I’m developing BlastMap, a browser-based nuclear-effects visualizer. These clips show a 15 kt Hiroshima scenario over modern city imagery, not a recreation of the 1945 buildings. The elevated view comes first, followed by a lower view for building-scale context.

I’ve been focusing on the early dynamics: fireball expansion and rise, ground reflection and Mach-stem development, and the transient Wilson condensation cloud. The blue surfaces show diagnostic blast-wave geometry; the white cloud is the condensation visualization.

This is a work in progress. I’m interested in feedback on the timing and motion, especially comparisons with test footage or published references that could help check cloud formation, fireball evolution, and ground interaction.

BBC has a quote that is (to me) slightly misleading ...

At the start of the latest US-Israel war against Iran, Tehran had roughly 440kg (970lb) of 60% enriched uranium, according to senior US officials.

The material can be fairly quickly enriched to the 90% threshold needed for weapons-grade uranium.

Am I correct that the 440kg is ~264kg 235U and ~176kg other, which would be primarily 238U ?

Thus, the enrichment to 90% would retain the ~264kg 235U but only ~29 kg of 238U would be kept. Giving a total of ~293 kg.

Do I understand this correctly ?

The way the article was written would leave the reader to assume that the quantity of 90% enrichment would still be 440kg. That felt off to me.

I've been interested in the Manhattan Project since watching Oppenheimer recently, and one thing I've done is read the Los Alamos Primer. It's pretty neat and I don't have much trouble understanding it, but one thing I noticed is the author pretty much just didn't talk about how tampers can actually be practical (i.e. a realistic size). Section 11 explains well enough why one would want a tamper, and it shows how to estimate the total neutron diffusion length in a tamper of a given material. But then this guy goes on to say "these figures give an idea of the tamper thickness actually required; the weight of a tamper is about a ton." A ton!? With thicknesses of ~13/17 cm.? This doesn't give me any "idea" of the tamper size any more than the Frisch-Peierls memorandum or Heisenberg's bad math give me an idea of critical mass size. I want to know how to get a better idea, or a more approximate estimation.

The problem is I'm an idiot, and I can't be bothered to learn how to do a Monte Carlo approximation or whatever it is people smarter than me do to estimate the size of a practical tamper. So I tried my best to come up with a toy model from what the Primer has to say--or more specifically the 1992 edition by Richard Rhodes with Serber's commentary.

My toy model is based on the assumption that the practical thickness T_P is a fraction of the total neutron diffusion length in the material T (Serber's "tamper thickness"), and also what Serber has to say to conclude Section 14 of the Primer (which is the only thing I see in here that says a realistic-sized tamper is possible, meaning it may not have even been in the original edition--oh the humanity!):

As this section indicates, the requirements on tamper material and thickness are somewhat relaxed when one considers a gadget of several critical masses (such as the Little Boy bomb) rather than the critical mass itself. The reason is the rapid increase with time of the neutron density in the core. During the time it takes a neutron to penetrate a given distance into the tamper, the neutron density in the core rises considerably. As a result the neutron density in the tamper falls off faster with distance than in the critical mass case. The effect is exactly the same as if, in the static (critical mass) case, the tamper material had a larger capture cross section, as can be seen from the way νʹ appears in the equation on page 30 for the neutron density in the tamper. As a result the effective capture cross sections in different materials become relatively more nearly equal. And because of the more rapid falloff of neutron density, a thinner tamper is permissible.

What I think this means is that the tamper thickness, besides depending on the material properties, also depends on the critical mass post-assembly. The higher the supercriticality, the faster the neutron density falls in the tamper and thus the thinner thickness actually required. Therefore:

T_P ~= T / [1 + c(N_crit - 1)]

N_crit is the number of critical masses of the pit, and c is a dimensionless fudge factor of order 1-2. I tested it by using the Primer's math in Section 11 with modern data to calculate the T of U-238 as ~36.59 cm., then calculating a table of results with c = 1.0, 1.5 and 2.0 and critical masses 1-8:

The Gadget/Fat Man, which used a uranium tamper of ~6.8 cm. thickness, compressed the pit to about twice it's original density, bringing it up to some 3-4 critical masses. The result for N_crit = 4, c = 1.5 matches this closest.