Nuclear Energy, Stability, Fisson & Fusion

Transcription

1 Nuclear Eergy, Stability, Fisso & Fusio Ordiary chemical reactios ivolve oly the valece electros. I a chemical reactio chemical bods are formed broke ad reformed. The differece i eergy is usually expressed as heat. Apart from those chages that affect the valece electros, the atoms themselves are uchaged. Sice atoms are either created or destroyed there are the same umber of each kid of atom preset after the reactio as before. I uclear reactios chages occur i the ucleus of the atom. The uclei of a atom may be trasformed ito uclei of a differet kid of atom. A ew elemet or a ew isotope of the origial elemet may form as a result. Nuclear reactios occur aturally i radioactive elemets whose uclei are ustable. High speed particles as well as eergy are emitted from the ucleus of a radioactive elemet. Nuclear trasformatios ca also be made to occur artificially as the result of bombardmet of atoms with protos, eutros or other high eergy particles. Nuclear Equatios. Nuclear equatios are used to show uclear trasformatios. Balaced uclear equatios require that both the atomic umber ad the mass umber must be balaced. Example : Whe Beryllium 9 is bombarded with alpha particles (helium uclei) a eutro is produced. The balaced uclear reactio is give as: 9 Be + 4 He à + C 4 6 The sum of the atomic umbers ad the sum of the mass umbers must be equal o both sides of the equatio. Example : Whe itroge 4 is bombarded with eutros protos are produced. The balaced uclear equatio ca be writte as: 4 N + à p + 4 C 7 6. Radioactive Particles The Frech scietist Heri Becquerel accidetally discovered t radioactivity. Radioactivity is the ability of some isotopes to udergo reactios ivolvig uclear trasformatios. I aturally occurrig radioactive substaces, radioactive decay results i the coversio of ustable uclei to more stable uclei. There are three basic types of radiatio that occur aturally, alpha particles, beta particles, ad gamma radiatio. Their characteristics are described below Name Symbol Mass Charge Descriptio Alpha α 4 + Helium Nuclei Beta β - Electros Gamma γ High eergy radiatio

2 3. Peetratig Power Alpha Particles travel at about 5-% of the speed of light. Sice they are relatively slow ad massive their peetratig power is low. Several sheets of paper ca block them. Beta Particles travel at velocities of up to 9% of the speed of light. They have moderate peetratig power. They ca pass through ski but ca be stopped by mm thick sheet of alumium. Gamma Rays travel at the speed of light. Their peetratig power is very high, requirig at least a eight mm thick piece of lead to stop them. Because of their relatively low peetratig power alpha ad beta particles are relatively harmless, although high eergy beta particles ca cause ski burs. Gamma rays are very dagerous due to their great peetratig power ad their high eergy. Exposure to gamma rays ca cause tissue damage resultig i various forms of cacer, cataracts, ad ay geetic damage to reproductive cells that lead to mutatios. 4. Examples of Radioactive Decay: Uraium 38, 38 U, is a alpha emitter 38 U à 4 He à 34 Th 9 Neptuium 39, 39 Np, is a beta emitter 39 Np à e à 39 Pu Durig beta emissios a eutro splits ito a proto ad a electro. The electro is emitted as a beta particle, while the proto remais i the ucleus. The et effect is the coversio of a eutro to a proto. The atomic umber icreases by oe while the mass umber remais the same. I atural radioactive decay gamma rays are always emitted with alpha or beta emissio, ever by itself. Radium 6 for example emits both alpha particles ad gamma rays. 8 Ra è R + 4 He + γ There is o differece i the balaced uclear equatio if the gamma rays are ot icluded. 5. The Half Life The half life for a radioactive substace is the time required for oe-half of the iitial radioactive material to decay. Sice it is a first order process, the half life is idepedet of the amout of the radioactive sample preset. Each radioactive isotope has its ow uique half life. These rage from a few secods to billios of years. The radioactive isotope of carbo, C-4, has a half life of 573 years. Plutoium, Pu-39 has a half life of 4, years. Plutoium is a highly toxic material that is used as a fuel i uclear power plats. Because of its log half-life, waste plutoium will be aroud for a log time. It will take approximately 7, years for plutoium ow preset to decay to % of its curret level ad more like 5, years to reach a safe level.

3 The half lives of some radioactive isotopes Isotope Half life Type of emissio Strotium 9 Uraium 38 Rado Plutoium 39 Cesium years Beta 4.5 x 9 years Alpha 3.85 days Alpha 4, years Alpha 3 years Beta The first order rate laws ca be used to predict the amout of a radioactive material remaiig after or a give time, or the time it would take for a give amout of radioactive material to drop to a particular cocetratio. 6. First Order Rate equatios The same set of equatios that are used to describe first order chemical reactio rates ca also be applied to uclear reactios. I a first order process the rate is idepedet of the iitial cocetratio. Therefore the half life is oly a fuctio of the first rate costat. For a first order uclear decay t / =.693 k Where t / = the half life ad k = first order rate costat ad l ( X o/ X ) = kt Where k = first order rate costat, t = the time elapsed X o = iitial cocetratio of the isotope ad X = cocetratio of the isotope at time, t Example After hours a solutio cotaiig.3 x -6 mol dm -3 of 4 AmCl 3 cotaied oly.7 x -6 mol dm -3 of the radioactive substace. What is the half-life for 4 Am? l (.3 x -6 ) (.7 x -6 ) = k (. hours) k =.7 x - hours - the t / = x - hours - = 59 hours

4 7. Nuclear Stability Spotaeous radioactivity ivolves ustable uclei which decay with the evetual formatio of a stable uclei. Thus some uclei are stable ad others are ot, for example C ad 4 C, 3 H ad 4 K. The isotopes i these examples have the same umber of protos but differ i the umber of eutros. Two importat factors determie uclear stability () The mass umber (which is the total umber of ucleos i the ucleus) ad () the eutro to proto ratio. I the ucleus positively charged protos repel each other. As the umber of protos i the ucleus icreases the forces of repulsio betwee the protos icreases substatially. Therefore a greater umber of eutros is required for the ucleus to be stable. This is evidet from the graph of the umber of eutros agaist the umber of protos preset i stable uclei. A eutro to proto ratio of oe to oe holds true for the stable uclei of the first twety elemets i the periodic table. This ratio icreases to. to aroud umber 6 ad.5 to aroud atomic umber 8. Elemets with atomic umbers above 83 ad mass umbers greater tha 9 do ot exist as stable isotopes. Poloium for example has 84 protos. The repulsive forces that result from these 84 protos are great that the ucleus is ustable that regardless of the umber of eutros. All isotopes of poloium are radioactive. Wheever the eutro to proto ratio is too large or too small, the ucleus is ustable. It is the called radiouclide ad it udergoes radioactive decay. If a radiouclide has a higher eutro to proto ratio tha for a stable ucleus, It falls to the left of the stable ucleus To reduce the umber of eutros back to the stable ratio it udergoes beta decay. A eutro disitegrates with the emissio of a high speed electro kow as a beta particle: à p + - e As a result a proto is produced from the eutro. Sice this raises the umber of protos by oe ad reduces the umber of eutros by oe. The eutro to proto ratio is lowered util it reaches a stable value. The o further radioactive occurs If a radiouclide has a lower eutro to proto ratio, it has fewer eutros ad therefore to the right of the stable value. A proto ca be trasformed ito a eutro either by positio emissio or by electro capture: Positro emissio Electro Capture P à e + à P + e à -

5 If the total umber of ucleos exceeds 9, the limit for stable uclei, the ucleus always lies beyod the stable limit, ad the ucleus is always radioactive. Several kids of radioactive decay are ivolved i order to reach the stability. For example, the radioisotope 38 U udergoes a sequece of fourtee radioactive decay steps before formig the fial product 6 Pb. This sequece is called the 38 U decay series The tremedous amout of eergy that results from radioactive fissio comes from covertig small amouts of mass are coverted to eergy Eistei's equatio predicts that the eergy released is equal to the mass times the speed of light squared. E= mc where E is the eergy, m the mass ad c the velocity of light equal to.998x 8 m s -. A very small amout of matter should produce a tremedous amout of eergy. If the mass is measured i kg ad the velocity of light i meters per secod, the uits of eergy will be expressed i Joules.A uit of eergy ca therefore be cosidered equivalet to a uit of mass. Sice oe atomic mass uit is equal to.66x -7 kg, The E = mc = (.66x -7 kg) x (.998x 8 m s - ) =.493x - J Sice ev =.6 x -9 J E =.493x - J /.6x -9 J ev - = 93x 6 ev = 93 MeV Example : Thus atomic mass uit has a eergy equivalet of 93 MeV. Thorium 8 is a alpha particle emitter ad that produces radium-4: Th 9 à Ra 88 + He I this reactio the mass of the products = 8.8 amu (atomic mass uits ) the mass of the reactats = amu The differece equals =.596 amu I kilograms this is =.596 amu x.66 x -7 kg amu - The eergy released by oe atom of thorium decayig is E= (.596 amu x.66x -7 kg amu - ) x (.998x -8 m/s) = 8.847x -3 J per atom. Sice mole of 8 Th has a molar mass = 8 g mol - ad cotais 6.x 3 atoms, it would produce ( 8.847x -3 J per atom ) x (6.x 3 atoms) = 5.39x J or about 533 billio Joules of eergy! I a uclear explosive the critical mass is separated ito sub-critical masses util detoatio. It ca produce 5 to millio C o explosio! A atomic explosio does ot take place i a uraium mie because the fissioable isotope of uraium, 35 U is ot foud pure i ature.less tha % uraium foud i ature is fissioable 35 U. The rest is 38 U which is ot fissioable by slow eutros. Thus the fissioable 35 U has to be separated from 38 U before is ca be used for producig eergy.

6 8. Fissio ad Fusio Nuclear fissio is the splittig (by impact of eutros) of a heavy ucleus ito two or more lighter uclei (called fissio fragmets) with the simultaeous release of eutros ad large amout of eergy. The additioal eutros released ca iduce further fissio. Two commo fissioable substaces iclude uraium 35 ad plutoium U 9 + à 36 Kr + Ba Pu 94 + à 38 Sr + Ba Durig the fissio process tremedous amouts of eergy are released. The eergy, E, is produced is the result of the mass defect. The total mass of the fissio products i the above reactios is slightly less tha the fissio reactats. The differece i mass is coverted ito eergy The amout of eergy produced is give by Eistei s equatio: E = mc where c is the velocity of light. A certai cocetratio of fissioable material must be preset if the fissio reactio is to occur. The critical mass is defied as the mass of the fissioable material i a certai volume eeded to sustai a chai reactio. A chai reactio occurs whe the uraium sample is large eough for most of the eutros emitted, to be captured by aother ucleus before passig out of the sample. Also, for a chai reactio, there must be a balace betwee et productio of eutros ad the loss of eutros caused by: (i) the capture of eutros by uraium atoms without fissio happeig (ii) the capture of eutros by other materials i the sample ad (iii) the escape of eutros without beig captured. Thus critical mass is very importat. 9. Nuclear Fusio Nuclear fusio occurs whe the uclei of small or lighter elemets combie to form heavier elemets with the release of eergy. Because the heavier ucleus is more stable tha the lighter uclei that are fused together, there is a et loss of mass ad eergy is released: A example of a fusio reactio: 3 H + H à He + + eergy The eergy produced by the su is the result of by fusio of protos; Hydroge ad Helium comprise approximately 99% of the su s mass. Nuclei must overcome electric repulsio betwee the positive uclei i order to fuse. As a result, extremely high temperatures (approximately 8 K) are required. Fusio is also called a thermouclear reactio. The hydroge bomb is a example of ucotrolled uclear fusio. Cotrolled fusio is ot yet possible but scietists are predictig that the appropriate techology will be available soo after the year.techical problems with fusio are related to the fact that the positive uclei repel each other ad pushig these together ad holdig them together requires eormous amouts of force. Thus, the problems with fusio are: (i) the productio of plasma. At very high temperatures the atoms are stripped of their electros ad the itesely hot mixture of positive uclei ad free electros is called plasma. This process eeds upwards of 4 millio o C! (betwee 4 ad x 6 o C) (ii) that the plasma must be held together log eough (about secod) to fuse the uclei, (iii) that eough eergy must be produced to make the process profitable.

7 Obviously, a magic bottle is eeded to hold the plasma. Two possibilities are beig cosidered: a. Magetic Cotaimet Method The magetic cotaimet method uses a magetic field to cofie the plasma ad allows it to be heated electrically to achieve fusio. A super-cooled maget is used. Lithium is the used to absorb ad trasfer the fusio heat to a geerator. Thus a egieerig problem icludes havig a temperature i the regio of 4- millio o C at the ceter of the plasma ad yet just two meters away the temperature of the maget at close to 73 o C!. b. Laser Igitio Method I the secod method, focused laser light is used to cause the uclei to fuse ad agai molte lithium is used to absorb the eergy from fusio ad trasfer it as heat to a steam geerator to produce electricity. The choice of material for makig a fusio reactor has to be such that (I) it must ot react with extremely hot lithium (at about o C) ad it must last cotiuous eutro bombardmet because high eergy eutros are produced as part of the product! Noetheless, the advatages of fusio as a power geeratig source are may:. The fuel deuterium ( H or D) is abudat (there are about deuterium atoms i. dm 3 of sea water!). I fact, the fuel is limitless ad cheap.. Fusio is cosidered much less dagerous tha fissio with regard to radioactive materials. 3. Massive shipmets of radioactive fuel would ot be required as they are i fissio. Also, far less waste has to be stored. 4. Theft of fuel material is elimiated ulike i fissio (of plutoium). 5. Fusio eergy could be used to electrolyze water ito H (g. Hydtroge is a much cleaer replacemet for atural gas ad petroleum. Besides the techical problems at the momet, tritium, 3 H (a isotope of hydroge) is produced ad is very radioactive. If 3 H ca be isolated it ca the be used as a fuel as well: 3 4 H + H à He + + eergy