AMS-UK: A UK Accelerator Mass Spectrometry Facility for Nuclear Fission Research

Phase 2 of the National Nuclear User Facility is a significant investment in science and engineering facilities and apparatus to support nuclear fission research on radioactive samples in the UK. This proposal is submitted under this initiative and concerns a very sensitive technique for the assessment of a significant group of radioactive elements produced in nuclear reactors: the actinides. The actinides are amongst the heaviest known elements, formed as a result of neutron capture on uranium. They are all radioactive, to a greater or lesser degree, and several are very long-lived. The combination of their radioactivity and chemistry renders some significant radio toxins that have be managed and stored carefully. The most significant is plutonium, which is often present in the form of the isotope 239Pu and to a lesser extent, 238Pu, 240Pu, 241Pu, 242Pu and occasionally 244Pu.

Plutonium is effectively extinct on Earth as a natural product of the Big Bang because its half life is too short to have survived. However, minuscule quantities are known to have formed in geological deposits that are naturally rich in uranium, via natural neutron capture processes on the most abundant uranium isotope, 238U, in these ores. Plutonium has been re-introduced to the environment, predominantly as a result of atmospheric nuclear weapons testing in the 1950-1990 period (fallout), but also as a result of nuclear reactor accidents (Chernobyl and Fukushima) and the dispersion of effluents from nuclear reprocessing activities: in the UK this is thought to be most significant due to activities at Sellafield and Dounreay.

The high radio-toxicity of plutonium requires that materials contaminated by it are managed and stored very carefully, especially since large quantities are soils from contaminated land and building materials from contaminated structures. However, how do we discern what was there before, often in a wider context (from fallout and natural arisings in uranium-rich ores), from what has been dispersed locally? Simply 'detecting' plutonium is not sufficient because, whilst radioactive, it is usually dispersed at such minuscule levels there is not enough to provide enough radiation to detect it on a practical basis. Special samples can be made and the alpha radioactivity counted from these, but this does not allow individual isotopes to be discerned, which is an important requirement: fallout material is often rich in the heavier isotopes (242Pu and 244Pu) whereas material from nuclear reactors tends to be rich in 239Pu, 240Pu and 241Pu.

In this proposal, we recommend investing in a recently-established capability to measure plutonium isotopes by their mass rather than their radioactivity. The isotopes are accelerated from a sample into which the plutonium has been extracted by dissolution, and dispersed in a magnetic field. They are ionised and collected in a particle detector where their position (as a result of the magnetic field deflection) and their rate of energy deposition are used to identify them, usually as a ratio of the rare isotope to an abundant alternative, where the latter can be introduced artificially to highlight the rare variant. This approach is called accelerator mass spectrometry. Until recently, this relied on large machines at particle accelerator facilities and was very expensive. Now, commercial systems are available that are smaller and cheaper, but the UK does not have one despite being the custodian of the largest stockpile of civil-separated plutonium. This proposal recommends that one of these is installed at Lancaster University, for external usage by the whole nuclear fission community. This is an important proposal because the UK Government committed to an agreement, the 'nuclear sector deal', which requires that businesses reduce the cost of decommissioning by at least 20%. Improved plutonium assay of contaminated materials will make a significant contribution to this aim.

The primary focus of the impact agenda for the AMS-UK is on realising the 20% reduction in decommissioning costs of the UK nuclear legacy, as part of the UK Gov't nuclear sector deal. We intend to contribute to this important target specifically by supporting understanding in the nuclear clean-up sector associated with plutonium-contaminated material (PCM). PCM is a major challenge associated with the clean up of nuclear legacies, especially where the land and facilities involved have been part of a reprocessing activity or subject to a reactor accident. It is a challenge because of the high radio-toxicity of plutonium and because trace plutonium in dispersed form is very difficult to detect in bulk quantities of material (since the dominant alpha-decay signature cannot penetrate the bulk substance and its containers, and the signature gamma-ray emissions used in nuclear fuel assay are not sufficiently prominent in dispersed form). Often, in the absence of analytical data to prove otherwise, a suspicion of an association of a given facility or tract of land with plutonium-related activities can be sufficient for waste to be consigned as PCM when it might be low-level waste (LLW) and thus disposed of much more cost effectively if so (see for example: 'Sellafield Ltd. and LLW Repository Ltd. Joint Waste Management Plan' (September 2018), https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/751137/Sellafield_Ltd_JWMP_15_Issue_1_unsigned.pdf and, specifically, page 11 where the requirement is stated thus 'implement a programme of work to further segregate material from the alpha stream that can be safely managed as LLW').

Once wastes are consigned as PCM, it can be too expensive for materials to be retrieved from interim storage and segregated afresh. Hence, the focus of this project will be on wastes arising from subsequent phases of clean-up, i.e., the analysis and processing of land and building wastes suspected to be contaminated. The AMS-UK will not be able to analyse bulk wastes in entirety, but it will enable baselines against which the inherent contamination of these materials can be compared with which to inform plans to deal with these large quantities of material. The most important sites for this impact are Sellafield and Dounreay, because these sites offer the largest challenge because of their history of reprocessing, the dispersion of plutonium-containing effluents before restrictions on this were imposed and accidents in which plutonium may have been dispersed (c.f., the Windscale fire and the Dounreay shaft explosion).

The nature of this impact is not only likely to be economic, even though the forecast expenditure on this problem is huge and therefore a 20% saving is also likely to be very significant. The impact from the AMS-UK use will also be societal, as its use to establish baselines and the origin of dispersed plutonium at or near to licensed sites will provide workers on the site responsible for this programme and local residents' groups with increased confidence that the work is being done properly, cost-effectively and that sites are being returned to baseline status wherever practicable. The impact will also be educational because AMS provides isotopic ratio data that can be used to indicate the origin of plutonium contamination in addition to its quantity. For complicated sites, such as Sellafield, there might be a number of sources of trace actinide material, with some originating long ago at a time of heightened secrecy and sparse record-taking. It is also possible to impact this programme by taking samples and AMS readings after clean-up, because the extraordinary sensitivity of the AMS technique can also be used to assess the efficacy of bulk clean-up programmes after they are completed.