Safety-relevant damage in the safety injection systems of French nuclear power plants
Ultrasonic examinations revealed indications in the safety injection system that were attributed to cracks.
Purpose and function of the safety injection system
It is connected in several places to the loops of the primary system, through which the water heated in the reactor core is fed into the steam generator and then cooled and returned to the reactor core.
The safety injection system is used to control accidents in which coolant (water) escapes from the primary system due to a leak. This accident is technically referred to as a Loss of Coolant Accident (LOCA). In such a case, water has to be injected into the primary system by means of the safety injection systems. This is necessary to supplement the loss of coolant via the leak and to ensure the cooling of the reactor core. Without this system, the so-called residual heat could otherwise not be removed, even after the reactor has been shut down, and this could lead to core damage and even core meltdown.
Safety relevance of the damage
If the cracks reach a certain size during ongoing operation, a leak or rupture of one of the affected lines may occur - especially if there is an additional mechanical load. Since there is a direct connection between the location of the crack indications and the primary system, a loss-of-coolant accident would then occur.
Scenarios are conceivable in which mechanical loads act simultaneously on all affected lines of the safety injection system, e.g. in the event of a severe earthquake. If, in such a case, several of these lines were to rupture simultaneously, core damage up to and including core meltdown could only be prevented if further emergency measures for reactor cooling could be successfully implemented.
Findings to date on the nature and cause of the damage
The pipe damage found are cracks in circumferential direction, i.e. they run transverse to the pipe direction. They are located in the base material next to weld seams. The operator has cut out affected pipe sections in order to have the cause of the damage determined by a laboratory. According to initial metallographic and microscopic examinations, the damage is due to intergranular stress corrosion cracking.
Stress corrosion cracking typically only occurs when three adverse conditions come together: high mechanical stresses, aggressive ambient conditions, and a material that is sensitive under these conditions. The exact cause of stress corrosion cracking can only be speculated at this point in time. The material concerned, a non-stabilised chromium-nickel steel, is, according to previous experience, not very sensitive to intergranular stress corrosion cracking under normal chemical conditions, i.e. practically oxygen-free hot water, in the primary system.
Effects to date and measures taken
When the damage found at the Civaux-1 plant became known, the operator also took Unit 2 off the grid for investigations; the Chooz B-1 and B-2 reactors, which are of an identical design, were also shut down - this means that all N4 reactor units, the most powerful French reactors with a gross rating of 1560 MW, are off the grid.
In addition, cracks were also found during overall maintenance inspection work in the safety injection system at the Penly nuclear power plant. This is a P'4-type reactor; however, the crack indications found there are not as pronounced as those in the N4 reactors. At the present time, it cannot be ruled out that further plants of this type will have to be examined in the short term. In France, a total of twelve of these reactor units are operated at five different locations.
EDF is currently still carrying out in-depth investigations on the cut-out pipeline sections. At the same time, the manufacturing documents are being evaluated, operating conditions are being analysed, and tests carried out so far are being re-examined for any indications. The investigations, the repair work and the inspections by the supervisory authorities and expert organisations will in any case take several months.