NBSLM 03 E (2010) Low Carbon Technologies and Solutions: Sections 6 - 8 N. K. Tovey (杜伟贤 ) M. A, Ph. D, CEng, MICE, CEnv Nuclear Power Norwich Business School 1
Section 6: NBSLM 03 E (2010) The Basics Nuclear Power: Low Carbon Technologies and Solutions N. K. Tovey (杜伟贤 ) M. A, Ph. D, CEng, MICE, CEnv 6. Nature of Radioactivity • Structure of the Atom • Radioactive Emissions • Half Life of Elements • Fission • Fusion • Chain Reactions • Fertile Materials 7. Fission Reactors 8. The Nuclear Fuel Cycle 9. Fusion Reactors (in notes) 2 Norwich Business School 2
Section 6: Nature of Radioactivity (1) Structure of Atoms. • Matter is composed of atoms which consist primarily of a nucleus of: – positively charged PROTONS – and (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 mass + + + 3 p 4 n • ELECTRONS are about 0. 0005 times the mass of the PROTON. • A NUCLEON refers to either a PROTON or a NEUTRON Norwich Business School Lithium Atom 3 Protons 4 Neutrons 3
Section 6: Nature of Radioactivity (2) Structure of Atoms. • Elements are characterized by the number of PROTONS present – HYDROGEN nucleus has 1 PROTON – HELIUM has 2 PROTONS – OXYGEN has 8 PROTONS – URANIUM 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 NEUTRONS – DEUTERIUM (heavy hydrogen) with 1 NEUTRON – TRITIUM with 2 NEUTRONS. Symbol D Symbol T • only TRITIUM is radioactive. • Elements up to Z = 82 (Lead) have at least one isotope which is stable Norwich Business School 4
Section 6: Nature of Radioactivity (3) Structure of Atoms. • URANIUM has two main ISOTOPES • 235 U • 238 U which is present in concentrations of 0. 7% in naturally occurring URANIUM 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%. Norwich Business School 5
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. Norwich Business School 6
Section 6: Nature of Radioactivity (5) Radioactive emissions. • FOUR types of radiation: • 1) ALPHA particles ( ) - large particles consisting of 2 PROTONS and 2 NEUTRONS the nucleus of a HELIUM atom. • 2) BETA particles (β) which are ELECTRONS • 3) 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. Norwich Business School 7
Section 6: Nature of Radioactivity (6) β - particles are stopped by a thin sheet of paper β – particles are stopped by ~ 3 mm aluminium - rays CANNOT be stopped – they can be attenuated to safe limits using thick Lead and/or concrete Norwich Business School 8
Section 6: Nature of Radioactivity (7) Radioactive emissions. • UNSTABLE nuclei emit Alpha or Beta particles • If an ALPHA particle is emitted, the new element will have an ATOMIC NUMBER two less than the original. e • If 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. Norwich Business School 9
Section 6: Nature of Radioactivity (8) Radioactive emissions. consisting of 92 PROTONS and 143 NEUTRONS is one of SIX isotopes of URANIUM • decays as follows: • 235 U alpha URANIUM 231 Th THORIUM beta 231 Pa PROTACTINIUM alpha 227 Ac ACTINIUM • Thereafter the ACTINIUM - 227 decays by further alpha and beta particle emissions to LEAD - 207 (207 Pb) which is stable. • Two other naturally occurring radioactive decay series exist. One beginning with 238 U, and the other with 232 Th. • Both also decay to stable (but different) isotopes of LEAD. Norwich Business School 10
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: • • • 238 U 4. 5 billion years 235 U 0. 7 billion years 232 Th 14 billion years • All 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 lives – the remaining number of atoms is less than one millionth of the original Norwich Business School 11
Section 6: Nature of Radioactivity (10) HALF LIFE. From a radiological hazard point of view • short half lives - up to say 6 months have intense radiation, but • decay 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 consequence • For intermediate half lives - these are the problem - e. g. Strontium -90 – has 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 Norwich Business School 12
Section 6: Nature of Radioactivity (11): Fission Some very heavy UNSTABLE elements exhibit FISSION e. g. 235 U n 235 U 93 Rb n n 13 140 Cs This reaction is one of several which might take place. In some cases, 3 daughter products are produced. Norwich Business School 13 13
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 reactions • Fragments 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 - 235 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. Norwich Business School 14
Section 6: Nature of Radioactivity (13): Fusion of light elements e. g. DEUTERIUM and TRITIUM produces even greater quantities of energy per nucleon are released. 3 H Deuterium – Tritium fusion Tritium 4 He (3. 5 Me. V) 2 H n Deuterium (14. 1 Me. V) In each reaction 17. 6 Me. V is liberated or 2. 8 pico. Joules (2. 8 * 10 -15 J) 15 Norwich Business 15/03/2018 School 15
Section 6: Nature of Radioactivity (14): Binding Energy Atomic Mass Number Binding Energy per nucleon [Me. V] 0 50 100 150 200 250 -2 -4 Fusion Energy release per nucleon 1 Me. V per nucleon is equivalent to 96. 5 TJ per kg -6 -8 -10 Iron 56 Range of Fission Products Uranium 235 Fission Energy release per nucleon Redrawn from 6 th report on Environmental Pollution – Cmnd. 6618 - 1976 1) The energy released per nucleon in fusion reaction is much greater than the corresponding fission reaction. 2) In fission there is no single fission product but a broad range as indicated. Norwich Business School 16
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 onwards • One or two demonstration commercial reactors in 2030 s perhaps • No radioactive waste from fuel • Limited radioactivity in power plant itself • 8 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 Norwich Business School 17
Section 6: Nature of Radioactivity (16): Chain Reactions n Fast Neutrons are unsuitable for sustaining further reactions fast neutron 235 U n n Slow neutron 235 U n fast neutron n Norwich Business School Slow neutron 18
Section 6: Nature of Radioactivity (17): Chain Reactions CHAIN REACTIONS • FISSION of URANIUM - 235 yields 2 - 3 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). Norwich Business School 19
Section 6: Nature of Radioactivity (18): Chain Reactions • IT IS VERY DIFFICULT TO SUSTAIN A CHAIN REACTION, • Most Neutrons are moving too fast • TO CREATE A BOMB, THE URANIUM - 235 MUST BE HIGHLY ENRICHED > 93%, • Normal Uranium is only 0. 7% U 235 • Material 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. Norwich Business School 20
Section 6: Nature of Radioactivity (19): Fertile Materials • FERTILE MATERIALS • Some elements like URANIUM - 238 are not FISSILE, but can transmute: n fast neutron e e 238 U Uranium - 238 +n 239 U 238 Pu 239 U 239 Np Uranium - 239 beta 239 Np beta 239 Pu Neptunium - 239 Plutonium - 239 PLUTONIUM - 239 is FISSILE and may be used in place of URANIUM - 235. Materials which can be converted into FISSILE materials are FERTILE. Norwich Business School 21
Section 6: Nature of Radioactivity (20): Fertile Materials • URANIUM - 238 is FERTILE as is THORIUM - 232 which can be transmuted into URANIUM - 233. • Naturally occurring URANIUM consists of 99. 3% 238 U which is FERTILE and NOT FISSILE, and 0. 7% of 235 U which is FISSILE. Normal reactors primarily use the FISSILE properties of 235 U. • 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 235 U is only decades • But using a Breeder Reactor Plutonium can be produced from non-fissile 238 U producing 239 Pu and extending the resource base by a factor of 50+ Norwich Business School 22
n Section 6: Nature of Radioactivity (21): Chain Reactions Sustaining a reaction in a Nuclear Power Station fast neutron Fast Neutrons are unsuitable for sustaining further reactions 235 U n fast neutron n Slow neutron n 235 U n fast neutron Insert a moderator to slow down neutrons n Norwich Business School Slow neutron 23
NBSLM 03 E (2010) Section 7: Fission Reactors Low Carbon Technologies and Solutions N. K. Tovey (杜伟贤 ) M. A, Ph. D, CEng, MICE, CEnv 6. Nuclear Power – The Basics 7. Nuclear Power: Fission reactors a) b) c) d) e) f) g) h) i) j) 8. 9. 24 General Introduction MAGNOX Reactors AGR Reactors CANDU Reactors PWRs BWRs RMBK/ LWGRs FBRs Generation 3 Reactors Generation 3+ Reactors Nuclear Fuel Cycle Fusion Reactors Norwich Business School 24
Fission Reactors (1) FISSION REACTORS CONSIST OF: i) iii) a FISSILE component in the fuel a MODERATOR a COOLANT to take the heat to its point of use. The fuel elements vary between different Reactors • Some reactors use unenriched URANIUM – i. e. the 235 U in fuel elements is at 0. 7% of fuel – e. g. MAGNOX and CANDU reactors, • ADVANCED GAS COOLED REACTOR (AGR) uses 2. 5 – 2. 8% enrichment • PRESSURISED WATER REACTOR (PWR) and BOILING WATER REACTOR (BWR) use around 3. 5 – 4% enrichment. • RMBK (Russian Rector of Chernobyl fame) uses ~2% enrichment • Some experimental reactors - e. g. High Temperature Reactors (HTR) use highly enriched URANIUM (>90%) i. e. weapons grade. Norwich Business School 25
Fission Reactors (2): Fuel Elements PWR fuel assembly: UO 2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~200 AGR fuel assembly: UO 2 pellets loaded into fuel pins of stainless steel each ~ 1 m long in bundles of 36. Whole assembly in a graphite cylinder Magnox fuel rod: Natural Uranium metal bar approx 35 mm diameter and 1 m long in a fuel cladding made of Mag. Nox. Burnable poison Norwich Business School 26
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 circuit 2) The steam cycle 3) The cooling water cycle. • ONLY the REACTOR COOLANT will become radioactive • The 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 10 o. C. Norwich Business School 27
Fission Reactors (4) REACTOR TYPES – summary 1 • MAGNOX - 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 31 st 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 decommissioned • AGR - 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. Norwich Business School 28
Fission Reactors (5) REACTOR TYPES - summary • SGHWR - 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. Norwich Business School 29
Fission Reactors (6) REACTOR TYPES - summary • CANDU - A reactor named initially after CANadian De. Uterium 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 1975. 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 238 U – extends resource base of URANIUM over 50 times. Mostly experimental at moment with FRANCE, W. GERMANY and UK, Russia and JAPAN having experimented with them. Norwich Business School 30
MAGNOX REACTORS (also known as Gas Cooled Reactors (GCR) • • FUEL TYPE - unenriched URANIUM METAL clad in Magnesium alloy MODERATOR - GRAPHITE COOLANT - CARBON DIOXIDE DIRECT RANKINE CYCLE - no superheat or reheat efficiency ~ 20% to 28%. DISADVANTAGES: • CANNOT LOAD FOLLOW – [Xe poisoning] • OPERATING TEMPERATURE LIMITED TO ABOUT 250 o. C - 360 o. C limiting CARNOT EFFICIENCY to ~40 50%, and practical efficiency to ~ 28 -30%. • LOW BURN-UP - (about 400 TJ per tonne) • EXTERNAL BOILERS ON EARLY DESIGNS. 31 ADVANTAGES: • LOW POWER DENSITY - 1 MW/m 3. Thus very slow rise in temperature in fault conditions. • UNENRICHED FUEL • GASEOUS COOLANT • ON LOAD REFUELLING • MINIMAL CONTAMINATION FROM BURST FUEL CANS • VERTICAL CONTROL RODS - fall by gravity in case of emergency. Norwich Business School 31
ADVANCED GAS COOLED REACTORS (AGR) • • FUEL TYPE - enriched URANIUM OXIDE - 2. 3% clad in stainless steel MODERATOR - GRAPHITE COOLANT - CARBON DIOXIDE SUPERHEATED RANKINE CYCLE (with reheat) - efficiency 39 - 41% DISADVANTAGES: • MODERATE LOAD FOLLOWING CHARACTERISTICS • SOME FUEL ENRICHMENT NEEDED. 2. 3% OTHER FACTORS: • MODERATE FUEL BURN-UP - ~ 1800 TJ/tonne (c. f. 400 TJ/tonne for MAGNOX, 2900 TJ/tonne for PWR). • SINGLE PRESSURE VESSEL with presstressed concrete walls 6 m thick. Prestressing tendons can be replaced if necessary. 32 ADVANTAGES: • MODEST POWER DENSITY - 5 MW/m 3. slow rise in temperature in fault conditions. • GASEOUS COOLANT (40 - 45 BAR cf 160 bar for PWR) • ON LOAD REFUELLING under part load • MINIMAL CONTAMINATION FROM BURST FUEL CANS • RELATIVELY HIGH THERMODYNAMIC EFFICIENCY 40% • VERTICAL CONTROL RODS - fall by gravity in case of emergency. Norwich Business School 32
CANDU REACTOTS (PHWR) ADVANTAGES: • MODEST POWER DENSITY - 11 MW/m 3. • MODERATOR - HEAVY WATER • HEAVY WATER COOLANT - low neutron COOLANT - HEAVY WATER absorber hence no need for enrichment. DISADVANTAGES: • ON LOAD REFUELLING - and very efficient indeed permits high load factors. • POOR LOAD FOLLOWING CHARACTERISTICS • MINIMAL CONTAMINATION from burst fuel can - defective units can be removed • CONTROL RODS ARE HORIZONTAL, and without shutting down reactor. cannot operate by gravity in fault conditions. • MODULAR: - can be made to almost any • MAXIMUM EFFICIENCY about 28% size OTHER FACTORS: • MODERATE FUEL BURN-UP - ~ MODEST FUEL BURN-UP - about 1000 TJ/tonne • FACILITIES PROVIDED TO DUMP HEAVY WATER MODERATOR from reactor in fault conditions • MULTIPLE PRESSURE TUBES instead of one pressure vessel. • FUEL TYPE - unenriched URANIUM OXIDE clad in Zircaloy 33 Norwich Business School 33
PRESSURISED WATER REACTORS – PWR (VVER) • • • FUEL TYPE - 3 – 4% enriched URANIUM OXIDE clad in Zircaloy MODERATOR - WATER COOLANT - WATER DISADVANTAGES: • • • ADVANTAGES: • GOOD LOAD FOLLOWING CHARACTERISTICS - claimed for SIZEWELL B. - most PWRs are NOT operated as such. • HIGH FUEL BURN-UP- about 2900 TJ/tonne – • VERTICAL CONTROL RODS - drop by gravity in fault conditions. 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. OTHER FACTORS: ON LOAD REFUELLING NOT POSSIBLE - reactor must be shut down. • LOSS OF COOLANT also means LOSS OF MODERATOR so reaction ceases - but SIGNIFICANT CONTAMINATION OF residual decay heat can be large. COOLANT CAN ARISE FROM BURST FUEL CANS - as defective units cannot be • HIGH POWER DENSITY - 100 MW/m 3, removed without shutting down reactor. and compact. Temperature can rise rapidly in fault conditions. NEEDS active FUEL ENRICHMENT NEEDED. - 3 -4%. ECCS. MAXIMUM EFFICIENCY ~ 31 - 32% • SINGLE STEEL PRESSURE VESSEL latest designs ~ 34% 200 mm thick. 34 Norwich Business School 34
BOILING WATER REACTORS – BWR ADVANTAGES: FUEL TYPE - 3% enriched URANIUM • HIGH FUEL BURN-UP- about OXIDE clad in Zircaloy 2600 TJ/tonne • MODERATOR - WATER • STEAM PASSED DIRECTLY TO • COOLANT - WATER TURBINE therefore no heat exchangers DISADVANTAGES: needed. BUT SEE DISADVANTAGES. . • ORDINARY WATER as COOLANT – OTHER FACTORS: but designed to boil: pressure ~ 75 bar. • CONTROL RODS MUST BE DRIVEN • LOSS OF COOLANT also means LOSS OF MODERATOR so reaction ceases - but UPWARDS - POWER NEEDED IN residual decay heat can be large. FAULT CONDITIONS. Water can be • HIGH POWER DENSITY - 100 MW/m 3, dumped in such circumstances. and compact. Temperature can rise • ON LOAD REFUELLING NOT rapidly in fault conditions. NEEDS active POSSIBLE - reactor must be shut down. ECCS. • SIGNIFICANT CONTAMINATION OF COOLANT CAN ARISE FROM BURST • SINGLE STEEL PRESSURE VESSEL 200 mm thick. FUEL CANS RADIOACTIVE STEAM WILL PASS DIRECTLY TO TURBINES. • FUEL ENRICHMENT NEEDED. - 3%. • MAXIMUM EFFICIENCY ~ 34 -35% • 35 Norwich Business School 35
RMBK (LWGR): - involved in Chernobyl Incident • FUEL TYPE - 2% enriched URANIUM ADVANTAGES: OXIDE clad in Zircaloy • ON LOAD REFUELLING • MODERATOR - GRAPHITE • VERTICAL CONTROL RODS which can drop by GRAVITY in fault • COOLANT - WATER conditions. DISADVANTAGES: NO THEY CANNOT!!!! • ORDINARY WATER as COOLANT flashes to steam in fault conditions OTHER FACTORS: hindering cooling. • MODERATE FUEL BURN-UP - ~ • POSITIVE VOID COEFFICIENT !!! MODEST FUEL BURN-UP - about positive feed back possible in some fault 1800 TJ/tonne conditions -other reactors have negative • LOAD FOLLOWING voids coefficient in all conditions. CHARACTERISTICS UNKNOWN • IF COOLANT IS LOST moderator will • POWER DENSITY probably keep reaction going. MODERATE? • FUEL ENRICHMENT NEEDED. - 2% • PRIMARY COOLANT passed directly • MULTIPLE PRESSURE TUBES to turbines. This coolant can be slightly radioactive. • MAXIMUM EFFICIENCY ~30% ? ? RMBK (LWGR): Norwich Business (involved in Chernobyl School incident) 36
FAST BREEDER REACTORS (FBR OR LMFBR) • FUEL TYPE - depleted Uranium or UO 2 surround PU in centre of core. All elements clad in stainless steel. • MODERATOR - NONE • COOLANT - LIQUID METAL DISADVANTAGES: • DEPLETED URANIUM FUEL ELEMENTS REPROCESSED to recover PLUTONIUM and sustain the breeding for future use. • CURRENT DESIGNS have SECONDARY SODIUM CIRCUIT • WATER/SODIUM HEAT EXCHANGER. If water and sodium mix a significant CHEMICAL explosion may occur which might cause damage to reactor itself. ADVANTAGES: • • LIQUID METAL COOLANT - at atmospheric pressure. Will cool by natural convection in event of pump failure. BREEDS FISSILE FUEL from non-fissile 238 U – increases resource base 50+ times. HIGH EFFICIENCY (~ 40%) VERTICAL CONTROL RODS drop by GRAVITY in fault conditions. OTHER FACTORS: • VERY HIGH POWER DENSITY - 600 MW/m 3 but rise in temperature in fault conditions limited by natural circulation of sodium. Norwich Business School 37
GENERATION 3 REACTORS: EPR 1300: 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 less • Construction is under way at Olkiluoto, Finland. Second reactor under construction in Flammanville, France Possible contender for new UK generation Norwich Business School 38
GENERATION 3: AP 1000: PWR • A development from SIZEWELL • Power Rating comparable with SIZEWELL Possible Contender for new UK reactors • Will two turbines be used ? ? • Passive Cooling – water tank on top – water falls by gravity • Two loops (cf 4 for EPR) • Significant reduction in components e. g. pumps etc. Norwich Business School 39
GENERATION 3: ACR 1000: Advanced Candu Reactor • A development from CANDU with added safety features less Deuterium needed • Passive emergency cooling as with AP 1000 See Video Clip of on-line refuelling Norwich Business School 40
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 design • Vertical control rods which must be driven upwards • Steam in turbines can become radioactive Norwich Business School 41
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: Helium • Connected directly to closed circuit gas turbine • Efficiency ~ 39 – 40%, possibility of CCGT? ? • Graphite/silicon carbide effective cladding • very durable at high temperatures Norwich Business School 42
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 Reactor Combustion Chamber Fuel In Compressor Turbine Generator Open Brayston Closed Brayston Cycle Air In Heat Exchanger Norwich Business School Exhaust 43
GENERATION 3+ REACTORS: the PBMR • • Efficiency of around 38 – 40%, but possibility of CCGT? ? ? Helium passes directly from reactor to turbine Pebbles are continuously fed into reactor and collected. Tested for burn up and recycled as appropriate ~ typically 6 times Norwich Business School 44
NBSLM 03 E (2010) Section 8: Nuclear Fuel Cycle Low Carbon Technologies and Solutions N. K. Tovey (杜伟贤 ) M. A, Ph. D, CEng, MICE, CEnv 6. Nuclear Power – The Basics 7. Nuclear Power: Fission reactors 8. Nuclear Fuel Cycle 9. Fusion Reactors 45 Norwich Business School 45
Section 8: Nuclear Fuel Cycle • • • TWO OPTIONS AVAILABLE: 1. ONCE-THROUGH CYCLE, 2. REPROCESSING CYCLE CHOICE DEPENDS primarily on: 1. REACTOR TYPE IN USE (more or less essential for MAGNOX), 2. AVAILABILTY OF URANIUM TO COUNTRY IN QUESTION, 3. DECISIONS ON THE POSSIBLE USE OF FBRs. 4. DECISIONS ON HOW RADIOACTIVE WASTE IS TO BE HANDLED. • Reprocessing leads to much less HIGH LEVEL radioactive waste, but more low level radioactive waste ECONOMIC CONSIDERATIONS done 10 years ago show little difference between two types of cycle except that for PWRs, ONCE-THROUGH CYCLE appeared MARGINALLY more attractive. Norwich Business School 46
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: 1. Transportation of Fabricated Fuel elements has negligible cost as little or no screening is necessary. 2. Special Provisions are needed for transport of spent fuel for both cycles. 3. For both ONCE-THROUGH and REPROCESSING CYCLES, the FRONT-END is identical. The differences are only evident at the BACK- END. Norwich Business School 47
Section 8: Simplified Fuel Cycle for a PWR (1) Cooling Ponds 1 T 0. 9 m 3 Fuel Rods Reactor Enrichment 1 PJ ~ 70000 homes 60 x 2 MW wind turbines 9 kg Plutonium REPROCESSING UF 6 Liquid 5 m 3 0. 96 t Uranium Concentrate 0. 5 m 3 Mining Spoil 1500 m 3 Storage HL Waste U 3 O 8 Ore Once Through 0. 7 m 3 LL waste 0. 8 m 3 IL waste Norwich Business School 0. 15 m 3 solid 0. 4 m 3 IL waste 0. 9 m 3 HL waste 48
Section 8: Simplified Fuel Cycle for a PWR (2) • MINING - ore > 0. 05% by weight of U 3 O 8 to be economic. – Typically at 0. 5%, 500 tonnes (250 m 3) must be excavated to produce 1 tonne of U 3 O 8 ("yellow-cake") which occupies about 0. 1 m 3. • URANIUM leached out chemically – resulting 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 (UF 4) • UF 4 converted into URANIUM HEXAFLOURIDE (UF 6) or "HEX" if enrichment is needed. Norwich Business School 49
Section 8: Simplified Fuel Cycle for a PWR: Enrichment (1) ENRICHMENT. proportion of URANIUM - 235 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. • 235 U diffuses faster than 238 U through this membrane. • Outlet gas from lower pressure is slightly enriched in 235 U (by a factor of 1. 0043) 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. Norwich Business School 50
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 0. 2 0. 3% 235 U, – 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. Norwich Business School 51
Section 8: Simplified Fuel Cycle for a PWR: Enrichment (3) ENRICHMENT. GAS CENTRIFUGE ENRICHEMENT • similar 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. Norwich Business School 52
Simplified Fuel Cycle for a PWR: Fuel Fabrication (1) • FUEL FABRICATION • 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 PLUTONIUM – URANIUM METAL bars are about 1 m in length and about 30 mm in diameter. • OXIDE Fuels for Other Reactors – Because 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 Elements – Little 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 making Norwich Business School 53
Simplified Fuel Cycle for a PWR: Fuel Fabrication (2) PLUTONIUM Fuel 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 elements • a 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. Norwich Business School 54
Simplified Fuel Cycle for a PWR: Fuel Fabrication (3) • 1 tonne of enriched fuel for a PWR produces ~1 PJ 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. Norwich Business School 55
Simplified Fuel Cycle for a PWR: BACKEND (1) BOTH ONCE-THROUGH and REPROCESSING CYCLES • SPENT FUEL ELEMENTS from the REACTOR • FISSION PRODUCTS mostly with SHORT HALF LIVES. heat is evolved – spent fuel elements are normally stored under water – at 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 - 131 and XENON - 133 -half-lives (8 days and 1. 8 hours respectively). • CAESIUM - 137 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. Norwich Business School 56
Simplified Fuel Cycle for a PWR: BACKEND (2) BOTH ONCE-THROUGH and REPROCESSING CYCLES • SPENT FUEL ELEMENTS stored under 6 m of water – also acts as BIOLOGICAL SHIELD. – Water may become radioactive from corrosion of fuel cladding causing leakage - so water is conditioned – – kept at p. H of 11 - 12 (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. Norwich Business School 57
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 - 238. Thus fuel cannot be used again. • VOLUME OF HIGH LEVEL WASTE MUCH GREATER (5 - 10 times) than with reprocessing cycle. • SUPERVISION OF HIGH LEVEL WASTE needed for much longer time as encapsulation is more difficult than for reprocessing cycle. Norwich Business School 58
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (1) • ADVANTAGES: • MUCH LESS HIGH LEVEL WASTE - therefore less problems with storage • UNUSED URANIUM - 235, PLUTONIUM AND URANIUM - 238 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. Norwich Business School 59
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (2) Dealing with liquid effluents • At SELLAFIELD the ION EXCHANGE plant • SIXEP (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+ times • Further 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'). Norwich Business School 60
Simplified Fuel Cycle for a PWR: Reprocessing Cycle (3) Fuel stored in cooling ponds to allow further decay Fuel decanned Dissolve Fuel in Nitric Acid The Chemistry add tributyl phosphate (TBP) in odourless ketone (OK) further treatment with TBP/OK cladding to intermediate level waste storage High Level Waste medium level waste reduced with ferrous sulphamate ** PLUTONIUM – converted for storage or fuel fabrication for MOX or FBR **Pipes in this area are of small diameter to prevent CRITICALITIES. URANIUM – converted into UO 3 and recycled Norwich Business School 61
Radioactive Waste Disposal – An Introduction LOW LEVEL WASTE. • CONTAINS MATERIALS CONTAMINATED WITH RADIOISOTOPES – either 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. Norwich Business School 62
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 10 m 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) Norwich Business School 63
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 - 10000 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. Norwich Business School 64
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 INCLUDE – burial in deep mines in SALT; – burial 1000 m BELOW SEA BED and BACKFILLED with SMECTITE; – burial under ANTARCTIC ICE SHEET, – shot INTO SPACE to the sun! Norwich Business School 65
Radioactive Waste Disposal – An Introduction – A Dilemma UK processes waste from Overseas Countries. • Should we send back exact quantities of each of: – High Level Waste – Intermediate Level Waste – Low Level Waste • Or should we: – Send back same amount of radioactivity • i. e. a larger amount of a small volume of High Level Waste • and no Intermediate and Low Level Waste? Norwich Business School 66
Fission Reactors: Fuel Elements (MAGNOX) Magnox fuel rod: Natural Uranium metal bar approx 35 mm diameter and 1 m long in a fuel cladding made of MAGNOX. Norwich Business School 67
Fission Reactors: Fuel Elements (AGR) AGR fuel assembly: UO 2 pellets loaded into fuel pins of stainless steel each ~ 1 m long in bundles of 36. Whole assembly in a graphite cylinder Burnable poison Norwich Business School 68
Fission Reactors: Fuel Elements (PWR) PWR fuel assembly: UO 2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~200 Norwich Business School 69
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