Why heavy Nuclei U-238 do not spontaneously undergo fission?

To understand this, one has to realize that the surface area is considerably increased when one large liquid drop breaks into two small liquid drops. In case of U-238, about 6MeV is required to overcome the surface forces and get separated into small fragments before energy is released. This situation is described in terms of potential barrier through which fission fragments have to pass.

The quantum mechanical probability for such a barrier penetration is considered. Because of large masses involved, this probability is extremely small and for U-238 we get a half life of approx. 10^16 years, for decay of spontaneous fission.

Actually for U-238, gamma rays of energy approx 6 MeV are able to induce fission reaction. This reaction is called photo fission for which threshold energy is 6MeV approx. So for all practical purposes, U238 is stable against decay by spontaneous fission.          

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