Splitting the Atom: Nuclear Fission

Fission Enrico Fermi, an Italian-born scientist working in the United States, was conducting one such experiment in He bombarded uranium nuclei with neutrons and obtained some unexpected results. Some of the uranium nuclei absorbed the neutrons and split in two! Fermi was the first to observe nuclear fission, although the mechanics of this process was not determined until 1938.

Fission When a fissile nucleus absorbs a neutron, the nucleus splits in two, releasing a number of neutrons. This is an example of nuclear fission.

Fission Nuclides that are capable of undergoing nuclear fission after absorbing a neutron are said to be fissile. Fissile nuclides are very uncommon. Uranium-235 and plutonium-239 are readily fissile. A typical fission reaction for uranium-235 is: Krypton-91 and barium-142 are known as the fission products or fission fragments. Three neutrons are freed from this uranium nucleus when it splits.

Fission When a uranium-235 nucleus absorbs a neutron, nuclear fission occurs. The uranium nucleus splits in two, forming in this example krypton-91 and barium-142. Three neutrons are released.

Aspects of Fission In the years after the discovery of nuclear fission, an intensive scientific and military endeavour was undertaken to investigate all of the ramifications. The two main aspects of this investigation related to the use of nuclear fission for nuclear weapons and in the generation of electricity.

Properties of Uranium-235 Uranium-235 is most likely to undergo fission when struck by a slow- moving or thermal neutron with energy as low as 0.01 eV. A slow-moving neutron can be absorbed into a uranium-235 nucleus, forming the highly unstable uranium-236 isotope. This then undergoes fission and releases energy.

Properties of Plutonium-239 Plutonium-239 is fissile in the same manner as uranium-235. However, plutonium nuclei require fast neutrons to bring about nuclear fission. Slow-moving, or thermal, neutrons do not cause fission in plutonium- 239 nuclei.

Chain Reaction

The large surface area of the flat piece of uranium-235 enables a large proportion of neutrons to escape into the air, causing the chain reaction to die out. (b) In the spherical piece of uranium-235, a sufficient proportion of neutrons remains inside the material to maintain the chain reaction, leading to an explosion.

Nuclear Weapons The design of a fission bomb is really quite simple. It has to contain separate subcritical masses of a fissile material, which are then combined at the desired time to make one supercritical mass when the explosion is required.

Little Boy The nuclear bomb that was dropped on Hiroshima contained two hemispherical subcritical pieces of 95% pure uranium-235. This bomb, known as Little Boy, was dropped by a B29 bomber called the Enola Gay on 6 August When it reached an altitude of 580 m, an explosive charge fired one piece into the other creating one supercritical mass of uranium-235, which exploded within one-millionth of a second after this. The bomb contained 64kg or uranium- 235, of which less then a kilogram underwent fission, to a total of approximately 18kt.

Little Boy

Fat Man Three days after the bombing of Hiroshima, a second bomb was dropped over Japan. The target city was originally Kokura, but low cloud and fog resulted in the attack being changed to Nagasaki. This bomb, nicknamed Fat Man, contained many small subcritical pieces of plutonium-239. It used a spherical implosion to force these pieces together and make one supercritical mass. The bomb contained about 6kg of plutonium-239, of which 1kg underwent fission, to a total of approximately 21kt.

Fat Man

Blast Radius

Questions 12.1 ~ 1,2,3,4,5,7, ~ 1,3,8