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.
When something happens at a U.S. nuclear power reactor like an unplanned shut down or problem that prevents operation at full power, it can be helpful to place that incident in context. One means of providing that context is to examine that reactors’ operating performance. The Nuclear Regulatory Commission posts information online that can help. Each work day, the NRC posts a data set containing the daily power levels reported for each nuclear power reactor in the country during the past year.
Each entry in the data set contains a date, the reactor’s name (e.g, Indian Point 2) and the percentage of rated power (i.e., 0 to 100).
The data set can be easily downloaded as a text file and imported into a spreadsheet program like Excel to allow the data to be examined. Those steps for Excel are:
- Go to http://www.nrc.gov/reading-rm/doc-collections/event-status/reactor-status/
- Right click on the “Power Status (raw data text file) for the last 365 days” link and use “Save target as …” to download this file to your computer
- Start the Excel program
- Select the “Data” tab across the top
- Doubleclick on the “From Text” icon at the upper left and select the text file downloaded to your computer in Step 2
- Verify that the first step of Text Import Wizard seeks to import Delimited data (rather than Fixed Width data) starting at row 1, then click the “Next” button
- Change the Delimiters option in the second step of the Text Import Wizard from using “Tabs as Delimiters” to “Other”, and enter a vertical line (| which appears on a standard keyboard as a capital backslash, \) in the white box to the left of “Other”, then click the “Next” button
- Change the Column data format for the first column in the third step of the Text Import Wizard from “General” to “Date (MDY)” and then click the “Finish” button
- Place the data into the existing spreadsheet by clicking the “OK” button
The data will be sorted from the power levels for each reactor 365 days ago at the top and the power levels from today at the bottom. Excel can be used to resort the data by reactor (column B) and date (column A) to facilitate reviewing individual reactor performance.
For example, the figure shows the daily power levels reported for the Fermi Unit 2 reactor in Michigan between April 18, 2012, and April 17, 2013. The plot shows a number of times when the reactor was shut down as well as prolonged periods when the reactor operated at reduced power levels. Although the data set doesn’t provide an explanation, other sources indicate that a broken feedwater pump prevents Fermi Unit 2 from achieving full power.
The plot for Peach Bottom Unit 2 shows the reactor was shut down in September and October 2012 and generally operating at full power otherwise. Again, other sources reveal that a refueling outage explains the downtime in fall 2012. Circumstantial evidence from the plot reinforces this fact – the steady decline in reactor power level from late July 2012 until the reactor was shut down in September is suggestive of a coastdown as the consumption of uranium and plutonium atoms in the nuclear fuel no longer enabled the reactor to achieve 100 percent power.
A future NEAT post will cover the monthly operating reports available in the NRC ADAMS online library that can be consulted to explain why reactors are not operating at full power.
The UCS Nuclear Energy Activist Toolkit (NEAT) is a series of post intended to help citizens understand nuclear technology and the Nuclear Regulatory Commission’s processes for overseeing nuclear plant safety.