|Waste and Fuel Management|
|Dr Chris Jones|
As of 2013 the UK currently stores 120,000 tonnes of Intermediate Level Waste (ILW) nationwide, with a further 190,000 tonnes predicted from decommissioning existing sites. This waste is reprocessed prior to storage in 500 litre stainless steel drums backfilled with concrete. These drums weigh in at ~1.2 tonnes each (Figure 1) and are housed and maintained at the Sellafield Ltd site in Cumbria.
A number of these canisters have started to exhibit external deformation after extended periods in storage. Therefore, examining the internal corrosion state and structural integrity of the drums while in storage is of paramount importance. However, this is particularly challenging due to the high radioactivity present, and the large scale dense material being impenetrable to traditional x-ray imaging techniques.
In order to penetrate the large containers, x-rays with a far greater energy than those used in medical imaging are required. This can be achieved by firing a pulse of petawatt (1015W) laser light onto a tantalum target. A bright flash of very high energy x-rays is produced with energies ranging up to several MeV. These then travel through the waste container and are detected in the same way traditional x-rays are captured producing a comprehensive picture of the inside of the nuclear drum.
An initial proof of concept study was performed on small scale simulated waste packages (Figure 2) which were imaged using a 10ps single pulse acquisition generating x-rays with a peak at ~200keV energy and compared to those obtained on a commercial x-ray tomography system. This was sufficient to identify features such as grout cracking caused by the corrosion within our sample and feature sizes as small as 100µm. In addition, on a parallel beam-line, neutron generation was achieved, this extends the capability of the proposed analytical device by enabling isotopic quantification of the fissile material present in the drums. The key enabling property of this approach is the small source size, since this enables high-resolution imaging with high-energy x-rays. Figure 3 demonstrates the potential of this novel approach; a uranium penny was imaged to sub-millimetre resolution behind 400mm of grout and metal. In this instance, a 200µm tantalum foil was used as the target which sacrificed lateral resolution for increased flux of x-rays.
Current development aims to produce a diode pumped high-energy laser system for rapid analysis of waste drums. These systems are capable of a 10Hz firing rate which in conjunction with a drum rotating stage will enable tomographic reconstruction of large waste containers (Figure 4). In addition, neutron measurements will be capable of identifying the quantity of fissile material from a single laser pulse. The current project is funded by a 3-year Science and Technology Facilities Council (STFC) Industries Partnership Scheme (IPS) to continue feasibility testing at the Central Laser Facility using the Vulcan laser. This project includes allocation of funding to develop a business case to begin development of this high-end equipment with Sellafield Ltd as the primary end-user. The ultimate aim is to incorporate this technology as the key component in a scanning facility to identify problematic containers prior to storage in a geological disposal facility (GDF).
The technology itself is highly versatile, with the ability to identify phase-changes in material, freeze ultra-fast motion (due to the femto-second pulse width) and penetrate large, highly dense, containers whilst providing sub-mm imaging resolution. The potential for this technology is wide reaching, extending far beyond the nuclear sector.