Nuclear Fission

Who Discovered Fission? Concepts Who Discovered Fission? What is Fission? The Process of Fission Energy Release Types of Fission Uncontrolled Controlled Summary

Who Discovered Fission? Almost 70 years ago, in 1938, Enrico Fermi (p 391)accidentally discovered nuclear fission when he was experimenting. During his experimenting, he found that when he bombarded uranium with a neutron, the uranium nucleus split into two roughly equal portions. This was the first nuclear fission and a huge step forward in nuclear science.

What is Fission? (p 391) In nuclear fission involves the splitting of the nucleus into roughly two equal parts using a slow moving neutron. Fission is a type of nuclear reaction that results in the release of a huge amount of energy.

What is Fission? (p 391) It has been discovered that Uranium-235 is the only isotope that can have a fission reaction The equation for nuclear fission of Uranium-235: Fission Reaction Uranium-235 3 neutrons produced Slow moving neutron Roughly two equal products Both heat and light energy

The Process of Fission Bombard an unstable nucleus with a low energy neutron. The unstable nucleus is than split into two roughly equal parts. Resulting in the release of 2-3 neutrons and energy.

The Process of Fission The neutrons produced from the fission than in turn bombard other nuclei, leading to a chain reaction. This picture shows a fission reaction where each neutron releases two more neutrons. (p392) if this is allowed to continue a lot of energy is released quickly (like in an atomic bomb)

Nuclear Reactor (p 392) At the heart of a nuclear reactor is the core which contains the fuel rods, made of UO2. A moderator slows down the neutrons so they can cause the uranium to split There must also be a mechanism to control the amount of energy produced by the reactor. Control rods absorb neutrons A fission reaction requires a minimum amount of fissionable matter (called the critical mass) in order to create a sufficient chain reaction. Anything below the critical mass is called sub critical. If the amount is sub critical, then the reaction will just fizzle, and the chain reaction will not occur.

Nuclear Reactor The CANDU reactor used heavy water as the moderator (p392/393)The Canadian reactor is called the CANDU reactor which is short for Canadian deuterium and uranium The CANDU reactor used heavy water as the moderator Heavy water (D2O) contains deuterium which is an isotope of hydrogen There are 2 reasons why the Candu reactors uses heavy water: It Uses natural rather than expensive enriched uranium It is safer as the heavy water can be drained to stop the reaction

Nuclear Reactor (p 393) There are some problems with any reactor: The reactor core can get to hot and cause a melt down. This can result in radioactivity being releases into the environment The spent fuel rods contain radioactive material that must be disposed of in an environmentally friendly way All nuclear reactors produce a lot of pllutonium-239 which could be extracted to produce nuclear weapons There is also thermal waste ( p 392)There are also benefits of using nuclear energy. It is a reasonably efficient energy source

Energy Release (p 394)Where does this large amount of energy come from? If you look at the equation of fission, you will see that the mass you start with is less than the mass of the products. This loss of matter during the reaction is called the mass defect. The mass defect is transformed into energy. This can be proved with Einstein’s mass-energy equivalence: E = mc² Even though the mass defect is small, it will be multiplied by (3.00 x 108)², which will result in a huge amount of energy. Speed of light Energy produced Mass defect

Types of Fission Generally speaking, there are two main types of nuclear fission: Controlled fission Uncontrolled fission Nuclear Reactors Nuclear Bombs

Nuclear Fission:the nuclear bomb A nuclear bomb uses the chain reaction of a fission reaction to release a huge amount of energy, destroying everything in the radius. Because the reaction is uncontrolled, it will grow very rapidly. Thus, resulting in a lot of energy being released very quickly.

Uncontrolled Fission

Controlled Fission:nuclear power Nuclear power plants, or reactors, use a fission reaction for energy use. In order to do this, a reactor controls the fission reaction by controlling the neutrons that are produced. This stops the rapid growth of the chain reaction, thus allowing the energy released, to be moderated. This allows nuclear power plants to harness the large amounts of energy released from the reaction.

Summary Nuclear fission is the splitting of an unstable nucleus by the bombardment of a low energy neutron It can be uncontrolled (nuclear bombs) or controlled (nuclear power plants and reactors. The mass defect is transformed into a large amount of energy during the fission reaction.

Nuclear Fusion (p 394) When 2 small nuclei join together to form a larger nucleus it is called fusion In fusion, like fission, the products have less mass than the reactants. (there is a loss of mass called a mass defect)

Nuclear Fusion (p 394)According to Einstein’s theory of special relativity, mass is a form of energy Therefore, like fission reactions, fusion reactions produce energy There are a series of fusion reactions occurring on the sun One of theses is: H + H He + n +energy

Nuclear Fusion There are a series of fusion reactions occurring on the sun (p394) On the sun the temperature is very hot so the particles have a lot of kinetic energy and when they collide they can fuse and become one