Why can't nuclear energy be extracted from ordinary non-radioactive elements using the principle of E equals mc squared instead of having to use radioactive elements?

Answer 1

First, most elements have radioactive isotopes. And you can get lots of energy from nonradioactive elements. So radioactivity is not the issue.

Two basic processes for getting atomic energy are fusion and fission. Let's look at fusion.

Fusion is when very light elements like hydrogen and it's isotopes are squeezed together to form helium. The process generates lots and lots of energy. Heavier elements can be fused, too, and all the fusion reactions up through fusion to create the element iron release energy. Stars do this in a process called stellar nucleosynthesis. Elements beyond iron are only created in supernova explosions where the gravitational collapse of certain types of stars results in the fusion of elements to create all the trans-iron elements. The elements thus created include everything up through uranium. The problem is that to cause fusion to happen, you need temperatures on the order of 120 million Kelvins. Generating that is tough, and containing it isn't possible with current technology. So what about fission?

Fission involves the "breaking" of heavy elements into lighter elements, of larger atoms into smaller atoms. You can break any atom, but it takes a lot of energy to do that. And it generally takes more energy than you can get out, which is not practical, except for experiments. Only a few of the very heavy elements are practical energy producers. And they are the radioactive elements that are what we call fissile. They will fission when they are hit by a neutron. It turns out that we can actually set up to do that, and we use uranium or plutonium as nuclear fuel. When we get enough of the element together, it fissions spontaneously and releases both energy, which we can use, and also neutrons, which will go cause other fissions to occur. Basically, only the fissionable isotopes of uranium and plutonium are practical for use as nuclear fuel in fission reactors.

We just can't use any old fuel and create nuclear energy. There are limits imposed by physics and also by our current technology.