Pre-nuclear modelJJ Thomson proposed the ‘plum pudding model’. Image: mhhe.com (This model is not correct)In the model, the mass and charges in the atom are spread uniformly throughout the atom.
Geiger and Marsden In 1911, they ﬁred alpha particles at gold foil. Image: mhhe.com Gold foil Alpha emitter Round screenThe theories of the time predicted that the particles would easily pass through the thin foil since the gold was not dense enough to stop them.
Unexpected result Some alphas bounced right back!Most alphas passed straight through as predicted.Some were slightly deﬂected. But one in 8 000 was bounced right back. Rutherford said: “It was quite the most incredible event that has ever happened to me in my life ... It was then that I had the idea of an atom with a minute massive centre, carrying a charge.” Image: PHET
Rutherford’s model He discovered the nucleus The nucleus of the atom is 20 000 times smaller than the atom itself but contains nearly all of its mass and the positive charge. Empty Nucleus space The alpha particles are repelled by the nucleus (bothpositive charges). A larger nucleus repels more strongly.The faster the alpha particles, the less they are deﬂected.
Nuclearsplitting an unstable nucleus.Huge amounts of energy from ﬁssion Can you balance the 1n neutron nuclear equation? Solution over page neutron ﬁssions U-235 into 2 smaller daughter nuclei gammaThe products are: 141Ba★two nuclei eg krypton-92 and 92Krbarium-141 both with a lot of KE★three neutrons with KE★gamma rays at the speed of light 3 1nImage: PHET
Chainproduce energy from ﬁssion How to continually reaction The fuel is enriched U-235, since U-238 does not ﬁssion. The ﬁssion reaction produces fast neutrons. When these are slowed down, they can cause new ﬁssion which produces more fast neutrons ... Scientists realised that if at least one neutron caused new ﬁssion, there would be a chain reaction. U 235 92 1 141 92 1 + n 0 → Ba 56 + Kr 36 + 3n 0 plus loads and loads of energyImage: PHET
Nuclear ﬁssion reactorThe KE of the products makes the core extremelyhot. This heat energy is transferred to the exchanger;steam turns the turbine and electricity is generated. Image: Adrian Watt, Absorb Physics
Parts of a nuclear reactorPART OF FUNCTION POSSIBLEREACTOR OF THE PART MATERIALSFuel rods Chain reaction Enriched uranium oxide in zirconium alloy Slow down fast Graphite orModerator neutrons heavy water Absorb excess Boron or cadmiumControl rods neutrons Water or carbon dioxideCoolant Transfer heat energy or liquid sodium Thick lead, steelContainment Radiation protection or concrete
Nuclear accident 2011? What happened at Fukushima, Japan in MarchWhen the earthquake struck, the A were Fill in the blanks chainpushed into the B to absorb neutrons and reactionstop the C reaction in the D. But the controlearthquake had damaged the cooling rodssystem, so super-hot E built up and reacted corewith the F round the fuel rods to produce ﬁssionhydrogen which exploded. The hot core fuel rodsbroke through the G containment vessel neutronsand released H materials to the radioactivesurroundings. To stop the reaction, acidcontaining boron was added to absorb I and steamstop the J. steel zirconium
INTERESTING, BUT NOT FOR THE IGCSEThe interior of nuclear reactors glows bluebecause of Cherenkov radiation which is causedby electrons travelling in matter at speeds fasterthan light can travel in that medium. Image: wikipedia
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