sign in sign up
<<
Home , Radioactive decay

Nuclear Classification

Chapter 31: • = # of = # of Radioactivity & – number = p + n • – Isotopes have the same atomic number but different mass numbers • Notation mass# Brent Royuk – atomic # 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 discovers radioactivity, 1896 • Radioactive nuclei are unstable. They – 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. • – A radioactive nucleus spits out an and becomes a daughter nucleus – e.g. 238 234 4 92U → 90Th + 2He • – A inside the radioactive nucleus changes into a and spits out an € – 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

• Radioactive decay occurs because of instability. Fusion 2 2 4 • A basic fusion reaction: 1 H + 1H → 2He + and fission are nuclear reactions that can occur because – VERY high temps– occurs only in core of 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 ? – 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 for fusion reactors? – How do you contain such a hot fire? 9 10

Nuclear Fusion

• 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 is • 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

• 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 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 as a 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 levels •

24

3