Uranium in Nuclear Reactors: Why Use It?

[otomanb]

As we know that usually and mostly Uranium is used in Nuclear Reactors. So ma question is why Uranium is used as compared to thorium and Radium.
Second thing that Uranium is found in solid, powder or which form it's found.
thanks

 

[Astronuc]

Uranium has a natural fissile isotope (U-235), which constitutes approximately 0.7% of natural uranium, with the remainder being U-238 (and traces of U-234).

Thorium is essentially all Th-232, which is fertile, not fissile. In order to use thorium in a fuel cycle, one must add a fissile isotope, e.g. U-235 or Pu-239, initially until sufficient U-233 is produced for recycle back into the Th-fuel. Pu-239 is a by product of neutron capture in U-238.
http://www.nndc.bnl.gov/chart/reCenter.jsp?z=90&n=142 (use Zoom 1)

Radium does not have a fissle isotope.

Uranium is found as an oxide or phosphate in various ores.
http://www.world-nuclear.org/info/default.aspx?id=446 - uranium deposit geology
http://www.world-nuclear.org/info/default.aspx?id=26789 - phosphate ores