Formation of an Unusually Stable Crystal Found in the Ashes of a 1963 Nuclear Weapons Test in Australia.
I've been reading through my son's Ph.D. Thesis proposal - which he's been advised to narrow, although I think he should remember what he cuts - and in doing so, I finally understood some materials science issues in nuclear engineering to which he's been alluding all these years that kind of passed through my brain, one ear to another, without sticking.
Cool. Sometimes reading is better than listening. That little brat is a smart kid.
This set me off wandering around the internet thinking about certain types of crystal structures - he's focused on cubic symmetry crystal structures, for reasons I now understand - and I ended up thinking about hexagonal structures exhibited by certain metals including a metal I love, technetium - a fission product that does not otherwise exist (except on an atomic scale in uranium ores) naturally on Earth - that can be isolated from used nuclear fuel as a substitute for the applications of the rare metal rhenium, including improving the properties of tungsten as an alloy.
One thing I learned from reading his work is that it is possible to make alloys with structural features that are only accessible in additive manufacture (3D printing) procedures; it is too difficult to make some of them via another route.
Wow! That is interesting.
Well, one thing leads to another. Here's how I think I ended up at the paper I'll discuss in this post: Some interesting phases that offer a mixture of properties characteristic of ceramics - high melting points and chemical resistance - and metals - chiefly machinability are known as MAX phases, which exhibit hexagonal crystal structures. I'm sure I've written about them before. (The world leader in MAX phase chemistry is Michel Barsoum, who I once took to dinner, whereupon he told me that the industrial manufacture of these phases is problematic. He's a wonderful person, a very nice man.)
Since I am interested in certain properties of nuclear fuels, I recalled a long ago speculation I'd had - which I abandoned as ridiculous - about actinide MAX phases. I decided to check up on how ridiculous I was being and came across this open sourced paper:
Etschmann, B., Missen, O.P., Conradson, S.D. et al. Environmental stability of a uranium-plutonium-carbide phase. Sci Rep 14, 6413 (2024).
The paper is open for reading, but I'll point to some cool stuff in it anyway with a few excerpts:
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 tests6,7, 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 239Pu being scattered over the area8. 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 Pu6. 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.
Uranium- and Pu-carbides are usually pyrophoric at µm-grain size, i.e. they oxidise rapidly in contact with oxygen or water10,11; yet this particle survived in the regolith for approximately 30 years under near-surface semi-arid conditions before being collected by remediation crews in the 1980s7,8. Thereafter, the particle was stored under ambient conditions until it was examined with synchrotron radiation in October 2018, and sliced open with a focused ion beam (FIB) in May 2019, exposing the U–Pu-carbide phase (Fig. 1A). Further FIB-scanning electron microscope (FIB-SEM) investigations in March 2020 highlighted that this Pu–U-carbide phase showed no signs of oxidation upon direct exposure to atmosphere for approximately 10 months...
It was, in other words, surprisingly stable.
Some more stuff:
In June 2022, a FEI Quanta 3D FIB-SEM was used to extract a ~ 32 µm3 ‘chunk’ of phase-A from Bruce (Fig. 1B,D). This sample was stored under ambient conditions for 3 months before access to synchrotron beamtime, and again there was no discernible oxidation of phase-A during this time. The single-crystal X-ray diffraction study was carried out at the micro-focus macromolecular MX2 beamline of the Australian Synchrotron13. Crystal data and details of data collection and refinement are given in Table 1. The crystal was maintained at 100(1) K in an open-flow nitrogen cryostream during measurement. The crystal structure refinement indicates that the phase has the structural formula (U,Pu)(Al,Fem3C3. The final model converged to R1 of 0.0297 for 288 independent reflections (1747 measured reflections), and chemical formula (U0.59(5)Pu0.41(5))(Al0.54(2)Fe0.46(2) 3C3 (Table 1)...
A little further on:
And for a last excerpt, that I find really cool:
(U,Pu)(Al,Fe)3C3 and UAl3C3 are “derivative-MAX phases”, with the general formula (MC)n(Al3C2)m and n = m = 1. Derivative-MAX phases contain metal ions (M) in [6 + 6]-fold coordination (e.g., MC6Al6), compared to sixfold coordination in MAX phases. As a result, derivative-MAX phases frequently contain high-Z metals on the M-site, including Y, Gd-Tm, Yb, Lu and U14,18,19. The Al layers in MAX phases can be described as centred hexagonal rings, but the Al3C3 layers in derivative MAX phases lack centring.
An interesting thing about these phases, described in the paper is that in general, pressure is not required to make them, they can be made apparently only by heating - which suggests to my mind wonderful possibilities.
It appears from the paper as well that depleted uranium tank shells of the type we all hate around here from the days of the Iraq oil wars, also can give rise to interesting phases.
This is, I know, esoteric, and while I hate weapons of war as a conditional pacifist - if that's not an oxymoron I don't know what is - it is wonderful that despite the moral hollowness of such efforts, there are scientific silver linings.
These things, I believe, have implications of special types of nuclear fuel that can help save the world.
Have a nice weekend.