Physics 30 Modern Physics Unit: Fission and Fusion Nuclear Energy For years and years scientists struggled to describe where energy came from. They could see the uses of energy and the results of energy acting upon objects, but where it came from was a mystery. Albert Einstein gave an explanation. He stated that mass and energy were interchangeable. (That is mass is really a form of energy. Mass can be changed into energy and vice versa.) Einstein developed an equation to describe this idea: E = mc 2 Where:E = energy in joules m = mass in kilograms c = speed of light in meter/second (3.00x10 8 m/s) For example, if we converted 1 kg of mass into energy we would get: E = m c 2 E = (1.0 kg) (3.00x10 8 m/s) 2 E = 9.00 x 10 16 J This amount of energy is equal to the total electrical energy requirements of Canada for a twoyear period. o The exact same mass of coal would produce 3 x 10 7 J of energy. Turning mass into energy seems very promising, but it can be difficult to do. Most of the mass in an atom is in the nucleus (protons and neutrons), so to look at how mass can become energy, we must look at the nucleus of an atom. When a nucleus is formed, the mass of the nucleus is LESS than the sum of the mass of the protons and neutrons that make up the nucleus. o Ex. A helium nucleus has 2 protons (2 x 1.0073) and 2 neutrons (2 x 1.0078) so the total mass of the nucleus should be 4.0320 o The actual mass of a helium nucleus is 4.0017 o There is a loss of 0.0303 when the nucleus is formed This loss of mass is called the mass defect the reason this happens is because energy is given off when the particles are settling down to become the nucleus (this stabilizes the nucleus and allows it to stay together). This energy that is given off is known as the binding energy it is the same amount of energy that is needed to break the nucleus apart. This binding energy is the energy equivalent to the difference between the actual mass of the nucleus of an isotope and the sum of the masses of the protons and neutrons that compose it, expressed as energy. When the protons and neutrons combine together it looks as follows: A certain amount of energy is needed to hold together particles of the nucleus. This certain amount of energy is more for larger nuclei. When a nucleus splits apart, this extra energy (binding energy) needed to hold the nucleus together is released. o This means the mass of protons and neutrons VARIES depending upon how many are grouped together. o Ex. a proton or neutron in Uranium has a mass of 1.00 u, while a proton or neutron in a Nickel or iron nucleus (which is smaller than Uranium) will have a mass of 0.999 u.
o This means that if a Uranium nucleus could be split into a nickel/iron nucleus, each nucleon (proton or neutron) would have a mass of 0.999 u, and therefore some energy would be released because of the loss in mass. In normal uranium there are 235 nucleons, therefore there would be a theoretical release of 200 Mev (million electron volts) if U-235 split into something where the nucleons have slightly less mass. This splitting of a heavier nucleus (U-235) to form two lighter nuclei (Fe-56, or Ni-58) is possible. o This process is called Fission. Nuclear Fission In 1939 Otto Hahn and Fritz Strassman bombarded uranium metal with neutrons then analyzed the products that were left over. What they found was that these newly created radioactive elements were identical to a number of different elements all near the center of the periodic table. o From this they determined that a uranium atom seemed to be catching a neutron and splitting into two nearly equal fragments. What happens: o Nucleus captures a slow moving neutron. o Nucleus becomes unstable starts to separate into two parts. This resembled cell division in biology, so it was called fission. o Other smaller particles are also ejected (alpha & beta particles, neutrons, gamma rays). o Energy released is so great that two new nuclei fly apart with great speed. o Fission, then, can be defined as the splitting apart of the nucleus of a heavy metal with neutrons into two or more nearly equal parts with the liberation of large amounts of energy and additional neutrons. (Largest amount of released energy is in the form of gamma rays). o If we were to represent this in the form of a chemical reaction, it would look as follows: Followed by: (Ba is barium and Kr is krypton) The excited uranium nucleus could have split into other types of elements as well: The three neutrons that are released in this reaction are now available to interact with more uranium atoms to cause more fissions, with the release of even MORE NEUTRONS. This creates reactions that keep going and are therefore called a chain reaction.
Neutrons released during this process have lots of energy, enough to cause another uranium atom to fission. o Because of their great speed, it may be necessary to slow down a neutron in order for fission to occur. If a fission reaction takes place in a small sample of uranium, the released neutrons may escape before the chain reaction is possible. To combat this, the size of the uranium sample is increased. o Increased sample size = increased collisions. It is possible to calculate how much material is necessary for a chain reaction to occur. This MINIMUM amount of fissionable material is called the critical mass. In order for a FISSION chain reaction to happen there must be: o A supply of fissionable material larger than the critical mass. o Adequate supply of slow neutrons. Example: Atomic bomb o In an atomic bomb, there are 2 or more pieces of uranium in the bomb that are both under the critical mass size. o When the bomb is detonated, these pieces of uranium are forced together, making a supercritical mass (anything greater than the critical mass). o Once this is achieved, the process of fission starts immediately, and this produces very large quantities of energy in a short period of time. o A very high temperature is reached and there is rapid expansion of the vaporized bomb components and the surrounding air the out-rushing of gas once the bomb detonates is what causes the blast damage of an atomic bomb explosion. o After a nuclear explosion, radioactive fragments fall onto the earth. This is called radioactive fallout. It can have damaging effects to our bodies if it enters through the food we eat, etc. Nuclear Reactors A nuclear reactor is a place where nuclear fission can be maintained as a self-supporting yet controlled chain reaction. It is a type of furnace where uranium is used as a fuel and the following products are produced: o Heat o Neutrons o Radioactive isotopes A Nuclear Power Reactor uses the heat energy to make steam, and then in turn to make electricity. A Nuclear Research Reactor is where neutrons are used as the tools of research.
Nuclear Energy Nuclear energy was first developed in the USA during WWII for the military. The 1 st controlled reaction occurred in 1942. It was done in a device that consisted of a huge pile of carbon blocks laid together to form a solid mass. Lumps of pure uranium within thin walled Aluminum cylinders, were inserted into the mass at regular intervals. Control rods and detecting devices were also inserted into long cylindrical holes that were drilled through the blocks. The Nuclear Reaction In the U-235, a few nuclei undergo fission and release fast moving neutrons. These fast moving neutrons enter the carbon and collide elastically with the carbon molecules and slow down. Carbon acts as the moderator (slows down neutrons). These now slow moving neutrons are captured by uranium nuclei to continue the chain reaction. Rising temperatures in the reactor were controlled by cadmium rods. Cadmium absorbs neutrons thereby decreasing the fission rate. To further lower the temperature, the cadmium rods are inserted further, thereby absorbing more neutrons and effectively slowing the amount of fission taking place. To raise the temperature, the cadmium rods are pulled out. In a power reactor, the released energy is transferred from the reactor by a coolant which circulates through the reactor. The coolant carries this heat energy to the heat exchanger where it converts water into steam. The resulting steam pressure drives a turbine which turns an electrical generator (which develops electricity). To review, a nuclear power reactor needs: o Fuel o Moderator o Absorber-control rods o Coolant o Heat exchanger o Turbine and generator
CANDU Reactor There are many reactors in the USA and Britain that use ordinary water as a moderator and enriched U-235 as fuel. o This is a very expensive way to produce energy because the cost of separating U-235 from natural U-238 is very high. Natural Uranium is 99.3% U-238 On the other hand, chain reactions using heavy water as a moderator are capable of using natural uranium as a fuel. This combination of moderator and fuel is used in the Canadian CANDU (Canada Deuterium Uranium) reactor. The cost of heavy water is high (this is water containing deuterium) but the cost of natural uranium is low because of the abundance in Canada. The only difference between producing electricity with a nuclear reactor and a fossil fuel plant is the way the heat is generated to create steam. o A fossil fuel plant uses a chemical combustion of fuel to create heat. o The nuclear power plant creates heat from nuclear fission. The uranium that is used in the CANDU reactor is formed into uranium oxide pellets. o These pellets are kept in special metal tubes and then formed into a fuel bundle. The metal tubes are sealed so that fuel does not come into contact with the heavy water. o Each bundle contains 37 sealed metal tubes. o 12 fuel bundles go in one pressure tube that is put into a calandria (reactor vessel).
o The calandria is placed into a concrete vault that is surrounded by the reactor building (one meter thick concrete walls). o Fuel bundles are inserted into the calandria by the fuel machine while the reactor is still operating. In the CANDU reactor, heavy water is used as both a moderator and coolant. o Heavy water surrounds the pressure tubes containing fuel elements. Heat from the fuel is transferred by the heavy water under pressure to ordinary water boilers. o Heat transfer causes steam that causes a turbine to work and produce electricity. Problems with Nuclear Fission Waste Heat: o Reactors are always built near large bodies of water. o Reactors need water to cool steam as it passes through the turbine this heated water gets returned to the lake. o This can raise the temperature of the lake it affects the aquatic life in the lake can make it uninhabitable. Safety: o There have been accidents that have happened. Some notable accidents include: NRX reactor at Chalk River, ON 3-Mile Island at Pennsylvania, USA Chernobyl, Russia o There are 3 stages to nuclear energy, all which have their own safety issues. o Mining: Uranium comes from open-pit mining. After uranium is removed, tailings are left (which are radioactive). There is a threat of having tailings contaminate our biosphere. o Using fuel in the reactor: If accidents happen at a reactor, radiation could be absorbed through skin, or consumed in food and water that is contaminated. CANDU Reactors have the following safety features. Uranium contained in metal tubes (cannot escape unless there is a huge temperature increase). Cadmium control rods can stop the reaction. Neutron poison (neutron absorbing solution) used as a backup system to stop reaction if required. Heavy water is available to prevent overheating. Reactor building kept at slightly lower pressure to slow materials from escaping. o Managing spent fuel: Bundles of spent fuel are highly radioactive. Plutonium is also produced in the CANDU Reactor this can be used to make nuclear bombs. We only have temporary storage right now (large underground water tanks). Economics: o The cost of building a nuclear power plant is very high. Cost of production is relatively cheap. Reactors must be dismantled and handled safely after 3 or 4 decades.
Security: o CANDU reactors produce plutonium this can be used to make bombs (perhaps by terrorists). o Plutonium may some day be used as a fuel in other nuclear reactors. o Canada does not harvest its fuel bundles to get the plutonium produced. Nuclear Fusion In the fusion process, two small nuclei fuse together to produce a larger nucleus. In this case, the mass of the product nucleus is less than the sum of the masses of the fusing nuclei. The mass which is lost is converted into energy by the equation E = mc 2. In the fusion process a higher percentage of energy is converted into energy than with fission. The sun is an example of energy being produced by the fusion process. It produces energy as follows: The energy released when 4 protons and 2 electrons combine to make one helium nucleus is 4.50 x 10-12 J of energy. When we try to fuse positively charged nuclei, the electrostatic repulsion comes into play. The only way to achieve fusion would be to heat the reactants to a very high temperature so that the collisions could take place and the reactants could fuse. At such a high temperature the fusing gases get completely ionized and change into an electrically conducting mixture called Plasma. Trying to confine the reactants at such a high temperature is extremely difficult. Two methods are currently being tried to confine the fuel gases in the fusion reactor. o Inertial confinement o Magnetic confinement Inertial Confinement: o In this type of fusion reactor, the frozen fuel pellets injected into the reactor are heated with intense laser beams aimed at them from several directions. o The fuel pellets change into plasma then implode the temp gets increased to allow fusion to occur. o The energy produced is transferred by the coolant to produce steam (same as other types of electricity producing plants).
Magnetic Confinement: o In this method, the ionized plasma is confined to a small space with the help of strong magnetic fields. o This principle is used in a Tokamak (donut shaped magnetic chamber). o Huge copper coils around the donut chamber carry electric current and so produce strong magnetic fields. o Due to the magnetic field, the plasma accelerates in the chamber and high speeds are achieved. o The temp of the plasma rises and it is expected that fusion will create more energy than what is consumed in the Tokamak to operate it. o Released energy shall heat the walls of the chamber from where energy can be taken out by water this will produce steam, drive a turbine, and produce electricity.