Presentation on theme: "Low Carbon Technologies and Solutions: Sections 6 - 8"— Presentation transcript:
1 Low Carbon Technologies and Solutions: Sections 6 - 8 NBSLM03E (2010)Low Carbon Technologies and Solutions: Sections 6 - 8N.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnvNuclear Power
2 Low Carbon Technologies and Solutions NBSLM03E (2010)Low Carbon Technologies and SolutionsN.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnvSection 6: Nuclear Power:- The Basics6. Nature of RadioactivityStructure of the AtomRadioactive EmissionsHalf Life of ElementsFissionFusionChain ReactionsFertile Materials7. Fission Reactors8. The Nuclear Fuel Cycle9. Fusion Reactors (in notes)2
3 Section 6: Nature of Radioactivity (1) Structure of Atoms.Matter is composed of atoms which consist primarily of a nucleus of:positively charged PROTONSand (electrically neutral) NEUTRONS.The nucleus is surrounded by a cloud of negatively charged ELECTRONS which balance the charge from the PROTONS.PROTONS and NEUTRONS have approximately the same massELECTRONS are about times the mass of the PROTON.A NUCLEON refers to either a PROTON or a NEUTRON3p4n+Lithium Atom3 Protons 4 Neutrons
4 Section 6: Nature of Radioactivity (2) Structure of Atoms.Elements are characterized by the number of PROTONS presentHYDROGEN nucleus has 1 PROTONHELIUM has 2 PROTONSOXYGEN has 8 PROTONSURANIUM has 92 PROTONS.Number of PROTONS is the ATOMIC NUMBER (Z)N denotes the number of NEUTRONS.The number of neutrons present in any element varies.3 isotopes of hydrogen all with 1 PROTON:-HYDROGEN itself with NO NEUTRONSDEUTERIUM (heavy hydrogen) with 1 NEUTRONTRITIUM with 2 NEUTRONS.only TRITIUM is radioactive.Elements up to Z = 82 (Lead) have at least one isotope which is stableSymbol DSymbol T
5 Section 6: Nature of Radioactivity (3) Structure of Atoms.URANIUM has two main ISOTOPES235U which is present in concentrations of 0.7% in naturally occurring URANIUM238U which is 99.3% of naturally occurring URANIUM.Some Nuclear Reactors use Uranium at the naturally occurring concentration of 0.7%Most require some enrichment to around 2.5% - 5%Enrichment is energy intensive if using gas diffusion technology, but relatively efficient with centrifuge technology.Some demonstration reactors use enrichment at around 93%.5
6 Section 6: Nature of Radioactivity (4) Structure of Atoms.Protons have strong nuclear forces to overcome the strong repulsive forces from the charges on them. This is the energy released in nuclear reactions++++++Stable elements plot close to blue line.Those isotopes plotting away from line are unstable.For elements above Lead (Z = 82), there are no stable isotopes.
7 Section 6: Nature of Radioactivity (5) Radioactive emissions.FOUR types of radiation:-1) ALPHA particles ()large particles consisting of 2 PROTONS and 2 NEUTRONSthe nucleus of a HELIUM atom.2) BETA particles (β) which are ELECTRONS3) GAMMA - RAYS. ()Arise when the kinetic energy of Alpha and Beta particles is lost passing through the electron clouds of atoms. Some energy is used to break chemical bonds while some is converted into GAMMA -RAYS.4) X - RAYS.Alpha and Beta particles, and gamma-rays may temporarily dislodge ELECTRONS from their normal orbits. As the electrons jump back they emit X-Rays which are characteristic of the element which has been excited.
8 Section 6: Nature of Radioactivity (6) β - particles are stopped by a thin sheet of paperβ – particles are stopped by ~ 3mm aluminium - rays CANNOT be stopped – they can be attenuated to safe limits using thick Lead and/or concrete
9 Section 6: Nature of Radioactivity (7) Radioactive emissions.UNSTABLE nuclei emit Alpha or Beta particlesIf an ALPHA particle is emitted, the new element will have an ATOMIC NUMBER two less than the original.eIf an ELECTRON is emitted as a result of a NEUTRON transmuting into a PROTON, an isotope of the element ONE HIGHER in the PERIODIC TABLE will result.
10 Section 6: Nature of Radioactivity (8) Radioactive emissions.235U consisting of 92 PROTONS and 143 NEUTRONS is one of SIX isotopes of URANIUMdecays as follows:-alphabetaalphaURANIUM235UTHORIUM231ThPROTACTINIUM231PaACTINIUM227AcThereafter the ACTINIUM decays by further alpha and beta particle emissions to LEAD (207Pb) which is stable.Two other naturally occurring radioactive decay series exist. One beginning with 238U, and the other with 232Th.Both also decay to stable (but different) isotopes of LEAD.
11 Section 6: Nature of Radioactivity (9) HALF LIFE.Time taken for half the remaining atoms of an element to undergo their first decay e.g:-238U billion years235U billion years232Th billion yearsAll of the daughter products in the respective decay series have much shorter half - lives some as short as 10-7 seconds.When 10 half-lives have expired,the remaining number of atoms is less than 0.1% of the original.20 half livesthe remaining number of atoms is less than one millionth of the original
12 Section 6: Nature of Radioactivity (10) HALF LIFE.From a radiological hazard point of viewshort half lives - up to say 6 months have intense radiation, butdecay quite rapidly. Krypton-87 (half life 1.8 hours)- emitted from some gas cooled reactors - the radioactivity after 1 day is insignificant.For long half lives - the radiation doses are small, and also of little consequenceFor intermediate half lives - these are the problem - e.g. Strontium -90has a half life of about 30 years which means it has a relatively high radiation, and does not decay that quickly.Radiation decreases to 30% over 90 years
13 Section 6: Nature of Radioactivity (11): Fission Some very heavy UNSTABLE elements exhibit FISSION e.g. 235Un235U93RbnnThis reaction is one of several which might take place. In some cases, 3 daughter products are produced.140Cs1313
14 Section 6: Nature of Radioactivity (12): Fission Nucleus breaks down into two or three fragments accompanied by a few free neutrons and the release of very large quantities of energy.Free neutrons are available for further FISSION reactionsFragments from the fission process usually have an atomic mass number (i.e. N+Z) close to that of iron.Elements which undergo FISSION following capture of a neutron such as URANIUM are known as FISSILE.Diagrams of Atomic Mass Number against binding energy per NUCLEON enable amount of energy produced in a fission reaction to be estimated.All Nuclear Power Plants currently exploit FISSION reactions,FISSION of 1 kg of URANIUM produces as much energy as burning 3000 tonnes of coal.
15 Section 6: Nature of Radioactivity (13): Fusion Fusion of light elements e.g. DEUTERIUM and TRITIUM produces even greater quantities of energy per nucleon are released.3HDeuterium – Tritium fusionTritium4He2HDeuterium(3.5 MeV)n(14.1 MeV)In each reaction 17.6 MeV is liberated or 2.8 picoJoules (2.8 * 10-15J)28/03/201715
16 Section 6: Nature of Radioactivity (14): Binding Energy Atomic Mass Number-2-4-6-8-10Binding Energy per nucleon [MeV]Iron 56Uranium 235Range of Fission ProductsFusion Energy release per nucleonFission Energy release per nucleon1 MeV per nucleon is equivalent to 96.5 TJ per kgRedrawn from 6th report on Environmental Pollution – CmndThe energy released per nucleon in fusion reaction is much greater than thecorresponding fission reaction.2) In fission there is no single fission product but a broad range as indicated.
17 Section 6: Nature of Radioactivity (15): Fusion Developments at the JET facility in Oxfordshire have achieved the break even point.Next facility (ITER) will be built in Cadarache in France.Commercial deployment of fusion from about 2040 onwardsOne or two demonstration commercial reactors in 2030s perhapsNo radioactive waste from fuelLimited radioactivity in power plant itself8 litres of tap water sufficient for all energy needs of one individual for whole of life at a consumption rate comparable to that in UK.Sufficient resources for 1 – 10 million years
18 Section 6: Nature of Radioactivity (16): Chain Reactions Fast Neutrons are unsuitable for sustaining further reactionsfast neutron235UnSlow neutronnn235Unfast neutronnSlow neutron
19 Section 6: Nature of Radioactivity (17): Chain Reactions FISSION of URANIUM yields free neutrons.If exactly ONE of these triggers a further FISSION, then a chain reaction occurs, and continuous power can be generated.UNLESS DESIGNED CAREFULLY, THE FREE NEUTRONS WILL BE LOST AND THE CHAIN REACTION WILL STOP.IF MORE THAN ONE NEUTRON CREATES A NEW FISSION THE REACTION WOULD BE SUPER-CRITICAL(or in layman's terms a bomb would have been created).
20 Section 6: Nature of Radioactivity (18): Chain Reactions IT IS VERY DIFFICULT TO SUSTAIN A CHAIN REACTION,Most Neutrons are moving too fastTO CREATE A BOMB, THE URANIUM MUST BE HIGHLY ENRICHED > 93%,Normal Uranium is only 0.7% U235Material must be LARGER THAN A CRITICAL SIZE and SHAPE OTHERWISE NEUTRONS ARE LOST.Atomic Bombs are made by using conventional explosive to bring two sub-critical masses of FISSILE material together for sufficient time for a SUPER-CRITICAL reaction to take place.NUCLEAR POWER PLANTS CANNOT EXPLODE LIKE AN ATOMIC BOMB.
21 Section 6: Nature of Radioactivity (19): Fertile Materials Some elements like URANIUM are not FISSILE, but can transmute:-nfast neutronee239Np239Pu238U239U+nbeta238UUranium - 238239UUranium - 239239NpNeptunium - 239beta239PuPlutonium - 239PLUTONIUM is FISSILE and may be used in place of URANIUMMaterials which can be converted into FISSILE materials are FERTILE.
22 Section 6: Nature of Radioactivity (20): Fertile Materials URANIUM is FERTILE as is THORIUM which can be transmuted into URANIUMNaturally occurring URANIUM consists of 99.3% 238U which is FERTILE and NOT FISSILE, and 0.7% of 235U which is FISSILE. Normal reactors primarily use the FISSILE properties of 235U.In natural form, URANIUM CANNOT sustain a chain reaction: free neutrons are travelling fast to successfully cause another FISSION, or are lost to the surrounds.MODERATORS are thus needed to slow down/and or reflect the neutrons in a normal FISSION REACTOR.The Resource Base of 235U is only decadesBut using a Breeder Reactor Plutonium can be produced from non-fissile 238U producing 239Pu and extending the resource base by a factor of 50+
23 Section 6: Nature of Radioactivity (21): Chain Reactions Sustaining a reaction in a Nuclear Power StationnFast Neutrons are unsuitable for sustaining further reactionsfast neutron235UnnnnSlow neutronfast neutronn235Unfast neutronInsert a moderator to slow down neutronsnSlow neutron
25 Fission Reactors (1) FISSION REACTORS CONSIST OF:- i) a FISSILE component in the fuelii) a MODERATORiii) a COOLANT to take the heat to its point of use.The fuel elements vary between different ReactorsSome reactors use unenriched URANIUMi.e. the 235U in fuel elements is at 0.7% of fuele.g. MAGNOX and CANDU reactors,ADVANCED GAS COOLED REACTOR (AGR) uses 2.5 – 2.8% enrichmentPRESSURISED WATER REACTOR (PWR) and BOILING WATER REACTOR (BWR) use around 3.5 – 4% enrichment.RMBK (Russian Rector of Chernobyl fame) uses ~2% enrichmentSome experimental reactors - e.g. High Temperature Reactors (HTR) use highly enriched URANIUM (>90%) i.e. weapons grade.
26 Fission Reactors (2): Fuel Elements PWR fuel assembly:UO2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~200AGR fuel assembly:UO2 pellets loaded into fuel pins of stainless steel each ~ 1 m long in bundles of 36.Whole assembly in a graphite cylinderMagnox fuel rod:Natural Uranium metal bar approx 35mm diameter and 1m long in a fuel cladding made of MagNox.Burnable poison
27 Fission Reactors (3) No need for the extensive coal handling plant. In the UK, all the nuclear power stations are sited on the coast so there is no need for cooling towers.Land area required is smaller than for coal fired plant.In most reactors there are three fluid circuits:-1) The reactor coolant circuit2) The steam cycle3) The cooling water cycle.ONLY the REACTOR COOLANT will become radioactiveThe cooling water is passed through the station at a rate of tens of millions of litres of water and hour, and the outlet temperature is raised by around 10oC.
28 REACTOR TYPES – summary 1 Fission Reactors (4)REACTOR TYPES – summary 1MAGNOX - Original British Design named after the magnesium alloy used as fuel cladding. Four reactors of this type were built in France, One in each of Italy, Spain and Japan. 26 units were built in UK.They are only in use now in UK. On December 31st 2006, Sizewell A, Dungeness A closed after 40 years of operation leaving Oldbury with two reactors is now continuing beyond its original extended 40 year life. Wylfa (also with 2 reactors) will close this year or next. All other units are being decommissionedAGR - ADVANCED GAS COOLED REACTOR - solely British design. 14 units are in use. The original demonstration Windscale AGR is now being decommissioned. The last two stations Heysham II and Torness (both with two reactors), were constructed to time and have operated to expectations.
29 REACTOR TYPES - summary Fission Reactors (5)REACTOR TYPES - summarySGHWR - STEAM GENERATING HEAVY WATER REACTOR - originally a British Design which is a hybrid between the CANDU and BWR reactors.PWR Originally an American design of PRESSURIZED WATER REACTOR (also known as a Light Water Reactor LWR). Now most common reactor.-BWR BOILING WATER REACTOR - a derivative of the PWR in which the coolant is allowed to boil in the reactor itself. Second most common reactor in use.RMBK LIGHT WATER GRAPHITE MODERATING REACTOR (LWGR)- a design unique to the USSR which figured in the CHERNOBYL incident. 16 units still in operation in Russian and Lithuania with 9 shut down.
30 REACTOR TYPES - summary Fission Reactors (6)REACTOR TYPES - summaryCANDU A reactor named initially after CANadian DeUterium moderated reactor (hence CANDU), alternatively known as PHWR (pressurized heavy water reactor). 41 currently in use.HTGR HIGH TEMPERATURE GRAPHITE REACTOR - an experimental reactor. The original HTR in the UK started decommissioning in The new Pebble Bed Modulating Reactor (PBMR) is a development of this and promoted as a 3+ Generation Reactor by South Africa.FBR FAST BREEDER REACTOR - 'breeds' PLUTONIUM from FERTILE 238Uextends resource base of URANIUM over 50 times. Mostly experimental at moment with FRANCE, W. GERMANY and UK, Russia and JAPAN having experimented with them.
31 MAGNOX REACTORS (also known as Gas Cooled Reactors (GCR) FUEL TYPE - unenriched URANIUM METAL clad in Magnesium alloyMODERATOR - GRAPHITECOOLANT - CARBON DIOXIDEDIRECT RANKINE CYCLE- no superheat or reheat efficiency ~ 20% to 28%.ADVANTAGES:-LOW POWER DENSITY - 1 MW/m3. Thus very slow rise in temperature in fault conditions.UNENRICHED FUELGASEOUS COOLANTON LOAD REFUELLINGMINIMAL CONTAMINATION FROM BURST FUEL CANSVERTICAL CONTROL RODS - fall by gravity in case of emergency.DISADVANTAGES:-CANNOT LOAD FOLLOW – [Xe poisoning]OPERATING TEMPERATURE LIMITED TO ABOUT 250oC - 360oC limiting CARNOT EFFICIENCY to ~ %, and practical efficiency to ~ 28-30%.LOW BURN-UP - (about 400 TJ per tonne)EXTERNAL BOILERS ON EARLY DESIGNS.31
32 ADVANCED GAS COOLED REACTORS (AGR) FUEL TYPE - enriched URANIUM OXIDE - 2.3% clad in stainless steelMODERATOR - GRAPHITECOOLANT CARBON DIOXIDESUPERHEATED RANKINE CYCLE (with reheat) - efficiency %ADVANTAGES:-MODEST POWER DENSITY - 5 MW/m3. slow rise in temperature in fault conditions.GASEOUS COOLANT ( BAR cf 160 bar for PWR)ON LOAD REFUELLING under part loadMINIMAL CONTAMINATION FROM BURST FUEL CANSRELATIVELY HIGH THERMODYNAMIC EFFICIENCY 40%VERTICAL CONTROL RODS - fall by gravity in case of emergency.DISADVANTAGES:-MODERATE LOAD FOLLOWING CHARACTERISTICSSOME FUEL ENRICHMENT NEEDED %OTHER FACTORS:-MODERATE FUEL BURN-UP - ~ 1800TJ/tonne (c.f. 400TJ/tonne for MAGNOX, 2900TJ/tonne for PWR).SINGLE PRESSURE VESSEL with pres-stressed concrete walls 6m thick. Pre-stressing tendons can be replaced if necessary.32
33 CANDU REACTOTS (PHWR)FUEL TYPE - unenriched URANIUM OXIDE clad in ZircaloyMODERATOR - HEAVY WATER COOLANT HEAVY WATERADVANTAGES:-MODEST POWER DENSITY MW/m3.HEAVY WATER COOLANT - low neutron absorber hence no need for enrichment.ON LOAD REFUELLING - and very efficient indeed permits high load factors.MINIMAL CONTAMINATION from burst fuel can - defective units can be removed without shutting down reactor.MODULAR: - can be made to almost any sizeDISADVANTAGES:-POOR LOAD FOLLOWING CHARACTERISTICSCONTROL RODS ARE HORIZONTAL, and cannot operate by gravity in fault conditions.MAXIMUM EFFICIENCY about 28%OTHER FACTORS:-MODERATE FUEL BURN-UP - ~ MODEST FUEL BURN-UP - about 1000TJ/tonneFACILITIES PROVIDED TO DUMP HEAVY WATER MODERATOR from reactor in fault conditionsMULTIPLE PRESSURE TUBES instead of one pressure vessel.33
34 PRESSURISED WATER REACTORS – PWR (VVER) FUEL TYPE – 4% enriched URANIUM OXIDE clad in ZircaloyMODERATOR - WATERCOOLANT WATERADVANTAGES:-GOOD LOAD FOLLOWING CHARACTERISTICS - claimed for SIZEWELL B. - most PWRs are NOT operated as such.HIGH FUEL BURN-UP- about 2900TJ/tonne –VERTICAL CONTROL RODS - drop by gravity in fault conditions.DISADVANTAGES:-ORDINARY WATER as COOLANT - pressure to prevent boiling (160 bar). If break occurs then water will flash to steam and cooling will be less effective.ON LOAD REFUELLING NOT POSSIBLE - reactor must be shut down.SIGNIFICANT CONTAMINATION OF COOLANT CAN ARISE FROM BURST FUEL CANS - as defective units cannot be removed without shutting down reactor.FUEL ENRICHMENT NEEDED %.MAXIMUM EFFICIENCY ~ %latest designs ~ 34%OTHER FACTORS:-LOSS OF COOLANT also means LOSS OF MODERATOR so reaction ceases - but residual decay heat can be large.HIGH POWER DENSITY MW/m3, and compact. Temperature can rise rapidly in fault conditions. NEEDS active ECCS.SINGLE STEEL PRESSURE VESSEL 200 mm thick.34
35 BOILING WATER REACTORS – BWR FUEL TYPE - 3% enriched URANIUM OXIDE clad in ZircaloyMODERATOR - WATERCOOLANT WATERADVANTAGES:-HIGH FUEL BURN-UP- about 2600TJ/tonneSTEAM PASSED DIRECTLY TO TURBINE therefore no heat exchangers needed. BUT SEE DISADVANTAGES..DISADVANTAGES:-ORDINARY WATER as COOLANT – but designed to boil: pressure ~ 75 bar.CONTROL RODS MUST BE DRIVEN UPWARDS - POWER NEEDED IN FAULT CONDITIONS. Water can be dumped in such circumstances.ON LOAD REFUELLING NOT POSSIBLE - reactor must be shut down.SIGNIFICANT CONTAMINATION OF COOLANT CAN ARISE FROM BURST FUEL CANS RADIOACTIVE STEAM WILL PASS DIRECTLY TO TURBINES.FUEL ENRICHMENT NEEDED. - 3%.MAXIMUM EFFICIENCY ~ 34-35%OTHER FACTORS:-LOSS OF COOLANT also means LOSS OF MODERATOR so reaction ceases - but residual decay heat can be large.HIGH POWER DENSITY MW/m3, and compact. Temperature can rise rapidly in fault conditions. NEEDS active ECCS.SINGLE STEEL PRESSURE VESSEL 200 mm thick.35
36 RMBK (LWGR): - involved in Chernobyl Incident FUEL TYPE - 2% enriched URANIUM OXIDE clad in ZircaloyMODERATOR - GRAPHITECOOLANT WATERADVANTAGES:-ON LOAD REFUELLINGVERTICAL CONTROL RODS which can drop by GRAVITY in fault conditions.NO THEY CANNOT!!!!DISADVANTAGES:-ORDINARY WATER as COOLANT - flashes to steam in fault conditions hindering cooling.POSITIVE VOID COEFFICIENT !!! - positive feed back possible in some fault conditions -other reactors have negative voids coefficient in all conditions.IF COOLANT IS LOST moderator will keep reaction going.FUEL ENRICHMENT NEEDED. - 2%PRIMARY COOLANT passed directly to turbines. This coolant can be slightly radioactive.MAXIMUM EFFICIENCY ~30% ??OTHER FACTORS:-MODERATE FUEL BURN-UP - ~ MODEST FUEL BURN-UP - about 1800TJ/tonneLOAD FOLLOWING CHARACTERISTICS UNKNOWNPOWER DENSITY probably MODERATE?MULTIPLE PRESSURE TUBESRMBK (LWGR): (involved in Chernobyl incident)
37 FAST BREEDER REACTORS (FBR OR LMFBR) FUEL TYPE - depleted Uranium or UO2 surround PU in centre of core. All elements clad in stainless steel.MODERATOR - NONECOOLANT LIQUID METALADVANTAGES:-LIQUID METAL COOLANT - at atmospheric pressure. Will cool by natural convection in event of pump failure.BREEDS FISSILE FUEL from non-fissile 238U – increases resource base 50+ times.HIGH EFFICIENCY (~ 40%)VERTICAL CONTROL RODS drop by GRAVITY in fault conditions.DISADVANTAGES:-DEPLETED URANIUM FUEL ELEMENTS REPROCESSED to recover PLUTONIUM and sustain the breeding for future use.CURRENT DESIGNS have SECONDARY SODIUM CIRCUITWATER/SODIUM HEAT EXCHANGER. If water and sodium mix a significant CHEMICAL explosion may occur which might cause damage to reactor itself.OTHER FACTORS:-VERY HIGH POWER DENSITY MW/m3 but rise in temperature in fault conditions limited by natural circulation of sodium.
38 GENERATION 3 REACTORS: EPR1300: PWR Schematic of Reactor is very similar to later PWRs (SIZEWELL) with 4 Steam Generator Loops.Main differences? from earlier designs.Output power ~1600 MW from a single turbine(cf 2 turbines for 1188 MW at Sizewell).Each of the safety chains is housed in a separate building.Efficiency claimed at 37%But no actual experience and likely to be lessConstruction is under way at Olkiluoto, Finland.Second reactor under construction in Flammanville, FrancePossible contender for new UK generation
39 GENERATION 3: AP1000: PWR A development from SIZEWELL Power Rating comparable with SIZEWELLPossible Contender for new UK reactorsWill two turbines be used ??Passive Cooling – water tank on top – water falls by gravityTwo loops (cf 4 for EPR)Significant reduction in components e.g. pumps etc.28/03/2017
40 GENERATION 3: ACR1000: Advanced Candu Reactor A development from CANDU with added safety features less Deuterium neededPassive emergency cooling as with AP1000See Video Clip of on-line refuelling
41 Generation 3 ESBWR: Economically Simple BWR A derivative of Boiling Water Reactor for which it is claimed has several safety features but which inherently has two disadvantages of basic designVertical control rods which must be driven upwardsSteam in turbines can become radioactive
42 GENERATION 3+ REACTORS: the PBMR Pebble Bed Modulating Reactors are a development from Gas Cooled Reactors.Sand sized pellets of Uranium each coated in layers of graphite/silicon carbide and aggregated into pebbles 60 mm in diameter.Coolant: HeliumConnected directly to closed circuit gas turbineEfficiency ~ 39 – 40%, possibility of CCGT??Graphite/silicon carbide effective claddingvery durable at high temperatures
43 GENERATION 3+ REACTORS: the PBMR Unlike other Reactors, the PBMR uses a closed circuit high temperature gas turbine operating on the Brayston Cycle for Power. This cycle is similar to that in a JET engine or the gas turbine section of a CCGT.Normal cycles exhaust spent gas to atmosphere.In this version the helium is in a closed circuit.PBM ReactorFuel InCombustion ChamberCompressorTurbineGeneratorOpen Brayston CycleClosed Brayston CycleExhaustHeat ExchangerAir In
44 GENERATION 3+ REACTORS: the PBMR Efficiency of around 38 – 40%, but possibility of CCGT???Helium passes directly from reactor to turbinePebbles are continuously fed into reactor and collected.Tested for burn up and recycled as appropriate ~ typically 6 times
45 Low Carbon Technologies and Solutions Section 8: Nuclear Fuel Cycle NBSLM03E (2010)Low Carbon Technologies and SolutionsN.K. Tovey (杜伟贤) M.A, PhD, CEng, MICE, CEnvSection 8: Nuclear Fuel CycleNuclear Power – The BasicsNuclear Power: Fission reactorsNuclear Fuel CycleFusion Reactors454545
46 Section 8: Nuclear Fuel Cycle TWO OPTIONS AVAILABLE:-ONCE-THROUGH CYCLE,REPROCESSING CYCLECHOICE DEPENDS primarily on:-REACTOR TYPE IN USE (more or less essential for MAGNOX),AVAILABILTY OF URANIUM TO COUNTRY IN QUESTION,DECISIONS ON THE POSSIBLE USE OF FBRs.DECISIONS ON HOW RADIOACTIVE WASTE IS TO BE HANDLED.Reprocessing leads to much less HIGH LEVEL radioactive waste, but more low level radioactive wasteECONOMIC CONSIDERATIONS done 10 years ago show little difference between two types of cycle except that for PWRs, ONCE-THROUGH CYCLE appeared MARGINALLY more attractive.4646
47 Section 8: Nuclear Fuel Cycle NUCLEAR FUEL CYCLE divided into two parts:-FRONT-END - includes MINING of Uranium Ore, EXTRACTION, CONVERSION to "Hex", ENRICHMENT, and FUEL FABRICATION.BACK-END - includes TRANSPORTATION of SPENT FUEL, STORAGE, REPROCESSING, and DISPOSAL.NOTE:Transportation of Fabricated Fuel elements has negligible cost as little or no screening is necessary.Special Provisions are needed for transport of spent fuel for both cycles.For both ONCE-THROUGH and REPROCESSING CYCLES, the FRONT-END is identical. The differences are only evident at the BACK- END.4747
48 Section 8: Simplified Fuel Cycle for a PWR (1) Cooling PondsFuel Rods1 T0.9m3Reactor1PJ~ homes60 x 2MW wind turbinesEnrichmentREPROCESSINGOnce Through9 kg PlutoniumStorageUF6Liquid 5m3HL Waste0.96 tUraniumU3O8Concentrate 0.5m30.4m3 IL wasteOreMining Spoil0.9m3 HL waste0.15m3 solid0.7m3LL waste0.8m3IL waste1500 m34848
49 Section 8: Simplified Fuel Cycle for a PWR (2) MINING - ore > 0.05% by weight of U3O8 to be economic.Typically at 0.5%, 500 tonnes (250 m3) must be excavated to produce 1 tonne of U3O8 ("yellow-cake") which occupies about 0.1 m3.URANIUM leached out chemicallyresulting powder contains about 80% yellow-cake. The 'tailings' contain the naturally generated daughter products.PURIFICATION/CONVERSION- dissolve 'yellow-cake' in nitric acid and conversion to Uranium tetrafluoride (UF4)UF4 converted into URANIUM HEXAFLOURIDE (UF6) or "HEX" if enrichment is needed.4949
50 Section 8: Simplified Fuel Cycle for a PWR: Enrichment (1) proportion of URANIUM is artificially increased.GAS DIFFUSION - original method still used in FRANCE."HEX" is allowed to diffuse through a membrane separating the high and low pressure parts of a cell.235U diffuses faster than 238U through this membrane.Outlet gas from lower pressure is slightly enriched in 235U (by a factor of ) and is further enriched in subsequent cells.HUNDREDS / THOUSANDS of such cells are required in cascade depending on the required enrichment.Pumping demands are very large as are the cooling requirements between stages.5050
51 Section 8: Simplified Fuel Cycle for a PWR: Enrichment (2) ENRICHMENT: GAS DIFFUSION.Outlet gas from HIGH PRESSURE side is slightly depleted URANIUM and is fed back into previous cell of sequence.AT BACK END, depleted URANIUM contains only % 235U,NOT economic to use this for enrichment.This depleted URANIUM is currently stockpiled, but could be an extremely value fuel resource should we decide to go for the FBR.5151
52 Section 8: Simplified Fuel Cycle for a PWR: Enrichment (3) GAS CENTRIFUGE ENRICHEMENTsimilar to the Gas diffusion in that it requires many stages."HEX" is spun in a centrifuge, and the slightly enriched URANIUM is sucked off near the axis and passed to the next stage.ENERGY requirements for this process are only ~10% of the GAS DIFFUSION method.All UK fuel is now enriched by this process at Capenhurst.5252
53 Simplified Fuel Cycle for a PWR: Fuel Fabrication (1) MAGNOX reactors: URANIUM metal is machined into bars using normal techniques.CARE MUST BE TAKEN not to allow water into process as this acts as a moderator and might cause the fuel element to 'go critical'.CARE MUST ALSO BE TAKEN over its CHEMICAL TOXICITY although this is not a much a problem as PLUTONIUMURANIUM METAL bars are about 1m in length and about 30 mm in diameter.OXIDE Fuels for Other ReactorsBecause of low thermal conductivity of oxides of uranium, fuels of this form are made as small pellets which are loaded into stainless steel cladding in the case of AGRs, and ZIRCALLOY in the case of most other reactors.TRANSPORT of FUEL ElementsLittle screening is needed as URANIUM is an alpha emitter and even a thin layer of paper is sufficient to stop such particles.No special precaution are needed as even enriched fuel is unsuitable for bomb making5353
54 Simplified Fuel Cycle for a PWR: Fuel Fabrication (2) PLUTONIUMFuel fabrication presents much greater problems.Workers require more shielding from radiation.Chemically toxic.Metallurgy is complex.Can reach criticality on its own WITHOUT a MODERATOR.Care must be taken in manufacture and ALL subsequent storage that the fuel elements are not of size and shape which could cause a criticality.NOTE:-Transport of PLUTONIUM fuel elementsa potential hazard, as a crude atomic bomb could be made without the need for a large amount of energy cf enriched URANIUM.DELIBERATE 'spiking' of PLUTONIUM with some fission products is considered to make the fuel elements very difficult to handle.5454
55 Simplified Fuel Cycle for a PWR: Fuel Fabrication (3) 1 tonne of enriched fuel for a PWR produces ~1PJ of energy.1 tonne of unenriched fuel for a CANDU reactor produces about ~0.2 PJ in a single pass.However, because of losses, about 20-25% MORE ENERGY PER TONNE of MINED URANIUM can be obtained with CANDU if the spent fuel is reprocessed.5555
56 Simplified Fuel Cycle for a PWR: BACKEND (1) BOTH ONCE-THROUGH and REPROCESSING CYCLESSPENT FUEL ELEMENTS from the REACTORFISSION PRODUCTS mostly with SHORT HALF LIVES. heat is evolvedspent fuel elements are normally stored under waterat least in the short term.100 days, the radioactivity reduced to about 25% of its original value, and after 5 years the level will be down to about 1%.Early reduction comes from the decay of radioisotopes such as IODINE and XENON half-lives (8 days and 1.8 hours respectively).CAESIUM decays to only 90% of its initial level even after 5 years.accounts for less than 0.2% of initial radioactive decay, but 15% of the activity after 5 years.5656
57 Simplified Fuel Cycle for a PWR: BACKEND (2) BOTH ONCE-THROUGH and REPROCESSING CYCLESSPENT FUEL ELEMENTS stored under 6m of wateralso acts as BIOLOGICAL SHIELD.Water may become radioactive from corrosion of fuel cladding causing leakage - so water is conditioned –kept at pH of (i.e. strongly alkaline in case of MAGNOX). Other reactor fuel elements do not corrode so readily.Any radionucleides escaping into the water are removed by ION EXCHANGE.Subsequent handling depends on whether ONCE-THROUGH or REPROCESSING CYCLE is chosen.Spent fuel can be stored in dry caverns,drying the elements after the initial water cooling is a problem.Adequate air cooling must be provided, and this may make air - radioactive if fuel element cladding is defective. WYLFA power station stores MAGNOX fuel elements in this form.5757
58 Simplified Fuel Cycle for a PWR: No Reprocessing ADVANTAGES:-NO REPROCESSING needed - therefore much lower discharges of low level/intermediate level liquid/gaseous waste.FUEL CLADDING NOT STRIPPED - therefore less solid intermediate waste created. (although sometimes it is)NO PLUTONIUM in transport so no danger of diversion.DISADVANTAGES:-CANNOT RECOVER UNUSED URANIUM - 235, PLUTONIUM OR URANIUM Thus fuel cannot be used again.VOLUME OF HIGH LEVEL WASTE MUCH GREATER ( times) than with reprocessing cycle.SUPERVISION OF HIGH LEVEL WASTE needed for much longer time as encapsulation is more difficult than for reprocessing cycle.5858
59 Simplified Fuel Cycle for a PWR: Reprocessing Cycle (1) ADVANTAGES:-MUCH LESS HIGH LEVEL WASTE - therefore less problems with storageUNUSED URANIUM - 235, PLUTONIUM AND URANIUM can be recovered and used again, or used in a FBR thereby increasing resource base 50 fold.VITRIFICATION is easier than with spent fuel elements. Plant at Sellafield now operational although technical problems are preventing vitrification at full capcity.DISADVANTAGES:-Greater volumes of both Low Level and Intermediate Level Waste are created.Historically, routine emissions from reprocessing plants have been greater than storage of ONCE-THROUGH cycle waste.5959
60 Simplified Fuel Cycle for a PWR: Reprocessing Cycle (2) Dealing with liquid effluentsAt SELLAFIELD the ION EXCHANGE plantSIXEP (Site Ion EXchange Plant)commissioned in early 1986,substantially reduced the radioactive emissions in the effluent discharged to Irish Sea since that time by a factor of 500+ timesFurther improvements with more advance waste treatment have now been installed.PLUTONIUM is stockpiled or in transport if used in FBRs. (although this can be 'spiked').6060
61 Simplified Fuel Cycle for a PWR: Reprocessing Cycle (3) cladding to intermediate level waste storageFuel stored in cooling pondsto allow further decayFuel decannedDissolve Fuel inNitric Acidadd tributyl phosphate (TBP) in odourless ketone (OK)The ChemistryHigh Level Wastefurther treatment with TBP/OKmedium level wastereduced with ferrous sulphamateURANIUM – converted into UO3 and recycled** PLUTONIUM – converted for storage or fuel fabrication for MOX or FBR**Pipes in this area are of small diameter to prevent CRITICALITIES.6161
62 Radioactive Waste Disposal – An Introduction LOW LEVEL WASTE.CONTAINS MATERIALS CONTAMINATED WITH RADIOISOTOPESeither very long half lives indeed,or VERY SMALL quantities of short lived radioisotopes.FEW SHIELDING PRECAUTIONS ARE NECESSARY DURING TRANSPORTATION.PHYSICAL BULK MAY BE LARGE as its volume includes items which may have been contaminated during routine operations.Laboratory Coats, Paper Towels etc.Such waste may be generated in HOSPITALS, LABORATORIES, NUCLEAR POWER STATIONS, and all parts of the FUEL CYCLE.6262
63 Radioactive Waste Disposal – An Introduction OPTIONS FOR DISPOSAL OF LOW LEVEL WASTE.BURYING LOW LEVEL WASTE SURROUNDED BY A THICK CLAY BLANKET IS A SENSIBLE OPTION.If clay is of the SMECTITE type acts as a very effective ion exchange barrier which is plastic and deforms to any ground movement sealing any cracks.IN BRITAIN IT IS PROPOSED TO BURY WASTE IN STEEL CONTAINERS AND PLACED IN CONCRETE STRUCTURES IN A DEEP TRENCH UP TO 10m DEEP WHICH WILL BE SURROUNDED BY THE CLAY.IN FRANCE, THE CONTAINERS ARE PILED ABOVE GROUND AND THEN COVERED BY A THICK LAYER OF CLAY TO FORM A TUMULUS.Energy Field Courses in 1999 and 2001 visited the site at ANDRA near Cherbourg. (Agence National de Déchets Radioactive)6363
64 Radioactive Waste Disposal – An Introduction INTERMEDIATE LEVEL WASTE.contains HIGHER quantities of SHORT LIVED RADIOACTIVE WASTE,or MODERATE QUANTITIES OF RADIONUCLEIDES OF MODERATE HALF LIFE- e.g. 5 YEARS YEARS HALF LIFE.IN FRANCE SUCH WASTE IS CAST INTO CONCRETE MONOLITHIC BLOCKS AND BURIED AT SHALLOW DEPTH.IN BRITAIN, it was originally proposed to bury similar blocks at the SAME SITES to those used for LOW LEVEL WASTE.UNSATISFACTORY AS CONFUSION BETWEEN THE TWO TYPES OF WASTE WILL OCCUR.SEPARATE FACILITIES ARE NOW PROPOSED.6464
65 Radioactive Waste Disposal – An Introduction HIGH LEVEL WASTE.At Sellafield, high level waste is now being encapsulated and stored on site in specially constructed vaults.A building about the size of the UEA swimming pool house in area and about twice as high houses all the high level radioative waste from the UKs Civil Nuclear Program with space for decommissioning of all final fuel from MAGNOX.MOST RADIONUCLEIDES IN THIS CATEGORY HAVE HALF LIVES OF UP TO 30 YEARS, and thus ACTIVITY in about 700 years will have decayed to around natural background radiation level.PROPOSALS FOR DISPOSAL INCLUDEburial in deep mines in SALT;burial 1000m BELOW SEA BED and BACKFILLED with SMECTITE;burial under ANTARCTIC ICE SHEET,shot INTO SPACE to the sun!6565
66 Radioactive Waste Disposal – An Introduction – A Dilemma UK processes waste from Overseas Countries.Should we send back exact quantities of each of:High Level WasteIntermediate Level WasteLow Level WasteOr should we:Send back same amount of radioactivityi.e. a larger amount of a small volume of High Level Wasteand no Intermediate and Low Level Waste?6666
67 Fission Reactors: Fuel Elements (MAGNOX) Magnox fuel rod:Natural Uranium metal bar approx 35mm diameter and 1m long in a fuel cladding made of MAGNOX.6767
68 Fission Reactors: Fuel Elements (AGR) AGR fuel assembly:UO2 pellets loaded into fuel pins of stainless steel each ~ 1 m long in bundles of 36.Whole assembly in a graphite cylinderBurnable poison6868
69 Fission Reactors: Fuel Elements (PWR) PWR fuel assembly:UO2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~2006969
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