The Muon: A Particle that Punches above its Weight

Peter Walmsley explains the role of muon tomography in waste management at the Sellafield site

ONE of the highest hazard facilities in the Nuclear Decommissioning Authority (NDA) estate is the Magnox Swarf Storage Silos (MSSS)¹ on the Sellafield site. The facility was first constructed in the 1960s, and its 22 compartments are used to store a mixture of different types of higher activity nuclear waste. The majority is waste resulting from the reprocessing of spent fuel from Magnox reactors².

Concerns around the ability of the aging compartments to continually store this waste in a safe manner have led to a decision to retrieve the waste from the silo compartments and ultimately reduce the risk and hazards that the facility poses

Reprocessing is a key operation, recovering uranium-235 which can be reused to manufacture new fuel. It is the first stage in the reprocessing operation, removing the outer magnesium shell to expose the irradiated metal fuel³, which creates the magnesium “swarf” waste. The swarf is contaminated with uranium metal fuel, inadvertently removed during the shearing process. Following its removal, the swarf is stored under the cover of water, in the swarf storage silos. Swarf ceased to be deposited at MSSS following the turn of the 21st century but concerns around the ability of the aging compartments to continually store this waste in a safe manner have led to a decision to retrieve the waste from the silo compartments and ultimately reduce the risk and hazards that the facility poses⁴.

An integrated waste management process

While a single-step approach to waste retrieval and treatment is often preferable, the NDA recognises that there are scenarios in which an alternative, staged process is more suitable. The extent of the characterisation information available, the condition of the raw waste storage facility and the deliverability/affordability of the programme play a role in determining the selected approach. Adopting a staged process, where retrieved waste is held in an interim state before final conditioning and disposal, creates the opportunity to understand more about the form of the waste and prepare the most effective means to condition it for disposal whilst minimising near-term risks to a tolerable level.

This progressive risk and hazard reduction strategy (see Figure 1) involves waste being retrieved from the legacy facility, for interim storage within robust containers within a heavily-shielded modern storage facility. Following an extended period of storage, waste conditioning is expected to be undertaken ahead of disposal in a geological disposal facility (GDF)⁵, which will be a highly engineered structure, capable of providing protection for thousands of years.

Condition monitoring and inspection (CM&I)

Whilst a staged approach may result in the delivery of immediate risk reduction, there are still considerable risks and consequences which must be considered throughout the waste lifecycle. During the interim storage period, there is a requirement to understand how the waste and waste container are evolving, to ensure that the waste is behaving as modelled and that there is no risk of a loss of containment from the storage package. The process by which information is gathered on a waste package is called condition monitoring and inspection (CM&I). In addition to alerting against the key unmitigated risks, CM&I also provides regulatory confidence, informs the requirements of the final treatment or conditioning plant, and demonstrates that delays to the delivery schedule of the GDF are tolerable.

During the interim storage period, CM&I can be conducted in-situ (with the waste container located in its stored position within the modern storage facility) or ex-situ (with the waste container moved away from its stored location). Regardless of this, the dose rates from the packages prevent personnel access to the package, thus the technology must be deployed remotely.

Technologies to deliver monitoring capability

A range of technologies with varying readiness levels, from basic visual inspection to complex novel imaging techniques, is being considered by Sellafield Ltd to fulfil the site’s CM&I requirements. Where technology is at a lower readiness level, funding initiatives such as the Game Changers nuclear innovation programme have assisted in developing these tools.

While this article mainly focuses on muon tomography, one CM&I technique which has potential across multiple sectors is Raman spectroscopy. The Fraunhofer Institute of Applied Photonics received funding to develop a Raman Spectroscopy system, from the proof-of-concept stage, to measure the concentration of hydrogen within Sellafield’s waste interim stores. The waste packages are expected to generate hydrogen during interim storage as a result of the reaction of magnesium and water, which will be released into the surrounding environment through filters on the waste container. Hydrogen gas is highly flammable, and its concentration is therefore a parameter of interest (it may also be possible to use concentration to infer the rate of reaction and therefore consumption of reactants).

Raman spectroscopy is a long-established lab-scale technique which uses the intrinsic nature of light’s interaction with a material to determine its chemical and structural information and evolution of the waste form. The Fraunhofer Institute aims to develop a system which can obtain range-resolved measurement of hydrogen concentration in a large storage environment. This is to be achieved by using a pulsed laser to excite a sample and, by measuring the wavelength, time-of-flight and intensity of the scattered photons, the concentration of hydrogen and its range can be inferred. The use of single-photon detection techniques allows such measurements to be made over extended (10 m+) range. A practical deployment has already been achieved at the Sellafield site⁶ and, in collaboration with REACT Engineering, options for deploying this tool during interim storage (in the next 5 years) are being developed. There may be other applications for Raman spectroscopy within the nuclear industry, as hydrogen generation due to the decomposition of water in contact with ionising radiation (radiolysis) is a common problem.

Here though, we’ll focus on the challenges faced in deploying muon tomography, and the work done to develop a system suitable for a highly-constrained nuclear environment. Similar to the more commonly used techniques of X-ray, muon tomography is an imaging technique which can be used to identify regions of differing densities and define their shape and dimensions. In the context of CM&I, it can be used to identify deformations within objects, distribution of waste types, and the measurement of phase boundaries. Lynkeos Technology, a 2016 spinout from the Nuclear Physics group at the University of Glasgow, and the National Nuclear Laboratory (NNL) are developing such a system. (This work was made possible via a successful Sellafield-funded R&D programme in 2009 and later commercialisation under an Innovate UK contract in 2017.) Note that while Raman spectroscopy lends itself to in-situ deployment, the size of the muon imaging system means ex-situ deployment is the only feasible option. Lynkeos is aiming for deployment within the next 5 years.