How did it begin?

During an overhaul on Doel 3 in 2012, a periodic inspection was performed on certain parts of the reactor vessel. During the inspection a new kind of ultrasound device was used that delivers better, more detailed inspection results. The inspections revealed the presence of certain impurities in the Doel 3 reactor vessel, and later in the Tihange 2 reactor vessel. Further inspection revealed that these were hydrogen flakes, a known metallurgical phenomenon. The hydrogen flakes were incorrectly referred to as 'cracks' in the media.

What are hydrogen flakes?

Hydrogen flakes occur when certain gases are present in the steel during the casting and forging process. Most such gases are eliminated when the forging process is executed correctly. During the forging of the Doel 3 and Tihange 2 reactor vessels 40 years ago, not all hydrogen was eliminated from the steel. It remained in the steel in the form of microscopic flakes. These are like small, flattened bubbles found inside the 20cm-thick steel wall. They are 12 to 16 mm long on average and about as thick as cigarette paper. They do not change over time and run parallel to the inner wall of the reactor vessel. Consequently, they are subject to limited mechanical stresses in the reactor vessel. Hydrogen flakes have no impact on the reactor vessel material.

What are the conclusions?

The discovery of hydrogen flakes led to an inspection that was unprecedented at that time. A team of experts from ENGIE Electrabel, Laborelec and Tractebel Engineering worked together with various well-known external organisations in Belgium and other countries. After tens of thousands of hours of inspection, they came to the conclusion that both Doel 3 and Tihange 2 could be restarted safely. The Federal Agency for Nuclear Control (AFCN/FANC) concluded inter alia that the ultrasonic inspection method used is very reliable, that hydrogen flakes are created during the forging of the reactor vessel, that the hydrogen flakes do not change over time and that the strength of the reactor vessel will remain ensured under all circumstances, both when the power station is in operation and in the event of an incident - all with significant safety tolerances.

 

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