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It is not only the density, but also the purity of Uranium which makes a difference. Weapons grade uranium is 99.3% pure U235 or more by mass. Reactor grade ura

It is not only the density, but also the purity of Uranium which makes a difference. Weapons grade uranium is 99.3% pure U235 or more by mass. Reactor grade uranium could be between 90-94% at most (the last I had read). It isn't worth economically to enrich further. If the purpose is to reach sustained criticality, this purity is enough to heat the fuel-rods. Although it may not seem a lot, the 6% impurity is enough to act as a neutron damper (for runaway chain reactions). Consequently, you need an order or two higher density with reactor grade Uranium to make it go supercritical & have uncontrolled fission - something that does not happen in normal circumstances.

fsh 5y ago
These numbers are way off. Power reactors run with a few percent enrichment at most. 90% enrichment is deep in weapons territory. Reactors use slow neutrons that have thermalized in moderators. Due to the much higher fission cross section at low neutron velocities, they reach criticality at much lower 235U densities compared to nuclear weapons. In fact, this is a serious danger in nuclear fuel production. In 1999 there was a criticality accident in a nuclear fuel production facility in Japan that killed two workers [1]. [1] https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents#1999_accident https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents#19...
tgflynn 5y ago
Your numbers appear to be way off. Commercial reactor grade Uranium is enriched to less than 20% (typically about 5%) while weapons grade Uranium is closer to 90% (the first U-235 bomb was enriched to 80%). Some specialized reactors do use highly enriched Uranium however. Source: https://en.wikipedia.org/wiki/Enriched_uranium https://en.wikipedia.org/wiki/Enriched_uranium
srvmshr 5y ago
Replying here because I can no longer edit. Yes, my bad. I confused this with something I read a long while back (~10Y). Energy production uses enrichment in ~20% range. Thank you for generously correcting my answer. As for a follow-up, I genuinely mixed up with something on the general ballpark of '90-94%' -> Is it something about Thorium-based breeder reactors? What kind of processes use high enrichment in this range

fsh 5y ago

These numbers are way off. Power reactors run with a few percent enrichment at most. 90% enrichment is deep in weapons territory. Reactors use slow neutrons that have thermalized in moderators. Due to the much higher fission cross section at low neutron velocities, they reach criticality at much lower 235U densities compared to nuclear weapons. In fact, this is a serious danger in nuclear fuel production. In 1999 there was a criticality accident in a nuclear fuel production facility in Japan that killed two workers [1]. [1] https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents#1999_accident https://en.wikipedia.org/wiki/Tokaimura_nuclear_accidents#19...

tgflynn 5y ago

Your numbers appear to be way off. Commercial reactor grade Uranium is enriched to less than 20% (typically about 5%) while weapons grade Uranium is closer to 90% (the first U-235 bomb was enriched to 80%). Some specialized reactors do use highly enriched Uranium however. Source: https://en.wikipedia.org/wiki/Enriched_uranium https://en.wikipedia.org/wiki/Enriched_uranium

srvmshr 5y ago

Replying here because I can no longer edit. Yes, my bad. I confused this with something I read a long while back (~10Y). Energy production uses enrichment in ~20% range. Thank you for generously correcting my answer. As for a follow-up, I genuinely mixed up with something on the general ballpark of '90-94%' -> Is it something about Thorium-based breeder reactors? What kind of processes use high enrichment in this range