Uranium,Thorium,what next?

[Stanley514]

What periodic elements could be used in nuclear energetics to obtain energy profit?
It seems that every element heavier than Iron could undergo fission and give energy?
Could be such element as Tungsten or Tantalum used as fuel instead of Thorium?

[QuantumPion]

You can spallate any element you like but the process is so inefficient it is impossible to have a net energy gain. The only way to make energy efficiently is through neutron chain reactions, and this only works for the few isotopes which have a large probability of fissioning when absorbing a neutron.

Besides U and Th, Plutonium can be made and fissioned economically, but that's pretty much it. There are other isotopes which are fissionable, such as Am and Cf, but we can't make these in large quantities efficiently.

[Astronuc]

There is no effective fertile element lighter than (or Z less than) Th.

Some research is being done on Cm and Am fuels, and some exotic ideas with Cf. One problem with those transuranics is that they must be fabricated and inspected remotely because of the radation levels (gamma, beta and spontaneous fission).

Cm and Am are produced in significant quantities in MOX fuel.

[brian_palmer]

Besides U and Th, Plutonium can be made and fissioned economically, but that's pretty much it. There are other isotopes which are fissionable, such as Am and Cf, but we can't make these in large quantities efficiently.

Quite right, however even more restrictive is the need for the isotope to emit more than 2 neutrons per fission on average.