Fusion Joining Of Elements With Low Atomic Number. Energy

Fusion

Fission

Joining of elements with low atomic number. Energy is released due to the change in mass (mass defect), as the new nucleus is lighter than the two nuclei that joined. Example: JET (Joint European Torus) in Culhamin UK ITER (Internation Thermonuclear Experimental Reactor) to be built in Canarache in France

Splitting of elements of high atomic number. Energy is released due to the change in mass (mass defect), as the total mass of the two new nuclei is less than the mass of the original nucleus. Example: Bradwell Power Station in Essex (in process of decommissioning) Sizewell B Station in Suffolk

Triple Product Plasma (gas mixture/hot electrically charged gas): 100-200 million K Energy Confinement time: 4-6 s Central density in plasma: 1.2 x1020 particles m-3

Critical fission reactors: autocatalysis (neutrons produced by fission induce subsequent fission) Subcritical fission reactors: Non selfsustaining fission reactions (therefore use radioactive decay or particle accelerators to induce fission) Fuel Rod Made from uranium isotopes. Uranium -235 is the main fissile isotope. The fuel rod is the site of the fission reactions. Control Rod Used to maintain the reaction as critical, made from neutron-absorbing material. Lowered in reactor when reaction needs to be slowed, Raised from reactor to increase rate of reaction. Coolant Carbon dioxide gas is pumped around the reactor via circulators. The heated

Temperature A sufficient rate only occurs at very high temperatures (the positively charged plasma ions need to overcome their natural repulsive forces.(100 million Kelvin is used in JET), as in plasma form electrons are stripped from the atoms to form ions.

Energy Confinement time Fusion reactions per unit volume is approximately proportional to the

square of the density. Therefore the density of fuel ions must be sufficiently large for fusion reactors to take place at the required rate. Waste products must be removed as the fusion power generated is affected by this accumulation. Density Definition- ratio of thermal energy contained in the plasma and the power input required to maintain these conditions. Energy is retained in JET with the use of magnetic fields as it confines the very hot plasma. Large volumes of plasma are optimal at heat retention. Reaction on Earth Between deuterium (1 neutron) and tritium (two neutrons) to form a helium nucleus and an uncharged neutron.

Tokamak: Reduces the conductive heat loss through the vessel, they must be contained in the central region and also minimises the release of impurities from the vessel walls into the plasma and cool the plasma by radiation. When in a magnetic field the plasma particles are forced to spiral along the magnetic field lines. Toroidal field: produces field around torus. Poloidal field: produces a field around the plasma cross section

gas is then passed through exchangers, containing water, and subsequently boils the water. The super heated steam is used to drive turbines which subsequently generate electricity.

Moderator Materials used to slow down the neutrons (which previously had kinetic energies in the region of MeV). Materials chosen include Deuterium (heavy water) and Carbon (graphite)

Fuel: Uranium oxide, purified to “yellowcake,” then reduced and undergoes an enrichment process. 3kg of uranium oxide is required to supply the energy needs for one “Westerner” a year. Radioactive waste: The half life of the radioactive waste produced by nuclear fission, ranges between a hundred thousand years and a million years, and therefore in order to prevent the environment and biological community it needs to be stored firstly in spent fuel pools (underwater for ten to twenty years) and subsequently stored in dry cask stores (steel cylinder welded or bolted shut and then further surrounded by more steel and concrete

Advantages as a power: Abundant fuels (deuterium, tritium etc.) Safe (any malfunction would cause a rapid cooling and termination, cannot become uncontrollable) “Green” (no contribution to global warming) The waste (active energetic neutrons) cause radiation but can be dealt with in a time scale of approximately 50 years

Advantages as a power: Low-emission, low-cost power supply capable of replacing fossil fuels.