Thorium is the answer

The first atomic bomb dropped on Hiroshima contained 141 pounds of uranium but only .001 pounds actually exploded and that created an explosion the equivalent of 30,000 pounds of TNT. The explosion occurs when enough enriched uranium is squeezed into a small space. Thorium cannot do that. No matter how concentrated it is and no matter how much is squeezed into a small spot it will not react. Thorium is fertile but not fissile meaning it cannot react without a starter. Thorium reactors remain quiet and stable until a reactor neutron is introduced and then the reaction starts up. Once the thorium is used up the reaction stops unless more thorium is added. If the reaction gets too hot it stops and will start up again when the reaction cools. Since water is not used to cool the reactor it operates at normal pressure and there is no need to build a dome to protect against explosions. Thorium is more available than uranium and less costly and does not adversely effect the environment when mined.