What is the Condition of Nuclear Fission?

[SANKET HAQUE]

According to the experimental curve of Binding Energy per nucleon vs Mass no. , we have come to know that heavier nuclei having less B.E. are fissionable. We have also learned from Neutron vs Proton curve that those nuclei having N/P>1 can show radioactivity. But my question is why not all heavy nuclei are fissionable? Why we only take Uranium as our fission element?

 

[DEvens]

A variety of isotopes are fissionable.

http://en.wikipedia.org/wiki/Fissile_material#Fissile_vs_fissionable

A smaller number of isotopes are fissile. To get a chain reaction you want fissile isotopes.

To get fissile isotopes there are not many choices. You can start with U235 (roughly 0.7% of natural Uranium). These days this is the most common way of running a nuclear reactor. It can either happen by enriching natural Uranium. Usually this is done to something around 5% U235 (a little more or less depending on the reactor design.) Or it can happen using heavy water as a moderator and using natural Uranium, such as in a CANDU.

Or you can produce a fissile isotope by breeding it in a reactor. There are some choices on the wiki page. There are several possible designs for such reactors, with a variety of features. A major attractive feature of such is that you would be feeding in fissionable material, such as U238 or Thorium. Such materials are much more common than the U235 in natural Uranium.

 

[mathman]

Most nuclei that are unstable are radioactive (alpha decay, beta decay, etc.) rather than fissionable.

 

[Astronuc]

≥

According to the experimental curve of Binding Energy per nucleon vs Mass no. , we have come to know that heavier nuclei having less B.E. are fissionable. We have also learned from Neutron vs Proton curve that those nuclei having N/P>1 can show radioactivity. But my question is why not all heavy nuclei are fissionable? Why we only take Uranium as our fission element?

Elements of Z ≥ 90 have fissionable isotopes, and a few are fissile, i.e., readily fission with thermal neutrons. U-235 occurs naturally, but as a small fraction (0.0071) of uranium, in certain ores. We use Pu-239 and Pu-241 as fuel in thermal and fast reactors, and in a few systems, we have used U-233. Fissile isotopes of elements heavier than uranium have to be produced in reactors, and most radionuclides heavier than Pu have shorter half-lives and many undergo spontaneous fission.

 

[snorkack]

Most nuclei that are unstable are radioactive (alpha decay, beta decay, etc.) rather than fissionable.

Indeed. The longest lived isotope for which the most common decay path is fission rather than alpha or beta decay is curium 250, with total half-life of 8300 years, of which 80 % is fission.

However... the lighter nuclei, from polonium to actinium, are much less stable than the nuclei on the island of stability, from thorium to curium.
Why does, e. g. radium undergo only alpha decay? Seeing how unstable radium is, why does radium not undergo fission?

 

[DEvens]

Indeed. The longest lived isotope for which the most common decay path is fission rather than alpha or beta decay is curium 250, with total half-life of 8300 years, of which 80 % is fission.

However... the lighter nuclei, from polonium to actinium, are much less stable than the nuclei on the island of stability, from thorium to curium.
Why does, e. g. radium undergo only alpha decay? Seeing how unstable radium is, why does radium not undergo fission?

The generic answer is easy to give and easy to understand. You just have to swallow the glib factor. It goes a little like this. The nucleus sits behind a potential energy barrier. For the decay to happen it has to quantum tunnel through this barrier. For isotopes that preferentially decay by an alpha decay, the barrier is smallest for an alpha particle to get out. This may be due to the specific details of the nucleus having an alpha particle lightly bound and so ready to go. On the other hand, a fission requires that the nucleus has to break up into two large chunks and each move away from the other. This may require the nucleus to move through a much larger potential energy barrier. So while the end state may be energetically favorable, the intermediate states are more difficult. So the rate is lower. For reactions of this kind the rate is very strongly sensitive to small changes in the barrier.

More specific details would require a detailed understanding of the characteristics of the nuclei of different isotopes. Sadly I am unable to provide this understanding.