Question

What is a conventional nuclear fission reactor and how does it work? What are products of nuclear fission and why are these an environmental problem? Do natural nuclear reactors exist, and if so, why?

Answer 1

A nuclear fission power plant uses the heat generated by a nuclear fission process to drive a steam turbine which generates usable electricity. The underlying physical process is the same for all power plant designs, what differs is the way in which the nuclear reaction is controlled. This is an engineering problem which must take into account factors such as:

• Control -- Keeping the nuclear reaction from dying out or exploding.
• Safety -- If something goes wrong it can be contained.
• Refueling -- Adding more nuclear fuel without stoping the reactor.
• Waste production -- The byproducts of the reaction must be manageable.
• Efficiency -- Capture as much of the heat as possible.

Control is the most important aspect to a design. When an atom of nuclear fuel (uranium) absorbs a neutron, the uranium will fission into two smaller atoms (waste) and release one to three neutrons. The kinetic energy of the waste is used to heat the water for the steam turbine. The neutrons are used to fission the next lot of uranium atoms and the process continues. If none of these neutrons are absorbed by another uranium atom then the reaction dies out. If too many neutrons are absorbed then the reaction grows extremely quickly and could explode. Current reactor designs are most usefully classified by how they ensure this nuclear reaction is kept at a level which produces power without getting out of hand.

Fast reactors

Neutrons emitted in the fission of uranium have a lot of kinetic energy and so are moving very fast. A fast reactor uses these neutrons by letting them be absorbed directly by the next uranium atom. A problem is that the isotope U-238 (which is not part of the power producing reaction) absorbs these fast neutrons with higher probability than U-235 (which produces the power). To get around this problem, the fuel for fast reactors must be enriched with a much larger fraction of U-235.

The fast reactor design is not currently used in large scale nuclear power plants.

Slow reactors

A slow reactor, also known as a thermal reactor, slows down the neutrons produced in the uranium fission to a thermal speed (the same speed as the unused fuel). This allows the neutrons to be more readily absorbed by the correct uranium isotope, U-235.

Uranium fuel

Uranium, like all other atomic elements, occurs in several different forms, known as isotopes. The most common isotope of uranium is U-238, which makes up 99.28% of all uranium atoms. The second-most common isotope, U-235 (0.71%), is the one used to generate electricity, because it can easily undergo fission.

Because nuclear fission is a very efficient source of energy, nuclear reactors require very little fuel. A single 20-gram uranium fuel pellet can produce the same amount of energy as 400 kilograms of coal, 410 litres of oil, or 350 cubic metres of natural gas.

Nuclear fission

Uranium (both U-235 and U-238) is relatively stable before entering the reactor: it emits such a small amount of radiation that unused fuel pellets are safe to be near. However, when a U-235 atom is bombarded with a neutron, it often splits (or