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Unit 1 PhysicsDetailed Study 3.3Chapter 12.2: Aspects of
Fission
Properties of uranium-235, uranium-238 and plutonium-239.∗Uranium-235 is likely to undergo fission when it is struck by a thermal neutron.∗A thermal neutron is a slow moving neutron (slow, ~1380ms-1, relative to the speed of light, 3x108 ms-1).∗The absorption of a neutron in the nucleus of Uranium-235, produces the unstable isotope Uranium-236.∗This highly unstable isotope undergoes fission and releases energy.∗Uranium-235 is unable to absorb a fast neutron, this is due to the fact that the neutron is not in close enough proximity for long enough for the strong nuclear force to take hold.
Section 12.2Aspects of Fission
Properties of uranium-235, uranium-238 and plutonium-239.∗Uranium-238 is only slightly fissile, it requires a lot of energy to induce fission.∗It is however, able to absorb a thermal neutron and become Uranium-239.∗Uranium-239 will then undergo decay, and in the process becomes Plutonium-239, which is fissile substance.∗For this reason, we say that Uranium-238 is fertile, because it is able to become a fissile substance after absorbing a neutron.
Section 12.2Aspects of Fission
Properties of uranium-235, uranium-238 and plutonium-239.∗Unlike Uranium-235, Plutonium-239 needs a fast neutron to bring about a fission reaction.∗A thermal neutron will simply be absorbed into the Plutonium-239 nuclei.
Section 12.2Aspects of Fission
Chain Reactions∗When Uranium-235 undergoes fission, it releases 2 or 3 Neutrons, these neutrons are then able to release a further 2 or 3 and so on.∗This causes the number of neutrons released to increase rapidly, this is known as a Chain Reaction. ∗If uncontrolled, these chain reactions can have devastating effects, this is the principle that Nuclear weapons rely on.
Section 12.2Aspects of Fission
Chain Reactions∗A single fission reaction only releases energy of order 10-13J.∗In an uncontrolled chain reaction however, a huge number of reactions occur in such a short period of time that an explosion results.∗1kg of Uranium-235 can release 8 x 1013J in just one-millionth of a second.
Section 12.2Aspects of Fission
Nuclear Fuel∗The two most common Uranium isotopes in the earths crust are Uranium-235, which has a half-life of 710 million years, and Uranium-238, which has a half-life of 4.5 billion years.∗As a result, there is far less uranium-235 available for mining∗0.7% Urainium-235 compared to 99.3% Uranium-238∗This means we need to increase the proportion of Uranium-235 compared Uranium-238 to be a useful nuclear fuel. This is known as Enrichment.∗Nuclear weapons require Uranium-235 to be enriched to around 90% purity, while nuclear reactors only require around 3% purity.
Section 12.2Aspects of Fission
Critical Mass∗The scientists of the Manhattan project in WW2 found that the explosiveness of a sample of fissile material depended on the size and purity of the sample.∗If the concentration of Uranium-235 or Plutonium-239 is too low, a chain reaction cannot be produced, as there is a small chance of the neutrons being absorbed by a fissile nuclei.∗ The surface area and shape of a sample also effects how a nuclear fuel explodes. ∗A flat sheet (large surface area) will lose heaps of neutrons to the air, thus causing chain reactions to die.∗While a sphere has a smaller surface area and a larger proportion of neutrons stay inside, continuing the chain reaction.
Section 12.2Aspects of Fission
Critical Mass∗The term subcritical refers to a fuel source whose surface area is quite large when compared to its volume. For example a marble, has a small volume but a large surface area comparatively.∗Critical Mass is the minimum amount of enriched fissile material in a spherical shape that leads to a sustained fission reaction.∗Supercritical is if the fissile material is above critical mass, this is capable of causing a nuclear explosion.
Section 12.2Aspects of Fission
