COVER STORY | RADWASTE MANAGEMENT
Keeping a close eye on waste
A new inspection system should increase the speed and effectiveness of visual inspection of waste at Sellafield
image and hence a fulfilled measurement of displacement, strain shear and correlation coefficient are generated. NPL adds that one of the challenges of DIC measurement is controlling the position of the camera to reduce parallax effects and to provide sufficiently reproducible position and orientation of the camera to allow successful DIC for interrupted deployment. DIC has been used since the mid 1980s, but it has only been within the last 15 years that cheap high-resolution imaging and appropriate computing resources have been available to make measurements within a sensible time. DIC is not widely used outside of the laboratory and it has been work at NPL that has identified the uses for this technology in diverse civil engineering structures like tunnels and nuclear stores. The HAIS system is a precision robotic platform that
Above: HAIS is being used to monitor waste stores at Sellafield in Cumbria, UK Photo credit: Sellafield Ltd
THE UK’S NATIONAL PHYSICAL LABORATORY (NPL) is the country’s national metrology resource. It is a public corporation owned by the Department of Business, Energy and Industrial Strategy (BEIS), and it develops and maintains the national primary measurement standards, provides a national measurement infrastructure and delivers the UK’s ‘Measurement Strategy’ on behalf of BEIS. It aims to use science and engineering to provide the measurement capability the country needs. Dr Nick McCormick, NPL’s principal research scientist, says, “At NPL we are interested in developing techniques to make measurements less subjective and to minimise human variability. The automation ensures the inspector can concentrate on areas of potential concern and use their skills to efficiently make an accurate assessment of conditions.” This year NPL has deployed a ‘High Accuracy Inspection
System’ (HAIS) to Sellafield Ltd to carry out regular inspections of nuclear waste stores. The HAIS was conceived by NPL, and it uses digital image
correlation (DIC) technology to analyse image sets and quantify changes over time. This imaging technique has previously been used in sectors including rail, aerospace and oil & gas, but NPL says it is particularly well suited to monitor the integrity and conditions of different materials and using it to examine evolution of nuclear materials allows extremely small changes to be detected more rapidly than using traditional methods. NPL explains that DIC examines two images — before and
after — at the pixel level by creating small subset images. These images are compared for cumulative differences after applying x/y displacement, strain, shear and exposure differences. Using optimised techniques, the best fit for these parameters are found for each small subset of the
34 | October 2021 |
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allows high precision repeatable image capture with high quality images, so it can be used to inspect nuclear waste products in storage.
Addressing the problem For many years, encapsulated waste products have been stored at the Sellafield Ltd site and over the next decade retrievals from legacy ponds and silos will generate more. As Sellafield Ltd evolves into a waste management and remediation site the range of waste products is also expected to increase. It is vital, for safe storage, that a programme of inspection of the waste stores and their key properties is undertaken. However, with many thousands of packages located in engineered stores, it is time-consuming to extract and inspect them all. Therefore, in-situ measurements and deployment techniques are required to demonstrate control, allow appropriate mitigating action when necessary, and reduce dose to workers who currently provide ad-hoc measurement capability. HAIS can carry out regular inspections up to 16m deep
into low-level waste stores. HAIS deploys a camera vertically into an inspection port and takes a series of images inside the waste storage at pre-determined points. The HAIS system used at Sellafield has been designed
for this particular environment, where WIFI is not possible and hence it is cable operated, using digital transmission between components. It provides its own illumination as the environment is unlit and has to be able to traverse up to 16 metres down into an environment whilst capturing high quality images.
During the inspection the HAIS uses DIC to analyse
previous image sets and quantifies changes over time, including corrosion, movement, vibration and dirt or water ingress.
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