Asked in PhysicsChemistryNuclear Energy
What do nuclear reaction result in?
October 19, 2009 5:07PM
There are actually three types of events that are commonly referred to as "nuclear reactions".
The first, and simplest, is RADIOACTIVE DECAY. Some atoms have nuclei which are unstable; generally these are isotopes of very heavy elements. To achieve stability, the nucleus of such an atom will emit radiation, which can be in the form of particles or energy. The four most common emissions are alpha particles, beta particles, neutrons and gamma rays.
Alpha particles are extremely large; they consist of two protons and two neutrons - basically a helium atom with no electrons. They carry a charge of 2+. Because they are so massive and energetic, alpha particles can cause serious damage to material. However, they have very little penetrating power - a sheet of paper will stop them, so alpha emitters are only a problem when one is exposed to them internally.
Beta particles are basically stray, highly energetic electrons. They carry a 1- charge. They can be damaging, but also have little penetrating power - a sheet of aluminum foil will stop the average beta. However, they can be a concern in skin exposure - "beta burn" is much like serious sunburn.
Gamma rays are highly penetrating. Like light, x-rays and other electromagnetic radiation, gammas consist of little packets, or "quanta" of energy. These packets behave like waves of energy under most conditions (although they will occasionally behave like particles, just to keep physicists on their toes). Gamma radiation can be extremely dangerous, and is shielded against with dense materials like lead.
Neutrons are stray subatomic particles. Because they are massive, and carry no electric charge, they can penetrate for great distances, and do serious damage to materials & tissue. Neutrons can combine with nuclei of atoms when they collide, producing radioactive isotopes. This process is referred to as "activation". Neutrons are shielded against with materials that contain many hydrogen atoms, such as water or polythene. These provide many little nuclei for the neutron to bounce off of & slow down.
Neutrons are also essential to the most used nuclear reaction, FISSION. When neutrons from radioactive decay of massive atome collide with nuclei of other massive atoms, sometimes they will cause the nucleus to split in two or more parts. This produces (a) several smaller atoms, (b) lots of radiation, including (1) alpha, beta, and gamma (2) more neutrons [important later, and [most useful to us] (3) heat - which can be used to create steam and drive turbines to create power.
The new neutrons from each fission can then go on to produce new fissions - sort of like when billiard balls hit other billiard balls to continue the action on a pool table. The trick is to achieve "criticality", which is the point at which you are producing & using up neutrons at the same pace. A "subcritical" reaction produces too few neutrons, and eventually dies out; a "supercritical" reaction produces too many, and speeds up until it runs away [this is a _bad thing_, trust me on this].
Criticality is controlled by using control rods, which are inserted into or withdrawn from the reactor as needed. These rods absorb neutrons, allowing the number running around the reactor to be controlled.
The third type of nuclear reaction is FUSION. We would really like to be able to produce a controlled, efficient fusion reaction since it appears to be (a) safer and less polluting in the long run than fission and (b) a nearly limitless source of energy. Unfortunately, laboratory experiments have only reached a point where we can retrieve 50% or less of the energy it takes to start a reaction.
In fusion, light atoms (like Hydrogen) are slammed together with sufficient force that their nuclei combine, creating (a) a bigger atom, and (b) a lot of energy. Fusion is what powers the sun and other stars. Most scientists think that all matter in the universe started out as Hydrogen atoms (or simpler particles), and that everything that is made out of heavier elements (like the planets, our biosphere, and YOU) is the result of fusion in stars that died a long time ago, spreading their atoms into space when they exploded as novae. So we are all made of star-stuff, which certainly makes _me_ feel pretty special.
In the laboratory, fusion is initiated by zapping pellets containing Hydrogen with high energy lasers. In hydrogen bombs (unfortunately, our only "practical" application of fusion) the reaction is started by a fission bomb packed around the hydrogen. The sun's reactions are driven by its tremendous force of gravity. Solar energy applications are a way we can indirectly take advantage of the enormous potential of fusion energy *right now*.