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I understand that adding flux does not change criticality. I'm not looking for K values here, just an estimate of the added neutron flux required to disable a typical ICBM warhead. I would like to compare it to flux levels in reactors and other neutron sources.

As noted in comments, neutrons will cause fissions to occur. If enough fissions occur over a short time period, the fissile material will heat up and possibly start to melt the warhead and/or the sudden thermal expansion would cause it to blow apart. Everything depends on the magnitude of flux added and the time involved.

A useful and understandable answer led to some useful hits. https://www.nuclear-power.net/nuclear-power/reactor-physics/reactor-dynamics/subcritical-multiplication/subcritical-multiplication-factor/

I think I understand the time dependence and asymptotic value of the neutron flux.

I'm still wondering what flux would be required to melt Pu from the heat of induced fission. Pu heat capacity= 0.13 J/(gK), and heat of fusion = 2.84 kJ/mol, and heat released from fission = 198.5 MeV. The fission rate = Neutron flux)(microscopic cross section)*(number density of nuclei).

I think I can work it out from that. Corrections appreciated. I'll assume adiabatic because I want a rapid heatup. I think neutron fluxes ranging from 10^10 to 10^14 neutrons/cm2/s (depending on the position within the reactor) are available.

Doesn't this make you wonder why I want to know?

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  • $\begingroup$ I don't think there's enough info in that diagram to give a solid answer. We don't know what the outer shell of the warhead is, and how it reacts to neutrons. Or if there are neutron absorbers, reflectors, or moderators in there that aren't mentioned. $\endgroup$
    – PM 2Ring
    Jan 29, 2021 at 1:32
  • $\begingroup$ Only those who know total aspects of the bomb , could , perhaps , give a good answer. It needs very complex and precise neutronics calculations in 3d , using a High Performance Computer .For you , for instance , you must know the external source neutron spectrum . $\endgroup$ Jan 29, 2021 at 7:53
  • $\begingroup$ Fluence is the time integrated value of the flux, so they should be easy to compare. Note that the question as written now doesn't make sense without the context of the previous edits. $\endgroup$ Jan 31, 2021 at 18:36
  • $\begingroup$ Adding neutrons will not make the warhead critical (criticality is more a function of the geometry). However, neutrons will cause fissions to occur. If enough fissions occur over a short time period, the fissile material will heat up and possibly start to melt the warhead and/or the sudden thermal expansion would cause it to blow apart. Everything depends on the magnitude of flux added and the time involved. What exactly are you looking for? $\endgroup$ Jan 31, 2021 at 18:40

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The warhead fissile material is in a subcritical configuration. If exposed to an external neutron source, the configuration is a "subcritical mass with an external neutron source", and the flux $(neutrons/cm^2/sec)$ approaches a constant value proportional to the strength of the external neutron source and inversely dependent on one minus the subcritical multiplication factor, $1 - k$. The actual calculation is complicated, and analytical approximations are given in the text, Lamarsh Nuclear Reactor Theory. An accurate calculation considering the exact geometry, and all the materials present besides the fissile primary, would require applying weapons neutron kinetic codes (on massively parallel computing machines) developed by Los Alamos and Lawrence Livermore National Laboratories, and would be different for different warhead nuclear packages, such as the W78 and the W87 for the current ICBMs. There have been calculations done to assess the effectiveness of hardening measures in the warhead to protect against extraneous neutron sources from enemy nuclear weapon detonations during delivery of the warhead; these studies are classified, so I cannot give you any more detailed information.

Except for certain conditions the warhead is inside a reentry vehicle, inside the missile, inside a silo, so it would require a massive external neutron source to affect the warhead to any significant extent.

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