People in glass houses30 August 2001
BNFL brought on line its third waste vitrification plant at Sellafield, the culmination of seven years work By: LIAM PUZZAR and STEVEN TROILET
High level waste arises from the reprocessing of irradiated nuclear fuel. At least 97% of irradiated fuel is recovered as reusable uranium and plutonium, with the remaining 3% being high level liquid waste.
Converting the liquid waste into a solid form by trapping the liquid in glass blocks – vitrification – is considered internationally as the optimum method of storing high level waste. In addition, vitrification reduces the volume of the liquid to one third of its original volume.
The existing two-line Vitrification Plant began operation at Sellafield in 1990. However, a number of problems have been identified which reduced their efficacy. The main constraints to throughput were:
•Melter crucible life was shorter than expected, leading to more frequent crucible changes and increased demand on maintenance facilities and equipment.
•Difficulties in cutting up failed melter crucibles for disposal leading to waste accumulating in the Breakdown Cell, which could impair the routine remote rebuilding of glass-making equipment.
•In-cell cranes were used more than planned, in part due to the reduced melter life and other maintenance problems.
•Master Slave Manipulators (MSMs) were less reliable than anticipated, resulting in increased remote maintenance operations and bottlenecks in the MSM repair cycle.
The third Waste Vitrification Plant (WVP) aimed to solve these problems.
The vitirification process
The high level liquid waste is transferred into the WVP via a pipebridge into an active liquor storage tank. It is then fed into a rotating tube inside a heated furnace. here, the liquid is evaporated and dried into powder known as calcine. The calcine is fed into a melting pot together with glass-making material. The melting pot is an elliptical vessel heated to 1150°C by an induction furnace. The glass and the calcine fuse in a ratio of 75% glass/25% calcine. Molten vitrified waste is then poured into containers, allowed to cool, and is fitted with a lid and closed by an automatic fusion welding technique. The weld is checked before the container is moved to the decontamination area, where high pressure water jets remove loose surface contamination. This container is then inspected remotely before being transferred inside a shielded flask to the Vitrified Product Store.
The project’s design engineers were using a CAD system as a design tool. In this instance, AutoCAD was used, mainly working in 2D, but occassionally to produce 3D representations of specific items of equipment, such as the calciner.
In addition, much of the specialist pipework planning, as well as 3D modelling was carried out in the Plant Design Management System (PDMS). PDMS was networked, with engineers sharing alterations and revisions. The massive increase in computer processing power has benefitted the usability, not just of PDMS, but of all CAD, CAM and simulation systems. PDMS models are now in colour and in 3D, with features such as clash detection. The fact that computers can now work with bigger models than ever before, has allowed the designer to model elements of the WVP down to individual nuts and bolts.
However, experience with viewing angles in the previous WVP lines led the engineers working on the project to get in contact with the mechanical department at Risley. They needed to be certain that the vessels within the main line were easily removable remotely via the MSMs. For the removal process to be easily achievable, the design engineers had to be certain that the couplings between the pipe and the vessel could be clearly seen by the operator viewing through a heavily lead-shielded window.
The design team thought that the answer might be found in a VR-based package called IGRIP, developed by Delmia, part of the Dassault Group. IGRIP-trained engineers at Risley were commissioned to check the requisite viewing angles and to ensure that the MSMs could reach not only the couplings, but also the platines which hold all the pipes into each vessel. As IGRIP at that stage could not account for the refractions that occur in glass of that thickness, Steve Troilet, the engineer responsible for the project, decided to err on the side of caution and to place the MSM operator close up against the viewing window with a small angle of sight into the WVP line.
This small scale piece of work showed unexpectedly that the couplings needed to be repositioned by 100mm if they were to be easy to work with. The main WVP design team was impressed by this technology, which was new at the time, and decided to commission more studies on other equipment requiring maintenance. Now the entire WVP line 3 exists virtually in IGRIP, and the designers found the system to be useful when for example, working out where to position switches for optimal viewing. Eventually, one operator was seconded onto the project full-time, carrying out further detailed studies on welding machines, carousels, lid placing machines, and so on.
IGRIP has also been used to prove the route for importing large pieces of equipment, such as the dust scrubber vessel and the pour cell carousel during maintenance operations.
The ergonomics element of the package was an element of design which had been almost impossible to consider in a detailed way previously. The entire control room and all the work benches were eventually modelled in a virtual environment, which enabled engineers and operators to come together and rehearse operations before construction had even begun. The model allowed the positioning of all the viewing monitors, keyboards and annunciators, so they were easily visible, with all the controls within reach of an operator. While AutoCAD and PDMS are specialist engineering
systems, Delmia software can be modelled to create a virtual world that anyone can understand. In this way, the virtual model acted as a communication tool across departments.
All three WVP lines feature a wash cell at each end. This is to facilitate crane and other equipment maintenance. Equipment is first moved out of the line into this wash cell, where it is stripped down and washed prior to manual maintenance. However, lines 1 and 2 experienced difficulties with this cell, especially with the jacks within it. Designers also realised that with more equipment in line 3, and with the site limited in size, there might not be sufficient space for the operators to carry out the various stripping down and washing procedures efficiently.
By modelling this cell within the Delmia environment, the designers were able to see exactly what was achievable within the space allocated to the cell, as well as examine a few new options. In the end, the jacks were removed entirely, and a solid rail system with a movable bridge implemented in their place, thus solving two limitations, space and unreliable jacks. Furthermore, it was possible to check that the operators of the wash cell’s Cartesian crane were able to reach all six of the main cell’s modules as well as all the electrical plugs. However, it quickly became apparent that the solid rail crane transport system was severely hindering the sight lines of the operators. This problem was easily solved by swapping the virtual rail for a lattice work version.
Videos have been created using IGRIP to demonstrate equipment operational sequences to operators, and to demonstrate to manufacturers and designers how their equipment will interact with the rest of the plant. This has resulted in improved equipment designs. The use of the Delmia environment for operator training was an initially unlooked-for benefit of the system. IGRIP was also used to check the performance of MSMs, detecting clashes and ensuring that the in-cell equipment can be reached with collisions. It is envisaged that when the plant is fully operational, if a piece of equipment breaks down in an area with poor visibility or accessibility, the IGRIP model will be deployed to try out a solution virtually, thus avoiding potential damage to plant and equipment in the real world.
IGRIP is an advanced 3D digital simulation systems. It is essentially a robotic simulation tool for design, evaluation and off-line programming of robotic workcells. Incorporating real world robotic and peripheral equipment, motion attributes, kinematics, dynamics and I/O logic, IGRIP produces accurate simulations and programs. The system allows the optimisation of robot locations, motions and cycle times, and eliminates costly collisions between robots, parts, tools, fixturs and surroundings.
TablesWVP Line 3 statistics