Importance of fusion stage in thermonuclear weapon

[mheslep]

... Unlike U-235, U-238 cannot sustain a chain reaction, so each U-238 nucleus which fissions does not release other solitary neutrons which can continue to cause fission in other U-238 nuclei. ...

Well U-238 fission does release neutrons, just not enough (1.7) and the energy of those neutrons produced by fission is too low to fission other U-238 nuclei. The result being exactly as you say, that U-238 can not sustain a chain-reaction.

The result then of a U-238 jacket around the fusion portion of weapon is that every 14 MeV neutron produced by fusion, that then causes a fission event in the jacket, 'boosts' the overall yield to ~200 MeV per event.

These circumstances are the motivation behind the renewed interest so called hybrid fusion-fission designs for controlled fusion power reactors (see LIFE) The use of U-238 in hybrids would be unique and unavailable to fission-only reactors.

 

[delsaber8]

interesting article here about pits...

http://cryptome.org/2014/03/nuke-pits.pdf

I only managed to skim the article but does it mention anything about the explosive lenses used in the pit, to compress the plutonium into a super critical mass?

 

[jim hardy]

I only managed to skim the article but does it mention anything about the explosive lenses used in the pit, to compress the plutonium into a super critical mass?

no it doesn't. (caveat - i didn't scrutinize it either, was just interested in the parts about longevity and makeup)You can see Klaus Fuchs' hand-made sketches(the ones he sent to Stalin) in Rhodes' "Dark Sun", with dimensions removed of course.
That was plenty for me.

 

[SteamKing]

interesting article here about pits...

http://cryptome.org/2014/03/nuke-pits.pdf

This brings up an interesting side issue: for all the efforts thru the years at devising various disarmament treaties, the US seems bent on disarming by neglect. Much of the nuclear weapons infrastructure which was created in the 1940s and 1950s has either been demolished or closed due to safety and environmental hazards. (The massive K-25 gaseous diffusion plant which once dominated Oak Ridge, TN is now gone.) The human capital which formerly worked at these installations has long since retired or died, so that much institutional knowledge associated with weapons production has been lost. In another 25 or 30 years, will there be enough technical capability remaining to replenish or renew even the current stockpiles permitted under arms control agreements?

 

[mheslep]

Are there reports that weapons in the US stockpile are in need of replacement? I have not seen anything near term.

All published US weapon designs rely on Pu not HEU for the primary.

 

[SteamKing]

Are there reports that weapons in the US stockpile are in need of replacement? I have not seen anything near term.

All published US weapon designs rely on Pu not HEU for the primary.

Like everything else, these devices don't last forever. With testing reduced or discontinued entirely (the US has not held a nuclear test since 1992), one is less certain of correct device operation with the passage of time.

HEU is required to make plutonium and other radioisotopes, and it can be used as fuel for commercial and military reactors. The uranium doesn't enrich itself. There are other critical nuclear materials besides HEU.

After the Savannah River nuclear reactors were shut down in 1988, there was no US production of tritium until a facility came on-line at Watts Bar. Radioisotope production for commercial or medical use also depends on having access to reactors where this material is bred; some isotope supplies now can only be obtained from Canadian reactors.

 

[mheslep]

Plutonium can be produced with natural uranium; indeed the Pu for the Nagasaki weapon was made this way. That is, neutrons are required to productively make uranium into plutonium which are generated by any moderated critical reaction. HEU, i.e. weapons grade uranium, could be used instead of the common LEU in power reactors, perhaps it is, but it not is required.

Similarly, I don't see any physics driven need for stored tritium in modern thermonuclear weapons. I know the early thermonuclear weapons used stored Tritium, but given the short half life it seems that reliance on production at detonation time from Li-6 would be imminently more practical.

From what I have read and gathered, much of the expertise in the US nuclear weapon program was engaged in building "advanced new types" of nuclear weapons up until the 1990s and the signing of the CTBT, which effectively restricts new weapons ( http://www.fas.org/bethecr.htm). In any case, I would expect a switch from new design and development to stockpile stewardship would bring about a reduction in visible staffing and facilities.