2Warm-up There are 1200 students in a hall. A student starts talking. If each talking student starts 3 others to talk each second, ...… what is the minimum time required for all students to be talking?
3Warm-up The time required = 6 s How to stop it from being out of control?Sec. No. of talking students0 1 3 = 42 4 + ____ 3 = ____3 ____ + ____ 3 = ____4 ____ + ____ 3 = ____5 ____ + ____ 3 = ____6 ____ + ____ 3 = ____41616166464256Ask teachers responsible for discipline to patrol around the hall.256102410244096
5Nuclear Reactions When a heavy nucleus splits up, huge amount of energy is released.E.g. atomic bombWhen 2 light nuclei join together to form a heavy nucleus, huge amount of energy is released.E.g. the Sun
6Nuclear FissionFission is the process of splitting a nucleus into two nuclei with smaller masses.Large amount of energy is releasesA chain reaction is an on-going series of fission reactions.Critical mass is the amount of fissionable material required to continue a reaction at a constant rateExothermic Reaction
7Nuclear fission U 235 92 + n 1 144 56 Ba + Kr 90 36 + n 1 + n 1 + uncharged not repelled onapproachingnucleiBa-144 nucleusneutronneutronneutronU-235 nucleusKr-90 nucleusU23592+n114456Ba+Kr9036+n1+n1+energy
8Nuclear fission U 235 92 + n 1 144 56 Ba + Kr 90 36 + n 1 + n 1 + 14456Ba+Kr9036+n1+n1+energyThe total mass of the fission products is found less than the total mass before fission takes place.Naturally-occurring uranium consists of 2 isotopes, U-238 (99.3%) & U-235 (0.7 %). It is the rare isotope (U-235) which undergoes fission.
9Chain reactionWhen a uranium nucleus splits, 2 or 3 neutrons are emitted.neutronU-235 nucleus splitsIf these neutrons carry on splitting other uranium nucleus...
13Neutrons may:1 - Cause another fission by colliding with a U235 nucleus2 - Be absorbed in other material3 - Lost in the systemIf sufficient neutrons are present, we may achieve a chain reactionCreates two smaller nuclides and free neutronsThe free neutrons potentially collide with nearby U235 nucleiMay cause the nuclide to split as wellEach split (fission) is accompanied by a large quantity of E-N-E-R-G-Y
15Nuclear FusionFusion occurs when two nuclei with low masses are combined to form one nucleus of larger mass.Can only happen when the nuclei are moving fast enough to get “close” to each otherTemperature in stars are high enough for fusion to occur (millions of degrees Celsius)FUSION video clip
16Nuclear fusion This process is called nuclear fusion. A HUGE AMOUNT of energy is also released when two light nuclei join together to form a heavy nucleus.This process is called nuclear fusion.
17Nuclear fusion H 2 1 + H 3 1 He 4 2 + n 1 + energy fusion hydrogen-2 nucleushydrogen-3 nucleusfusionneutronhelium-4H21+H31He42+n1+energy
18Nuclear fusion H 2 1 + H 3 1 He 4 2 + n 1 + energy +energySince nuclei carry “+” charges, they repel each other.For fusion to occur, the 2 hydrogen nuclei must approach each other with very high speed. hydrogen gas of high temperature(108 C)!
19Nuclear fusionNuclear fusion occurs in the core of the Sun, giving out heat and light. The reaction takes place continuously for billions of years.
20Q1 Which of the following… Which of the following elements is used in nuclear fission?A U-235B U-238C HydrogenD Helium
21Q2 According to Einstein’s… According to Einstein’s mass-energy equation E = mc2, what is the amount of energy produced if 1 kg of a certain element completely changes into energy?A 3 108 JB 3 1016 JC 6 1016 JD 9 1016 J
22Q3 In nuclear… (U) fission In nuclear ___________, nuclei of uranium are split into smaller pieces and large amountof energy will be released. In order tomaintain the ___________ reaction, mass ofuranium used should exceed the __________mass.fissionchaincritical
23Q4 In nuclear… (H) fusion In nuclear __________, nuclei of hydrogen are joined together and____________ nucleus is formed.helium
24THE SUNEvery second, the sun converts 500 million metric tons of hydrogen to helium. Due to the process of fusion, 5 million metric tons of excess material is converted into energy in each second. This means that every year, 157,680,000,000,000 metric tons are converted into energy.
25Complete the Reaction 4Be 1H element 4Be 2He 6C 2He 2He 6C 8O 2He atomic number (protons)4Be1H2He6C2He1H1H6C2He4Be8O2He2He
26Nuclear Fission Neutron induced in U235 Fission is Exothermic The sum of the masses ofthe resulting nuclei is lessthan the original mass (about 0.1% less)The “missing mass” is converted to energy according to E=mc2
27U.S. Electrical Power Production by Source The United States has 103 nuclear power reactors in 31 states.Nuclear energy provides electricity for one of every five homes and business … the second-largest source of electricity after coal.Some states are clearly are more dependent on nuclear energy than others. Vermont gets 76 percent of its electricity from nuclear reactors … New Hampshire, 58 percent … and South Carolina, 55 percent.In Virginia, nuclear power provides more than 40 percent of the electricity used.Source: EIA(2004)
28Nuclear Fuel Costs Nuclear Fuel Costs Include UraniumEnrichmentManufacturingWaste DisposalTotal Nuclear Fuel Cost is Only About 0.5 cents per kilowatt-hourUranium accounts for only about 20% of this cost or 0.1 cents per kilowatt-hourIncreasing Uranium Cost has Minimal Impact
29Review Nuclear fission: A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this processNuclear fusion:Several small nuclei fuse together and release energy.