Norwich Business School Nuclear Power 1 NBSLM03E (2010) Low Carbon Technologies and Solutions:...

Norwich Business School


Nuclear Power

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NBSLM03E (2010) Low Carbon Technologies and Solutions: Sections 6 - 8

N.K. Tovey ( 杜伟贤 ) M.A, PhD, CEng, MICE, CEnv



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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)

Section 6: Nuclear Power:- The Basics

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NBSLM03E (2010)Low Carbon Technologies and Solutions


Norwich Business School 3

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

• ELECTRONS are about 0.0005 times the mass of the PROTON.

• A NUCLEON refers to either a PROTON or a NEUTRON

+++

3p

4n

Lithium Atom

3 Protons 4 Neutrons

Section 6: Nature of Radioactivity (1)


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.

• only TRITIUM is radioactive.

• Elements up to Z = 82 (Lead) have at least one isotope which is stable

Symbol DSymbol T



Structure of Atoms.

• URANIUM has two main ISOTOPES

• 235U which is present in concentrations of 0.7% in naturally occurring URANIUM

• 238U 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%.


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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.

+

+

++

+ +



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.



- particles are stopped by a thin sheet of paper

β – particles are stopped by ~ 3mm aluminium

- rays CANNOT be stopped – they can be attenuated to safe limits using thick Lead and/or concrete

β



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.

U23592

• 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.

Th23190

He42

Np23593

e



Radioactive emissions.• 235U consisting of 92 PROTONS and 143 NEUTRONS is one of SIX

isotopes of URANIUM • decays as follows:-

URANIUM

235Ualpha

THORIUM

231ThPROTACTINIUM

231PaACTINIUM

227Ac

• Thereafter the ACTINIUM - 227 decays by further alpha and beta particle emissions to LEAD - 207 (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.

beta alpha



HALF LIFE.

• Time taken for half the remaining atoms of an element to undergo their first decay e.g:-

• 238U 4.5 billion years • 235U 0.7 billion years • 232Th 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



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




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This reaction is one of several which might take place. In some cases, 3 daughter products are produced.

n

n

n

140Cs

93Rb235U

Some very heavy UNSTABLE elements exhibit FISSION e.g. 235U

Section 6: Nature of Radioactivity (11): Fission

13 13


• 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.

Section 6: Nature of Radioactivity (12): Fission



15 10/04/23

n

4He 2H

3H

Deuterium

Tritium

Deuterium – Tritium fusion

(3.5 MeV)

(14.1 MeV)

In each reaction 17.6 MeV is liberated or 2.8 picoJoules (2.8 * 10-15J)

Fusion of light elements e.g. DEUTERIUM and TRITIUM produces even greater quantities of energy per nucleon are released.

Section 6: Nature of Radioactivity (13): Fusion

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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.

0 50 100 150 200 250 Atomic Mass Number

-2

-4

-6

-8

-10

Bin

ding

Ene

rgy

per

nuc

leon

[M

eV]

Iron 56

Uranium 235Range of Fission

Products

Fusion Energy release per nucleon

Fission Energy release per nucleon

1 MeV per nucleon is equivalent to 96.5 TJ per kg

Redrawn from 6th report on Environmental Pollution – Cmnd. 6618 - 1976

Section 6: Nature of Radioactivity (14): Binding Energy


• 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 2030s 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

Section 6: Nature of Radioactivity (15): Fusion


n

n

n235U

n

n

n

235

U

Slow neutron

Slow neutronfast neutron

fast neutron

Fast Neutrons are unsuitable for sustaining further reactions

Section 6: Nature of Radioactivity (16): 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).



• 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% U235

• 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.



• FERTILE MATERIALS• Some elements like URANIUM - 238 are not FISSILE, but

can transmute:-

n

238U

fast neutron

239U

238UUranium - 238

239UUranium - 239

+n

ee

239NpNeptunium - 239

239PuPlutonium - 239

beta beta

239Np239Pu

PLUTONIUM - 239 is FISSILE and may be used in place of URANIUM - 235.

Materials which can be converted into FISSILE materials are FERTILE.

Section 6: Nature of Radioactivity (19): Fertile Materials


• URANIUM - 238 is FERTILE as is THORIUM - 232 which can be transmuted into URANIUM - 233.

• Naturally 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 decades

• But using a Breeder Reactor Plutonium can be produced from non-fissile 238U producing 239Pu and extending the resource base by a factor of 50+

Section 6: Nature of Radioactivity (20): Fertile Materials


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n

n

n235U

n

n

n

235

U

fast neutron

Slow neutron

fast neutron

fast neutron

n

Fast Neutrons are unsuitable for sustaining further reactions

Slow neutron

n

Insert a moderator to slow down neutrons

Sustaining a reaction in a Nuclear Power StationSection 6: Nature of Radioactivity (21): Chain Reactions



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6. Nuclear Power – The Basics7. Nuclear Power: Fission reactors

a) General Introductionb) MAGNOX Reactorsc) AGR Reactorsd) CANDU Reactorse) PWRsf) BWRsg) RMBK/ LWGRsh) FBRsi) Generation 3 Reactorsj) Generation 3+ Reactors

8. Nuclear Fuel Cycle9. Fusion Reactors

Section 7: Fission Reactors

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FISSION REACTORS CONSIST OF:- i) a FISSILE component in the fuel

ii) a MODERATOR

iii) 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 235U 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.

Fission Reactors (1)


PWR fuel assembly:

UO2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~200

Magnox fuel rod:

Natural Uranium metal bar approx 35mm diameter and 1m long in a fuel cladding made of MagNox.

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 cylinder

Burnable poison

Fission Reactors (2): Fuel Elements


• 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 10oC.



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 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 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.



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.



REACTOR TYPES - summary

• CANDU - 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 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 238U

– 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.



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• 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%.

ADVANTAGES:-• LOW POWER DENSITY - 1 MW/m3.

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.

DISADVANTAGES:-• CANNOT LOAD FOLLOW – [Xe

poisoning]

• OPERATING TEMPERATURE LIMITED TO ABOUT 250oC - 360oC 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.

MAGNOX REACTORS (also known as Gas Cooled Reactors (GCR)

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• FUEL TYPE - enriched URANIUM OXIDE - 2.3% clad in stainless steel

• MODERATOR - GRAPHITE • COOLANT - CARBON DIOXIDE• SUPERHEATED RANKINE CYCLE (with

reheat) - efficiency 39 - 41%

ADVANTAGES:-• MODEST POWER DENSITY - 5 MW/m3.

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.

DISADVANTAGES:-• MODERATE LOAD FOLLOWING

CHARACTERISTICS

• SOME FUEL ENRICHMENT NEEDED. - 2.3%

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.

ADVANCED GAS COOLED REACTORS (AGR)

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• FUEL TYPE - unenriched URANIUM OXIDE clad in Zircaloy

• MODERATOR - HEAVY WATER COOLANT - HEAVY WATER

ADVANTAGES:-• MODEST POWER DENSITY - 11

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 size

DISADVANTAGES:-• POOR LOAD FOLLOWING

CHARACTERISTICS• CONTROL 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/tonne

• FACILITIES PROVIDED TO DUMP HEAVY WATER MODERATOR from reactor in fault conditions

• MULTIPLE PRESSURE TUBES instead of one pressure vessel.

CANDU REACTOTS (PHWR)

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• FUEL TYPE - 3 – 4% enriched URANIUM OXIDE clad in Zircaloy

• MODERATOR - WATER • COOLANT - WATER

ADVANTAGES:-• 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. - 3-4%.

• MAXIMUM EFFICIENCY ~ 31 - 32%

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 - 100 MW/m3, and compact. Temperature can rise rapidly in fault conditions. NEEDS active ECCS.

• SINGLE STEEL PRESSURE VESSEL 200 mm thick.

PRESSURISED WATER REACTORS – PWR (VVER)

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• FUEL TYPE - 3% enriched URANIUM OXIDE clad in Zircaloy

• MODERATOR - WATER

• COOLANT - WATER

ADVANTAGES:-• HIGH FUEL BURN-UP- about

2600TJ/tonne • STEAM 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 - 100 MW/m3, and compact. Temperature can rise rapidly in fault conditions. NEEDS active ECCS.

• SINGLE STEEL PRESSURE VESSEL 200 mm thick.

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BOILING WATER REACTORS – BWR


• FUEL TYPE - 2% enriched URANIUM OXIDE clad in Zircaloy

• MODERATOR - GRAPHITE• COOLANT - WATER

ADVANTAGES:-• ON LOAD REFUELLING• VERTICAL CONTROL RODS which

can drop by GRAVITY in fault conditions.

NO THEY CANNOT!!!!

RMBK (LWGR): (involved in Chernobyl incident)

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/tonne

• LOAD FOLLOWING CHARACTERISTICS UNKNOWN

• POWER DENSITY probably MODERATE?

• MULTIPLE PRESSURE TUBES

RMBK (LWGR): - involved in Chernobyl Incident


• FUEL TYPE - depleted Uranium or UO2 surround PU in centre of core. All elements clad in stainless steel.

• MODERATOR - NONE• COOLANT - LIQUID METAL

ADVANTAGES:-• 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 CIRCUIT

• WATER/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 - 600 MW/m3 but rise in temperature in fault conditions limited by natural circulation of sodium.

FAST BREEDER REACTORS (FBR OR LMFBR)


• 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.

Construction is under way at Olkiluoto, Finland.

Second reactor under construction in Flammanville, France

Possible contender for new UK generation

• Efficiency claimed at 37%• But no actual experience

and likely to be less

GENERATION 3 REACTORS: EPR1300: PWR


10/04/23

• A development from SIZEWELL

• Power Rating comparable with SIZEWELL

• 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.

Possible Contender for new UK reactors

GENERATION 3: AP1000: PWR


•A development from CANDU with added safety features less Deuterium needed

•Passive emergency cooling as with AP1000

See Video Clip of on-line refuelling

GENERATION 3: ACR1000: Advanced Candu Reactor


• 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

Generation 3 ESBWR: Economically Simple BWR


• 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

GENERATION 3+ REACTORS: the PBMR

• Efficiency ~ 39 – 40%, possibility of CCGT??

• Graphite/silicon carbide effective cladding

• very durable at high temperatures


• 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.

Fuel In

Air In

Combustion Chamber

Exhaust

CompressorTurbine

Generator

PBM Reactor

Heat Exchanger

Open Brayston CycleClosed Brayston Cycle



• 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



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6. Nuclear Power – The Basics7. Nuclear Power: Fission reactors

8. Nuclear Fuel Cycle

9. Fusion Reactors

Section 8: Nuclear Fuel Cycle

45




• 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.



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.



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Liquid 5m3

HL Waste

Concentrate 0.5m3

0.15m3 solid

Once Through

Storage

0.9m3 HL waste

0.4m3 IL waste

0.8m3 IL waste

0.7m3 LL

waste

REPROCESSING9 kg Plutonium

0.96 tUranium

U3O8

UF6

Enrichment

Cooling Ponds

Reactor

Ore Mining Spoil

1PJ

Fuel Rods

1 T0.9m3

1500 m3

~ 70000 homes60 x 2MW wind turbines

Section 8: Simplified Fuel Cycle for a PWR (1)


• 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 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 (UF4)

• UF4 converted into URANIUM HEXAFLOURIDE (UF6) or "HEX" if enrichment is needed.

Section 8: Simplified Fuel Cycle for a PWR (2)


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.

• 235U diffuses faster than 238U through this membrane.

• Outlet gas from lower pressure is slightly enriched in 235U (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.

Section 8: Simplified Fuel Cycle for a PWR: Enrichment (1)


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% 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.



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.



• 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 1m 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

Simplified Fuel Cycle for a PWR: Fuel Fabrication (1)


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.



• 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.



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.

Simplified Fuel Cycle for a PWR: BACKEND (1)


BOTH ONCE-THROUGH and REPROCESSING CYCLES

• SPENT FUEL ELEMENTS stored under 6m of water – also acts as BIOLOGICAL SHIELD. – Water may become radioactive from corrosion of fuel cladding causing

leakage - so water is conditioned – – kept at pH 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.

Simplified Fuel Cycle for a PWR: BACKEND (2)


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.

Simplified Fuel Cycle for a PWR: No Reprocessing


• 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.

Simplified Fuel Cycle for a PWR: Reprocessing Cycle (1)


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').



**Pipes in this area are of small diameter to prevent CRITICALITIES.

The Chemistry

Fuel stored in cooling ponds to allow further decay

Fuel decanned cladding to

intermediate level waste storage

Dissolve Fuel in Nitric Acid

add tributyl phosphate (TBP) in odourless ketone (OK)

further treatment with TBP/OK

reduced with ferrous sulphamate

High Level Waste

medium level waste

URANIUM – converted into UO3 and recycled

** PLUTONIUM – converted for storage or fuel fabrication

for MOX or FBR



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.

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)



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.



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 1000m BELOW SEA BED and BACKFILLED with SMECTITE;

– burial under ANTARCTIC ICE SHEET, – shot INTO SPACE to the sun!



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?

Radioactive Waste Disposal – An Introduction – A Dilemma


Magnox fuel rod:

Natural Uranium metal bar approx 35mm diameter and 1m long in a fuel cladding made of MAGNOX.

Fission Reactors: Fuel Elements (MAGNOX)


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 cylinder

Burnable poison

Fission Reactors: Fuel Elements (AGR)


PWR fuel assembly:

UO2 pellets loaded into fuel pins of zirconium each ~ 3 m long in bundles of ~200

Fission Reactors: Fuel Elements (PWR)

Norwich Business School Nuclear Power 1 NBSLM03E (2010) Low Carbon Technologies and Solutions:...

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