Balgium’s Federal Agency for Nuclear Control (FANC) on 14 June published on its website the results of inspections carried out on the Tihange 2 and Doel 3 nuclear power plant. Tihange 2 and Doel 3 were taken offline in 2012 after ultrasound tests suggested the possibility of cracks in their reactor vessels. The further investigation attributed the defects to “hydrogen flakes” in the steel introduced during the manufacturing process. After comparing those studies with more recent tests done in 2014 and again in 2016/17, Fanc concluded that no new flakes had been detected the existing flakes had not increased in size.
“When steel components are manufactured, there is a risk of excess hydrogen building up in the cast steel during the process of cooling and casting. When the hydrogen evaporates, it leaves behind tiny bubbles in the steel. In this case, the bubbles were rolled flat into tiny hairline cracks during the forging process,” Fanc explained.
Following the original discovery of the problem in 2012, Fanc ordered that Doel 3 and Tihange 2 should be shut down and not restarted until owner/operator Electrabel had conclusively proved that the hydrogen flakes did not pose a risk. Fanc allowed Electrabel to restart the units in May 2013 for one operating cycle, after carefully analysing case documents submitted by Electrabel and consulting with various national and international experts. However, Fanc required Electrabel to answer before the end of the cycle some long-term safety concerns and to undertake mechanical testing of irradiated steel components affected by hydrogen flakes.
When this testing produced unexpected results, the units were closed again for further investigations. Fanc once again sought the opinion of eminent international experts and asked Electrabel to conduct another round of ultrasonic tests on the problem areas. The reports of the various external experts were sent to the Fanc over the following months and compared with the conclusions of the Fanc's own experts. Doel 3 and Tihange 2 were then authorised to restart in November 2015. Both units were restarted in December but Electrabel was required to carry out further inspections of reactors vessels during their next refuelling outages and at least every three years after that.
New inspections were performed during refuelling outages last November at Doel 3 and in April this year at Tihange 2. Although these revealed additional flaw indications these were attributed to changes in the inspection conditions.
“Since we have been able to find scientific explanations for all these newly reported hydrogen flakes, or they have been accounted for by signals recorded in previous inspections, the analysis of these results allows us to conclude that no new hydrogen flakes have appeared and that there has been no change in the size of the hydrogen flakes already detected,” Fanc said. The units were restarted in December (Doel-3) and May (Tihange-2). However, Fanc stipulated that a full analysis had to be completed within three months of restarting. In the case of Doel 3, the results of the full analysis confirmed the results of the provisional analysis. The complete analysis results for all hydrogen flaw indications for Tihange 2 is due by September 2017.