Nuclear reactions


Ionizing radiation

Radiation in the form of a fast-moving particle is dangerous to life forms like us because each particle can ionize a lot of molecules. When a radioactive nucleus decays, the alpha, beta, or gamma particle released generally has an energy of hundreds of keV or even MeV. Ionizing a molecule takes only a few eV, so a fast-moving particle can easily ionize thousands of molecules.

Ionized molecules inside a cell are bad news because they change the chemistry, which can seriously affect how the cell behaves. A little radiation is generally fine; all of us receive some radiation exposure. A lot of radiation is generally to be avoided, however.

From a biological perspective, it makes most sense to talk about radiation from the point of view of absorbed dose. This is the absorbed energy divided by the mass of the material that is exposed to the radiation.

absorbed dose (Gy) = absorbed energy (J) / mass (kg)

The SI unit for absorbed dose is the gray (Gy). A more commonly used unit is the rad, which is a hundredth of a gray.

1 rad = 0.01 gray

Different kinds of radiation have different levels of effectiveness when it comes to ionizing molecules in living tissue. The absorbed dose really needs to be corrected for this, and the correction is an easy one. Different types of radiation are measured relative to 200 keV x-rays, and assigned a multiplication factor known as the relative biological effectiveness (RBE) based on how effective they are relative to 200 keV x-rays. The biologically-equivalent dose is measured in rem, and is given by:

Biologically-equivalent dose (rem) = Absorbed dose (rad) x RBE

Different RBE values are given here:

Fission and nuclear reactors

If a uranium atom absorbs a neutron it will be unstable, and will generally split into two fragments. This process, the splitting of a large nucleus into two smaller ones, is known as nuclear fission.

Many nuclear reactors use uranium as fuel to generate electricity. Although radioactive by-products are produced in the reactor, generating electricity in a nuclear reactor is much more efficient than using a chemical process, such as burning oil, gas, or coal. The chemical processes that occur during burning produce a few eV of energy per molecule. Splitting a uranium nucleus into two pieces produces an average of 200 MeV per nucleus, a factor of about 10^8 more energy per nucleus than you get from burning something.

Uranium-238 is by far the most naturally-abundant isotope of uranium; uranium-235, however, is much more likely to absorb a neutron and break apart. For this reason many reactors use enriched uranium, uranium with about 3% of U-235 (about 4 times as much as the natural abundance).

U-235 is most easily split by a slow-moving neutron. Such neutrons, with kinetic energy of 0.04 eV or less, are known as thermal neutrons. When a thermal neutron hits a U-235 nucleus and is absorbed, the nucleus generally splits into two big pieces and a few neutrons. Two of the possible reactions are:

As many as five neutrons can be released in a reaction, but the average number is 2.5. These neutrons are used to sustain the chain reaction in the reactor: neutrons don't have to be sent in continually because they are produced in the reactions. These neutrons have kinetic energies of several MeV, however, and this energy must be removed so the neutron becomes a thermal neutron and can be used to break apart another uranium nucleus. The neutrons are slowed to thermal energies by a moderator, which is often water.

A nuclear reactor is designed to safely sustain the chain reaction of fissioning nuclei in its core. To keep a reactor operating safely it must be ensured that, on average, each reaction produces one thermal neutron that is used to split apart another nucleus. If less than one, on average, carried on the chain, the chain would soon die out; this is known as subcritical. If exactly one neutron per reaction goes on take part in the chain reaction, the reactor is critical, meaning its operating at exactly the right level. The danger comes if more than one neutron per reaction goes on to sustain the chain; in this case the reactor would be supercritical, the rate of reaction would spiral out of control, and a meltdown could occur.

The system used to control the reaction rate is a set of rods that can be moved into or out of the reactor core. These rods absorb excess neutrons. If the reaction rate is too high, the rods are moved further into the core so more neutrons are absorbed, slowing the reaction rate to a safe level; a rate too low and the rods are moved out of the core so that more neutrons are available.

To sum up, then, a nuclear reactor requires these components:

  1. fuel with nuclei that can be split using thermal neutrons
  2. a moderator to slow down the high-energy neutrons produced in the reactions
  3. control rods to control the rate at which the reaction occurs

Elementary particles

There is life beyond the nucleus; atoms may be made up of electrons, neutrons, and protons, but protons and neutrons are themselves made up of quarks. There are also plenty of exotic particles (muons, pions, etc. ). If you'd like to learn more about these things, one place to start is by going to The Particle Adventure home page on the web.