The art of shutting down a nuclear plant

February the 8th at 1:45 p.m., the nuclear reactor at Mühleberg automatically stopped upon detection of an anomaly. Although not frequent, such events are not exceptional. Gaëtan Girardin, a researcher in nuclear engineering and head of the Crocus reactor at EPFL, will participate on February 22nd in “Science! On tourne”—a series of public dialogues about science. The first lecture is entitled "One Year Later: has Fukushima really changed the game?" He provides us with some techniques to better understand the operation of nuclear plants and, more particularly, the shutdown procedure that was made impossible in Japan.

After Fukushima, people have realized that shutting down a nuclear reactor is not as simple as turning out the lights. Things are a little more complex.
Gaëtan Girardin: That is correct; we need to stop a chain reaction. A nuclear reactor is a tank filled with metal bars about the diameter of a pen that contain uranium. This is the nuclear fuel itself. As it decays, the uranium nucleus releases neutrons, which will collide with its neighbors, and so on ... The heat produced by the fission reactions will be used to produce steam, which then turn an electric turbine. This is common to all power plants. To stop the reactor, that is to say to stop the chain reaction, we must act on the production of neutrons, or capture them.

Specifically, how does one proceed?
Between the fuel rods, or sometimes in place of some of them, we introduce bars called "control rods". In Mühleberg, for example, there are ceramic rods inside metal sheaths, which have the property of absorbing neutrons. In other reactors, the system may vary. But the principle remains the same whatever the technology.

In Fukushima, the reaction had yet been properly stopped... 
And this is not sufficient. We know that once the chain reaction stopped, a reactor still produces about 7% of its operating energy as heat. This may seem trivial, but if you consider the power of a nuclear facility, and the fact that the whole is confined in a small space, there is the potential to seriously damage the materials. We must understand one thing: the heat does not magically disappear. Just as when you turn off an electric stovetop, its temperature does not drop instantly to 20 degrees. We therefore use a water circuit to cool the reactor, especially after it has been stopped. If this is not done correctly, there is a danger that the materials melt: first the fuel rods, then the tank and, finally the concrete containment unit. That is precisely what happened at Fukushima. Following the earthquake, the reactors were shut down correctly, but the tsunami flooded the cooling system and this led to a core meltdown.

At Mühleberg, it was a simple routine check which caused the shutdown.
It appears that the technicians on duty conducted a water test in the wrong place, compared to what was originally planned. I do not know the details of this incident. Maybe this caused a slight change in flow or pressure in one part of the water cooling supply circuits. Whatever the reason, an anomaly was detected by the monitoring system and the reactor was automatically switched off. This is a normal procedure in such circumstances.

Under what circumstances are these systems programmed to shut down?
A reactor is equipped with many security systems that monitor the vital parts of the machine. There are sensitive points, such as the fuel temperature or the flow of water in the cooling circuit. Any damage, however minor, will be detected and analyzed by security systems, and cause a reactor to shut down. It is absolutely essential.

This is not the first time a reactor is shut down automatically. The same phenomenon is quite common.
We should not exaggerate either. The last time in Mühleberg was in 2007—this does not happen every day. In 2011, none of our five reactors were automatically interrupted by the operating system. What strikes me is that these cases are perceived as a problem. In my opinion, it is reassuring that our plants are conceived so that the reactor shuts down by itself at the detection of the slightest abnormality. This shows that safety is never compromised.

Provided by Ecole Polytechnique Federale de Lausanne