Thousands of cracks have been found in the steel reactor pressure vessels in nuclear reactors Doel 3 and Tihange 2 in Belgium - vessels contain highly radioactive nuclear fuel cores.

The failure of these components can cause catastrophic nuclear accidents with massive release of radiation.

The pervasive - and entirely unexpected - cracking could be related to corrosion from normal operation, according to leading material scientists Professor Walter Bogaerts and Professor Digby MacDonald.

Speaking on Belgian TV, Professor MacDonald said: "The consequences could be very severe ... like fracturing the pressure vessel, loss of coolant accident. This would be a leak before break scenario, in which case before a fracture of a pipe occurred ... you would see a jet of steam coming out through the insulation.

"My advice is that all reactor operators, under the guidance of the regulatory commissions should be required to do an ultrasonic survey of the pressure vessels. All of them."

Professor Bogaerts added: "If I had to estimate, I would really be surprised if it ... had occurred nowhere else ... I am afraid that the corrosion aspects have been underestimated."

Jan Bens, Director-General of the Belgian nuclear regulator the Federal Agency for Nuclear Control (FANC), has said that this could be a problem for the entire nuclear industry globally - and that the solution is to begin the careful inspection of 430 nuclear power plants worldwide.

An unexplained embrittlement

The problem was discovered in the summer of 2012. Both the Doel 3 and Tihange 2 reactors have been shut down since March 24th, 2014 after additional tests revealed an unexplained advanced embrittlement of the steel of the test sample.

At the time the reactors' operator, Electrabel, dismissed the cracks as being the result of manufacturing problems during construction in the late 1970's in the Netherlands - but provided no supporting evidence.

FANC also stated that the most likely cause was manufacturing - but added that it could be due to other causes. Following the further tests FANC has now issued a statement confirming that the additional 2014 tests revealed 13,047 cracks in Doel 3 and 3,149 in Tihange 2.

"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."

Immediate action needed to prevent another catastrophe

On February 15th the nuclear reactor operator, Electrabel (GDF / Suez parent company) announced that it would be prepared to "sacrifice" one of its reactors to conduct further destructive tests of the reactor pressure vessel in order to study this poorly understood and extremely concerning damage phenomenon.

Electrabel's findings will be submitted to FANC which will organize a new meeting of the international panel of experts to obtain their advice on the results of the new material tests and on the new data.

According to Electrabel, the findings constitute a "Level 1 occurrence on the International Nuclear Event Scale (INES)" but the company emphasises that the event "has no impact whatsoever on the wellbeing or health of the employees, thelocal residents, or the surrounding area."

But Glorieux dismisses such complacency: "As we approach the fourth anniversary of the Fukushima-daiichi nuclear disaster, evidence has emerged that demands immediate action to prevent another catastrophe. Thousands of previously unknown cracks in critical components of two reactors point to a potentially endemic and significant safety problem for reactors globally.

"Nuclear regulators worldwide must require reactor inspections as soon as possible, and no later than the next scheduled maintenance shutdown. If damage is discovered, the reactors must remain shut down until and unless safety and pressure vessel integrity can be guaranteed. Anything less would be insane given the risk of a severe nuclear accident"

There are 435 commercial nuclear reactors worldwide, with an average age of 28.5 years in mid 2014. Of these, 170 reactors (44 percent of the total) have been operating for 30 years or more and 39 reactors have operated for over 40 years. As of 2015, Doel 3 has been operating for 33 years; Tihange 2 for 32 years.

Oliver Tickell edits The Ecologist.