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

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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.