Engineering an inlet5 September 2018
Building information modelling (BIM) is playing an important role in the conceptual design of an unshielded intermediate-level-waste handling cell for the UK’s geological disposal facility. NEI learns how this approach has the potential to accelerate cleanup and reduce lifecycle costs.
THE UK HAS ACCUMULATED A legacy of radioactive waste and continues to produce radioactive wastes from various industries and programmes. UK government policy for management of these wastes is safe and secure interim storage, followed by geological disposal, as set out in the 2014 Implementing Geological Disposal White Paper. Radioactive Waste Management Limited (RWM), a wholly owned subsidiary of the Nuclear Decommissioning Authority (NDA), is responsible for delivering a geological disposal facility (GDF) and managing the UK’s higher activity radioactive waste inventory.
As disposal of certain types of radioactive waste moves a step closer following the completion of public consultations and recent progress on the planning framework for siting and constructing a GDF, RWM continues its research to develop engineering solutions which will be put to use in handling and transporting radioactive waste destined for disposal in a GDF.
Waste disposal units (DUs) containing unshielded intermediate-level waste (UILW) are currently planned to be transported to a GDF and transferred underground in standard waste transport containers (SWTC). An inlet cell, a shielded facility assumed to be located underground, would allow the remote unpacking and handling of disposal units from their containers for transfer and placement in designated disposal areas.
During 2016 RWM prepared an updated generic design for the GDF. The design was bounded to a range of geological environments (higher strength rock, lower strength sedimentary rock and evaporite rock) suitable for higher activity waste disposal in the UK.
The generic GDF inlet cell design has a planned receipt capacity of 2500 containers per year for the UILW emplacement period. The throughput for some periods will be modified to match the container receipt rate by changing the operational shift pattern, for example going from a three-shift (24 hour) working pattern to two-shift operations. The GDF generic design waste receipt schedule has a planned capacity of 2300 containers per year for 24 years, followed by 1500 per year for about 44 years.
The generic design was classified by RWM using the NDA’s technology readiness level (TRL) methodology. This methodology has three components:
- Technology, which refers to a technological process, method, or technique such as machinery, equipment or software needed for the plant, facility or process to achieve its purpose.
- Readiness for operations at the present time.
- Level of maturity of equipment.
Equipment that is already being used for the same function in the same environment has a higher level of maturity than equipment being developed. The level is a nine-point scale based on a qualitative assessment of maturity.
The generic design was estimated to be at level two, the ‘invention and research’ stage. The aim of this phase of the design development work was to progress it to level three, ‘proof of concept’ or ‘demonstration, in principle, with the potential to work’. This would provide confidence that an inlet cell could be constructed, operated to meet the throughput target and be decommissioned in the range of geological environments suitable for hosting a geological disposal facility in the UK.
The approach to the design utilised existing proven engineering, knowledge sharing and systems engineering all fully integrated with building information modelling (BIM) technology. A summary of the objectives is shown in Table 1.
Initial review and simplification
An initial process flow diagram was produced showing the sequential steps of the generic design process: importing containers to the inlet cell, removing the disposal units from containers, sentencing the units to the disposal areas for emplacement and finally exporting the containers for reuse. The flow diagram is a basis for sharing of best practice, legislative and guidance requirements. Experience suggested some initial simplifications. For example, relocating the off-gas pressure equalisation of the containers to the surface and incorporating off-gas monitoring. This gave the benefit of registering any significantly different radiation readings before moving containers underground. Potential defects can be identified at the surface level, avoiding contamination while underground and on the return to the surface. Experience from existing UK facilities enabled the team to simplify the generic design by removing 14 individual process steps.
Systems engineering and optioneering
A systems engineering approach was applied to produce a functional specification for the inlet cell.
It comprises two hierarchical levels of requirements at increasing levels of detail. The first level covers user requirements (where the user is deemed to be the operator of the inlet cell). The second includes system requirements, structured by functionality and interfaces. The third level, associated sub-system requirements will be derived during the detailed design phase.
The specification provided a baseline to ensure the options derived during the optioneering study and subsequent design development remained aligned to the project aims and objectives. This proved to be a significant advantage, providing the design team with a point of focus throughout the design process and a basis for simplification and refinement, as illustrated in Figure 1.
The next phase of the design requirements was the selection of the most appropriate design options, based on the simplified process flow diagram and the specification.
The design team was fortunate to have a considerable amount of design and operations experience from the Sellafield encapsulation suite. In addition to this, Sellafield Ltd shared its experience of automated and manual unbolting equipment, and provided the team with an opportunity to visit the Magnox encapsulation facility and associated product stores. This gave the team a fascinating insight into the operation of a facility which in principle was very similar to what the GDF inlet could look like in reality.
Street Crane Express of Sheffield also offered guidance on various mechanical lifting options and associated costings. The optioneering study was conducted over three stages; high-level, functional and configuration.
High-level optioneering considered both above and below ground options, based on international good practice. This included examination of the Swiss (Nagra) conceptual facility design for receipt and transfer of UILW as a surface-located alternative example. At the functional stage, options were considered against the system requirements, for example considering how the containers would be transported through the process steps. The options included an endless chain drive, a conveyor system and a bogie.
The configuration stage, supported by operational experience and the visit to Sellafield, considered two top level options. Either containers would enter and exit the inlet cell from the same location, or they would follow a ‘production line’, entering the inlet cell from one location, passing through a series of process steps, and returning to the start via an independent route.
The option chosen was a series of processing steps at different locations, using the production line principle. A simple configuration for achieving a production line facility was developed. This consisted of tunnels connecting orthogonally, which returned the bogies to a location where the containers could be loaded and off-loaded and reduced the cycle time of a container travelling through the facility.
In the operational sequence a container is posted into the facility in the bottom left hand corner utilising an 80t electric overhead travelling crane. The container is placed on a waiting bogie on one of two traversers and transported from the start point through the roller shutter door to the start of the processing tunnel. On reaching the start of the processing tunnel the bogie is driven off traverser number 1 to the unbolting zone.
The container is unbolted in the unbolting zone and driven through a shield door into the disposal unit transfer zone where the lid is removed by a 12t electric overhead travelling crane with a dedicated grapple. The lid is placed on a seismically qualified steel stand. The container is then moved forward to a dedicated location where a second 12t package removal crane, fitted with a dedicated grapple, engages with the disposal unit. The disposal unit is raised to the upper level of the disposal unit transfer zone and placed on a bogie for transport to final disposal. The container lid is replaced and it is driven through a second set of shield doors to the rebolting zone. After rebolting the SWTC is remotely monitored and, once cleared, driven onto a second traverser to the start of the return tunnel. On reaching the return tunnel the container disembarks the second traverser and returns to the start point via the return tunnel. On reaching the start point the container is removed from the inlet cell and returned to the surface.
There were two means chosen to move the container within the facility, a bogie and traversers. The primary engineering consideration for the bogie was the bogie length to rail wear and wheel stress. The team was able to overcome this challenge by utilising a three-axle design, which with subsequent modification to the suspension, ensured the load was equally distributed across all the three axles. By optimising the length, the structural size of the cell, and subsequently the distance between processing stages, could be reduced.
The traversers proved a simple means of accommodating the orthogonal interfaces of the proposed configuration (see Figure 3).
The traversers consist of a wide platform bogie, with the bogie and container load distributed across a number of rails and axles. Busbars have been chosen as a means of distributing power to the transporters. The busbar distribution integrates well with the third, fourth and fifth power rail power philosophy and enables on-board control devices to be utilised reducing the amount of electrical cabling and necessary penetrations into the inlet cell.
The integrated use of a bogie and traversers underpinned what may be regarded as a relatively novel step for nuclear design. The design team proposed an operating philosophy using multiple bogies (a total of four) in operation simultaneously under PLC control. Since no operators would be permitted into the inlet cell under normal operations, this enabled an optimum solution to powering the bogies though the zones and in the return tunnel. The solution was a third, fourth and fifth set of electrical power rails.
Kuka Systems provided robotic equipment expertise in support of the remote bolting and rebolting of the container lid. The solution is based on proven technology, meets the system requirements and was dimensionally integrated into the inlet cell design (see Figure 5).
The structure of the UILW inlet cell facility fully incorporates the shielding requirements as specified by RWM. However, the design only utilises the required shielding thickness in the area where the disposal unit is exposed within the process. The rest of the facility civil structural design complies with UK and ISO standards to ensure the necessary strength and structural stability.
Designing for effective maintenance is a critical part of modern design. The UILW inlet cell has dedicated, shielded crane maintenance areas. In support of this there are dedicated maintenance cranes along with designed transfer routes and equipment for the export and import of crane components. The design incorporates an outline breakdown recovery strategy implemented from outside of the cell, enabling the package to be safely manipulated in the event of component failure on the package handling crane.
Building Information Modelling
Building information modelling (BIM) played a large part in the design, ensuring it remained aligned to requirements. The data input into the BIM system was from the technical specification, which had technical data on all of the equipment within the inlet cell. By default, since the technical specification was fully aligned to the requirements, the BIM data were completely aligned. This ensured that all the visual representations, the 3D model, drawings and animation were fully integrated with the systems engineering approach.
As the design is developed in the future the BIM model has the capability to integrate sub-system requirements, all new components, interface details, materials and dimensions, fits and tolerances. They can be continually reviewed, managed and aligned to the original requirements forming an integrated management system.
Another significant advantage in utilising BIM is the ability of the system to interface with similar software packages across UK industry. Examples include crane manufactures who could export their detailed design, in response to a project specification directly into the overall BIM system.
BIM has the capability to store all the necessary information supporting design substantiation reports and the nuclear safety case. The benefit of utilising BIM in this role is the system’s ability to control and manage individual access to the entire portfolio of stored design information.
This represents enormous potential to help support the entire GDF design. During the detailed design phase, there will be a requirement to rationalise, refine and develop ideas and control and manage supply-chain data and information. BIM will support management control of the design in the detailed development phase.
Throughout the project, the use of systems engineering and BIM facilitated effective and efficient communication of a ‘single source of the truth’ during multiple iterations and across a project team which was dispersed widely across the UK. Using 3D modelling and animations ensured the design was aligned to requirements, and enabled identification of safety and throughput enhancements during technical review and effective communication of design progress to the client. This provided a platform to assimilate and transfer knowledge of nuclear design and operational experience from a range of experts. This result was:
- The potential to process UILW at a GDF at an increased rate by using an automated operation to enable the use of multiple bogies within a production-line type configuration. This enhancement provides a theoretical annual throughput of 9882 containers assuming a 50-week, 24-hour continuous working pattern. Based on this, an availability figure of 25% is required in order to meet a plant throughput of 2500 units per year. This offers the potential benefit of removing disposal units from waste producer sites more quickly, accelerating decommissioning.
- Demonstration of UILW inlet cell technical feasibility by basing the design on standard industrial equipment and remote operations proven in a nuclear environment.
- The foundation for an asset management approach during maintenance and decommissioning to manage, maintain and replace equipment components over the lifetime of a GDF. Such a strategy would potentially provide a further opportunity for financial saving, by increasing equipment effectiveness and utilisation across the entire GDF project.
The inlet cell design is now of sufficient detail to support RWM’s illustrative designs and demonstrate feasibility in the near term. To underpin aspects of the design further in the medium term, RWM will need to consider workshop based trials. As the design of the inlet cell is closely linked to the design of the transport container, RWM is working with its supply chain to consider further development of this item.
Author information: Mike Farrer, Technical lead and lead mechanical engineer on the design at Arup; Ken Cowell, Used his EC&I experience on the GDF project for Arup; Ally Clark, Managing director of MCM Environmental; Stine Norskov, Structural Engineer at Ove Arup & Partners; Neil Robertson, Senior technician working in engineering design at Arup; Richard Hardy, Engineering manager at Radioactive Waste Management Limited; Steven Lesser, Project director at Arup