The Highly Active North Outer cell (HANO) in the former Magnox Primary Separation Plant has been successfully filled with 505m3 of light-weight foaming grout to the required level for the PBI, 8 days ahead of schedule.
The completion of the HANO grouting concludes a significant piece of work that was aimed at stabilising the internal vessels and pipework in the cell. Now the cell has been fully grouted, it mitigates a significant safety risk of having vessels collapsing in the cell and leaves the cell in a fit state for care and maintenance pending full decommissioning.
The Primary Separation Plant was constructed in the early 1950s to carry out the first stage of reprocessing fuel from the pile reactors. It was subsequently modified to accept oxide fuels following the opening of the Magnox Reprocessing Plant before ceasing operations in the 1970s. During operations, the 11 storey, 61 meter high, High Active North Outer cell was used as a ventilation route, however after 20 years of use many of the mild steel beams within the cell were severely corroded by the acidic ventilation effluent. Hence the cell was in need of stabilisation of to allow safe decommissioning operations in the MAN (Medium Active North) cell below.
Following extensive research and development, the project team worked with the University of Dundee, Concrete Technology Unit and Westlakes Engineering to develop a lightweight foam grout. With an air bubble consistency to minimise the weight and seismic vulnerability on the building, the grout can be crushed to 30% of its original volume when full decommissioning of the building takes place.
The first phase of grouting, filling the cell shaft, was completed in late 2007, however because of the ‘inverted L’ shape of the cell, additional substantiation work had to be undertaken to confirm the integrity of the roof above the Medium Active Cells and its ability to support the weight of the grout. This led to the development and use of an even lower density grout at 300kg/m3 to minimise weight loading and a further safety case and off site trials.
Grouting of the top 12 metres of the cell, consisting of floors 7 to 10, started on 8 June, and the last of the 55 pours was completed on 6 September. The maximum depth of each pour was 25 cm and each layer took 24 hours to complete and set. Strict quality controls were followed to ensure that the grout was of the correct density and strength to ensure that each layer did not collapse under its own weight until it had time to fully cure and harden.
Preparatory work also included the rerouting of the HANO ventilation extract route and the stabilisation of a section of brickwork wall between the Inner and Outer High Active cells.
Jeremy Hunt, Senior Project Manager for HANO Stabilisation praised the efforts of the team saying “The preparation and execution of this project has been to the highest standard, with innovative development of a simple solution.”
Steve Slater, Head of Projects, Decommissioning and Demolition said “The HANO Stabilisation project is an important part of hazard reduction. We are proud to able to deliver on our commitments to the NDA and contribute to another Decommissioning success.”