I got a great offlist note of correction on my sloppy description of why U-235 undergoes fission with neutron bombardment. It is posted below. There is good website listed at the end that details some of this graphically. There are also informative references to Iran and N. Korea, that I'll post later.
CG
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actually, increased numbers of neutrons makes a nucleus more, not less, stable. the reason is that the neutrons kind of nestle themselves in between the protons, lessening the electrostatic repulsion between the positively charged protons whilst simultaneously adding an equal increment of nuclear (strong) binding force. so if you look at a curve of the # of neutrons and # of protons in stable elements in the periodic chart, the curve leans over towards more neutrons than protons as the atomic weight increases.
there are particularly stable brands of neutron-proton mixtures. a neutron-proton pair is obviously more stable than two protons (no electrostatic repulsion), two protons are more stable than three protons (nuclear spin effects, up-down spin pairs are more stable), and two pairs of neutron-protons are exceedingly stable (alpha particle / helium nuclei). in heavy nuclei, a similar process makes pairs of neutron-protons more stable. U238 nuclei thus have components that are bound more tightly together (even #'s of neutrons and protons), whilst U235 (odd #'s of neutrons) slightly less so.
now what happens when each absorbs a neutron? answer: depends on how fast the neutron is moving. while both U235 and U238 can absorb the neutron, in the former case, the 235 switches to the intermediate product 236, which is stabler (even-even binding energy), meaning there is a re-arrangement of the nuclear constituents which *releases* some energy that is available for propelling pieces of the nucleus apart. it turns out this can happen *even* for the slowest absorbed neutrons: the activation energy required for fission is smaller than the released energy of the deeper binding potential for that added neutron. in contrast, the 238 switches to 239 (odd-even binding), so the re-arrangement does not release as much energy available for separating components of the nucleus, *unless* the energy of the neutron can supply the deficit amount. so U238 will fission most likely fission only at sufficiently high neutron speeds. since the energy of released neutrons *after* fission generally lies below this limit, U238 cannot be explosive.
naturally, in this scenario the odd-neutron-numbered plutonium will also fission with slow neutrons, (239-240 re-arrangement).
the topmost figure at http://www.uic.com.au/uicphys.htm tells the story in a nutshell.