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Half of the nuclear reactors in France are now shut down, some of them due to a corrosion defect in the safety system. Explanations.
The vessel that houses the nuclear reaction is located behind the reactor’s double concrete enclosure. Inside, the water is heated to 330 degrees. It is sent under high pressure in large cylinders. This is where the heat is recovered by another independent circuit, to produce electricity. The radioactive water goes back into the tank to be reheated. This loop is called “the primary circuit”. But it is not totally closed: for safety, emergency circuits are connected to it. In an emergency, this allows water to be injected from a reservoir to cool the reactor.
A year ago, anomalies were detected very close to the connection with the primary circuit, at a place where the pipes form a bend. It was at the welds of this elbow that microcracks were detected. These can measure up to 5 mm. Since the steel of the pipe is 3 cm thick, there is no risk of leakage, but it is impossible to let these cracks grow.
For the experts, the problem comes from the elbow shape itself, which would have generated sufficient temperature differences to fatigue the steel. This defect affects the 16 most recent reactors, whose plans have been adapted from the less powerful reactors. Repair is possible but you have to stop the reactor, replace the cracked part and resolder, while paying attention to the radioactivity. A very technical task, mastered by only a few dozen people in the world.