Reactor vessel cracks revealed in Belgium earlier this month spark international inspections

Бельгийская АЭС Doel расположена на берегу Западной Шельды на границе с Нидерландами
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Publish date: August 30, 2012

Written by: Charles Digges

As inspectors investigate possible manufacturing shortcomings in the reactor pressure vessel of the No 3 reactor at the Doel Nuclear Power Plant in Belgium – which earlier this month was discovered to have cracks – nuclear safety authorities in other countries are conduction their own test to root out similar flaws.

The reactor vessel at the Doel No 3 plant was manufactured by the now-bankrupt Dutch firm Rotterdam Drydock Company, which sold a number of such vessels to a variety of international customers. The discovery of the cracks at Doel’s Unit 3 by the use of a new ultrasound measuring technique, sent a nervous ripple through the international nuclear industry.

Rotterdam Drydock Company had sold 21 reactor vessels to nuclear power plants in the US, France, Switzerland, the Netherlands, Germany, Spain, Sweden and the UK, according to the Associated Press, though that information was not released until later in the month.

Flaw rates on INES

The flaws revealed at Doel 3 have provisionally been rated as Level 1 out of seven levels on the International Nuclear Event Scale (INES)  – constituting it as an “anomaly.” FANC may change that rating depending on what further inspections reveal.

Some 9 million people live within a 75 kilometer radius of the plant.

The ultrasound technique that discovered the cracks at Doel’s Unit 3 scanned the whole surface of the reactor rather than just around the weld zones. The initial tests were conducted in June. 

They showed indications that “could be assimilated to potential cracks,” according to World Nuclear News. Additional tests confirmed the presence of these flaws, which are believed to be manufacturing defects in the steel vessel.

The reactor was shut down for further inspections to determine whether the reactor vessel could be salvaged.

Belgium’s Federal Agency for Nuclear Control (FANC) said in a statement that: “Numerous flaw indications in the basic steel material of the reactor vessel were detected in late June, in particular in the bottom-most ring,” adding that, “these are ‘laminar’ flaws parallel with the surface of the walls and, as such, theoretically not dangerous, as they are normally not subject to stress.”

Is it really a manufacturing flaw?

Nils Bøhmer, Bellona’s general director and nuclear physicist, was not as quick as the FANC to blame manufacturing defects in the reactor vessel, and predicted that similar problems will begin to appear worldwide due to ageing nuclear reactors, whose average lifespan is about 30 years.

The wear and tear observed in Belgium, said Bøhmer, “is something that comes of neutron radiation from power generation” within the reactor core that is housed by the vessel.

“This will, as reactors get older, cause cracks,” in reactor vessels said Bøhmer.

Another Belgian reactor, Tihange 2 – whose vessel was also produced by Rotterdam Drydock –  was stopped on August for a maintenance outage and will undergo the same examinations as Doel Unit 3.

The FANC on August 16 called a meeting of nuclear safety authorities of the countries that had purchased reactor vessels from Rotterdam Drydock.

FANC director general Willy De Roovere said the purpose of the meeting was “to give information on the situation at Doel 3 and not to make a decision about its future,” WNN quoted him as saying. “Furthermore, this international contact made it possible to share expertise on reactor vessel integrity and inspections.”

A second meeting of the nuclear safety authorities is planned for October.

International inspections of the Dutch reactor vessels

The meeting has spurred a series of international inspections.

The Spanish Nuclear Safety Council (CSN) is analyzing the documentation and the fabrication process of reactor pressure vessels of the Cofrentes and Garoña plants, which were both manufactured by Rotterdam Drydiock.

CSN said that preliminary investigations show that the vessel of the Cofrentes plant is not affected by the same defects found in Belgium as a different manufacturing process was used. Cofrentes’ vessel was reported to show none of the same defects as the Belgian reactor which CSN attributes to a different welding procedure used on that Vessel.

Garoña’s vessel was constructed using the same process as that for Doel 3, but CSN has not reported defects similar to those discovered at Doel 3.

The Swedish Radiation Safety Authority (SSM) has asked that a host of planned inspections of the vessel at the No 2 reactor at its Ringhals plant be increased to monitor for similar manufacturing defects. Sweden purchased that reactor vessel from Rotterdam Drydock in the1970s, WNN said.

Ringhals 2 will commence its annual refueling and maintenance outage on September 15, said SSM in a release. Plant owner Ringhals AB is to present an action plan to SSM by 1 June 2013 should the same defects be discovered.

The French nuclear safety authority, the Autorité De Sûreté Nucléaire (ASN), said that checks carried out in France showed that no French vessel presents defects corresponding to those found at Doel 3. Swiss nuclear authorities made a similar announcement, but added they will carry out additional ultrasound tests, said WNN.

Are the Rotterdam Drydocks vessels safe for operation?

The 1,006 MW Doel No 3 reactor, operated by GDF Suez unit Electrabel, is scheduled to close in 10 years according to the nuclear exit plan the Belgian government adopted in July. A pressurized water reactor, it went into service in 1982.

The suspected fractures at the reactor, 25 kilometers north of Antwerp near the Dutch border, which provides a sixth of Belgium’s nuclear-generated power, do not pose any health and safety threat, said FANC.

GDF Suez unit Electrabel has pushed for putting Doel 3 back online once the issue of the cracks is addressed. This begs the question of whether continues operation of cracked reactor vessels, which may be discovered in the course of the international inspections, is safe.

Bellona’s Bøhmer said that would be ill-advised without a full rehabilitation of the vessels in question.

“Any cracks in a reactor vessel must be taken seriously and lead to a proper rehabilitation of the vessel before the reactor is put back online,” he said.

Rehabilitation would mean, said Bøhmer, repairing the vessels cracks if possible. Another possible route of restoring power generation, he said, would be to place restrictions on how much energy the affected reactors are allowed to generate, thus reducing the possibility of further cracking.

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