Scientists Get In-Person Demonstration of Station Blackout
Aug. 15, 2013—Three senior reactor operators this week presented to a National Academy of Sciences panel a table-top simulation of a station blackout scenario, similar to what happened at Fukushima Daiichi after the March 2011 earthquake and tsunami.
The exercise—and an accompanying video of a similar drill in a station simulator—were designed to give the panel a real-time experience of how decisions are made in the control room of a U.S. nuclear energy facility as it recovers from an event entailing the loss of all off-site power and emergency diesel generators.
The NAS is continuing an ongoing technical study of the Fukushima accident.
NAS panelists listened intently as the reactor operators demonstrated their roles during the exercise, giving and following commands that would be used in a control room.
Three reactor operators are the minimum staffing contingent in a control room at any time. The scenario was based on current operating procedures and equipment and did not include post-Fukushima enhancements like the FLEX strategy.
The script began with a report from Glen Morrow, regulatory assurance manager and senior reactor operator at Exelon Generation’s Dresden Nuclear Power Station, in his role as a reactor operator.
“Attention for update,” Morrow began his report. “Reactor scram. Reactor mode switch is in shutdown. Reactor water level is 145 inches and lowering slowly. Reactor pressure is 1,100 pounds [per square inch] cycling on SRVs [safety relief valves] … all control rods are fully inserted. End of update.”
Philip Amway, Fukushima response technical lead and a former senior reactor operator with Constellation Energy Nuclear Group, in his role acknowledged the report and began issuing commands to Morrow and to Derwood Tootle, severe accident management project manager and senior reactor operator at Southern Nuclear’s Hatch nuclear energy facility, who played the role of the third reactor operator.
Amway explained each step in the script, peeling away the jargon to explain what actions were taken. Amway emphasized that the basic goals were to preserve backup electrical power, control reactor pressure and maintain containment.
“We always talk in terms of power, pressure level and containment,” said Amway. “And in an event like this … those are your focus items, and as long as you respond to those, you are going to be able to stabilize the plant.”
Amway added that after the site loses power and the reactor automatically shuts down, the shift manager would assign and prioritize all actions based on emergency operating procedure (EOP) flowcharts. These are on the control room walls but have been committed to memory by the operators. The flowcharts have well-defined situation entry and exit points that reactor operators follow in certain situations.
“As soon as you meet any of those entry conditions, you enter this flowchart at the top, you execute, you drop down through your steps,” Amway said. Copies of the flowcharts were also presented to the NAS panel.
One of the questions from the panel in the question-and-answer session that followed the demonstration was about the guidance that controls venting to reduce containment pressure.
“All the authority to do that venting remains with the shift manager. No outside authority is required to do that,” said Morrow. “All that guidance is proceduralized.”
The NAS panelists also wanted to know what enhancements in procedures were being considered post-Fukushima.
“We are looking at changes to our severe accident management guidelines (SAMGs) and EOPs to incorporate things like beyond-design-basis capability and containment venting capability so we can prolong reactor core isolation cooling operation and not damage it due to overheating,” Amway said.
Tootle added that work was continuing on integrating the FLEX strategy with existing procedures.
FLEX is a strategy initiated by U.S. nuclear energy facilities that calls for additional portable emergency equipment, such as diesel generators, pumps and hoses, to be positioned at strategic locations around a plant site and at regional depots in Tennessee and Arizona.