Why is fuel for nuclear power plants commonly enriched in uranium-235?

Answer 1

The reason 235U is used in nuclear reactors as a fuel is because it supports an increasing-rate nuclear (fission) chain reaction; it is fissile. It will release neutrons when it fissions, and these neutrons, if they can be slowed a bit, will cause more 235U atoms to fission and a chain will quickly build. This isotope of uranium is capable of crating a sustainable nuclear chain reaction with the least "tinkering" as regards geometry, moderators and other reactor components or systems. A link will be found below to the Wikipedia post this isotope of uranium.

Answer 2

There are only two elements found in any quantity in nature that can be used easily for fuel in nuclear plants based on fission. These elements are uranium, found as 235U and 238U, and thorium, found as 232Th. Each of these has different properties. 232Th is fertile, meaning it can be rendered into a synthetic isotope that is capable of fission, or divide. 238U is fissionable, meaning that it will undergo fission if it is hit hard enough by a neutron. 235U is fissile, meaning it undergoes fission all on its own.

It happens that when 235U divides, it produces neutrons that are moving fast enough to cause other 235U and 238U atoms to divide. The neutrons from 238U, when it divides, are not going fast enough to cause other 238U atoms to divide.

Since this is the case, when a mass contains a sufficient amount of 235U in it, the spontaneous fission of atoms is sufficient to cause a chain reaction, which goes faster and faster until it explodes, unless it is controlled. The sufficient quantity is called critical mass, and is not found in nature on Earth today. Since 235U is the only natural isotope that can have critical mass, getting a critical mass reactor to work with naturally occurring materials requires increasing the percentage of 235U in the fuel, or enrichment.

When it is controlled, the chain reaction from critical mass makes a lot of heat, which can be used to drive a steam turbine. There are other ways of driving a reactor, but this is the oldest and best understood, so it is the most commonly used. One way to drive the reactor is to use synthetic isotopes, such as plutonium isotopes, to provide the fast neutrons. Another approach is to drive the reaction with very fast protons that have been accelerated to the point that their impact on substances such as lead produces fast neutrons through spallation. These technologies are relatively new, however, and have not yet come into common use.

Answer 3

Uranium-235 is the isotope that undergoes nuclear fission, which is the process that generates energy in nuclear power plants. By enriching the fuel to increase the concentration of uranium-235, the likelihood of a fission reaction occurring is enhanced, leading to more efficient energy production.