The nature of Pu-bearing particles from the Maralinga nuclear testing site, Australia

Abstract

The high-energy release of plutonium (Pu) and uranium (U) during the Maralinga nuclear trials (1955–1963) in Australia, designed to simulate high temperature, non-critical nuclear accidents, resulted in wide dispersion µm-sized, radioactive, Pu–U-bearing ‘hot’ particles that persist in soils. By combining non-destructive, multi-technique synchrotron-based micro-characterization with the first nano-scale imagining of the composition and textures of six Maralinga particles, we find that all particles display intricate physical and chemical make-ups consistent with formation via condensation and cooling of polymetallic melts (immiscible Fe–Al–Pu–U; and Pb ± Pu–U) within the detonation plumes. Plutonium and U are present predominantly in micro- to nano-particulate forms, and most hot particles contain low valence Pu–U–C compounds; these chemically reactive phases are protected by their inclusion in metallic alloys. Plutonium reworking was observed within an oxidised rim in a Pb-rich particle; however overall Pu remained immobile in the studied particles, while small-scale oxidation and mobility of U is widespread. It is notoriously difficult to predict the long-term environmental behaviour of hot particles. Nano-scale characterization of the hot particles suggests that long-term, slow release of Pu from the hot particles may take place via a range of chemical and physical processes, likely contributing to on-going Pu uptake by wildlife at Maralinga.

Environmental stability of a uranium-plutonium-carbide phase

“…As described by Cook et al 6 , Bruce is a complex particle, with textures indicative of formation via cooling of at least two immiscible polymetallic melts that formed in the explosion environment: Melt1 is Al-rich, and Melt2 is Pu–U–Fe(Al)-rich; Melt2 often forms globules within Melt1 (Fig. 1 A).…”

“…1 in Derimow and Abbaschian 12 ), Melt2 first crystallized a U–Pu–(Al)–C phase (Fig. 1 C), labelled Phase A by Cook et al 6 . These acicular crystals account for ~ 50 vol% of bulk Melt 2, and carry most of the Pu in the Bruce particle.…”

“…Burns et al 7 noted that during the Vixen B trials, “the plutonium melted and burned as it was ejected vertically to heights of up to 800 m and dispersed by the prevailing winds”. Particles experienced temperatures well in excess of 2000˚C, as demonstrated for example by evidence of (Pu,U)O 2 vapor coexisting with (Pu,U)O 2 solid in other particles 6 . These temperatures are consistent with the high-T synthesis of MAX and derivative-MAX-type phases 15 , which are typically produced at temperatures between ~ 1500 and 1600 °C under a reducing atmosphere.…”

“…A putative U–Pu-carbide phase was recently identified in a hot particle from the North-east plume associated with the Taranaki Test Site at Maralinga, South Australia. This particle, referred to as ‘Bruce’, was most likely from the Vixen-B sub-critical nuclear tests 6 , 7 , a part of the British nuclear weapon testing program conducted in Australia in 1952–1963. The Vixen-B trials (1960–1963) were ‘safety’ tests designed to investigate the performance of nuclear components subjected to a crash or a fire.…”

“…This involved the use of conventional explosives (TNT) to detonate Pu-containing nuclear warheads, which resulted in over 22 kg of 239 Pu being scattered over the area 8 . The U–Pu-carbide phase was identified on the basis of semi-quantitative Energy Dispersive Spectrometry (EDS) data showing U, Pu, Al, Fe, and C as major components, and X-ray Absorption Near-Edge Structure spectroscopy (XANES) results showing the presence of low valence (metallic-like) U and Pu 6 . Uranium, and by inference Pu, in carbide phases have metallic-like electronic structures as demonstrated by Butorine et al 9 using a combination of high-energy-resolution-XAS and the Anderson inclusion model.…”