The “killer app”, the sine qua non, is the implosion system and associated non-nuclear parts.

I neither said nor implied (quoting John:) you can interchange Pu and U at will throughout.

As I specifically said:

The details of the amount and type of driver explosives, tamper/reflector choices, core configuration, use of different fissiles, etc, are all managable details for early builders.

There is no implication that quite important details must not be analysed and designed around. However, these are relatively straightforward.

If you have the hardest part, the implosion system, then you can (as the US did), use it as a standardized unit, and just vary the specifics of the other components.

There is no implication in my post that you can just toss a ball of U235 or Pu239 in the center and it will work equivalently.

BTW – your point about not knowing when to initiate if you don’t bother determining the EOS (something I never said nor implied), it is automatically dealt with if one uses an internal initiator. Just as the US did for it’s entire initial atomic arsenal for over a decade (and as the current proliferators seem to be doing).

As a second BTW –

The equation-of-state of Pu and U are not as different as you seem to say, just as Ted Taylor who actually KNEW) correctly pointed out.

As Sublette more recently stated (discussing compression) points out:

Compression figures for plutonium are classified above 30 kilobars, but there is every reason to believe that they are not much different from that of uranium. Although there are large density variations from element to element at low pressure, the low density elements are also the most compressible, so that at high pressures (several megabars) the plot of density vs atomic number becomes a fairly smooth function. This implies that what differences there may be in behavior between U and Pu at low pressure will tend to disappear in the high pressure region.

Actually, even in the low pressure region the available information shows that the difference in behavior isn’t all that great, despite the astonishingly large number of phases (six) and bizarre behavior exhibited by plutonium at atmospheric pressure. The highest density phases of both metals have nearly identical atomic volumes at room pressure, and the number of phases of both metals drops rapidly with increasing pressure, with only two phases existing for both metals above 30 kilobars. The lowest density phase of plutonium, the delta phase, in particular disappears very rapidly. The amount of energy expended in compression at these low pressures is trivial. The compression data for uranium is thus a good substitute for plutonium, especially at high pressures and high compressions.

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In any event, the people building bombs will have determined the relevant characteristics of the materials they use, and will tailor the configuration to the implosion system.

yale

I can see good reasons for using composite pits especially as primary triggers, but that is not the same thing as saying that you can interchange Pu and U at will throughout.

Jeffrey said, “A job worth doing is a job worth doing right.”

It is NOT the fissile core (HEU or Pu) that is hard.

It is the non-nuclear implosion system that is the heart of the technology.

BTW – that physicist who said 8 kg of Pu or 25 kg of HEU is W..A..Y.. off the mark.

Only an extremely advanced and experienced and sophisticated design team attempting to achieve either very high yield or a very compact and light (but inefficient) bomb would dare to build with that much material.

Even Fat Man only used ~6 kg of Pu and it was a pretty touchy and quite unsafe device.

Anyway, The ability to achieve a converging shockwave is what it is all about. The details of the amount and type of driver explosives, tamper/reflector choices, core configuration, use of different fissiles, etc, are all managable details for early builders.

The bleeding-edge, very narrow margin, boosted primaries of US missile warheads is not relevant to a nation building an arsenal. Even the US is wanting to robust-up its designs.

A converging shockwave at sufficient velocity pushing a sufficient tamper will create a city-buster from HEU or Pu.

The US arsenal was originally designed to use HEU, Pu and varying composites blends along with all sorts of tampers, configurations, etc WITH THE IDENTICAL IMPLOSION SYSTEM.

The venerable Mk 4 came in a rainbow of flavors for the pits – using whatever the production facilities had available.

The equation-of-state of HEU and Pu are similar enough for guv’mint work.

yale

Website:
Comment: Iran is presumably able to develop a HEU implosion device based on information from the Khan network. The document spotted in Iran by the IAEA on hemipherical casting of U suggests as much. So do the Chinese HEU implosion device blueprints supplied by the Libyans to the IAEA.

The stability of the electronic structure has to do with the state mixing – say for example d-shell with f-shell. As the pressure increases, the energy levels of different states cross and create strong interations that lead to the phase transitions at the crossing pressures.

Pu is so complex because it is naturally at the crossing point at ambient pressure. This complexity seems to me to be inherent in the actinides generally, and U will manifest similar properties – only at extremely high pressures which are difficult to access experimentally. Therefore, the high pressure equation of state may be MORE uncertain and difficult to obtain than that for Pu, even though the ambient pressure state date might be more complex in Pu.

Ted Taylor, in his Curve of Binding Energy, remarked that the compressibility of all the actinides were the same – yet, I think this comment needs to be viewed with suspicion at least. He was talking about improvised weapons. Perhaps he meant the same insofar as a low performance bomb could achieve, or maybe he was just being very coarsely approximate.

It may be possible to alloy the U to move around these phase transitions in pressure space and thereby achieve desirable properties.

I freely admit this could be much ado about nothing. Maybe this is not a problem of concern, but it seems to me that a potential bombmaker would need to know, not guess, the correct answer to these questions…and a Pu test wouldn’t answer them adequately.

The last sentence in my comment was merely a reflection on how our thinking may be wrong because my little community of estimators frequently assume countries with limited technical means will always seek the simplest technical solution.

Sharam Chubin argued this “simple solutions for poor countries” assumption was a natural extension of the cultural and social divide between the developed countries of the Northern Hemisphere and the underdeveloped countries of the Southern Hemisphere.

On another tack, however, what about the possibility (regularly raised by Jeff over at ACW) that Iran just might be trying to develop a parallel Pu track?