alternating current phases, each offset slightly in time. An open circuit or ground fault on one or two of the three phases of grid power cannot be detected by current power protection systems at US nuclear plants, NRC said in a February report. Such systems are designed to detect a loss of voltage on all phases or a complete loss of power, licensees have told NRC.

The agency said licensees without the ability to detect and protect from such an open phase condition may not be meeting regulatory requirements.

The Nuclear Energy Institute, which represents nuclear plant owners, builders and suppliers, has coordinated industry response to the problem, which has also recently affected nuclear plants in Canada and Sweden. US plants took interim steps to protect against the problem, mostly involving new procedural steps, NEI said last year.

All US nuclear plant operators, by a formal vote of their chief nuclear officers, agreed in September to a binding industry initiative that will resolve the issue by making permanent plant modifications by the end of 2016, NEI's senior director of engineering John Butler told NRC in November. Those modifications, which may involve the addition of relays that can detect the open phases, could cost $1 million or more per site, he said.

Exelon, which has begun installing the new relays, has said costs to protect seven of its nuclear plants could be $27 million.

Sweden's 1,208-MW Fosmark-3 on May 30 experienced the loss of two phases of offsite power as a result of an error during maintenance. Systems providing cooling to the reactor core briefly lost power and stopped, and some equipment was damaged, officials of operator Vattenfall said in June. Swedish regulators have also asked nuclear plant operators in that country to submit information on steps to prevent a similar event in the future.

--William Freebairn, william.freebairn@platts.com
--Edited by Annie Siebert, ann.siebert@platts.com

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