DDopson Ars Tribunus Militum
22y
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Doc12 said:
If someone has access to industrial centrifuges then HALEU seems like a convenience rather than a necessity. Could use un-enriched uranium in your centrifuges and would just take longer to get to the end point?
Yes, quite a bit longer. If you are a nation-state that can build an industrial scale centrifuge operation, then any reactor fuel is a proliferation risk. It takes several times more effort to concentrate to 5% than from 5% to 90%.

There's a concept called Separation Work Units (SWUs) that can be used to quantify how much centrifuge time is needed to separate x kg of uranium at y% enrichment into an enriched stream at z% enrichment and a tailings stream with <<y% enrichment. And there's a calculator here. https://www.urenco.com/swu-calculator

Let's say you want to produce 1 kg of 90% enriched U235.

Starting from 176 kg of natural uranium, it takes 227 SWUs of effort if you are exhausting tailings at 0.2%. Or if you exhaust tailings at 0.5% it only takes 154 SWU, but then you need a whopping 424 kg of natural Uranium feedstock. Either way, it's a lot of work.

Starting with 20 kg of 5% enriched reactor fuel, drained down to 0.5% tailings, is 48 SWU. Or if you can afford be wasteful of 5% enriched reactor fuel, 30 kg + 30 SWU gets you to the same place with 2% tailings, something that only makes sense if you've stolen the reactor fuel rather than enriching it from natural uranium.

Starting with 5 kg of 20% enriched HALEU drops the separation effort to 12 SWU, assuming 2% tailings. Or if you stole a lot of it, and only need to extract half the U235 content, 10% tailings is 7.4 SWU per kg of weapons-grade HEU.

The key point is that the greatest investment of separation work comes in the early enrichment stages where vast quantities of natural uranium are concentrated into relatively smaller quantities of moderately enriched uranium. It takes several times more effort to concentrate to 5% than from 5% to 90%.

So this risk isn't unique to HALEU. Regular reactor fuel is almost as problematic. //

DDopson Ars Tribunus Militum
22y
1,933
Subscriptor

clewis said:
Timed to the µs? I'll just use a Raspberry π and a real time operating system. If we need ns, I might have to buy some speciality expansion boards.

While modern technology has rendered trivial some aspects of the problem that were much harder in the pre-transistor era, it's still not quite as simple as that. You need high quality explosive initiators that will trigger detonation with a predictable lag, versus all of the default solutions have unacceptably high variability such that one side goes before the other, creating an asymmetric implosion. The solution is often found by driving enormous currents through wires or foil embedded in relatively sensitive explosives, and in 1945, achieving such a rapid current rise was a major challenge. Today, someone developing similar high-precision initiators would benefit tremendously from the plethora of off-the-shelf power electronics (much more important even than the Raspberry pi), but it's still something that a real engineer needs to spend some time testing very carefully or your weapon will dramatically underperform it's designed yield. //

DDopson Ars Tribunus Militum
22y
1,933
Subscriptor

Wickwick said:
Sandia National Labs has (at least) one group which specializes in analyzing the risk of improvised industrial goods for explosive yield. Many years ago I helped adapt some lab equipment so it could be used in a cloud of muriatic acid as if someone had detonated a railcar full of the stuff. Just last year at a conference I was speaking to one of their researchers about the threats posed by things like diesel tankers, LNG ships, etc. None of those things are viewed particularly highly on the threat matrix compared to something like ANFO (Beirut port explosion). Fuels have a lot of stored chemical energy, but you can't couple that to the environment in a manner that creates a large explosion without having an oxidizer intimately mixed in. Achieving the near-nuclear-level blast of a massive fuel-air burst bomb is a non-trivial engineering feat. //

+1, and note that weaponized fuel-air bombs don't just use any old fuel laying about. They have a specifically engineered mix of a highly volatile liquid with a wide detonation range (better tolerance for uneven dispersion), such as ethylene oxide, with a powder like aluminum, which boosts the energy release per unit of oxygen consumed.

Poorly dispersed commercial fuels badly underachieve their on-paper energetic value.