Budge the sludge

Sellafield’s Magnox Waste Storage Facility was constructed for the underwater storage of Magnox swarf (generated in Magnox fuel decanning operations) together with large quantities of miscellaneous beta-gamma waste (MBGW). It was constructed in four phases between 1960 and 1984, and has 22 compartments, each about 21ft² (2.0m²) and with 50ft (15.2m) of working depth. It was gradually filled over the period 1964-2000, when it was closed to further waste tipping. The plant is currently in a quiescent phase.

The Magnox Waste Storage Facility is an ageing plant and therefore a decommissioning project is underway comprising of the following elements:

• Care and improvement of the existing asset.

• Initial retrievals using Swarf Retrieval Facility (SRF).

• Preparation for retrievals.

• Retrievals to a ‘dark and dry’ state.

The team tasked with decommissioning this building faces several challenges including establishing accurate historical inventory information and retrospectively engineering a means of retrieving waste from an ageing building. The location of the building, its condition, high background radiation plus the very radioactive and heterogeneous nature of its contents present considerable additional challenges.

Over time, the Magnox alloy corrodes to form a sludge. Since cessation of waste tipping, much of the uncorroded Magnox from the more recent and relatively MBGW-free compartments (C19-C22) has been retrieved using SRF and transferred to an encapsulation plant for treatment followed by storage in modern facilities.

However, to remove the sludge material plus any uncorroded swarf with associated uranium (as metal at the time of tipping) and MBGW from the older compartments has required the development of three purpose built silo emptying plants (SEPs) These are essentially mobile shielded cells or caves equipped with the grabs, manipulators and other features necessary to extract the sludge and transfer it to skips inside shielded flasks for transport to waste treatment facilities.

SPECIAL PURPOSE MACHINES

Process design work, optioneering studies, R&D and HAZOP reviews by BNFL engineers confirmed the use of SEPs as the preferred option for emptying of the silos. In 1996, Rolls-Royce Nuclear Engineering Services won the first of two BNFL contracts for the detailed design, procurement, manufacture, assembly and works testing of three SEPs. The contract for SEPs 1 and 2 was the highest value plant and equipment (P&E) supply contract awarded by BNFL.

SEP1 and 2 are identical machines except for being ‘mirror images’ of each other. They are designed to operate in parallel within the building with each on its own set of rails to facilitate the emptying of 12 waste compartments (C1 through to C12) with SEP1 on the northern (odd numbered) compartments and SEP2 on the south (even numbered).

The SEP3 contract was awarded to Rolls-Royce some two years later. It shares around 70% of the same design as SEPs 1 and 2 and is designed to operate on its own rails to empty the remaining five pairs of waste compartments (C13-C22). When required it can transfer from one set of rails to the other by a north-to-south transfer feature unique to SEP3 and it is also rotated through 180º to prevent clashes with building furniture. It also has more shielding than the other two plants.

SEP2 is the lead machine, with SEP1 planned to follow after nine months and SEP3 a further nine months later. Numerous installation constraints within the Magnox Waste Storage Facility associated with limited access routes, restricted build areas, rigid build sequences and the need to maintain safety at all times, have dictated this planned sequence of installation.

All three mobile caves are supported on seismic isolation bearings at all times except when being moved. For this purpose their weight will be transferred to a set of four skates, which allow the cave to be pushed along the rails by a dedicated drive system.

The three SEPs are designed to grab waste from within a compartment and transfer it into a skip which, when full, will be loaded into a flask and transported to a dedicated waste management facility being built at Sellafield. A SEP will operate on a compartment for several weeks until the level of waste remaining dictates the movement to another compartment.

COMMISSIONING EXPERIENCE

All three SEPs have been designed to enable the fully works tested machines to be dismantled down to a minimum number of sub-assemblies, bearing in mind the available lifting capabilities and the size of the building access openings in the Magnox Waste Storage Facility. This design feature also minimises workforce radiation exposure during re-assembly.

All three have been built at the Rolls-Royce build facility with full replication of the appropriate building furniture. The works crane has been designed to replicate the crane in the Magnox Waste Storage Facility. All the equipment was brought into the build facility using a mock-up of the designated hoist well in the storage facility.

In order to comply with ALARP (as low as reasonably practicable) radiation dose uptake requirements all build and maintenance activities were timed and the number of personnel used during each task was noted. A video record was also made. This data will be used for ALARP assessments when determining the potential dose uptake during future site installation.

SEP2 works testing

The SEP2 mobile cave mechanisms have been subjected to a range of tests including endurance testing of selected items over their equivalent operational life expectancy.

The ‘Hoist & Hose System’, which lowers the grab into the silo and retrieves the waste, was subjected to 40,000 lower and raise cycles over a six-month period. A range of different operational conditions were applied to the system during the tests that were performed over a working height of 22m. A number of minor modifications were implemented on all three mobile caves as a result of these tests.

The ‘Skip Bogie & Serapid Chain Drive System’, which moves the skip between the import and the waste retrieval position, is currently undergoing endurance testing based upon a design life expectancy of 81,500 cycles. The system has currently achieved 15,000 cycles with only minor problems reported.

The planned testing programme covered all aspects of setting-to-work, static and dynamic plant item testing through to full system testing under dry and wet retrieval conditions. In total some 13,400 individual tests were successfully completed.

The first phase of the functional tests on the SEP2 mobile cave covered proving the dynamic operation of all the machines integrated systems without actually retrieving any simulated waste materials. The second phase of functional testing focused on the automated retrieval of simulated waste and MBGW items from a mock-up silo. A total of six skips were filled and exported in the flask as part of the testing programme.

After completion of the main testing programme, a programme of throughput trials was implemented in order to underpin estimated throughput times for the numerous processes that were involved in the import and export of a skip. A total of 14 skips were imported and exported with a total of 25t of simulated waste retrieved from the mock silo.

Fifteen major plant items were also subjected to maintenance demonstration exercises to prove the viability of the associated maintenance instruction documents. These involved major strip-down and re-assembly of fully built plant items that had been ‘final’ assembled to the SEP mobile cave. All the associated tasks were recorded and timed for use in future operator training and for ALARP assessments.

SEP3 works testing

The current project objectives for SEP3 are to functionally test only the key differences from the SEP2 mobile cave. The SEP3 mobile cave was only partially built to be safe in use, provide adequate functionality and the correct all-up weight.

All the lift and rotate elements of the 370t SEP3 mobile cave and the full supporting rail systems were fully built and tested using temporary cabling and temporary control systems. The whole lift and rotate exercise took some 10 days to complete involving major plant item movements/reconfigurations as part of the planned 180º transition (and return) process.

SEP2 remaining work

Over the next 30 months the SEP2 mobile cave will be modified to incorporate the design changes resulting from improvements identified during the testing and maintenance demonstration phase. A number of previously deferred and new items of equipment will also be incorporated into the mobile cave build during this period.

During this same period the working documents for defining the dismantling process and for controlling the physical operations involved will be produced. This exercise will be particularly demanding of module/equipment lay-down areas, as the first item of equipment that is dismantled at the works is essentially the last item to be installed in the silo building.

The documentation that will define the complex logistics involved when transporting some 400t of plant and equipment in sequences to the Sellafield site will also be addressed during this period. The project team at NESL will also produce the detailed installation method statements that will define the processes and procedures to be adopted during the installation of the mobile cave in the Magnox Waste Storage Facility.

The three-month SEP2 dismantling period at the contractor’s works will be followed by a 12-month installation period within the Magnox Waste Storage Facility and both the dismantling and re-assembly activities will be supervised by the same personnel. On completion of the re-assembly work all the major functional elements of the SEP will be re-tested over the following three months, but without accessing the silo contents.

At the end of this period of setting-to-work the mobile cave will be handed over to a specialist BNFL commissioning team. A further period of around 18 months of active commissioning tests will be required prior to handover to the plant operators.

SEP1 remaining work

In parallel with the work described above on the SEP2 mobile cave, the SEP1 mobile cave will be progressively brought to the same status of design, build and ‘documentation’ production as the lead SEP2 mobile cave. All previously held and deferred work, all improvements resulting from tests on SEP2 and the new work that has subsequently been identified by the project team will be implemented.

At various stages of its planned build and test programme the SEP1 mobile cave will be used to perform additional ‘endurance tests’ of new or improved equipment designs. It will also be used for the practical demonstration of new or revised maintenance instructions to operators before it is dismantled and shipped to the Magnox Waste Storage Facility.

The SEP1 mobile cave will follow the dismantling and site installation plan as that described above for SEP2 but will lag by some nine months.

SEP3 remaining work

The SEP3 mobile cave will initially be used as a research and development rig for proving new mobile cave equipment designs that are still in the early stages of development. The existing system functionality of the mobile cave will also be improved and enhanced.

The pit below the SEP3 mobile cave is to be partitioned off from the other two mobile caves with the intention of filling the area under SEP3 with water. The provision of ‘mock-up’ compartment furniture within this water filled area will provide a near full replication of the internals a typical compartment.

Over the next 12 months the above SEP3 enhancements will be incorporated into the build and the mobile cave systems will be commissioned using temporary power and controls.

Upon completion of the R&D trials the SEP3 mobile cave will be dismantled and all temporary equipment removed. Dismantling is necessary to allow the ‘final build’ of the majority of its plant items to take place (from their current temporary build status).

Thereafter the machine build will be progressed through to completion of works testing. This process will also involve the incorporation of any applicable improvements resulting from tests on SEP2 and the SEP3 lift & rotate trials, plus any new work that has subsequently been identified by the project team.

The SEP3 mobile cave will follow the dismantling and site installation plan as that described above for SEP1 but will lag by some nine months.

BUILDING MODIFICATIONS

Whilst the work on the three mobile caves is progressing at the contractors works, as described above, the Magnox Waste Storage Facility itself is undergoing a long-term programme of improvements, enhancements and modifications to the plant, its features and infrastructure to enable each of the mobile caves to be accepted in the programmed time-scale. All the above is to be performed under difficult conditions of background radiation and constrained space requiring considerable skill in the design, planning, on site supervision and management to ensure safe execution.

The building modifications are a significant and technically challenging part of the whole project. In particular there is a need to maintain all essential services during the replacement and relocation of key elements of plant and equipment that need to be moved to accommodate the mobile caves.

THE FUTURE

Once bulk retrieval of the material from the silos has been completed the plant will be put into a state of surveillance and maintenance. This is currently seen as involving among other tasks:

• A gradual shutdown of systems as the process and safety functions they provide are no longer required.

• Sealing the individual compartments.

• An active building area radiological monitoring system.

• Ventilation to ensure that the compartment pressures follow atmospheric conditions.

• Periodic inspection and maintenance.

Steve Barlow, Project Engineer, British Nuclear Group, Sellafield, Seascale, CA20 1PG, UK