generalfusion

How a nuclear fission reaction works

Nuclear fission involves breaking large atoms apart based on the principle of radioactive decay. Certain isotopes with large nuclei, such as 235Uranium and 239Plutonium, decay spontaneously very slowly, but can be induced to decay rapidly when bombarded with another particle such as a neutron. The resulting decay breaks the original nucleus into two or more intermediate-sized nuclei, releasing energy and further free neutrons in the process. These free neutrons can trigger yet more fission events, which in turn release more neutrons and energy in a chain reaction.

Nuclear fission is the basis of all operating nuclear power plants. Fuel is loaded into a nuclear reactor and a fission chain reaction is induced. The energy from the reaction is released as electromagnetic radiation and kinetic movement of the fission products and fragments. This energy is converted to heat as the particles and radiation collide with the atoms that make up the reactor and its working fluid, which is usually water or heavy water. The heated water is used in a steam turbine to generate electrical power.

Typical fission events release about two hundred million electronvolts of energy for each fission event (an electronvolt, or eV, is a unit of energy commonly used to describe atomic-scale reactions;

1 eV = 1.6×10−19 J). By contrast, most chemical oxidation reactions (such as burning coal) release at most a few electronvolts per event, so nuclear fuel contains at least ten million times more usable energy than does chemical fuel.

Problems with nuclear fission

Several problems plague nuclear fission reactors. Firstly, extracting energy from a nuclear fission reaction is tantamount to controlling the rate of the nuclear chain reaction. If this control fails, the reaction can exceed the capability of the power plant to extract the heat, and a meltdown can occur, with subsequent release of radioactive materials into the environment.

Secondly, fission reaction products have, on average, about the same ratio of neutrons and protons as their parent nucleus, and are subsequently unstable and radioactive. These products are more highly radioactive than the parent fuel material and are intimately mixed with the residual fuel. Some of these fission products have half-lives of tens of thousands of years, creating long-term radioactive waste storage problems.

Thirdly, there is a limited supply of suitable fuel. Present uranium production accounts for only about half of the world’s uranium demand; the remainder comes from de-commissioned nuclear weapons. If all of today’s world electrical energy were generated using fission power plants, known uranium reserves would deplete in less than 100 years. (New fuel breeding technology under development capable of using the more abundant 238U isotope as opposed to the current and much more rare 235U isotope would allow reserves to last for about a thousand years.)