DU emerges as a long term pollutant in the environment whose very decay products serve to increase its persistence.
Campaigners have long argued that DU residues from conflict present a long-term risk to civilians.
This is due in part to the rate of radioactive decay from the isotopes in DU - and indeed DU gets more radioactive for thousands of years due to the in-growth of radioactive decay products.
But as two new studies reveal, it is also because of the rate at which DU dust particles and intact or fragmentary DU penetrators corrode in the environment - and the corrosion products.
To the horse's mouth - the UK's live DU firing ranges
The two UK studies have now shed more light on the processes that affect DU's environmental persistence.
The studies were undertaken at the UK's DU firing ranges. The first, 'The corrosion of depleted uranium in terrestrial and marine environments' by C. Toque et al, was performed at Kircudbright in Scotland, where DU rounds are fired into the sea.
The second, 'Microanalytical X-ray Imaging of Depleted Uranium Speciation in Environmentally Aged Munitions Residues' by Daniel E. Crean et al, was carried out at Eskmeals in Cumbria, England. Here DU was fired into hard targets to examine its effectiveness against different types of armour.
Eskmeals: considerable volumes of DU dust
The testing at Eskmeals produced considerable quantities of DU dust, some of which was found to have spread 6km from the site to the nearby village of Milom.
Researchers took samples of DU particles from two separate areas on the site and analysed them to assess how they had changed over the 30 years since they were first produced.
The two sites comprised an area of open surface soil, and a disposal area for DU-contaminated wood. "U speciation was different between the two areas", the scientists found.
And given the high rainfall and oxidising conditions at the site, they may have been surprised to discover that:
"Surface soil particles showed little extent of alteration, with U speciated as oxides U3O7 and U3O8. Uranium oxidation state and crystalline phase mapping revealed these oxides occur as separate particles, reflecting heterogeneous formation conditions."
Low DU solubility and mobility in soil
They conclude that "The persistence of U oxide phases such as U3O7 and U3O8 reflects the low solubility and mobility of the primary species in surface soils at the Eskmeals site."
Furthermore, "the presence of primary impact particles results in the persistence of health risks associated with inhalation, should these particles be disturbed."
No less surprisingly, "Particles recovered from the disposal area were substantially weathered, and U(VI) phosphate phases such as meta-ankoleite (K(UO2)(PO4)·3H2O) were dominant.
"Chemical imaging revealed domains of contrasting U oxidation state linked to the presence of both U3O7 and meta-ankoleite, indicating growth of a particle alteration layer.
However meta-ankoleite, a mineral also known as Hydrated Potassium Uranyl Phosphate, is highly insoluble under basic, neutral or mildly acidic conditions. The mineral's formation indicates once again the persistence of DU in the environment.
"This study demonstrates that substantial alteration of DU residues can occur, which directly influences the health and environmental hazards posed by this contamination", warn the researchers.
30 years, on, DU particles are still an inhalation hazard
In other words, the uranium oxide particles from the testing were found to be strongly resistant to further corrosion; and the corrosion product meta-ankoleite is itself highly insoluble. So even 30 years after firing the particles would still present an inhalation hazard if resuspended.
This finding fits with the results of a study around a former speciality metals factory in Colonie north of New York. Researchers there showed that particles produced between the 1960s and 1980s were still intact and present in the environment.
In that case the particles had been produced through the incineration of DU, not by its use in weapons. However this new study demonstrates that DU residues from munitions use are similar in composition and persistence.
That these particles can survive for so long in the comparatively wet conditions of the UK and northern US suggest that particles in the arid conditions of Iraq may be even more long-lived.
DU corrosion - a complex picture emerges
Meanwhile a second study, this time of fragments of DU, has again highlighted the significant gaps in our ability to predict the future behaviour of solid contamination.
During test-firing at Kircudbright, intact or partially intact DU rounds have ended up both in the sea and on the range due to firing malfunctions.
The scientists therefore studied the behaviour of DU in both soils and the marine environment. The results showed that DU corrosion in soil is highly complex and subject to apparently random variations due to the protective effect of corrosion products.
Incorrect estimates can all too easily arise
"The rate of mass loss was found to vary through time in one soil environment and this is hypothesised to be due to pitting increasing the surface area, followed by a build up of corrosion products inhibiting further corrosion.
"This indicates that early time measurements of mass loss or corrosion rate may be poor indicators of late time corrosion behaviour, potentially giving rise to incorrect estimates of time to complete corrosion.
"The DU alloy placed in apparently the same geochemical environment, for the same period of time, can experience very different amounts of corrosion and mass loss, indicating that even small variations in the corrosion environment can have a significant effect."
Marine corrosion also poorly understood
The researchers were more confident about predicting the behaviour of DU in the marine environment, where chemical conditions are less variable. But even here it is difficult or impossible to replicate the conditions in a laboratory setting:
"The marine environment at the experimental site was very turbulent. This may have caused the scouring of corrosion products and given rise to a different geochemical environment from that which could be easily duplicated in laboratory experiments."
In conclusion whole picture is unexpectedly complex and unpredictable:
"The experiments highlight that the corrosion of DU is controlled in the environment by a number of factors that are not fully understood. It is therefore difficult to undertake laboratory experiments to truly replicate the conditions in real corrosion environments."
Downplaying of DU concerns unsupported by science
The findings support ICBUW's view that attempts by the UK and US government to downplay concerns based on the findings from a limited number of contaminated site assessments in the Balkans are not supported by the available science.
ICBUW has long argued that the variability of conditions at different sites requires that each is individually assessed and the risks they may pose to civilians and the environment calculated.
Following its assessments in the Balkans, the UN Environment Programme suggested that intact or fragmentary penetrators in soils may have completely corroded in 25 years.
These new studies suggest that the actual picture may be far more complicated than originally assumed. But more than that, DU emerges as a long term pollutant in the environment whose very decay products serve to increase its persistence.
Also in The Ecologist: Chris Busby on 'Uranium - the 'demon metal' that threatens us all'.
Doug Weir coordinates the International Coalition to Ban Uranium Weapons.
Oliver Tickell edits The Ecologist.
This is an extended version of an article first published by ICBUW: New study finds that depleted uranium particles are long-lived in environment