Valve malfunctions at PPL's Susquehanna Plant

 

A turbine valve failed in mid-position at the PPL's Susquehanna Steam Electric Station nuclear power plant near Berwick, Pa., on the morning of Aug. 18. 

 

According to a memo from PPL to the Nuclear Regulatory Commission, plant inspectors performing a weekly functionality test found the high pressure coolant injection (HPCI) valve stem in the plant's Unit 1 nuclear reactor "was not in the full closed position." 

 

The HPCI pump was declared inoperable after the discovery was made, the document stated. 

An investigation is underway to determine why the valve was stuck open. PPL reported the incident to the Nuclear Regulatory Commission, as required, calling it "an event or condition that could have prevented fulfillment of a safety function required to mitigate the consequences of an accident." 

 

Another valve malfunction occurred at the Three Mile Island nuclear power plant in November 2006. Two steam generator safety valves remained open in the Unit 1 reactor, sending information about steam pressure, temperature and flow to the control room; the unusual numbers triggered a reactor shutdown. News sources at the time reported the two main steam safety valves (MSSVs) remained open while operators brought pressure down to a level that would cause the valve to close after being"stuck open." Then-owner AmerGen and the NRC later withdrew their use of the words "stuck open" to describe the incident. The NRC agreed with AmerGen that the term used to report the   shutdown was "not technically accurate."

 

The word "stuck open" are often employed in accounts of the March 1979 accident at Three Mile Island's Unit 2 reactor, when a power-operated steam relief valve remained open and pressure dropped, turning coolant to steam and helping form a large steam bubble in the reactor vessel. Instruments in the control room failed to show operators that the valve was stuck open, and the temperature in the reactor rose to a level that began melting the uranium fuel and its cladding. An explanation published in March 2009 by the National Energy Institute, an nuclear trade organization, states that "operators did not replace the water that was lost as a result of the open relief valve," and explains that this began the meltdown, but the words "stuck open" are not used by the NEI.