Nuclear Energy – Learning Outcomes  Describe the principles underlying fission and fusion.  Interpret nuclear reactions.  Discuss nuclear weapons.  Describe the structure and operation of a nuclear reactor.  Discuss the environmental impact of fission reactors.  Discuss the development of fusion reactors.  Discuss fusion in the Sun.  Discuss mass-energy conservation in nuclear reactions.  HL: Solve problems about mass-energy conservation. 1

Nuclear Fission  Nuclear fission is the splitting up of a large nucleus into two smaller nuclei, releasing energy and neutrons.  e.g. uranium-235 will fission if it is given an extra neutron becoming uranium-236).  It produces a krypton-92, a barium-141, three neutrons, and lots of energy. 2 by fastfission – public domain

Chain Reaction  Since fission reactions produce neutrons, they can cause further reactions.  Not all neutrons will cause fission.  If at least one neutron from each reaction causes another reaction, the process is called a chain reaction.  The minimum amount of material needed to cause a chain reaction is called the critical mass. 3 by fastfission – public domain

Atomic Bomb  If each reaction causes more than one reaction on average, it is called supercritical.  Fission weapons use supercritical reactions to release large amounts of energy to devastating effect.  The ignition mechanism brings two subcritical masses together quickly with chemical explosives, starting the uncontrolled chain reaction. 4 by Charles Levy – public domain

Fission Reactor 5

Fission Reactors  Uranium found in ore is mostly uranium-238, which is not fissionable. A little is uranium-235, which is fissionable.  Enriched uranium is processed uranium where the amount of U-235 is increased to almost critical levels.  U-235 releases fast neutrons, but requires slow neutrons to start another reaction. U-238 captures fast neutrons.  Graphite moderators slow neutrons down so the U-235 captures them instead. 6

Fission Reactors  To create a sustainable reaction, there must be a critical mass.  This is dangerous however.  Uranium fuel is separated into multiple subcritical rods, while the total amount is critical.  Cadmium control rods absorb neutrons and can be raised or lowered to control the rate of chain reaction between them.  The energy resulting from the reaction is used to boil water and the steam runs a turbine. 7

Environmental Impact AdvantagesDisadvantages No CO 2 emissionsRadioactive waste products No greenhouse gasesAccidents can be catastrophic High energy output Mostly safe 8  Around 450 nuclear reactors in operation with more on the way.  13 countries use nuclear power to generate more than ¼ of their energy.  France uses nuclear power for over ¾ of its energy.  Nuclear power supplies over 10% of the world’s energy. Unnecessary facts

Nuclear Fusion 9 by Wykis – public domain

Nuclear Fusion  Nuclei are positively charged, so the coulombic force between them is repulsive and only gets stronger as they get closer.  To overcome this, the nuclei need huge energies to get close enough to each other to fuse. 10 by Panoptik – CC-BY-SA-3.0

Fusion Reactors  They don’t exist yet.  Temperatures > 10 8 K are required.  This takes a huge amount of energy.  To date, reactors are unable to get more energy out than they put in.  Some projects use magnetism to try to compact samples so they fuse easier.  Others use lasers to implode samples. 11

Fusion vs. Fission  Fusion produces less radioactive waste than fission.  Fusion cannot cause a runaway reaction (because it’s so difficult to get any reaction at all). Fission definitely can.  Deuterium can be easily extracted from seawater. Tritium can be manufactured from lithium. Most fissionable materials are difficult to get or process.  Fusion is way cooler. 12

Fusion in the Sun  Stars use a number of different reactions to produce energy depending on their type.  Our Sun primarily gets its energy from fusing hydrogen.  Elements up to iron are produced during this fusion.  Heavier elements are not produced in our Sun, but in other stars that died violently (supernova), using the extreme energy to produce heavier elements. 13

Mass-Energy Equivalence 14

Mass-Energy Equivalence 15 Higher Level