Following switchyard problems that led to the momentary loss of the reactor coolant pumps at the Byron 2 reactor, the Nuclear Regulatory Commission is requesting information from U.S. nuclear plants regarding a potential design vulnerability.On Friday, the NRC issued a bulletin and a press release detailing a Jan. 30 reactor trip at Byron, when plant safety systems shut down the reactor because of an under-voltage condition on the busses that power two of the pumps. According to NRC bulletin 2012-01, the under-voltage condition was caused by a broken insulator stack in the conductor that supplied the unit's station auxiliary transformers. The conductor broke off from the power line disconnect switch, creating a high-impedance ground fault.Once the reactor tripped, the other two coolant pumps automatically switched to auxiliary transformers. But because the conductor in question was an open circuit, the flow of current to coolant pumps increased, prompting all four of them to trip because of phase overcurrent. With no coolant pumps functioning, the NRC reported, operators performed a natural circulation cooldown.Within eight minutes, plant personnel isolated the problem and started emergency diesel power, declaring an unusual event that lasted until switchyard repairs and restoration of offsite power were complete.The event revealed a generic problem that necessitated an NRC notice and a request for a response from all 104 operating U.S. power reactors and the four under construction. In describing the condition in a release Friday, the NRC said the "degraded offsite power source potentially could have damaged the plant’s emergency core cooling system."The NRC indicated responses from plants will be used to determine if further regulatory action is warranted.