Further flaws found in Belgian reactor vessels

Additional inspections at Electrabel's Doel 3 and Tihange 2 power reactors have revealed more extensive flaking within their reactor pressure vessels (RPVs) than previously identified, the Belgian regulator has announced.

The Federal Agency for Nuclear Control (FANC) said that Electrabel - a subsidiary of GDF Suez - has now completed ultrasonic inspection of the units' RPVs. The company used a technique originally developed for checking the welding and cladding of the RPV. However, Electrabel slightly modified it to check for flaws in the wall of the RPV itself. For this, "the detection threshold of the probes had to be lowered to ensure the proper detection of all flaw indications," FANC said.

An inspection carried out by Electrabel in 2014 - based on the improved procedure and the modified settings of the machine - resulted in the detection of a greater number of flaw indications than were measured in 2012 and 2013.

FANC said 13,047 flaw indications have now been found at in the RPV of Doel 3 and 3149 in that of Tihange 2. Previously, some 8000 flaw indications had been identified at Doel 3 and about 2000 at Tihange 2. These additional flaw indications, it said, "are similar to those which were previously considered and are located in the same area of the RPV".

Electrabel said, "The number of defects due to hydrogen is not in itself critical to assess the structural integrity of the vessels. We must also consider their position, their size, their inclination and the possible interaction between them." It added, "The analyses in this regard are ongoing."

The company has continued its research on the material properties of the RPV. A fourth irradiation campaign is currently underway at the BR2 research reactor using hydrogen-flaked samples of test material from France and Germany. The results of these tests are expected by April.

Belgium's nuclear safety authorities - FANC and Bel V - convened an international expert panel in early November to discuss Electrabel's tests. That meeting concluded the methodology used "was not yet sufficiently developed to make a well-grounded judgement". The panel suggested further actions and studies.

FANC said that, based on those suggestions and on documents already analysed, it and Bel V have passed a series of additional demands and suggestions to Electrabel so that it can "adjust its methodology and validate the underlying hypotheses of its arguments." The expert panel is scheduled to meet again in April to discuss the results of the new material tests and on the new data provided by Electrabel.

Doel 3 and Tihange 2 were taken offline in 2012 when ultrasound testing suggested the possible presence of cracks in their reactor vessels. Further investigations indicated that the defects are so-called hydrogen 'flakes' and were introduced during the manufacturing process. FANC allowed Electrabel to restart the units in May 2013, requesting that further tests be conducted to evaluate the effect over time of these flakes. The units were again taken offline last March after Electrabel reported that one set of tests to investigate the mechanical strength of irradiated specimens of similar material "did not deliver results in line with experts' expectations".

FANC and Bel V have requested that Electrabel submits a safety case to justify the restart of both reactors. FANC said the company must "demonstrate specifically and convincingly in its safety case that the flaw indications in the walls of the RPVs do not compromise its structural integrity".

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.

Researched and written
by World Nuclear News