The Seabrook nuclear plant in New Hampshire is located near the Atlantic Ocean. Large pipes extend offshore to draw in seawater to cool plant equipment. Because an earthquake might damage this piping, the plant has a cooling tower onsite near the Unit 1 containment building. Unlike the iconic concrete chimneys often associated with nuclear plants, Seabrook has a mechanical draft evaporative cooling tower. Motor-driven fans force air upward past warm falling water. The air cools the water which in turn is piped throughout the plant to cool safety equipment.
Water collects in the bottom of the cooling tower. The plant’s operating license includes a regulatory limit requiring the cooling tower basin’s water level to be at least 42.15 feet above mean sea level. Two separate instruments monitor the water level in the basin and transmit that information to the control room.
On November 2, 2012, operators in the control room noticed that one instrument (SW-LI-6139) showed the basin’s water level to be about one foot below the indication on the second instrument (SW-LI-6129). Both instruments monitor the same thing – water level in the cooling tower basin – and should provide the same result. Workers checked two other indications – a control room chart recorder and a plant computer point. These indications matched the value from SW-LI-6129, so they determined that instrument SW-LI-6139 was reading incorrectly.
On December 7, 2012, workers determined that the cooling tower basin’s level was below the minimum limit of 42.15 feet and took measures to add water to the basin until the level rose above that limit.
Subsequent investigation determined that instrument SW-LI-6139 had been reading correctly all along and that instrument SW-LI-6129 had been providing falsely high indications.
They further found that the control room chart recorder and the computer received information from instrument SW-LI-6129.
The nuclear industry talks often about conservative decision-making. It entails erring on the side of caution when making choices. Two control room instruments monitoring the same parameter output different values. Conservative decision-making, if actually employed rather than meekly being bantered about, would assume the output providing the least safety margin – the lower of the two water level indications in this case – is valid until demonstrated otherwise.
But here, as in way too many other instances, workers used convenient decision-making and assumed the instrument showing the least safety margin was bad. And they compounded their initial poor decision-making by deciding not to check about their guess for over a month. Imagine their surprise upon finding that the “bad” reading was good and the “good” reading was actually bad. At the same time, they should have realized that their “good” decision-making was actually bad, very very bad.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.