Nuclear Fission

How Fission Works

Nuclear fission is the reaction that occurs when the nucleus of an atom splits into two smaller nuclides having similar masses. This process releases one or more neutrons in the case of the fissioning of a nucleus with a high mass number. A neutron with a suitable energy may interact with a nucleus to induce fission. A nucleus may undergo spontaneous fission very rarely.

During the fission process, energy is released as less energy is now needed to hold together the nucleus of each of the smaller nuclides compared to the larger U-235 nucleus. The energy release in one fission is about a million times greater than that of the most energetic chemical reactions because it involves the nuclear strong force rather than the electromagnetic force. The energy release is sufficiently large that for a large number of fissions, it is possible to demonstrate the loss of mass based on Einstein’s equation: E=mc2.

Of the naturally-occurring nuclides, the uranium-235 atom is the most fissioned using low energy neutrons. When a neutron traveling at just the right speed interacts with the nucleus of a uranium-235 atom, the nucleus splits into two smaller parts, producing a pair of atoms of different elements. The fission process also releases two or three neutrons. The new atoms are radioactive isotopes.

Nuclear Fission
Nuclear Fission.

If the neutrons released during fission can be made to go on to split more uranium-235 atoms which in turn, create more radioactive isotopes and release more neutrons, this cycle is known as a nuclear fission chain reaction. This process will continue until either the uranium-235 is used up or until the flow of neutrons is stopped. Learning how to control the fission process in uranium-235 and produce other radioactive elements gave birth to the nuclear industry.

Energy from Fission

Man holding a CANDU fuel bundle
Man holding a CANDU fuel bundle.

The benefits from generating electricity using nuclear fission are enormous. The amount of energy released by the fission a single kilogram of uranium-235 produces the same energy as burning 1,300,000 kilograms of coal or 1,350,000 litres of fuel oil – equivalent to the energy used in all of Canada for three days! In Canada, our nuclear power stations presently use natural uranium, which is comprised of 0.72% uranium-235, and almost all of the rest being uranium-238. Most other nuclear power stations in the world use enriched uranium, which has slightly more than 0.72% uranium-235.

The man in the picture is holding a typical fuel bundle used in CANDU reactors to generate electricity. Inside each bundle are ceramic pellets that are uranium oxide. Each bundle has a mass of about 23 kilograms and can generate about 1,000,000 kilowatts-hours of energy. That’s enough energy to run a typical home for 100 years.

A Brief History of Nuclear Fission

Wilhelm Röntgen, of Germany, discovered x-rays in 1885 and by 1934 Enrico Fermi had split the nucleus of the uranium atom. At that time he was unable to explain the results. In 1938 Fermi’s experiment was repeated by Otto Hahn and Fritz Strassmann, and they were still unable to explain the results. One of their former colleagues, Lise Meitner was able to come up with the fission explanation, and her nephew Otto Frisch coined the word “fission” in 1939. By this time, physicists and chemists like Niels Bohr, Albert Einstein, James Chadwick, Ernest Rutherford, and Marie Curie were able to accurately describe the structure of the atom and properties of radioactivity. It was a period of intense and exciting scientific research.

After fission was described in 1939, the possibility of using nuclear fission as an energy source was widely discussed by scientists. Some physicists like Leo Szilard, Eugene Wigner, and Edward Teller could foresee the possibility of building an atomic bomb and urged Albert Einstein to write a letter to American President Franklin D. Roosevelt warning him that the technology to build such a weapon would soon exist and Nazi Germany might begin working on just such a weapon.

In the early 1940’s Allied research into nuclear fission focused on developing the first atomic weapon. Top secret facilities were built at Los Alamos, New Mexico in the United States and at Chalk River, Ontario, Canada (in cooperation with Britain and France). The first atomic explosion, code-named “Trinity”, occurred on July 16, 1945 at Los Alamos, New Mexico. On August 6, 1945 an atomic weapon was dropped on Hiroshima, Japan followed by a second atomic weapon being dropped on Nagasaki just three days later on August 9, ending the Second World War (in the Pacific).

The decision to drop atomic bombs remains controversial. Over 200,000 people were killed by the dropping of these weapons. This has convinced most people that they should never be used again.

After the Second World War, Canada focused its nuclear program toward using nuclear fission to generate electricity, create radioactive isotopes for medicine, manufacturing and conducting scientific research.