Nuclear Waste: A Serious and Growing Problem
Once a uranium or plutonium atom splits, it gives rise to at least two smaller “daughter” atoms, plus several subatomic particles. Some of the subatomic particles go on to have collisions with uranium or plutonium atoms. If they collide with the daughter atoms, these may split again, or absorb the subatomic particle and “transmutate” into a different species. Some of these new atoms are stable, some are unstable and radioactive.
In the end, spent nuclear fuel bundles contain original fuel which did not yet decay, plus a large number of new atoms, many radioactive. Some of these substances are dangerous because they are radioactive; others because they are chemically toxic; some are both radioactive and toxic. Some are highly radioactive species which therefore decay rapidly; others are less radioactive and very long-lived.
Radioactivity simply means that an atom is unstable, either decaying by spitting out a subatomic particle, or by splitting into two or more daughter atoms, or both. An alpha particle is a common decay product. This is a helium nucleus containing 2 protons and 2 neutrons. Because alphas are relatively heavy and large, they are slow-moving and cannot penetrate even materials such as paper. However, if they do hit something, because of their size and weight, they do a great deal of damage. On the other hand, a beta particle (an electron) is another common decay product. It has negligible weight and high velocity. Beta particles are stopped by heavy clothing, dense wood or metal sheeting. The third common type of radiation is gamma. Gamma rays carry a lot of energy, but have no mass. They penetrate thick concrete, but have reduced intensity passing through lead bricks. They tend to do less damage than equivalent doses of alpha or beta radiation, but are very hard to contain.
The long-lived species common in nuclear fuel like 239Pu, and 235U emit highly penetrating gamma rays and must be kept shielded. Plutonium is also a highly toxic substance.