Bradwell: leading the way4 July 2017
A new approach to managing fuel element debris at the Bradwell site in the UK has accelerated the cleanup programme by almost a decade.
Amajor decommissioning milestone has recently been achieved at the Bradwell nuclear site. Located on the Blackwater estuary in Essex, UK, the site has now completed work to manage its inventory of fuel element debris (FED), which totalled over 240t. Retrieving, processing and safe disposal of fuel debris is a key work programme at Bradwell.
Bradwell had previously been selected by the Nuclear Decommissioning Authority (NDA) to be a ‘lead and learn’ site for the Magnox estate, accelerating decommissioning wherever possible so the site could be the first power reactor site in the UK to enter a Care and Maintenance phase. The site is managed by Magnox Ltd (currently owned by Cavendish Fluor Partnership) working for the NDA, which owns the site.
Most Magnox fuel debris is from the outer fuel cladding of fuel elements used in Magnox reactors, made of magnesium alloy (the Magnox name was derived from this). Debris was produced when the alloy was removed from the fuel elements so they could be packed more densely in flasks for transportation to Sellafield for reprocessing.
At Bradwell fuel element debris was stored, along with other wastes, in 18 underground storage vaults at the site. The waste stream comprises: un-corroded metallic magnesium alloy; corrosion products, generated while in the vaults; gravel from the bottom of the storage vaults; and interface waste (a debris/gravel mix retrieved from the layer in the vaults where the two meet. The wastes were retrieved, monitored, characterised, and packaged into nearly 2900 drums.
A Best Practicable Environmental Option (BPEO) study was undertaken, which identified a dissolution plant at Bradwell as the best treatment option, followed by encapsulation of secondary wastes.
In the process the debris is dissolved in a nitric acid solution to separate radioactive materials from inert waste. It reduces the permanent disposal of harmless waste, cutting the volume of solid waste for disposal by more than 90%.
The radioactivity of the remaining effluent is minimised. The effluent is neutralised, so heavy metals precipitate out of solution.
Most of the radioactivity remains with the resulting solid sludge, which is filtered off, dried and stored as intermediate level waste (ILW). The remaining effluent is then filtered and passed through ion exchange columns to remove more of the residual radioactivity. What remains is monitored and discharged in accordance with permits granted by the Environment Agency.
Developing, installing and commissioning the dissolution plant proved to be a complex process, resulting in lower throughput volumes than originally envisaged. As a result of a concerted effort to fully commission and improve the plant, dissolution is now operating efficiently. However, the delays in commissioning the plant and achieving optimised throughput affect the overall site programme. As part of continuous improvement and optimisation, alternative strategies for managing the fuel debris were explored to mitigate delays to the Bradwell programme.
A desk-based feasibility study to explore whether any debris could be disposed of as low-level waste (LLW) at the Low-Level- Waste Repository (LLWR) in west Cumbria resulted in a change of strategy. FED was historically assumed by Magnox to be ILW, but based on improved characterisation data resulting from sampling of the materials retrieved at Bradwell, a significant proportion of the FED is now demonstrably LLW. This is partly as a result of radioactive decay while the FED has been stored in vaults, but also because of improved separation of the metallic debris and corrosion products from the high dose-rate items such as springs and fuel fragments.
Having demonstrated the feasibility that some debris could be sent to west Cumbria, an integrated project team was established including Magnox, LLWR and Tradebe Inutec, who were selected to treat and consign the waste in accordance with LLWR’s waste acceptance criteria and environmental permits, set by the Environment Agency.
It required close partnerships between the organisations, without prejudicing LLWR’s independence in assessing the submissions provided by Magnox/Tradebe Inutec to dispose of each consignment.
As a first-of-a-kind project, a number of challenges were posed which required significant technical underpinning in order to be addressed and ensure delivery of the project.
Having established the inventory of LLW debris, the work initially focused on ensuring it could be safely packaged for transport, first to the Tradebe-Inutec facilities (located at the Winfrith site in Dorset), then onwards to LLWR for disposal. This included the receipt and unloading of FED drums at the Tradebe- Inutec facilities, and storage while the drums were waiting to be processed.
At Tradebe Inutec, the drums underwent a super-compaction process to improve packaging efficiency. This process worked smoothly, with no evidence of drum failure, no release of particulate materials and a limited release of liquid to the compactor discharge systems. The super-compacted drums will be co-packaged with other LLW wastes in half-height ISO containers and transported to LLWR, where the containers will be grouted with cement to ensure a stable final disposal product.
Hydrogen evolution levels were monitored throughout the duration of the project, as this was a key part of the Tradebe-Inutec safety case and LLWR’s waste acceptance criteria and environmental safety case. Development work was performed to understand the nature of the hydrogen evolution process and the impact on each of the waste management phases. Co-packaging also ensured hydrogen evolution levels were managed within the transport and LLWR grouting plant safety cases.
The integrated project team had to be fully compliant with each other’s requirements and it was also crucial that Magnox Ltd could demonstrate to the Environment Agency, as the regulator, that the project was the Best Available Technique (BAT) for disposition of FED which has been classified as LLW, whilst the FED still classified as ILW continued to be treated through dissolution.
The project was successful and was delivered within the agreed timescale. The last shipment left Bradwell in March 2017. In total 90t of LLW FED and 55t of FED gravel and interface waste have now been consigned to Tradebe Inutec in two phases, for treatment and disposal at LLWR, representing over half of Bradwell’s FED inventory. This has saved approximately 90 weeks or two years of dissolution operations at the available throughput, better value for money than the original strategy.
Successful delivery relied on early engagement in the integrated project team, ensuring a clear and shared understanding of the scope of work needed to deliver the project, which in turn helped to address the safety case requirements at each stage.
Maintaining this engagement throughout the consignment process was equally important, given the change of strategy from agreed work plans. Taken together with a flexible approach to working, this allowed the technical and engineering challenges to be addressed in a responsive manner, ensuring the project progressed to the agreed schedule.
Magnox Ltd is now using this strategy to manage fuel element debris inventories at other sites.
The end of FED recovery and disposal operations means that Bradwell is predicted to enter the Care and Maintenance phase around nine years earlier than originally proposed.
The focus will now be on post-operational clean-out and decommissioning of the FED dissolution plant. The remaining critical path activity for Care and Maintenance is to complete ILW operations on the site, with all ILW conditioned and packaged ready for storage in the site’s interim storage facility. This is due to be completed by the end of 2017, when the remaining works will include the deplanting and decommissioning of the ILW facilities and remaining civil works.
Another recent example of progress is the demolition of buildings that were part of the site’s spent fuel ponds complex – the first time a Magnox reactor has achieved this. The buildings were decontaminated over a four-year period to levels that allowed the redundant buildings to be demolished using conventional demolition methods. The remaining buildings will now be enclosed in weatherproof cladding in preparation for when the site enters Care and Maintenance.
As it completes physical works, Magnox Ltd has been working with regulators to ensure the requirements for entering Care and Maintenance are fully defined and met by the site. Bob Nichols, Bradwell closure director, said: “We have proved with projects like these, and the weatherproof cladding of the reactor buildings we completed in 2016, that we are able to work collaboratively with our supply chain and with other parts of the NDA estate, to accelerate progress and deliver our desired outcomes without compromising our high safety standards. This will stand us in good stead as Bradwell approaches its Care and Maintenance phase.”