Safety checks on reactor vessels

While investigations continue into possible manufacturing defects in the reactor pressure vessel of unit 3 of Belgium's Doel nuclear power plant, nuclear safety authorities in other countries are assessing vessels of plants supplied by the same manufacturer for similar flaws.

A new ultrasound measuring technique was used for the first time in June 2012 over the whole surface of the Doel 3 reactor vessel, rather than just around the weld zones. This showed indications that "could be assimilated to potential cracks." Additional tests confirmed the presence of these flaws, which are believed to be manufacturing defects in the steel vessel.

Belgium's Federal Agency for Nuclear Control (FANC) said, "Numerous flaw indications in the basic steel material of the reactor vessel were detected in late June, in particular in the bottom-most ring." It added, "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."

Another Belgian reactor, Tihange 2, was stopped on 16 August for a maintenance outage and will undergo the same examinations as Doel 3 as its reactor pressure vessel was produced by the same manufacturer Rotterdam Drydock Company (Rotterdamsche Droogdok Maatschappij, RDM).

As many as 21 reactor pressure vessels made by RDM are located around the world. Therefore, FANC organized a meeting of nuclear safety authorities in Brussels on 16 August to discuss the issue. The meeting was attended by experts from the USA, France, Switzerland, the Netherlands, Germany, Spain, Sweden and the UK. Most of these countries have nuclear power plants whose vessels have been manufactured by RDM.

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." The safety authorities attending the meeting were also informed about the additional inspections requested by FANC and its technical subsidiary Bel V. These additional investigations are expected to be completed at the end of September. "Furthermore, this international contact made it possible to share expertise on reactor vessel integrity and inspections," De Roovere said. A second meeting of the nuclear safety authorities is planned to be held in October.

The Belgian government expects FANC to provide a full analysis and explanation of the established phenomenon and the possible risks. In a statement, the government said, "At this stage it is therefore too early to draw any conclusion whatsoever about the Doel 3 and Tihange 2 plants." It added that it cannot currently provide any estimated restart date for the two units. "Obviously, the two plants will not restart until the government has received guarantees from both the regulator and the panel of international experts that the flaw indications pose no risk to the public, workers and the environment." However, it reassured the population that Belgium would not be confronted with blackouts, especially during next winter, even if Doel 3 and Tihange 2 remain offline.

International response

The Swedish Radiation Safety Authority (SSM) has requested that the number of planned inspections of the reactor pressure vessel of Ringhals 2 - which was supplied by RDM in the early 1970s - should be increased to check for any similar manufacturing defects as found at Doel 3. The reactor is due to enter its annual refuelling and maintenance outage on 15 September. Plant owner Ringhals AB is to present an action plan to SSM by 1 June 2013.

Meanwhile, the Spanish Nuclear Safety Council (Consejo de Seguridad Nuclear, CSN) is analyzing the documentation and the fabrication process of reactor pressure vessels of the Cofrentes and Garoña plants, which were both manufactured by RDM. 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. The Cofrentes vessel is composed of semicircular clasps welded vertically, while the Doel 3 vessel is composed of forged rings welded horizontally. However, Garoña's vessel was constructed using the same process as that for Doel 3. CSN noted significant differences between the Garoña and Doel 3 vessels - including the size, thickness, number of forged pieces and the type of reactor. "Therefore the defects from Doel 3 cannot be extrapolated to Garoña," according to CSN.

CSN said that it will continue to review the manufacture of the vessels and the parameters determining the possible appearance of these defects, as well as analyzing the results of inspections conducted in different areas of the vessels.

The Swiss federal nuclear safety inspectorate ENSI said that no indications of manufacturing defects have been detected in the reactor pressure vessel of the Mühleberg nuclear power plant. However, further ultrasound examinations will be conducted to confirm this. The vessel of the Leibstadt plant, while featuring piping supplied by RDM, was made using rolled steel, not forged steel, from Japanese and French suppliers.

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.

The discovery of the flaws at Doel 3 has provisionally been rated as Level 1 on the International Nuclear Event Scale (INES), an 'anomoly'. However, FANC said that this rating may be reassessed when more information and the results of further analyses are available.

Researched and written
by World Nuclear News