Electrabel prepares restart of Doel 3 and Tihange 2

Belgium's Federal Agency for Nuclear Control (AFCN) yesterday announced it had approved the restart of the Doel 3 and Tihange 2 nuclear power plant units. Operator of the plants, Electrabel, said the regulator had concluded that "it is totally safe" to bring the reactors back into operation, following investigations into manufacturing flaws in their pressure vessels.

Electrabel shut down the units in March 2014.

"Shutting down a nuclear power plant as a precautionary measure is a general principle of nuclear safety," the company said yesterday.

On 17 July, Electrabel submitted the first version of its final reports, known as safety cases, to the AFCN. Definitive versions of these reports were submitted on 28 October. These documents "very meticulously set out the findings of the substantiated research carried out by a multidisciplinary team comprising dozens of internal and external experts, both in Belgium and abroad", Electrabel said.

The company listed the conclusions reached after "inspections, tens of thousands of hours of investigative work and more than 1500 material tests" as six main points. Flaws detected in the reactor vessels are hydrogen flakes that were produced during the forging process; they were thus present from the outset, it said. The hydrogen flakes are quasi-laminar, lying parallel to the interior wall of the reactor vessel. "As a result, they are marginally subjected to mechanical stresses and have no negative impact on the reactor vessel's structural integrity," it said. The ultrasonic inspection method was proved to be "perfectly capable of detecting, locating and sizing" each hydrogen flake.

A supplementary inspection in 2014 pinpointed all the hydrogen flakes and showed that they have remained stable and are therefore not "evolving", Electrabel said. Hydrogen flakes present in the reactor vessel's irradiated material have no impact on the "evolution of the fracture toughness of surrounding material", it said. Finally, the structural integrity of the reactor vessel "remains guaranteed under any circumstances, both during normal operation and in the event of an incident", it said.

All the calculations and analyses were based on "highly conservative hypotheses", it said, and the company can "thus confirm that the conclusions bear witness to the vessel's structural integrity, with very wide safety margins".

Following AFCN approval, Electrabel said it has started preparatory work to restart the units. This work will take around four weeks to complete and will be monitored by the supervisory authorities.

After the next fuel cycle the reactor vessels of the units will undergo a follow-up inspection using the qualified ultrasonic method, the company said.

Tihange 2 will reach the end of its 40-year lifetime in 2022 with Doel 3 following in 2023.

Researched and written
by World Nuclear News