A Designers View of Nuclear Energy - Department of Materials ...

A Designers View ofNuclear EnergyTony Roulstone March 2011Engineering - Energy, Fluid dynamics and Turbo-machinery

Summary• After 25 years of retrenchment, nuclear power is firmly on the agenda, both in UK andaround the world;• UK plans to build up to 12 large PWRs - will replace (at least twice over) the current~10GWe nuclear capacity;• Global potential for >1000 new large reactors in next 20 years – replacing the current400 reactors (~350 GWe) & growing the share of global electricity from 15% to ~30%• Topics covered:1. Thermal nuclear reactors design essentials;2. Some design safety & considerations focusing on Fukushima incident;2

The First Nuclear Reactor• Enrico Fermi and a team from Metallurgy Departmentof University of Chicago built and controlled the firstsustained nuclear reaction;• In a racquets court in Staggs Field athletics stadium ofthe University of Chicago on 2 Dec 1942;• Reactor constructed from Graphite blocks and Uraniummetal constructed in ‘pile’ in 30’ * 60’ room withCadmium coated control rods;• The team proved that sustained fission or amultiplication factor k >1 could be achieved , andthe measurements were made for the first practicalsustained nuclear reactor;• Three types of control rods:• Electric motor operated controlling rods;• Emergency ‘zip’ rod driven in by gravity;• Liquid Cadmium salts to be released into a controltube.• Sustained fission demonstrated by neutron countgrowing exponentially i.e. k eff >1CP1 – Chicago Pile 13

Simplified Reactor Physics - FissionThermal reactor fission:Uranium 235 is the onlynaturally occurring fissileatom - typical fission reactionn + U 235 Xe 140 + Sr 94 + ~2-3 n’s + 193MevthermalfastHigh thermal nabsorptionlow fast nabsorptionFission Product distributionUranium 235 fission cross sectionIntermediate energyresonances linked toquantum states - loss of n4

Neutron Fission Cycle100 fissionsn’s per fission &fast fission - ε * η259 neutrons59 n’s lost 100 absorbed no fission 100 n leading to fission45 absorbedby structure14 absorbedby fissionproducts100 absorbed by fuelwithout fission27 by Pu, 15 by U 23558 by U 23832 fissions in Pu,63 fissions in U 2355 fissions in U 238ActivationProductsProduce Pu, higher actinides& their decay products100 fissionneutronsModeratorabsorption - fFuel resonancecapture - p5

Light Water Reactors are DominantPressurised Water & Boiling Water ReactorsPWR• Derived from submarine propulsionreactors & widely installed around theworld ~ half of world capacity;• Low thermal efficiency ~33%;• Major problem was Three Mile Island in1981 where minor fault led to confusingsignals & operators damaged reactor;• Initial materials problems led to lowreliability - since rectified• Now preferred in EU, Russia & China,sharing market in US with BWRBWR• Simpler plant with integrated core coolingand power cycle, high radiation dose fromoperation;• Core and steam separation integrated inone vessel;• Activated Nitrogen 16 can limits access toturbine during operation;• Some doubts about safety containment;• More complex coolant chemistry;• Popular in US, Sweden and Japan.6

Pressurised Water ReactorPWR overview:• Operating conditions :• Pressure: 16 MPa• Average temperature: 280-290 o C• Reactor core consist of bundles of enriched~3% Uranium Oxide fuel in open bundles withZircalloy fuel clad tubes;• Vertical control rods operated from the top of thereactors;• Multiple loops (3 or 4) carrying sub-cooled water,flowing upwards through core;• Refuelling at 3 years intervals from top of core bymeans of removable vessel head;• Complex coolant injection and decay heatremoval systems – issue addressed by latestWestinghouse design AP1000;• Able to separate chemistry strategies of primaryand secondary/condenser water;• Most popular reactor design ~50% of installedcapacity: US, France, Germany, Spain & nowRussia & China – high availability, long fuelcycles & low operator dose.7

Boiling Water ReactorBWR overview• Operating conditions:• Pressure (saturated) ~7.3 MPa• Average temperature 310 o C• Reactor core consist of bundles of enriched ~3% UraniumOxide fuel in shrouded bundles with Zircalloy fuel clad tubes;• Vertical control rods operated from bottom of core – do not dropinto core to shut-down reactor;• Reactor cooling flow by augmented natural convection;• Steam separators above the core with direct feed to wet steamturbines – some carry over of contamination;• Complex power control and multi-level emergency core coolingsystems;• Refuelling at 2 year intervals from top of core by means ofremovable vessel head;• Coolant chemistry has to cover contamination from condenserwater as well as core requirements – not preferred for coastsites because of potential chloride contamination?• Defence against major accident must take account of reactor &turbine building – containment, aircraft crash etc.8

Some Design & Safety Considerations9

Nuclear Safety PhilosophyIn the real world things can go wrong:• Three Mile Island - minor leak withconfusing signals led to core beinguncovered and major damage but littleexternal radiation;• Chernobyl - prompt criticality followed bya core fire in 1986 – major externalradiation in Ukraine and across Europe.Safety Strategies:1. Preclude the event/accident by design;2. Provide defence in depth to ensure theeffects are protected, contained, orminimised;3. Independent evaluation of all design andoperating practices.Developed world safety approach:• Design base events which are always protected with asignificant margin of safety;• Large scale events rendered very unlikely by:o providing defence in depth/multiple barriers torelease;o including features and best practise in the design toextent that is As Low As Reasonably Practicable.Frequencypa10 -4Area ofacceptance ofdesign10 -7Complete Protection withhigh degree of certaintyLarger releases - bydesign made mostunlikelyALARP – best practice iscontinually being challengedProbabilisticsafety targetRelease10

Nuclear Safety approach affects the designEvents & HazardsOpened ended process foridentification of potential hazardsPlant Faults External Haz Internal Haz Normal OperationAnalyse event & accident sequences with frequenciesConsider primary & secondary means of protection---> Design basis of structures, containment & safety systems, including human factorsProbabilistic Risk AnalysisLow frequency < 10 -4 pa/high consequenceDesign Basis AnalysisHigh frequency >10 -4 paDemonstrate with high degreeof certainty11

Japanese Earthquake• Precursor shock: Sanriku offshore earthquake 9 Marchat 11.45am:7.3 on the Richter scale and originated 160 kilometresoffshore, some 8 kilometres underground.All Japan’s nuclear reactors operated normal throughthis event – Japanese design standard is 7.5 onRichter scale.• Earthquake facts: Friday 11 March 2.46 pm:9.0 on Richter scale under sea at depth of 24kmlocated 130 km (80 miles) E of Sendai, Honshu,Japan• Many post earthquake shocks in range 6-7 on theRichter scale12

BWR Reactor Cooling & Containment• Reactor vessel enclosed in a so calleddry well containment with pressuresuppression torus;• Dry well surrounded by concretesecondary containment and shieldingbuilding;System operation following an earthquake:1. Shut down nuclear reaction & isolate reactor from turbine;2. If grid connection lost, transfer onto battery/diesel power;3. Cool reactor by discharge to dry well & make up water;4. Heat exchangers remove heat from the dry well, ensuringpressure remains below 4 bar (60 psi);5. Long term cool-down ensured by heat removal exceedsdecay heat level.13

Decay Heat & Energy7.00%6.00%5.00%4.00%P/Po3.00%Decay Heat & EnergySec Min Hour Day Week MonthP/PoG/Po700060005000400030002.00%20001.00%0.00%1000G/Po0 MJ/MWDecay Heat (Beta & Gamma): P(t) = 0.0622P 0 (t -0.2 – (t+t 0 ) -0.2 )Integrated stored energy G(t) = 0.0622P 0 * 1/0.8 (t 0.8 - (t+t 0 ) 0.8 + t0.80 )For: P 0 = 3000MWt 0 = 2 years = 2*365*24*3600 = 6.3 * 10 7 EOLP (1day) = 0.0046 *3000 ~ 14 MW - equivalent to Latent Ht ~7 kg/s of waterG (1day) = 534 *3000 ~ 1600 GJ - equiv to LH of 800m 3 of water RPV ~500m 3- energy required to melt large civil core ~200GJ !!14

Steam Venting & Hydrogen Explosion Fukushima 3Daiichi Reactors - BWRUnit 1 439 MWe 1971 Unit 2 760 MWe 1974 Unit 3 760 MWe 1976 Unit 4 760 MWe 197815

Fukushima 1 IncidentQuake OccursFriday11 Mar 2.46pmAll threeoperatingReactors units1/2/3 shutdownSuccessful shutdownLoss of gridpowerTsunamiarrives11 Mar 3.45pmDieselsproviding powerSwamped by>7m waveNo significantelectrical powerCore HeatDamageReactorpressureincrease dueto decay heatEnergyrelieveddischarge todry wellRestoring watersupplies12 Mar 0000amLack of reactormake-up leadsto falling waterlevelsTemp powersuppliesPart of coreuncoveredDecay heat raisefuel temp above700cSteam-Zircalloyreaction producehydrogen -some coredamageHydrogenExplosion12 Mar 4.17pmDry well pressureincrease above4bar (60psi)Concern aboutdry well rupture –decision to ventdry well. Coolingby fire sprayusing sea waterRelease ofsteam, hydrogen& some fissionproducts (I, Cs)radiation levelsEvac orderedRadiationRelease12 Mar 4.17pmRelease ofhydrogen led toexplosion –spark + airDamage tooutercontainmentbuilding but notdry well orreactor vesselRadiation levelraised to 1000μSv for shortperiod - fallingto

Fukushima 2• Similar sequence of events to unit 1 until Monday and was of less concern than units 1/3 –unit being cooled by external fire pump;• On Sunday unit 2 viewed as under control with 3m of water over the core;• At some time its fire pump either ran out of fuel, or failed in some other way;• Monday 14 Mar:ooRising pressure in unit 2 lead to concerns about extended uncovering of core and loss of fuelcooling -> about larger release of radiation – Xenon & Krypton gases, plus Iodine & Caesium;Extension of the site exclusion zone.• Tuesday 15 Mar 6.10amooExplosion thought to failure of dry well containment vessel due to high steam pressure;Larger rise in radiation levels up to 8000μSV/hr rapidly falling to about 2000 μSV/hr – hazardto health for workers on site;• Also, confusion about a fire at unit 4, which was already shut down – fire caused byearthquake – fire now out - may have also led to radiation hazard from spent fuel at site??17

Fukushima – Emerging Design Points• Incident not terminated and hence no yet fully understood – particularly the level of radioactivecontamination;• We will not know the level of core damage until TEPCO can take off the head and examine thecore - views vary between minor fuel/clad damage & major fuel melting;• Reactor design considerations:• Design levels for Tsunami and earthquake resistance level;• Separation/segregation of safeguard systems – not all fail at once;• Design reliance on grid or diesel power for safe shutdown;• Clean-up of fission products if venting is required & Hydrogen recombination to preventexplosion;• Volumes of water available for cooling v time required for action;• Provision of more passive means (less dependence on power) of rejecting heat from:ooReactor;Drywell/containment.18

PWR Passive Safety• The cost and complexity of designs such as Sizewell B and subsequently EPR led tostudy of so called ‘passive safety’ systems based on ideas from BWR safety;• Design Objectives:ooKeep the core covered with water;Emphasis use of on large volumes of reserve water;o Use natural circulation as the means of cooling – both fromCore and form Containment;o Means to get primary circuit into a low pressure condition following all but thesmall lest leak, quicklyooSimplify systems to reduce complexity, footprint, maintenance and cost;Westinghouse AP1000 is the first system to be built embodying these ideas.19

PassivePWRLOCA ProtectSchematicContainment sprayWater to assistcoolingSteamDe-Pressurise3 stage SpargeCoremake-upFeedContainment -Concrete OuterSteel liner innerWaterstorageStage 4De-PressTo ColdLegNaturalCirculationAir InPressAccumNaturalCirculationAir InLow PressureRecirculation pumpSump20

Fukushima et al - The Broader Issues• There will be broader effects from this extreme incident:1. Find out with certainty what happened and why – much of current information is partialand perhaps incorrect -> lessons to be learned for Japan and their BWRs;-> lesson to be learned for other operating reactors2. How can such extreme events be protected against and handled? Gen III+ reactorhave address to a greater extent external threats like these?3. Fuels the debate whether nuclear power is too hazardous to be ever employed?‘Engineers measures and assurances always will fall short of the scale of hazard’‘ There is no current or likely alternative for the foreseeable future to nuclear, withrenewables, in an energy policy that addresses climate change & security of supply’or21

MPhil in Nuclear Energy2222

MPhil in Nuclear Energy - Programme Overview• MPhil programme in Nuclear Energy combines:Nuclear Technology + Business + Industry Practice• Taught 1 year MPhil in Nuclear Energy - October – August from October 2011:o Five core nuclear engineering modules;o One core nuclear policy module;o Four elective modules from MPhil & Tripos provision technical & management;o Long project/dissertation involving industry club members, or national labs.23

Core ScopeReactor PhysicsCore TopicReactor Engineering & HeatTransferFuel Cycle, Waste &DecommissioningFuel & Reactor MaterialsSafety & Advanced SystemsNuclear Technology PolicyScopeCore physics & shielding – steady state power & shapes,depletion control elements & use of poisons, core kinetics& system control.Coolant types, thermal cycles, heat transfer, thermal limits– reactor systems, their optimisation and operatingcharacteristics including normal operation & how toaddress main types of fault condition.Whole fuel cycle: mining to waste & how waste ismanaged, decommissioning principles.Fuel and reactor materials – including selection, safety andlife issues – radiation behaviour & damage, structuralintegrity & fracture mechanics, EAC.Safety philosophies, impact on design, justificationapproaches, control & reliability, advanced systemsincluding Gen IV, Thorium & FusionEnergy studies & climate change, economics of energy,nuclear politics, proliferation & physical security.24

Engagement with IndustryWho?EDF Energy Atkins NDAAREVA AMEC SercoAubert & DuvalFraser-Nash ConsultancyWhat?• Lectures which include current experience and practice;• Projects addressing real problems faced by the industry;• Scope for funding and career development.25

More Information• Applications being made now through BoGS website;• Limited numbers for 2011;• Cambridge Nuclear Energy Centre: www.cnec.group.cam.ac.uk• MPhil in Nuclear Energy website: www-diva.eng.cam.ac.uk/mphil_nuclear/• Course Director – Tony Roulstone: armr2@cam.ac.uk0775 362 763426

Endarmr2@cam.ac.uk2727