Belgian utility Electrabel has submitted an action plan to the country's nuclear regulator for carrying out additional tests to determine whether two of its reactors can safely be restarted following the discovery of manufacturing defects in their reactor pressure vessels last year.
|Doel (Image: Electrabel)
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. Another Belgian reactor, Tihange 2, was stopped in August for a maintenance outage examinations found similar flaws as Doel 3.
The flaw indications are described as "little fat flakes", with an average width of 10-15 mm and about 1 mm thick. Some 8000 have been detected in the surface of the steel wall of the reactor vessels of Doel 3 and about 2000 in the vessel of Tihange 2. These flaws are thought to originate from the casting and forging process when the vessels were manufactured. Both reactor pressure vessels were produced by the same manufacturer Rotterdam Drydock Company (Rotterdamsche Droogdok Maatschappij, RDM) in the late 1970s. Investigations suggest that these flaw have not worsened or been influenced by the subsequent operation of the units.
Prior to issuing its final decision on whether the reactors can restart, Belgium's Federal Agency for Nuclear Control (FANC) has requested that Electrabel carry out additional tests and provide further information. It has asked Electrabel to perform a so-called "load test" during which the resistance of the reactor vessel is investigated under more severe conditions than normal operating conditions by increasing the pressure in the vessel. After these tests have been conducted, ultrasonic inspections will be carried out to determine whether the flaw indications have evolved. As well as tests on the actual reactor vessels, FANC has requested that tests are also performed on samples of similar steel components that were manufactured for the use in a nuclear power plant but never used to see if they present similar flaw indications. They will also check the mechanical strength of the samples.
Electrabel has now submitted a detailed action plan for conducting these tests to FANC for its approval. The company anticipates completing the tests by the end of March and will then submit the results to FANC.
The regulator said that if the latest tests are conclusive, "we can recommend the government to decided in favour of a restart" of the reactors. The final restart decision, it noted, rests with the government. FANC added, "It is not possible to make any predictions with regard to the timing in this respect."
In a 15 January statement, FANC said that it "sees no elements that would have lead to a permanent shutdown of the nuclear power plants." However, it noted, "Only when having examined all the data, FANC will be able to determine whether the safety margin is not impaired."
Researched and written
by World Nuclear News