The big problem is that it can be from normal operation, not faulty construction.
"In carrying out tests related to theme 2 during the spring of 2014, a fracture toughness test revealed unexpected results, which suggested that the mechanical properties of the material were more strongly influenced by radiation than experts had expected. As a precaution both reactors were immediately shut down again."
As nuclear reactors age, radiation causes pressure vessel damage, or embrittlement, of the steel mostly as a result of the constant irradiation by neutrons which gradually destroys the metal atom by atom - inducing radioactivity and transmutation into other elements.
Another problem is that hydrogen from cooling water can migrate into reactor vessel cracks. "The phenomenon is like a road in winter where water trickles into tiny cracks, freezes, and expands, breaking up the road", says Greenpeace Belgium energy campaigner Eloi Glorieux.
"Iit appears that hydrogen from the water within the vessel that cools the reactor core is getting inside the steel, reacting, and destroying the pressure vessel from within."
He adds that the findings mean that "the safety of every nuclear reactor on the planet could be significantly compromised ... What we are seeing in Belgium is potentially devastating for nuclear reactors globally due to the increased risk of a catastrophic failure."
