Regulator warns of design vulnerability

Nuclear regulators have instructed licensees of US nuclear power plants to provide information about the designs of their electric power systems after an incident earlier this year revealed a potential weak spot.

Nuclear regulators have instructed licensees of US nuclear power plants to provide information about the designs of their electric power systems after an incident earlier this year revealed a potential weak spot.

Events at Exelon's Byron revealed the potential problem (Image: NRC)

The Nuclear Regulatory Commission (NRC) has alerted all licensees to a "potential design vulnerability" which it says could affect the operation of key safety equipment in the event of problems with off-site power. The flaw was revealed in a 30 January incident at Exelon's Byron 2, when the unit automatically shut down because of unbalanced voltage entering the onsite power distribution system from the transmission network.

Plant operators had to manually trip the necessary circuits to isolate the degraded external power source and switch to emergency power, because the protection scheme for the plant's electric power system was not designed to do this automatically. Plants require reliable off-site and on-site power systems with sufficient capability and capacity to operate safety-related systems, and the NRC says that the degraded off-site power source could potentially have damaged the plant's emergency core cooling system.

The NRC formally informed US licensees of the Byron event in March, and has now requested licensees provide information on their electric systems designs. The order applies to all the 104 currently operating commercial power reactors in the USA, and also to the four combined construction and operation licences issued for new reactors earlier this year. Licensees have 90 days to comply with the NRC's request.

Researched and written
by World Nuclear News