Lighting the way to a low-carbon UK26 April 2018
Philip Awty describes the graphite core inspection work that takes place at the UK’s advanced cooled gas reactors and the vital contribution it makes to supplying low-carbon electricity.
Cavendish Nuclear recently signed a five-year graphite contract to support the continued operation of EDF Energy’s fleet of advanced gas-cooled reactors in the UK. The nationwide agreement replaces a series of local contracts for the inspections.
EDF owns and operates seven advanced gas-cooled reactor (AGR) power stations, which are most of the UK’s current nuclear capacity. Each station has two reactors that are routinely shut down for refuelling and maintenance. During the shutdown period, essential maintenance and reactor core inspections are undertaken.
Each reactor core is 10m high, made up of around 6000 graphite blocks and weighs the equivalent of more than 100 double decker buses.
During normal reactor operation the structure of the graphite blocks will change; some cracking will arise in some of the blocks as they age or lose matter. The condition of the graphite is monitored and at a certain stage it will determine the end of generation. The reactor core inspections are crucial in maintaining the industry regulator’s confidence in the material state and the continued operation of the AGR stations.
An AGR statutory inspection programme requires EDF Energy to monitor and maintain detailed records of the geometric position and physical condition of the graphite bricks in the core.
There are two main types of inspection:
- Channel bore measurement (diameter and tilt) and visual inspection,
- Trepanning samples of graphite for analysis.
Cavendish Nuclear originally became involved in the inspection of AGR core graphite during the 1970s when the channel bore measurement units (CBMU) were designed, manufactured and tested at Whetstone. The CBMUs operated safely and efficiently, requiring only maintenance and periodic overhauls, until 2005.
Since 1999 Cavendish Nuclear has provided EDF Energy with dedicated teams of suitably qualified and experienced personnel to maintain and operate the core monitoring equipment systems and deployment hoists at all seven AGR sites in the UK.
Since 2000 Cavendish Nuclear has been involved in a continuous programme of design, development, prototype testing and modifications of EDF Energy’s fleet of inspection tools and associated deployment hoists, whilst making the equipment available for use each year.
Previously, the service was delivered on an ad-hoc basis, with contracts from the seven individual stations frequently issued “just-in-time”. This deterred long-term investment in skills, planning and equipment, leading to regular equipment breakdowns that could extend the outage.
A new approach
The embedded traditional thinking (of 15+ years) was challenged by the new contract. Cavendish Nuclear and EDF Energy agreed new plant and station requirements for the inspections.
The requirements identified improvements on the traditional methods, benefiting all involved. This included:
- Reducing the ‘roving’ element of teams’ work to improve station ‘ownership’. This reduces employee travelling requirements, changes people’s behaviour, improves equipment readiness, optimises resource utilisation and allows opportunistic reactor inspection capability.
- Changing from an outage-focused delivery to a five-year budget uniformly invoiced over 64 months, which gives EDF Energy improved cash flow, reduces overheads, increases the focus on operational activities and allows investment in plant and equipment across all seven station sites.
- Changing from single outage planning to a jointly developed annual programme with bi-weekly 26-week look-ahead programme briefs. This allows consolidation and alignment of maintenance and campaigns across all seven stations, improves station planning and resource optimisation, and leads to a reduction in non-value-added activities.
By committing to increased multi-year contract durations Cavendish Nuclear can support and plan long-term operations and improve resource, tools, training and equipment across the sites. This has not only changed how the team operates but had a direct effect on behaviours and attitudes for all those involved.
This contract has eliminated the inefficiencies inherent with transactional multi-site funding, focusing on a single delivery plan across all stations to improve resilience, flexibility and equipment reliability.
Previously, site inspections were undertaken by a team of seven, moving between the seven sites. The new strategy, piloted during the last 12 months, has increased the available personnel to 17, with two specialists at each station supported by three mobile team leaders.
The combination of site-based and mobile personnel improves the efficiency and safety of the teams by utilising the station contacts, to reduce the number of people who need to transfer between the sites. The three mobile team leaders serve to ensure continuity and transfer of knowledge between the sites.
The sustainability of these teams important. By moving away from a site-based, outage-by-outage set of contracts to a five-year planning cycle across the AGR fleet, Cavendish Nuclear can invest in succession planning and train additional personnel to meet an increased repair and inspection requirement. This provides assurance about the long-term sustainability of a pool of suitably qualified staff capable of delivering the service.
As part of the strategy, the team has improved the equipment maintenance programme, using an equipment tracker to trend faults, for configuration control and condition monitoring. This ensures the equipment is maintained and exercised on a regular basis and there is an accurate record of the its status at all times, even when an outage is not planned. This provides assurance that the equipment remains operational and minimises disruption when it is used during an outage. It also provides the agility to deliver minor repairs or inspections at short notice during unplanned outages.
Planning for outages is also improved. Previously, planning and checks would take place nine weeks before the outage. This now takes place 18 weeks in advance, followed by further checks two weeks before the equipment is due to be used. This has helped resolve any arising issues or faults and ensured the outage and inspection is delivered effectively and to time.
In 2005, Cavendish Nuclear designed and manufactured a new system to increase the operating range and improve reliability and accuracy of graphite measurements. The Mk2 New In-Core Inspection Equipment or NICIE is used to inspect and measure the fuel channels when the reactor is depressurised and in either an air or carbon dioxide environment.
The NICIE tool is used to take bore measurements and tilt readings of the fuel channels. It also has a camera that is capable of inspecting both the bottom and sides of the fuel channels.
The channel data recorded by the console is processed by EDF Energy and submitted to the graphite assessment panel as a report detailing the status of the core.
The Mk2 NICIE Tool has 360-degree rotation, a retractable wheel assembly, solid state tilt measurement, and down-channel and side-viewing cameras for performing visual inspection.
The tool is deployed from the Mk2 NICIE hoist once it is coupled to the reactor; its orientation is determined by an array of LEDs on top of the tool, which is seen by a down channel camera located at the top of the channel.
The Hybrid Trepanning Tool Unit (HTTU) is used to take physical samples of the graphite core when a reactor is depressurised and in an air environment. The HTTU, designed by Cavendish Nuclear, allows EDF Energy to confirm the stability of its graphite cores and therefore extend the lifetime of the AGRs.
The HTTU houses a trepanning tool cutter and the supplies, services and mechanisms required to support its various operations. The cutter is a thin-walled hollow tube with diamond tip, which can take graphite samples 19mm in diameter and 75mm long for inspecting the channel block’s properties. The HTTU also houses a camera for channel wall and trepanned hole inspections. Again, an LED array is used to determine the orientation of the tool.
The new contract and associated changes to the way graphite inspections are delivered across the AGR fleet has secured a more efficient, sustainable, flexible and resilient service. This is crucial in maintaining the continued operation of the AGR stations and keeping the UK supplied with low-carbon energy.
Sue Walker, who manages EDF Energy’s graphite team said: “Long term partnerships like this are an essential part of ensuring that EDF Energy’s nuclear stations continue to provide secure, affordable low carbon electricity for the UK. Historically, this [graphite inspection] service was delivered on an outage-by-outage basis under contracts with individual stations, which deterred long-term investment in skills, planning and equipment. This new contract delivers reduced cost and overheads compared to managing multiple small complex contracts across the fleet to cover planned work and emergent repairs.”
Philip Awty is Project Manager at Cavendish Nuclear