Nuclear flaws raise safety questions

Materials World magazine
,
1 Apr 2015

Defects found in two Belgian nuclear reactors have challenged ongoing operations. Simon Frost asks, what are the implications for the UK’s nuclear fleet?

Plans to reopen two offline nuclear power plants in Belgium hang in the balance after recent ultrasonic inspections found thousands of flaw indications in the steel reactor pressure vessels (RPVs).   

Doel 3 and Tihange 2, pressurised water reactors (PWRs) operated by Belgian energy company Electrabel, were shut down in March 2014 after inspections revealed embrittlement of steel test samples. But the country’s nuclear regulator, the Federal Agency for Nuclear Control (FANC), stated the facilities could resume operation in July 2015 if Electrabel could produce a Safety Case to justify the restart, using the most sensitive ultrasonic inspection techniques available alongside best-practice structural integrity methods.      

Those inspections revealed a 60% increase in the number of cracks measured in 2012 – a total of 13,047 cracks in the Doel 3 reactor and 3,149 in Tihange 2, and larger hydrogen flakes on average than previously measured. Tests and analysis of results are ongoing. 

FANC has stressed that the new flaw indications are in the same areas as those previously detected and that the greater number is due to the increased sensitivity of the MIS-B acoustic beam focusing technique. Likewise, it noted that the size of flakes is greater because separate flakes in close proximity were treated as one. 

A statement from Electrabel claims that ’the same indications are located in the same place with the same dimensions. Therefore, the hydrogen flakes formed during the forging process of the reactor vessel 40 years ago and detected in 2012 haven’t evolved.’ But given the huge improvement in inspection methods since the Belgian reactors’ commissioning in 1975, it’s hard to prove or disprove when the flaws originated.     

Question of origin

Professor Andrew Sherry, Chief Scientist at the UK’s National Nuclear Laboratory (NNL), says, ‘One possibility is that the flaws originated during manufacture and have sat there quite benignly throughout the operational life of the reactor. The alternative is that they have been developing during the lifetime of the reactor. Those scenarios have very different implications – if the flaws are developing during the operational life, you need to know how the materials’ properties are changing and at what point those defects may become an issue.’ 

Electrabel is insistent that the new flaw indications are a result of the increased sensitivity of analytical parameters, rather than deterioration. But the company has been criticised for an apparent lack of transparency, having not released the raw measurement data from the most recent tests. 

In a debate aired by Belgian national television broadcaster La Une, politician and environmentalist Jean-Marc Nollet leveled an accusation at Electrabel’s Chief Nuclear Engineer, Wim De Clercq. ‘You shut down the plants without even waiting for the order of the FANC – that’s how dangerous it was. Is that what prevents you from publishing such results?’ De Clercq replied that the results were yet to be correlated.  

Incredibility of failure

‘An RPV is what’s known as an incredibility of failure component,’ says Sherry. ‘In other words, it cannot be allowed to fail – a failure probability of 10-7 per reactor year of operation must be demonstrated.’ The RPV contains coolant and highly radioactive fuel cores, so defects of a critical size can lead to serious nuclear accidents. 

In an interview with La Une, Digby MacDonald, Professor of Materials Science and Engineering at the University of California, USA, advised that all reactor operators, under the guidance of regulatory commissions, should be required to immediately carry out ultrasonic surveys of all pressure vessels, echoing a call to action made by the Director-General of FANC, Jan Bens. 

A different breed

This would not greatly affect the UK’s nuclear facilities, as Dr David Armstrong, Royal Academy of Engineering Research Fellow at the University of Oxford, points out. ‘The majority of the UK’s reactor fleet are not PWRs but advanced gas-cooled reactors (AGRs), which have a concrete pressure vessel rather than the steel ones found in these RPVs in Belgium’. 

The only PWR in the UK is Sizewell B, in Suffolk, which was built and commissioned between 1987–1995. As Sherry explains, ‘The reactors in Belgium were commissioned in 1975, so they are from a different manufacturing vintage, too.’ Unscheduled shutdowns at Sizewell B have never been related to faults in the RPV.   

AGRs are not immune to cracking, though – in the past year, these have included findings at Hunterston B, in North Ayrshire, and Dungeness B, in Kent. But, Sherry notes, ‘Once indications of flaws are discovered, they have to be justified – for continual operation, the severity of flaws has to be assessed via structural integrity or fracture mechanics techniques, which are better developed in the nuclear industry than in any other. Those assessments have to be checked by an independent assessor and then the regulator. They call it a “triple-lock” – the safety management system in the nuclear industry is an extremely robust regime.’ 

The entire fleet of seven AGRs in the UK is scheduled to be retired over the next 10–15 years, and studies into materials for the next generation of nuclear plants are well underway. ‘The difficulty in improving the understanding of how radiation damage affects mechanical behaviour is that it is expensive and time-consuming to work with irradiated materials,’ says Armstrong. ‘The pressing need is to work with neutron-irradiated materials rather than ion-irradiation. New facilities such as the Materials Research Facility at the Culham Centre for Fusion Energy, Oxfordshire, will enable easier access to the equipment required for this, and accelerate our understanding of both the ageing of current reactor metals and the development of newer, more irradiation-resistant materials.’
As Sherry concludes, ‘Like all engineering sectors, the nuclear industry always looks to continually improve what it does.’

To read the full interviews with Andrew Sherry and David Armstrong, download the Materials World app