Repair welds detected in OL3 main coolant pipes have no safety significance

02/08/2010

According to the additional surveys and tests required by the Radiation and Nuclear Safety Authority Finland, the main coolant pipes of the reactor loop in the nuclear power plant being constructed in Olkiluoto comply with the requirements. Additional surveys were required due to certain shortcomings detected in the activities of the pipe manufacturer in autumn 2009.

The French subcontractor hired by the plant supplier Areva to manufacture the reactor circulating pipes did not document the repair welding of minor depressions and scratches generated on the pipe surfaces during various work phases as stipulated in the requirements for components critical to the safety of a nuclear plant. The repair welding was detected in conjunction with a regular quality control inspection carried out by Areva and TVO in autumn 2009. Areva interrupted the subcontractor's work immediately. The necessary additional surveys were initiated after STUK was informed of the case by TVO.

All repair welds of the main coolant pipes were analysed and documented thoroughly. It was detected in the survey that the repair welds were superficial, approximately 0 - 5 mm in depth, and located in areas where the load of the pipes is low. Most welds were found to be of good quality, and they could be left as they were. Some welds were slightly processed, but no new repair welds were needed. According to STUK's estimates, the repair welds do not compromise the integrity of the main coolant pipes with thick walls (70 - 100 mm) during plant operation.

Because the incident revealed significant shortcomings in the subcontractor's work, STUK also wanted reassurance of the quality of the load-bearing nozzle welds implemented by the same subcontractor. For this purpose, a large amount of assembly weld X-ray films were reviewed, and a number of other non-destructive retests were carried out under STUK's supervision. As the result of the tests, it was stated that the assembly welds and the related documentation complied with requirements. The final inspections of the pipes are still ongoing at the site of manufacture. Main part of them have been approved and transported to Olkiluoto for installation in the Olkiluoto 3 plant.

The same subcontractor has also manufactured some components of a lower safety classification for the Olkiluoto 3 plant, and TVO has, upon STUK's request, also reviewed the compliance of these components. In addition, TVO has issued a report on developing the control and quality assurance of other subcontractors in order to prevent similar incidents from occurring in the future.

Additional information:
Assistant Director, Martti Vilpas, tel. +358 9 759 88 655
Information Officer, Risto Isaksson, tel. +358 9 759 88 208

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