Explaining the Impossible
Depleted Deuterium is apparently at the core of some models of Bolter round used by the Imperium. At first this sounds like nonsense, and that there couldn't possibly be a logical explanation, but this may not be the case. Most assume that it was a mistake (and it is possible), that the writer meant 'Depleted Uranium' which is used in modern penetrator rounds. However, 40K is a sci-fi-fantasy hybrid and what if we assume the term is 'correct', and what hypothetical would explain this bizarre terminology?
What follows is a long winded explanation and an exercise in mental gymnastics. It is totally unofficial and just how my mind put it together to rationalise it all (it shouldn't be taken as gospel). I've posted it here to illustrate that even the bits that many fans consider silly could in fact make sense given a little creative thought. I write this because I love the idea that what seems primitive or nonsense in 40K can be flipped around and made reasonably sophisticated and then used to explain complicated terms in a fun way (education by stealth!). Read at your own peril 
= This is Heresy =
Uranium
First we need a premise for the terminology. Seeing the DU (Depleted Uranium) is often put forwards as 'what they really meant to say' I thought it would be fun to start with that! The element known as Uranium is actually a mix of several isotopes, the two most common being Uranium-235 and Uranium-238. Uranium can be processed to separate these two isotopes form one and another. The results of this processing yields one part with more Uranium-235, and the other with more Uranium-238. Neither part is 'pure' on isotope or the other. In general terms, the two parts are categories according to which isotope has increased: The part with more Uranium-238, and less Uranium-238, than naturally occurring is referred to as ‘depleted’, and the other part which has more Uranium-235, and less Uranium-235, than naturally occurring is referred to as ‘enriched’.
So really ‘depleted’ and ‘enriched’ relate to which isotope the material contains above normal in percentage. With weapons grade enriched Uranium it is more the 90% Uranium-235.
So it could be said that depleted Uranium is literally a name for mostly U-238 and enriched Uranium mostly U-235 (generally speaking, not 100% I know, but it will do for the next bit…)
This terminology is then taken and applied to the element Hydrogen...
Depleted Deuterium
Hydrogen has is three isotopes 1H (Hydrogen), 2H (Deuterium) and 3H (Tritium) and they are all present in hydrogen. So if Hydrogen is ‘depleted’ and 2H and 3H isotopes are removed, the part that is mostly a mix of 2H and 3H could be referred to (by the Adeptus Mechanicus) as ‘enriched Hydrogen’.
If this ‘enriched Hydrogen’ (2H and 3H) was further process and the 3H was removed; the 3H could be referred to as ‘super enriched Hydrogen’. While the bulk of the remainder which is mostly 2H could now be referred to as being depleted Hydrogen - but we already have ‘depleted Hydrogen’ (the first process where 2H and 3H was removed) so it is specifically referred to as ‘depleted 2H’. This would be 2H with very little tritium in it (if any).
This makes ‘depleted 2H’ a very specific name for specific product.
Gothic
In translation from High Gothic into English the words for ‘depleted 2H’ is translated as ‘depleted Deuterium’. This is technically inaccurate, but a correct translation in layman's terms.
Conclusions
It’s all to do with Adeptus Mechanicus processing systems and terminology.
A priest will instantly know that a product marked ‘depleted Deuterium’ (translation) is nearly pure 2H derived from a super enrichment process, whereas a product marked enriched Hydrogen will be mostly Deuterium but will also contain Tritium.
Deuterium would be the name of the element, but to the Adeptus Mechanicus this would never be used for a product because a product with always contain the other isotopes (and the Adeptus Mechanicus are very picky).
They would use ‘Hydrogen’ as an element name and as a product because all the isotopes have the same number of protons: One.
Terminology
The Adeptus Mechanicus could use the following terminology where describing these products.
- Hydrogen (Hydrogen with all isotopes)
- Depleted Hydrogen (Near pure 1H, with trace of 2H and 3H)
- Enriched Hydrogen (Contains mostly 2H and a smaller amount of 3H, and not much 1H)
- Super Enriched Hydrogen (Mostly 3H with a wee bit of 2H, and an even smaller amount of 1H)
- Depleted Deuterium (mostly 2H, with a small amount of 1H)
Depleted Deuterium can only happen because Hydrogen has three isotopes of interest, and so there is two steps instead of one. There would be no Depleted Tritium as there is nothing else to take out (I bet GW puts ‘depleted Tritium’ just to mess with this theory wow, my ego! )
Confusion
This concept also shows the Adeptus Mechanicus to be very precise when it comes to terminology. The way we use names would not be tolerated by the Cult! One of the reasons being that mixing element and product names causes confusion as borne out in multiple forums
Well that’s my (unofficial) thoughts on the matter!
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