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.

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. by fastfission – public domain 92 235 𝑈 + 0 1 𝑛→ 56 141 𝐵𝑎+ 36 92 𝐾𝑟 +3 0 1 𝑛 +𝑒𝑛𝑒𝑟𝑔𝑦

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. 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. by Charles Levy – public domain

Fission Reactor

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.

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.

Environmental Impact Advantages Disadvantages No CO2 emissions Radioactive waste products No greenhouse gases Accidents can be catastrophic High energy output Mostly safe 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 Nuclear fusion is the combining of two nuclei to form a larger one with the release of energy. e.g. deuterium and tritium fuse to form a helium nucleus and a neutron. 1 2 𝐻 + 1 3 𝐻 → 2 4 𝐻𝑒 + 0 1 𝑛 +𝑒𝑛𝑒𝑟𝑔𝑦 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. by Panoptik – CC-BY-SA-3.0

Fusion Reactors They don’t exist yet. Temperatures > 108 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.

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.

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.

Mass-Energy Equivalence Mass is a form of energy. We can calculate the energy of mass using Einstein’s famous formula: 𝐸=𝑚 𝑐 2 . The result is that when things lose energy (e.g. burning petrol), they get lighter; and when things gain energy (e.g. accelerating something), they get heavier. In nuclear reactions, energy needs to be input if the mass of the products is higher than the mass of the reactants. If the products are lighter than the reactants, energy is given out. Hence, fission only gives out energy for heavy elements and fusion only gives out energy for light elements.

Mass-Energy Equivalence 𝑐=3× 10 8 𝑚 𝑠 −1 Mass-Energy Equivalence e.g. Calculate the kinetic energy gained when a 1000 kg car accelerates from rest to 15 m/s. How much mass does it gain? e.g. The Sun gets 4 000 000 000 kg lighter every second due to emission of light. What is the power of the Sun? e.g. How much energy is given out from the fusion of deuterium and tritium? 𝑚 𝐷𝑒𝑢𝑡𝑒𝑟𝑖𝑢𝑚 =2.014102 𝑢 𝑚 𝑇𝑟𝑖𝑡𝑖𝑢𝑚 =3.016049 𝑢 𝑚 𝐻𝑒𝑙𝑖𝑢𝑚 =4.002602 𝑢 𝑚 𝑛𝑒𝑢𝑡𝑟𝑜𝑛 =1.008665 𝑢 1 𝑢=1.660539040× 10 −27 𝑘𝑔 Higher Level