Preparing for the Wylfa closure

Materials World magazine
,
3 Oct 2015

Ahead of the Wylfa nuclear power plant closure later this year, Natalie Daniels looks at the decommissioning process and the roadmap for the future.

In the UK, there are currently 16 nuclear reactors, generating around 18% of the country’s electricity. In September 2014, Magnox Limited received approval to extend the life of its Wylfa 1 nuclear reactor plant by 15 months, until December 2015, when the de-fuelling of Britain's oldest nuclear power reactor will begin.  

Wylfa is the last Magnox power station in the UK, located on the north coast of the island of Anglesey, Wales. Wylfa Reactor 1 became operational in 1971 and has since generated 232TWh. The site stands across 20.8 hectares and currently has only one running reactor and two operational turbo-generators, with an output of 437MW, which supplies more than 20% of Wales’ electricity. The Magnox site consists of pressurised reactors fuelled by natural and low-enriched uranium, cooled by carbon dioxide and a graphite moderator. At the time of construction, Magnox reactors were considered extremely safe due to their simple design, low power density and their use of a gas coolant – and it’s thanks to this that Wylfa has exceeded its predicted lifetime. 

Performance beyond expectations

The number of lifetime extensions has come as a blessing for the UK energy supply, Nuclear Decommissioning Authority (NDA), and the people of Anglesey. In the first years of its operation, the Nuclear Installations Inspectorate estimated Wylfa’s design life to be between 20–25 years – it is now more than 40 years old. The station’s lifetime was first predicted based on Wylfa’s high gas coolant pressure, noting concerns that it could cause more damage to internal steel and graphite components than in other Magnox stations. 

After demonstrating to the Office for Nuclear Regulation (ONR), that the site could still meet stringent safety requirements, Reactor 1 has continued to operate since 2013, taking partially used fuel from Reactor 2 – a process known as inter-reactor exchange of fuel. ‘Since its lifetime extension was granted, the site has delivered additional revenue for the UK taxpayer of up to £785 million since the plant’s original closure date in 2010,’ said Jonathan Jenkin, Stakeholder Relations Manager at the NDA. 

With its closure imminent, the NDA team is preparing for decommissioning. Nigel Monckton, Head of Communications at Magnox said, ‘We will be using the principles of “lead and learn” to ensure that lessons we have learnt from other sites are transferred to Wylfa. Although every site is different, the core challenges are the same.’ 

The Wylfa reactor core contains around 50,000 fuel elements. Each of these has to be individually removed using a defuelling machine and transferred to the dry fuel store for around six months, which allows radioactive decay to take place and will reduces heat output. They are then transferred to fuel flasks and transported to Sellafield by rail. Monckton adds, ‘The basic principle is to defuel the reactors, recover, passivate and store any operating wastes. Then we will remove any non-essential facilities such as turbine halls and seal the reactor building for some decades prior to returning to clear the site. This decommissioning is known as safestore – described by EDF Energy as when any high-level radioactive waste that can be removed is made safe elsewhere. The defueled power station is monitored for 40–80 years while any remaining radioactivity decays to a safe level. This makes decontamination and dismantling simpler and safer.’ 

The Wylfa site will be secured by the NDA and the process, according to Magnox, will comprise the following steps:

Radioactive and non-radioactive contamination will be reduced to meet the requirements of the regulatory regime for the next planned use of the site and the current use of the adjacent land. 

Where the next planned use no longer requires a nuclear site licence, radioactive contamination will be reduced to meet the criteria for de-licensing, with any remaining radioactive substances being subject to the relevant environmental permitting regime.

The physical state of designated land will be made suitable for the next planned use of the site – structures and infrastructure will be made safe or removed where necessary, having first explored opportunities for their re-use.

Not all plain sailing

To do this, the team faces a number of hurdles. According to Monckton, the team’s first challenge is ‘to safely maximise the electricity output of the remaining fuel up to the end of generation this year.’ They will then undertake the defuelling of the reactors. ‘This is a challenge in itself as, although the process is similar to defuelling during operations, it is a more intensive process, albeit one we have managed effectively and safely at other sites.’

The scale of the project is also a challenge. The ancillary buildings are large structures containing a considerable amount of equipment, such as turbines, which have to be safely disconnected and dismantled to allow the demolition of the buildings. ‘Ultimately, one of the biggest challenges will be segregating and dealing with all the waste arising from decommissioning activities.’ To do this, as much waste as possible will be categorised and disposed of as free-release waste, then a smaller volume as very low level waste (VLLW) or low level waste (LLW). Any intermediate level waste (ILW) will be safely stored in appropriate facilities until the UK National Repository is constructed. As Monckton explains, ‘There are also concerns about other hazardous non-radioactive materials, such as asbestos, which reflect the age in which the facilities were built.’ 

Learning from the past

Decommissioning and de-fuelling a nuclear plant requires a different approach to that used 20 years ago. Monckton notes, ‘Previously there would have been a more bespoke approach at each site – the use of programmes ensures more consistent and effective solutions to similar decommissioning challenges across the piece. As of April 2015, approximately 90 commercial power reactors, 45 experimental or prototype power reactors, more than 250 research reactors and around 90 fuel cycle facilities, have been retired from operation.

The NDA's funding settlement in future financial years will be confirmed this Autumn and Magnox is currently updating its site plans.

The key difference now is the technology involved, in particular at Wylfa. ‘We have a dry fuel route – the fuel is not immersed in water during storage. This means we have a dry store to decommission rather than a fuel storage pond. However, even in these circumstances we will be drawing on the lessons we have already learned in decontaminating and decommissioning major concrete structures.’ 

As the site clearance approaches, the process is scheduled to be complete by 2026. To some, this closure marks the end of an era for the UK nuclear industry but as Monckon explains, ‘There is nothing dramatic about this closure, it is just the end of a long process.’