Advanced gas-cooled reactors (AGRs), such as those at Hunterston B, which use carbon dioxide (CO2) as a coolant for their graphite neutron moderators, depend on the reliable delivery of stored CO2 to meet their requirements, especially in high-demand situations.

As a result of the life extension of the Hunterston B power station to 2023, EDF Energy wanted to develop a new CO2 storage and delivery system to meet this coolant supply need. To help ensure this system would adhere to revised and new safety and functional requirements they commissioned my company, Frazer-Nash Consultancy, to undertake the front end engineering design (FEED). Bringing together an integrated, multidisciplinary design and engineering team, we worked collaboratively with EDF Energy’s engineering and operations team to develop an optimised CO2 system design. This article outlines some of the benefits that this working approach delivered to the project. 

The project

With a challenging timescale and the additional complexity that arose from extensions to scope such as incorporating the auxiliary steam supply for the vaporisers into the design, the CO2 system was never likely to be a straightforward project. However, the complementary nature of EDF Energy’s Architect Engineering approach, and Frazer- Nash’s Systems Approach, meant that we were able to work together as a collaborative team to go beyond the original brief.

Why did the two approaches work so well together? EDF Energy’s Architect Engineering approach provides a framework
for the integration of design and safety case requirements to achieve engineering solutions for investment related work, in a consistent manner. Frazer-Nash’s Systems Approach applies a combination of analysis, human factors, and simulation and modelling techniques to develop innovative solutions to problems. The combination of these approaches meant that, working with EDF Energy’s architect engineering and safety case leads, we were able to obtain increased insight into the user and safety functional requirements and, as an outcome, to deliver an informed and value- engineered design criterion.

For the original brief we were provided with a functional specification and process flow diagram, and asked to do the preliminary design. Part of our Systems Approach, however, is that we respond to challenges holistically, working with clients to understand the range of issues surrounding their needs. So, to ensure we had the information to do this, we offered to capture the user and system requirements, and to provide both safety assessment and human factors analysis to the project.

Our breadth of knowledge, across systems engineering, design, safety and analysis expertise, and our experience in other fields such as defence, oil and gas and rail – in addition to the nuclear industry – allowed us to offer this multidisciplinary approach, involving teams from across the company. Combining structures, electrical, safety, fluids, process engineering, systems development and modelling expertise, we were able to bring best practice techniques from a range of sectors, to deliver added value to EDF Energy’s design analysis.

As part of the requirements capture we facilitated optioneering workshops with the EDF Energy team to tease out some of the detail, and help to optimise the design. These workshops were beneficial in helping to clarify issues and consider the most appropriate, fit-for-purpose options, coming up with solutions in a number of areas. One of these was in optimising the pipework layout. In this situation, the workshop process allowed the piping arrangement to be simplified when it was agreed that the pumps and tanks could effectively run in trains, rather than having a complex manifold arrangement between them. Other workshops looked at failure modes and failure scenarios, such as the considering whether valves should fail open or fail closed.

Through these workshops, and through working with the architect engineer and safety case lead to evaluate, in detail, the different options available, we were able to de-risk the design process for EDF Energy. Defining the project requirements, from front-end design to installation and commissioning methods helped EDF Energy to reduce its risk, and mitigated against delays to the schedule and effectively manage budgeted costs. The process, with the requirements capture and development being undertaken concurrently with the concept and preliminary design, was able to expand to incorporate any extensions to the scope.

Designing in-depth

As an example, for the new commercial boilers, which had already been purchased by Hunterston B, we were able to offer a gap analysis, to see what needed to be done to ensure that they met the safety case requirements. Through defining the complete system requirements for the commercial containerised auxiliary boilers, and conducting a gap analysis, we were able to identify modifications that could be made to meet the safety functional requirements, delivering a System Requirements Document and a gap analysis report. We were also able to identify what modifications were required to the control instrumentation of the boilers; and to validate the set-up for the pump control aspects.

The pump control system, which maintains the pressure of the CO2 ring main, needed to be controlled automatically, switching on and off to maintain CO2 pressure at all times. However the system set-up is complicated by a number of factors: the stringent reliability requirements for the control system; the need for each pump’s control system to operate independently; a system tolerance to the failure of any two pumps; and the sharing of demand equally between the pumps, while avoiding frequent starting and stopping which can cause overheating.

To assess the concept design for the pump control system we constructed a mathematical model of the whole CO2 plant, and of the proposed control system using Matlab, Simulink and Stateflow. We created a number of CO2 demand scenarios, based on the plant data and experience from by the Hunterston B technical team, to test the proposed control system and expose potential weaknesses in the design. The modelling and simulation approach enabled the performance of the proposed control system to be assessed, and we suggested a number of design improvements to develop an improved control strategy, simulating these to demonstrate that they satisfied the requirements. This rigorous assessment of the control system enabled EDF Energy to proceed to the detailed design phase with a greatly improved design, and a high degree of confidence that it would meet their performance requirements in maintaining a reliable supply of CO2 at all times.

The capture of user and system requirements also, of course, involved design reviews with the key stakeholders. Again, using EDF Energy’s operational excellence database, and the leads’ and stakeholders’ knowledge and experience of the Hunterston B site, was key to informing the development of a CO2 plant that could effectively meet their needs. Drawing on the expertise of EDF Energy’s architect engineer and safety case lead, we also integrated human factors into the CO2 system’s design: both in terms of ergonomics and usability, and in considering the operating philosophy within which the EDF Energy team were working.

Within the pump houses, for example, the area around the pumps is confined. Operators need to have adequate space to work in and around the pumps, to be able to maintain them, and to be able to exit from the space easily and quickly if needed. To help with this, Frazer-Nash used a NAVISWORKS ‘fly-through’ on the 3D CAD, which was interrogated by the human factors team to check dimensions and gaps. This ensured that the system complied with both Health and Safety Executive requirements, and the strict ONR safety regulations.

Stacks of safety

In providing the safety assessment to EDF Energy, tools used included hazard identification (HAZID), a hazard and operability study (HAZOP) and a failure mode and effects analysis (FMEA). Again, the collaborative approach was of great benefit here – using the operational experience of the maintainers to inform issues such as pipework lagging, supports and potential leak paths. With safety being of paramount importance to EDF Energy, our work with the ONR was of great benefit. Prior to the project commencement, a member of the team was working within the design authority, who developed the safety case strategy and safety case submissions, and was able to tie-in that knowledge and feed it back into the system design.

Our exposure to the wider stakeholders included, as well as the design authority, other EDF Energy subcontractors, where
we were producing piping schedules, valve schedules and other elements to feed into the pipework design of the CO2 plant. All this meant that, working collaboratively with the expert knowledge of EDF Energy’s architect engineer and safety case lead, the team was able to modify the original design concept to deliver a more effective requirements-based solution, which could be taken forward into detail design and build.

EDF Energy recognised that the outcomes, and learning process developed during this highly successful collaborative and multidisciplinary approach has enabled EDF Energy to develop best practice for similar projects. Angus Morrison, lead
architect engineer in EDF Energy’s Architect Engineering Branch, said: “The benefits of joint working have enabled a systematic approach to be applied, providing a method that can be applied across a portfolio of projects.”

But support did not stop there – while the FEED activity may have been completed, the project continued, and with an engineering lead working directly with EDF Energy, we were able to support the tendering process and the detailed design phase. It was Henry Ford who said: “If everyone is moving forward together, then success takes care of itself,” and through working together, sharing information and using their complementary approaches EDF Energy and Frazer-Nash were able to successfully achieve their goal, ensuring that the completed CO2 plant would fully meet the needs of Hunterston B.