Hunterson B reopening approved after graphite cracks assessed

Materials World magazine
,
27 Sep 2019

One of Hunterston B’s nuclear reactors has been given permission to resume generating electricity after hundreds of cracks were found in the bricks. Shardell Joseph founds out more.

The Office for Nuclear Regulation (ONR) gave EDF Energy permission to restart Reactor 4 at the North Ayrshire plant for a four-month period, after hundreds of cracks were discovered in the graphite bricks within the reactor cores. EDF Energy is seeking permission for Reactor 3 to reopen.

During inspection, it was predicted that there are more than 350 cracks on the graphite core of Reactor 3 – considered a breach of the operational limit for the plant – resulting in its closure in March 2018. Although an inspection confirmed that Reactor 4 had fewer cracks, due to the lower core burn up, it was closed soon after as it was deemed likely that it would also exceed the safety limit.

Since the closure, EDF Energy has developed a new safety case for Reactor 4, which justifies a further period of operation and increases the operational allowance of keyway route cracks from 350 to 700. The ONR has assessed and approved this safety case, stating that they were satisfied that the reactor could be safely shut down, including during a one in 10,000-year seismic event, allowing the return of service of Reactor 4 on 20 August 2019.

The cracking phenomenon

Nuclear technologies tend to be regional and the UK has a long history of using graphite as a moderator. According to ONR, they are now the only fleet of advanced gas-cooled reactors (AGR) in the world to have a graphite-moderated, CO2 cooled core.

The function of the graphite moderator slows down the speed of neutrons produced during nuclear fission, helping to sustain the chain reaction, allowing the heat to be used for electricity production. The reactor’s core is constructed of thousands of interlocking graphite bricks forming channels that contain nuclear fuel. The reactor control rods' function is to help control the nuclear reaction and they are the primary shut down mechanism for the reactor. For this reason their function must not be challenged, even in the event of major seismic activity in the areas surrounding the plant. 

Using graphite comes with problems that can compromise the safety of the reactor. ‘Over time during reactor operation, the graphite bricks age and their properties change due to the effects of radiation and the reactor coolant,’ ONR Superintending Inspector, Ian Bramwell, told Materials World.

‘These changes in material properties include the modulus, coefficient of thermal expansion, dimensional change, reduction in density through oxidation and changes in strength as a consequence of oxidation.’

According to Bramwell, the change in dimensions causes the graphite brick components to suffer internal stresses. ‘The mechanism of dimensional change in graphite is not fully understood but is related to rearrangement of the atomic structure and accommodation of those strains by the microstructure which brings about a macro change in the dimensions of graphite components,’ he added.

‘In the AGRs the highest stresses develop in components experiencing the greatest fluence – the graphite fuel bricks. Together with stress raising keyways machined in the fuel bricks, reduced strength as a result of oxidation and thermally induced stresses from changes in the coefficient of thermal expansion, means the fuel bricks are susceptible to cracking.’

Mentioned as a concern in the ONR report, the keyway root cracking of fuel bricks can distort channels within the graphite core that has the ability to undermine the fundamental safety requirements, which include:

  • Unimpeded insertion of control rods and unimpeded movement of fuel
  • Ensuring that gas flow will remain adequate to ensure adequate cooling of the fuel and core, and
  • That appropriate moderation – slowing neutrons to sustain the nuclear chain reaction – and thermal inertia – reducing the speed of temperature changes – are maintained.

Assessing the claim

To determine the safety of returning Reactor 4 to service, ONR was required to assess the claims made by EDF Energy, carried through by specialist inspectors with a high level of knowledge regarding graphite behaviour and the methods used. One of the claims stated that graphite core degradation does not undermine the reliability of the shut down system during normal operation and plant faults, nor does it prevent the graphite core from meeting the fundamental safety requirements.

The report affirmed that from a civil engineering perspective, the most significant risks addressed by the case relate to the justification that core distortion will not prevent successful insertion of the control rods during and following a seismic event. This justification is based on the revised seismic modelling of the pre-stressed concrete pressure vessel.

Working with Bristol University, and other leading consultants and expert academics, EDF Energy has completed what it claims to be the most extensive investigation of the reactor ever undertaken.

‘As a result we have demonstrated that even in the most extreme conditions our reactors operate within large safety margins,’ an EDF Energy spokesperson told Materials World. ‘In particular, all control rods would operate as they are designed to do and will safely shut down the reactor in all circumstances.’