According to Wikipedia, who, what, where, when, and why constitute a formula for getting the complete story on a subject. If that subject is an event like the unplanned shut down of a nuclear power reactor, these five questions will get answered. But it can take time to obtain these answers. This post illustrates the process followed in obtaining these answers for a recent unplanned reactor shut down.
At 9:41 am central standard time on January 4, 2013, the operators manually tripped the Unit 2 reactor at the South Texas Project nuclear plant. By regulation, this unplanned reactor trip was reported to the NRC. The NRC included this report in the notifications it posted on January 7, 2013.
This initial notification fully answered four questions about the event and partially answered the fifth:
WHO: The control room operators
WHERE: The South Texas Project Unit 2 reactor
WHAT: Manually tripped the reactor from 100 percent power
WHEN: 9:41 am CST on January 4, 2013
WHY: Two control rods in Shutdown Bank E (control rods D-8 and M-8) unexpectedly dropped into the reactor core as Shutdown Bank C control rods were being inserted during monthly testing
The why question was only partially answered because the reason for the two control rods dropping into the reactor core during testing was not yet known.
Background on this event:
South Texas Project has two pressurized water reactors. In this type of reactor, control rods containing neutron absorbing material are withdrawn vertically from the reactor core to allow the nuclear chain reaction to proceed, and lowered into the core to terminate the chain reaction. The control rods are individually labeled (e.g., D-1, D-2, etc.) and are assigned to different groups (e.g., Shutdown Bank C and Shutdown Bank C).
The control rods are held out of the reactor core by electro-magnets. A control system sends signals to de-energize the magnets and lower the control rods into the reactor core. Failure of the control system or failure of the power supply to the magnets can, and do, cause control rod(s) to drop unexpectedly into the reactor core. Depending on the number and location of the dropped control rod(s), the plant’s operating procedures guide the operators to return the control rod(s) to their initial position(s) or to manually shut down the reactor immediately by lowering the remainder of the control rods into the reactor core.
By regulation, the reactor’s owner submitted a written account – called a Licensee Event Report or LER – to the NRC within 60 days. The LER for this unplanned reactor trip was dated February 28, 2013. It reaffirmed the answers to the four questions answered by the event notification and provided the rest of the answer for the why question. Troubleshooting activities identified high resistance to electrical current flow between contacts of a switch in the control system for the control rods. This resistance blocked a signal that should have caused Shutdown Bank E Control Rods D-8 and M-8 to remain in place. Because this signal was blocked, these two control rods went to their fail-safe position, which was fully inserted within the reactor core. The high resistance was attributed to contamination.
By policy, the NRC’s resident inspectors assigned to the South Texas Project independently assessed the company’s answers to the who, what, where, when, and why questions and documented their review in an inspection report dated May 6, 2013.
These five questions get asked right away when someone dies unexpectedly, an airplane crashes, or an event occurs at a nuclear power plant. Some questions can be answered right away. But the why question generally takes longer. The why question is often a series of related questions like in this event at the South Texas Project. Why did the two control rods drop into the core? Why did switch fail to send the proper signal to the control rods? Why did the signal get blocked?
Often, each question has a number of possible answers. It takes time to identify the correct answer and to exclude the other candidates. Sometimes, identifying right answers and eliminated wrong answers requires sending material offsite for laboratory analysis. And while answering some questions, lab results can also raise others. Like, “how did that get in there?”
The answer to the why question is often the most interesting among the five answers. In that regard, the initial reports on events at nuclear power plants are like the promos for later TV new programs. They tell you who the key stories being covered and when they air, but you have to stay tuned to learn the full stories.
It can take time to get the answer to the why question about a nuclear power plant event. But a slow right answer is preferable to a quick wrong answer.
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.