Laser-driven multi-modal probe beams for nuclear waste inspection

This proposal aims to develop a revolutionary new tool for the stand-off inspection of nuclear waste packages using laser-driven x-rays and neutron beam pulses. This will be a collaborative project between the University of Bristol's Interface Analysis Centre (IAC), Sellafield Limited, Queen's University Belfast (QUB) and the STFC Central Laser Facility (CLF). By firing an extremely high-energy laser for a very short duration, an intense spot of x-ray radiation is generated and projected towards detector plates. In a similar manner to medical x-rays, any object placed between the bright source of x-rays and either photographic film or digital image plates, is captured in detail. However, because a very high energy is used, imaging of packages containing uranium waste, one of the densest materials on Earth, is possible.

Since 1952, Sellafield has been responsible for safe storage and reprocessing of all the UK's nuclear waste. Decades of research and development have resulted in more manageable forms of nuclear waste. However, a number of problems remain, particularly with the ageing 'legacy' nuclear waste that has been stockpiled since the 1960s. Before long term storage in a geological disposal facility is considered, the composition and degradation state of the waste material and containment vessels needs to be established. Due to the radioactivity and dangerous corrosion products formed during storage, a destructive investigation of the waste containers is considered far too hazardous to be performed. Therefore, a non-destructive, stand-off evaluation of the containers is proposed.

For a visual inspection of the internals of a nuclear waste package, high-energy x-rays are used to create an image of the sample. Typical means of producing x-rays do not achieve either the resolution required or the energy to penetrate through the large, dense waste containers. Therefore, it has been proposed that the petawatt Vulcan laser at the CLF is utilised to generate the necessary high-energy x-rays required for this analysis.

In addition, the Vulcan laser facility is capable of producing a beam of neutrons in parallel with the high-energy x-rays. By probing the waste containers with a neutron beam any fissile material contained inside will undergo a small amount of fission and emit secondary neutrons. Depending on the fissile material that reacts, the emitted neutrons will generate a unique signature which can be used to identify the particular isotope present in the sample. Analysis of this data holds the potential for isotopic quantification, thus identifying the exact quantity of highly radioactive uranium-235 compared to the isotopically different, and far less radioactive, uranium-238.

Whilst the initial aims of this proposal are for characterisation of samples via a single-shot approach, the end goal is the development of a system capable of firing up to ten times a second by construction of a small footprint, high-energy DiPOLE laser with the corresponding sensors capable of rapid data acquisition. In anticipation of such a system, one component of this project aims at improving existing detector technology with a focus on rapid image capture and neutron detection.

The final section of the project is the production of a business case to pursue the eventual development of a fast firing analysis system to form the basis of a nuclear waste package scanning facility. Much like CT scanning, by rotating the waste container during multiple image acquisition a 3D profile of the contents can be constructed. This technique would allow us to probe deep inside the waste containers and assess their contents in detail without any destructive investigation or disturbance to the potentially toxic, pyrophoric, and radioactive contents. We consider that this technology would have excellent global export potential to other nations producing nuclear waste.