Re: Why does U238 have a stable nucleus while U236 splits?

Hi Don:

I am going to take a shot at answering your question and at the same time ask any madscientists out there who can amplify on my answer to please do so.

You are correct about U-238. Even though it is radioactive (with a very long half-life) it is also very unlikely to fission. I assume that is what you mean by "relatively stable." U-235, on the other hand, is what I call "the fissionable uranium isotope." When it absorbs a neutron into its nucleus, it very quickly undergoes fission, splitting into two lighter nuclei and releasing energy plus additional neutrons. Why does U- 235 do this and U-238 and U-234 do not? I believe that is your question.

The answer to the question lies in nuclear quantum theory. Within the nucleus, the short-range nuclear force is controlled by nuclear quantum mechanics. Certain states are favored and others are "forbidden." Adding another neutron to the 92 protons and 143 neutrons of a U-235 nucleus changes the quantum numbers to a forbidden state, resulting in the "disintegration" of the nucleus into two fission fragments.

I guess it is obvious from my answer that I don't have a very good grasp of nuclear theory, but I know from studying the isotopes that are and are not radioactive, that certain combinations of neutrons and protons seem stable and others do not. I believe that many of the same short- range forces that control radioactivity also affect the probability of fissioning in the heavier isotopes. For instance, elements with even number of protons have (usually) many more radioactively stable isotopes than elements with an odd number of protons.

I hope this helps a little, and I invite others to correct and/or add to this explanation.

Jerry Gels