The original objective
When we set out to build a nuclear waste container to meet the challenge set by Sellafield back in 2017, we had safety, operability, ease of manufacture and cost-effectiveness in mind. There are a number of differences between the Capsa design and other waste containers on the market, but the key unique identifier is the lid arrangement; rather than using a bolted interface, the Capsa container instead uses a lid which is secured using a reverse iris mechanism.
One of the original premises in our objective to develop a new waste disposal technology was that the final design should be simple, and therefore easy and cost-effective to make. A key element of the brief was that the new generation of waste containers should use standard manufacturing equipment. This makes the Capsa solution more affordable whilst being comparable to other containers in terms of safety and function.
When we approached the design process for the Capsa waste container, we wanted to ensure that it would fulfil the safety and functional requirements stipulated by the UK’s Nuclear Waste Services without relying on complex, precision manufacturing. The resulting product maximises the efficiency of the waste material mass as a ratio to the container’s size and is designed so that basic manufacture techniques and Commercially Available Off The Shelf (COTS) parts can be used to keep costs down.
Ensuring that a product is designed not just for the intended job, but also for volume manufacture, avoids issues with rejects, re-work and specialist techniques needed to maintain tolerances – all of which affects the production cost. During the development phase of the project we partnered with Nuclear AMRC to specifically look at design for manufacture, ensuring the manufacture process is as cost-effective as possible whilst providing a robust and reliable product.
Key design features of the Capsa nuclear waste container
The Capsa technology uses a patented lid closure system; a reverse-iris mechanism. A standard lid configuration that requires bolting takes time and relies on QA checks, such as torque feedback, to confirm that the bolts have been tightened correctly, whereas our closure system can be handled remotely using a simple locking system. The Capsa design also avoids the issue of galling; existing lid systems tend to use stainless steel bolts in stainless steel tapped holes, and those bolts can seize up, preventing future access if required.
The reverse iris mechanism can be operated remotely, can be easily removed at a later date and doesn’t rely on QA checks to approve fitment. A simple visual check can be performed to confirm that the mechanism is correctly engaged.
The body of the container is based around the principles of pressure vessel design; a rolled tubular section with a welded seam down its length and a domed end attached. The domed end is available as a standard off-the-shelf part, whilst the body can be made using standard rolling and welding equipment. The hub ring at the top of the body interfaces with the lid and is made from a machined ring which is welded to the top of the tubular section.
The lid is a machined stainless-steel plate fitted with the closure mechanism which is made up of components laser cut from stainless steel plate. The lid and closure mechanism are straightforward and therefore economical to produce. A tortuous path around the interface between the body and the lid combines with a pre-loaded metal spring. This design means that tight manufacturing tolerances aren’t needed to provide a seal between the lid and body of the container or to ensure correct fitment. Again, this reduces the cost of production and makes the manufacture output more reliable.
Capsa as a practical solution
We took a holistic approach to the design process rather than focusing solely on the waste container itself. Alongside the design of the container, we’ve also looked in detail at the following areas:
- concept designs for lifting equipment with options for forklift or an overhead lifting grab
- a concept design for a remote lid removal grab
- how the container would behave in an impact scenario.
The design process has therefore been iterative and the container itself has evolved over the past 18 months as we’ve looked at how it will be used in the real world.
In parallel to the conceptual letter of compliance submitted to NWS in March 2023, we are also building prototypes of two representative sizes of container. We have worked in partnership with the Nuclear AMRC and other manufacturers, with Capsa Solutions doing the final assembly on site. The two sizes of prototypes are simply to demonstrate that the geometry of our nuclear waste container can be varied according to the requirements of the user.