6m3 Reinforced Concrete Box Supply for Radioactive Wastes
Following their strategic move from Ductile Cast Iron Containers to a reliance on the WAGR Concrete Box, Magnox, the company tasked with the safe and secure clean-up of 12 nuclear sites and operation of one hydro-electric plant, invited us to bid for a prototyping contract for a revised version of the WAGR box – the Magnox 6m3 Reinforced Concrete Box (RCB).
We were successful in our bid and were able to add value from the outset by developing new, superior concrete mixes. These mixes were then used to cast a prototype which consequently saw us win follow-on work to make an additional three prototypes, including the High-Density (HD) concrete mix variant.
We went on to secure the supply contract for 860 concrete/steel containers for Magnox through competitive tender. The contract was awarded in May 2017 for £20m and was delivered over a four-year period.
- Support the successful development of a prototype
- Add value where possible
- Provide expert opinion, advice and consultancy throughout the process
Although the contract was to be a “build to print” supply with some changes that would make the manufacturing process easier, such as making the body upper and lower casings in four quarters rather than one upper and one lower fabricated piece, the customer, as functional needs were adjusted, instructed multiple significant design changes. APL on all counts delivered these changes that altered every aspect of the tendered “build to print” design, effectively entering into a re-design phase.
Planning and implementation
Although the contract was to be a ‘build to print’ supply with some changes that would make the manufacturing process easier, such as making the body upper and lower casings in four quarters rather than one upper and one lower fabricated piece, the customer (as functional needs were adjusted) instructed multiple, significant design changes. We adapted to and delivered all of these changes and effectively entered into a re-design phase.
Some of these changes were:
- Introduction of engineered vents in wet poured lid
- Introduction of basket to accommodate waste form growth, allowing an annulus between the box wall and basket
- Additional furniture and package requirements introduced
- Introduction of soffit plate to prevent wet poured grout filling interspace between basket and box wall when pouring lid
- Concerns over how to seal the soffit plate to the box to prevent wet poured lid mix bypassing the soffit plate and entering the main box volume
- Introduction of extended twistlock assemblies to prevent vents being blocked on stacking of boxes and also because flatness accuracy for hand finished container body base and on-site finished wet poured lid
- The internal box finish was retarded to allow keying of grout backfill
- Addressing concerns that internal cover was now compromised by the addition of a vent for the long-term safety case and thus cover on the internal faces of the box was increased resulting in operational changes to provide appropriate finish and redesigning aspects of rebar to ensure volume was not compromised
- Revised internal surface finish gave rise to problems with release of mould core that cause plant damage which led to a further revision of operational arrangements
- Concerns over welded twistlock assemblies so we revised to have a machined body and threaded anchor bar construction. This led to concerns over thread tightness
- New load testing requirements on twistlocks for hold down and lifting as original stacking load path altered with the revised requirement for extended twistlock assemblies
- Updated requirements on concrete cover to support operational life requirements
- Clarified concrete material properties that introduced tighter mix control requirements
All of the above contributed to the change of the concrete mixes, rebar cage, form of the box body, concrete cover requirements and twistlocks. These changes introduced secondary concerns that had to be addressed such as the addition of vents, change to internal finish, change to cover requirements, change rebar placement etc.
We also undertook an ‘engineering for manufacture’ review where savings were realised. An example of this was the review associated with the rebar cage to facilitate staged assembly, accurately 3D modelled. The models were then used to produce the jigs ensuring correct placement at the sub-assembly and final assembly stages. This reduced total rebar cage assembly time from three days to around one day and provided shared savings.
For this particular container, we achieved the performance of a tightness Class 3 design with no pre-stressed or lined design. This was realised through the quality of manufacture, such as pumping concrete from the ground up and demonstrated the advancement of mix and casting technology. It should be noted that the predecessor container, the WAGR box, did not pass leak tests.
We successfully managed all changes and achieved customer design approval with the appropriate substantiations, where required, using our design subcontractor Frazer-Nash Consultancy (FNC). In addition, we were able to adapt all plant and equipment to accommodate these changes and we subsequently passed the pre-production phase and have now completed the manufacture of the over 700 6m3 RCBs.
The manufacturing rate set by the contract is six RCBs per week and this is achieved with the five installed moulds we have. We incorporated into the production design and logistics the ability to increase production further to ten RCBs per week should Magnox wish.
We’re proud to have produced Lifetime Quality Record (LTQR) packs that meet the customers’ requirements. Each box has a LTQR pack that is around 800 pages with a Magnox “golden thread” route map matrix extending to around 30 pages to help the reader find the specific data they need. We believe that this level of documentation is well in excess of other companies that supply ILW containers in the UK.
Magnox has accepted of over 600 units and has started active waste packaging at the nuclear decommissioning sites.