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?