More flaws found in Belgian RPVs

18 February 2015

The Belgian nuclear regulator has announced that additional flaws have been found in the reactor pressure vessels of Doel 3 and Tihange 2.

Flaws were first discovered in the walls of RPVs during planned maintenance in the summer of 2012. They were originally detected using an ultrasonic inspection technique designed for checking the vessel welding and the cladding, but it also proved to be able to detect flaw indications in the wall of the vessel.

Electrabel had to submit a safety case to justify the restart of both reactors, demonstrating specifically and convincingly that the flaw indications in the walls of the RPVs do not compromise its structural integrity. After an analysis of these safety cases, the Federal Agency for Nuclear Control and technical support organisation, Bel V, decided in May 2013 that Doel 3 and Tihange 2 could be restarted. But, it stipulated that Elecrabel had to perform a series of 'medium-term' actions to consolidate the hypotheses of its safety case.

As one of the actions, FANC required Electrabel to qualify the inspection technique, to prove that all hydrogen-induced flaw indications can be found and correctly measured using ultrasonic inspection. Electrabel found that the inspection procedure had to be slightly modified and that the detection threshold of the probes had to be lowered to ensure the proper detection of all flaw indications.

FANC announced in February that additional inspections carried out using the improved procedure had resulted in the detection of a greater number of flaw indications than was measured in 2012 and 2013. Electrabel now has to take into account 13,047 flaw indications for Doel 3, up from around 8000, and 3149 flaw indications for Tihange 2, up from 2000, in its safety case calculations. FANC said that the additional flaw indications are "similar to those that were previously considered and are located in the same area of the RPV."

At the same time, FANC said that Electrabel is continuing its research on the material properties of the RPV and the unexpected results of the previous fracture toughness test, which caused both reactors to be taken offline early, in March 2014. FANC said that the fourth irradiation campaign is being executed in the Belgian research reactor BR2, where, next to hydrogen-flaked samples of the French VB395 test material, other hydrogen-flaked samples of another test material of German origin are also being irradiated. The results of this irradiation campaign and of the subsequent material tests are expected by April 2015.

Also in April 2015, FANC will organise a new meeting of the international panel of experts to obtain their advice on the results of the new material tests and on the new data provided by Electrabel.

Last month, FANC said the process potentially to restart the two reactors had been extended from April to 1 July so that Electrabel could answer further questions from the independent panel.

 



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