Ansaldo NES – Silo Emptying Plant Mobile Caves
The MSSS facility at Sellafield was built just over 50 years ago as a facility for the underwater storage of Magnox swarf. This swarf was generated from Magnox fuel de-canning operations and a variety of other intermediate level solid wastes, termed miscellaneous beta-gamma waste (MBGW). Over the last 50 years, increased storage demands have seen the silo extended three times to give a total of 22 storage compartments. The operating floor of the building forms the roof of the compartments and spans all four parts of the building. Above the silos, a shop floor gave access to charge holes through which waste was deposited and is stored underwater.
In June 2015, Ansaldo NES, fully owned by Ansaldo Nucleare and part of the Ansaldo Energia Group, achieved another important milestone for Sellafield Ltd, completing, at the Wolverhampton site, the testing and commissioning of the first of three Silo Emptying Plant (SEP) mobile caves. The SEP machine has been dismantled into almost 30 separate modules and transferred to the Ansaldo NES Beckermet facility, three miles from the Sellafield main gate entrance, waiting to be reassembled inside the Magnox Swarf Storage Silos (MSSS) building and tested before initiating solid waste retrievals in 2017.
The MSSS building at Sellafield is one of the site’s four Legacy Pond and Silo facilities and has been described as one of the most challenging industrial decommissioning projects in Europe. The Nuclear Decommissioning Authority (NDA) and Sellafield Ltd are focused on safely decommissioning these buildings as part of the hazard and risk reduction programme.
Since 1996, Ansaldo NES (then owned by Rolls-Royce) have been working with Sellafield Ltd on a project to design, manufacture, test and commission a bespoke solution to resolve this challenge. Three SEP machines are currently being constructed, tested and trialled by Ansaldo NES at Wolverhampton and are at varying phases of their lifecycle. Each of the fully assembled SEP Caves weights approximately 500 tonnes with dimensions of 11m x 5m x 6m and is assembled from over 13,500 parts each. These machines will be used to extract Intermediate Level Waste (ILW) from these compartments ready for transfer and processing.
The SEP machines will have to operate in a radioactive environment where operator access is restricted due to the radiation dose levels. Because of this, the machines will need to work safely and continually for up to 25 years without any modifications or upgrades. The SEP mobile caves are shielded cells that control a suite of waste retrieval tools and equipment. The tools are deployed, as required, to retrieve, re-size, reduce and consign to skips, the waste stored in each of the silos. The SEP machines are mounted on a substantial rail system, allowing them to be moved between silos as the retrievals operations progress. The exported skips will then be transferred to the future new facility, the Sellafield Direct Encapsulation Plant (SDP), for further processing and immobilisation of the waste.
Ansaldo NES is delivering to the MSSS programme with a dedicated Integrated Project Team, deployed across its headquarters in Wolverhampton as well as sites in Risley (Warrington) and Beckermet (Cumbria). Pro/Engineer 3D solid CAD modelling has been used extensively during the design process to allow design iterations with automatic updating of General Arrangement and Detailed Drawings. The final models are some of the most complex that have been produced using the ProE tool.
The ProE software has the advantage that it provides virtual prototypes which identify any design interface problems prior to manufacturing, thereby avoiding unnecessary re-work. Produced to support the 3D solid CAD modelling was a 1/10th scale model of the retrievals process. This model was used to validate and trial the effectiveness of various end effectors on the Second Stage Deployment arm and also to identify rope twist and wear issues.
The concept of the shielded cell can be simply described as a shielded box with tunnel and access penetrations to enable an operator to remotely control the retrievals operation without being exposed to radiation or contaminated material. The tools used to remove the waste itself have been designed and developed specifically for the retrievals process.
To meet the unique challenge of the retrieval operation, innovative bespoke tooling has been developed by Ansaldo NES and is evident in numerous areas of the SEP Mobile Caves.
A critical part of the design requirement was to ensure craters are not formed within the compartments being excavated. Additional structural constraints meant that the charge hole has a small aperture size through which all waste must be retrieved and the necessity to keep track of the positioning and rotational location.
In order to achieve the operational constraints, two important stages to the removals process were required. At the beginning of retrievals, a grab is used to retrieve waste near the charge hole and a solution was required to ensure that this would not result in the formation of craters. Ansaldo NES developed a ‘silo rake’ with the ability to form a flat bed of waste throughout the lifetime excavation of each silo compartment. Telescopic rake arms consisting of a series of rectangular sections that slide within each other on bearing pads were created using a serapid chain to extend and retract the arms. It also features a slewing mechanism, allowing the rake to rotate incrementally in relation to the top plate assembly.
Early design of Telescoping Silo Rake
After the waste is removed from the upper section of the silos, a more direct approach is employed which removes the need for the rake and increases the efficiency of the retrieval process. Engineers at Ansaldo NES developed the Second Stage Deployment (SSD) unit, able to deploy a ‘rope hung grab’ to a multitude of predefined positions within the silo compartment. A system, using two deployment arms to rotate and extend from 1.2m to 3.3m in a controlled manner within a tightly controlled space-envelope, allows all areas of the compartment to be accessed. The arms are able to deploy a minimum mass of 410kg through their full range of travel whilst managing the hydraulic supply hoses and maintaining a constant tool height within ±50mm while in motion.
Able to deploy to four ‘rope hand-over’ positions with the use of bespoke end deflectors and having the ability to park in a position to allow unhindered waste retrieval operations.
Significant time and cost was saved during the design stages by the innovative use of a 1/10th scale 3D model, able to simulate the behaviour and operation prior to full manufacture. This enabled the design process to be accelerated and directly interfaced with finite element analysis (FEA), for stress, seismic and impact assessments. Ansaldo NES undertook comprehensive verification of the results using a variety of alternative calculation methods to achieve this correlation.