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Nuclear Reactions

by Ron Kurtus (revised 16 November 2000)

Radioactive substances give off energy as they decay into more stable elements. (See Radioactivity for more information.) When the decay in a substance like Uranium is accelerated, the result is a nuclear reaction. This may be something like the sudden uncontrolled decay of an atomic explosion or the controlled manufacture of energy in a nuclear power plant.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Radioactivity in Uranium

Most elements have some unstable isotopes mixed with the stable atoms. Some, such as Uranium have a sufficient number to be considered radioactive.

Uranium primarily consists of atoms with the atomic weight of 238, mixed with small amounts of the Uranium-235 isotope and other variations. Both are slightly unstable, such that it would take millions of years before half of them would have decayed (also known as their half-life).

What is unique about Uranium-235 is that it is very fissionable by low energy neutrons. In other words, if U-235 is hit by a neutron, its nucleus becomes extremely unstable and within 10-14 seconds will explode and break into a Krypton-92 nucleus and a Barium-141 nucleus, plus three neutrons and some gamma radiation. Everything flies apart with great energy. (Note that 92 + 141 + 3 = 236.)

Krypton-92 and Barium-141 are both unstable. They decay into other particles within days, giving off more radiation.

What is interesting about this is that you would think that adding a neutron to the nucleus of Uranium-235 should result in Uranium-236. But U-236 is fairly stable, with a half-life of 20 million years. The explanation comes from the fact that the nucleus is not as simple as we would like to believe--consisting of protons and neutrons. There are other sub-atomic particles that make up these basic particles, as well as the gluon particle that holds the nucleus together.

The Atomic Bomb

If there is a concentration of U-235 available and a neutron hits only one atom, that atom will split and give off 3 other neutrons. Those 3 neutrons will split 3 more atoms, resulting in energy and 9 neutrons. Those will split 9 more atoms, resulting in even more energy and 27 neutrons. As you can see, the number or explosions and amount of energy released increases rapidly. This is the basis for the atomic bomb.

Using nuclear energy to make electricity

Using some slowing material will control the rate that the U-235 atoms split, resulting in a chain reaction where only heat energy is given off. That heat can be harnessed as nuclear energy. If the chain reaction is not controlled, it results in an atomic explosion.


You have seen why nuclear reactions occur and how that energy can be made into either an atomic bomb or a controlled reaction to create the heat for making electricity.

Try to benefit mankind by constructively applying your knowledge of science

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