Demystifying Uranium ratios:
How they can provide evidence on weapons
Uranium has two main isotopes — U235 and U238. U235 is more radioactive and a lot less abundant than U238. As it occurs naturally, about 99.3% is U238, almost all the rest is U235. If Uranium is to be used for its fissile properties these proportions have to be altered because the concentration of U235 is too low to produce the super-critical mass needed to generate a fission chain reaction in a nuclear weapon. This is why we hear so much about "Uranium enrichment". Enrichment is a process to increase the concentration of U235.
"The extent of the enrichment depends on the purpose for which the Uranium is required. Some military reactors used to produce Plutonium for use in nuclear weapons are fuelled with natural Uranium and use no enriched Uranium. Commercial nuclear–power reactors use Uranium enriched to about 4% in Uranium-235. For use in a fission nuclear weapon Uranium is enriched to more than 90%."1
There are several ways of enriching Uranium. We don't need to discuss them here.
The other side of the coin is that once the Uranium has been enriched there is a lot of U238 left over. This is known as Depleted Uranium (DU) and is essentially a waste product. However, it has properties of hardness, density, cheapness and a pyrophoric propensity to catch fire at relatively low temperatures which make it useful for various purposes. All four combine to make it especially desirable to the military for making armour-piercing and bunker-busting weapons — the hardness and density make for good penetration, while the pyrophoric quality incinerates whoever and whatever was inside the tank or bunker. And all at minimum cost to the taxpayer.
The bad news (bad, that is, for the public image of the military) is that using DU as a weapon is illegal because it has indiscriminate effects. It leaves a radioactive residue of insoluble Uranium Oxide dust. Being persistent and highly mobile it presents an inhalation hazard to civilians and soldiers of both sides in the conflict. Detection of the residue after a conflict is therefore important as evidence of criminal activity. The main way of doing it is take environmental samples and measure the ratio of U238 to U235.
Here we have to look at the third isotope of Uranium — U234, which is a product of the decay of U238. U234 makes up 0.0055% of natural Uranium by mass. This leaves U238 at 99.2745%; U235 at 0.72%. So to get the natural ratio of U238 to U235 we have to divide 99.2745 by 0.72 = 137.88.
When organisations publish analysis results with a number higher than 137.88 it means there is less U235 than naturally occurs, indicating that DU has been used. A number lower than 137.88 indicates the presence of an unnaturally large proportion of U235 — i.e. it’s enriched. In practice it is usual to assume that ratios between 136 and 142 are inconclusive; ratios larger or smaller than these values are significant.
1We are indebted to Prof. Frank Barnaby for the quoted paragraph. It's from his 2003 book How to Build a Nuclear Bomb Granta Books ISBN 1-86207-677-4
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