4/16/2016 Option C: Energy C.3 : Nuclear Fusion and Fission What is a nuclear reaction? • A nuclear reaction is any reaction that involves the nucleus. • These reactions change the identity of an atom, as opposed to chemical reactions which only involve valence electrons. 1 4/16/2016 The nucleus • The nucleus is made up of protons and neutrons. • We know what the protons do – they provide an electrostatic attraction to the electrons close… but what about the neutrons? The Neutrons • The major function of the neutrons is to hold the nucleus together. • The neutrons provide a strong nuclear force of attraction within the nucleus, counteracting the repulsion between the positively charged protons. 2 4/16/2016 How is the nucleus held together? • In the 1930’s it was first observed that the mass of an atoms nucleus is less than the sum of the masses of the protons + neutrons…? • Some of the mass of the nucleus is converted into energy to hold the nucleus together. 3 4/16/2016 Mass Defect • The difference in mass of the nucleus and it’s parts is referred to as the mass defect, and the energy (e=mc 2) it provided is called the nuclear binding energy. 4 4/16/2016 Nuclear vs. Chemical Reactions • This nuclear binding energy is released during nuclear reactions (fission & fusion), and is ~1,000,000X greater than the chemical bond energy released during chemical reactions. What makes an isotope radioactive? • Elements are radioactive when their nucleus is unstable. • The stabilizing force of the neutrons is effective for smaller elements, though all elements above lead are radioactive. 5 4/16/2016 Band of Stability 6 4/16/2016 All Radiation is not the same • Radiation, the particles and/or energy given off by nuclei undergoing radioactive decay, comes in a number of forms. • The forms we need to know are: – Alpha emission – Beta emission – Gamma emission Alpha Emission (too many protons) Emits an alpha particle • Made up of 2 protons and 2 neutrons • Effect on parent: – Mass lost, resulting in a new element Danger Level Extremely ionizing but least penetrating 7 4/16/2016 Beta Emission (too many neutrons) • A neutron is converted into a proton and an electron. The electron is emitted as a β particle • Result: – parent changes slightly in mass (new element produced) Danger Level Less ionizing but more penetrating than alpha 8 4/16/2016 Gamma Rays • A form of EMR given off when nucleons rearrange themselves inside a nucleus • Don’t result in a change in the identity of the atom • Gamma rays typically come from the nucleus, Danger Level while x-rays come from Least ionizing but the electrons. much penetrating than alpha 9 4/16/2016 How long will radioactive isotopes decay? 10 4/16/2016 It depends 11 4/16/2016 Types of Nuclear Reactions Fission & Fusion Nuclear Fission • Involves the splitting of a heavy nucleus into smaller elements, releasing the nuclear binding energy. • This can be done for isotopes larger than Fe-56, though is primarily done with U-235 & Pu-239. 12 4/16/2016 Artificially Induced Fission • First discovered by German scientists in 1938 when they discovered barium after bombarding uranium with neutrons. A chain reaction • Assuming a critical mass of fissionable material is present, a chain reaction can occur as each fission releases additional neutrons. 13 4/16/2016 Controlling the Reaction • The fission reaction has to occur at an acceptable speed in power plants, or else the energy would be lost. • Moderators are used to slow down the neutrons, while control rods (often graphite) can absorb the neutrons 14 4/16/2016 Fuel Rods 15 4/16/2016 Control Rods Uranium Enrichment • U-235 is the most fissionable isotope, but… • Naturally occurring uranium is <1% U-235. • Uranium enrichment increases the percentage U-235, increasing the likelihood of fission. 16 4/16/2016 Breeder Reactors • As the supply of U-235 is limited, “breeder reactors” are used to convert the more common U-238 into fissionable Pu-239. 17 4/16/2016 The problem with breeders… • Plutonium is highly toxic and these reactors are more susceptible to accidents. • Plutonium can be concentrated from reactor grade to weapons grade very easily. Issues with nuclear energy • Nuclear waste is radioactive for a very long time. – High Level Waste (ex. Control rod) • high activity; long half-life – Low-level Waste (ex. Fuel containers, clothing) • low activity; short half-life • And of course there is always the potential for a major accident. 18 4/16/2016 Three Mile Island (1979) Chernobyl (1986) 19 4/16/2016 20 4/16/2016 Fukushima (2011) 21 4/16/2016 Fukushima (2011) Disposal of high-level waste • Requires a geologically-stable area with impervious rock and away from water supplies. 22 4/16/2016 Uses of Nuclear Power 23 4/16/2016 24 4/16/2016 Nuclear Fusion • Occurs when light nuclei are brought together to form heavier elements. • This typically involves the fusion of two hydrogens to form helium. 25 4/16/2016 The up-side of fusion • It releases 3-4X the energy of a fission reaction. • It produces almost no radioactive waste. • The supply of deuterium (H-2) is cheap and almost unlimited (from the ocean) The down-side of fusion • It requires way more energy than fission to begin (reactants must be in a plasma state). • It is much harder to control. 26 4/16/2016 Cold Fusion • Many attempts have been made to design a controllable fusion reactor (pg. 533) and will certainly continue, as success would mean electricity would cost us pennies per day. 27 4/16/2016 Fusion reactions power stars! • Fusion reactions generate the energy released from stars, which explains why absorption data shows us that the Sun is made up of primarily hydrogen and helium. 28 4/16/2016 The Electromagnetic Spectrum • Every object above 0 K emits EMR of different wavelengths. • The Sun is hot enough to emit all wavelengths of visible light (white) • Atmospheric gases absorb some wavelengths of this light, which allows us to identify them. 29 4/16/2016 The End No HW except reading C.4 (pages 542-550) 30 4/16/2016 BONUS COVERAGE!!! Natural Sources Terrestrial Cosmic 31 4/16/2016 Terrestrial • Consists of radioactive elements found here on Earth • By far the major contributor to terrestrial radiation is Radon-222, which is a naturally occurring decay product of Uranium-238, which is found in the soil Radon Gas • A colorless, odorless gas • The EPA estimates that whose concentration it causes 21,000 deaths can build up in poorly from lung cancer each ventilated homes. Not year. uncommon in our area. 32 4/16/2016 Stanley Watras • The build-up of radon gas in homes was made famous when an engineer repeatedly set off radiation detectors at a new nuclear power plant – that didn’t yet have radioactive material. Testing in his home revealed that his families cancer risk would be the same as if they smoked 135 packs of cigarettes daily. 33 4/16/2016 Radon Abatement Systems 34 4/16/2016 Natural Sources Cosmic Cosmic Radiation • Consists of a variety of high energy photons primarily from the sun and supernovas • Though mostly blocked by the earth’s atmosphere and magnetic field, it still increases our cancer risk 35 4/16/2016 Poses a serious stumbling block to any thoughts of space travel 36.