Nuclear Classification Chapter 31: • Atomic number = # of protons = # of electrons Radioactivity & – Mass number = p + n • Isotopes Nuclear Physics – Isotopes have the same atomic number but different mass numbers • Notation mass# Brent Royuk – atomic # X is a standard notation. • e.g. 238U Phys-112 92 • This is often just written as U-238. The Concordia University € superscript/subscript notation is most often seen in nuclear equations. € 2 Nuclear Classification Nuclear Classification • Most elements have several naturally occurring • Most elements have several naturally occurring isotopes. Isotopes can also be created artificially. isotopes. Isotopes can also be created artificially. • Many isotopes are radioactive, especially heavy ones. • Many isotopes are radioactive, especially heavy ones. 3 4 Radioactivity Nuclear Decay • Henry Becquerel discovers radioactivity, 1896 • Radioactive nuclei are unstable. They – Uranium exposes film • Three “Rays” break into pieces and change into – Alpha, Beta, Gamma something else. • Oops, Alpha particles are He-4 nuclei, Beta particles are electrons. • Alpha Decay – A radioactive nucleus spits out an alpha particle and becomes a daughter nucleus – e.g. 238 234 4 92U → 90Th + 2He • Beta Decay – A neutron inside the radioactive nucleus changes into a proton and spits out an € electron – e.g. 14 C 14 N 0e 5 6 → 7 + -1 6 € 1 Half-Life Half-Life • Radioactive decay is an utterly random process, governed by quantum physics. But large numbers of particles follow predictable patterns. • The half-life is the time during which 50% of some nuclei will decay. 7 8 Fusion and Fission Nuclear Fusion • Radioactive decay occurs because of instability. Fusion 2 2 4 • A basic fusion reaction: 1 H + 1H → 2He + energy and fission are nuclear reactions that can occur because – VERY high temps– occurs only in core of stars of external conditions applied to the nucleus. – But the reaction is self-sustaining, once it gets going. – I.e., they can be controlled. • Big problem 100 €years ago: What makes the fire in the sun? – The sun isn’t a chemical fire, it’s a nuclear fire. – Wood gets lighter because it vaporizes and loses part of its mass to gaseous products. Does the sun get lighter? • There’s only 10 billion years left • Fusion reactors are being designed and tested. We haven’t passed the “break-even” point yet. – Where will we get the fuel for fusion reactors? – How do you contain such a hot fire? 9 10 Nuclear Fusion Nuclear Fission • In fission, a large nuclei splits into two big daughters 235 1 140 94 1 • One typical equation: 92 U + 0n → 54 Xe + 38Sr + 2 0n • Energy is also released. The daughters are a little bit lighter than the parent. € 11 13 2 Nuclear Enrichment Chain Reactions • The only fissionable naturally occurring isotope is • Neutrons cause nuclei to fission, which release U-235. neutrons, which cause nuclei to fission, which… • U-235 is only 0.7% of mined Uranium. • Controlled fission vs. explosions • 3-5% enrichment for fuel, >90% for weapons – Cd, B control rods in reactor core 14 15 Chain Reactions Nuclear Power • Critical mass • Power plant fuel usage rates: – Feynman – Nuclear: 6.5 oz/minute – U-235: 15 kg (grapefruit); Pu-239: 5 kg (orange) – Coal: 10 tons/minute • Implications for bomb design & proliferation 16 21 Nuclear Waste • Spent nuclear fuel is high-level waste. Each full-size power plant produces about 75 m3/year. – low-level: things that are irradiated and become radioactive • A nuclear plant is refueled every 18 months • The amount of HLW worldwide is currently increasing by about 12,000 metric tons every year • A coal power plant releases 100 times as much radiation as a nuclear power plant of the same wattage. • Processing: diluted with twice volume of neutral material: block of ceramic • Within 600 to 1000 years, high level waste will be back to Uranium ore levels • Depleted uranium 24 3 .