Presentation on theme: "Fundamentals of Nuclear Power. Nuclear Fission We convert mass into energy by breaking large atoms (usually Uranium) into smaller atoms. Note the increases."— Presentation transcript:
Linear Chain Reaction Obviously, an expanding chain reaction cannot be sustained for long (bomb). For controlled nuclear power, once we reach our desired power level we want each fission to produce exactly one additional fission
Tricks of the trade Slow moving (thermal) neutrons are more effective at inducing fission, but, fissions produce fast moving electron. We need to slow neutrons down. Fissions typically produce several neutrons but a linear chain reaction only needs one. We need to get rid of a good fraction of our neutrons.
Moderator Neutrons are slowed down by having them collide with light atoms (Water in US reactors). Highest level of energy transfer occurs when the masses of the colliding particles are equal (ex: neutron and hydrogen)
Control Rods Control rods are made of a material that absorbs excess neutrons (usually Boron or Cadmium). By controlling the number of neutrons, we can control the rate of fissions
Basic Ideas The Uranium is both the fuel and the source of neutrons. The neutrons induce the fissions The Water acts as both the moderator and a heat transfer medium. Control rods regulate the energy output by “sucking up” excess neutrons
Practicalities Processing of Uranium Each ton of Uranium ore produces 3-5 lbs of Uranium compounds Uranium ore is processed near the mine to produce “yellow cake”, a material rich in U 3 O 8. Only 0.7% of U in yellow cake is 235 U. Most of the rest is 238 U which does not work for fission power.
Enrichment To be used in US reactors, fuel must be 3-5% 235 U. Yellow cake is converted into UF 6 and this compound is enriched using gaseous diffusion and/or centrifuges. There are some reactor designs that run on pure yellow cake.
NOTE: A nuclear bomb requires nearly 100% pure 235 U or 239 Pu. The 3% found in reactor grade Uranium CANNOT create a nuclear explosion!
Fuel Pellets The enriched UF 6 is converted into UO 2 which is then made into fuel pellets. The fuel pellets are collected into long tubes. (~12ft). The fuel rods are collected into bundles (~200 rods per bundle ~175 bundles in the core
Cladding The material that the fuel rods are made out of is called cladding. It must be permeable to neutrons and be able to withstand high heats. Typically cladding is made of stainless steel or zircaloy.
Other Options Other countries use different reactor designs. Some use heavy water (D 2 O) as a moderator. Some use Graphite as a moderator. Some are designed to use pure yellow cake without further enrichment Liquid metal such as sodium or gasses such as Helium are possibilities to use for coolants
Breeder Reactors A big problem with nuclear power is the creation of Plutonium in the reactor core. This is a long lived radioactive element that is difficult to store. Q: Why not use it as a fuel too?
Basic Idea Process that creates the Pu. During fission use one of the extra neutrons to create a Pu atom
Somewhat difficult in that we want fast neutrons to “breed” the 239 Pu out of the 238 U, but we want slow neutrons to induce the fission of 235 U. Requires a different design of reactor. Doubling time: Time required to produce twice as many 239 Pu atoms as 235 U destroyed. A good design will have a 6-10 doubling time. There are no currently operating breeder reactors in the US.
Nuclear Power in the US We currently generate approximately 20% of our electricity using nuclear power. No new nuclear power plants have been “ordered” since the late 1970’s. Even “new” plants are nearing 20 years old and will start to need replacing.