3Fukushima AccidentOn March 11th, 2011, a gigantic earthquake with a magnitude 9 on the Richter scale shook Japan. The earthquake triggered a tsunami, which was exceptionally high, reached the Fukushima coast about one hour after the earthquake.All reactors in operation at Fukushima shut down automatically. While the offsite external power source was lost due to the earthquake, emergency diesel generators (EDG) started up properlyEven though the earthquake was of a magnitude far greater than anticipated, there is today no evidence that it produced mechanical or structural damage which would have, in the absence of the tsunami, caused a severe accident. The seismic response analysis and the visual investigations conducted so far did not seem to show major damage to safety-related equipment.
4Fukushima Accident - contd The majority of the damage was caused by the tsunami. At Fukushima Daiichi it caused complete loss of AC power, loss of ultimate heat sink and serious degradation of DC power sources. This led to the loss of decay heat removal at three NPP units, to severe reactor core damage, to the loss of containment integrity and to significant radioactive releases to the environment. In addition, the upper part of the fourth unit reactor building was destroyed by hydrogen explosion and the spent fuel pool structures of that unit suffered mechanical damages.
5Some reassuring thoughts as far India is concerned Huge earthquakes and huge tsunamis are not commonplace
7Status of Seismicity – Indian NPPs Criteria - No Active fault within 5 kmSite Seismic ZoneNarora IVRawatbhata IIKakrapar IIITarapur IIIJaitapur IIIKaiga IIIKalpakkam IIKudankulam II
8Tsunamigenic locations for Indian coast TARAPURKALPAKKAMONLY FAR FIELD SOURCESKUDANKULAMTECTONIC PLATE BOUNDARIES18 March 2011
9How does this benefit us? FukushimaEarthquake knocked out Class 4 supplyTsunami knocked out other suppliesIndiaEQ and tsunami don’t occur togetherGround motion due to an earthquake causing tsunami is negligibleEarthquakes causing significant ground motion do not cause tsunamiWe get warning (~ 2 hrs)
10Fukushima Accident Lessons Learnt The key criterion of success:- recovery of power supply- water feed for the decay heat removalAs prompt as possible!Availability of undamageable portable engineering means for power and water supply in the conditions of NPP isolationAccident prevention and accident mitigation:- implementation of design fundamental;- emergency preparation;- Severe Accident management.Source: Prevention and Mitigation — Equal Priorities Prof. Vladimir Asmolov, WANO President
11ACCIDENT MANAGEMENT GOAL ACCIDENT MANAGEMENT MEASURESTo prevent the core melting(To keep the integrity of the Ist and IInd physical barriers – Fuel & Clad)The recovery of the core coolingTo retain melt inside the RPV(To keep the integrity of the IIIrd physical barrier - RPV)In-vessel coolingEx-vessel coolingTo prevent the containment failure(To keep the integrity of the IVth physical barrier - Containment)Core catcherHydrogen managementFiltered venting systemSource: Prevention and Mitigation — Equal Priorities Prof. Vladimir Asmolov, WANO President
13Securing energy for India’s future is a major challenge World OECD Non-OECD India India(developing world) of our dreamPopulation(billion)(stabilised)Annual av. per ~ ~ ~ ~capita Electricity (kWh)AnnualElectricityGeneration (trillion kWh)Carbon-di-oxideEmission ?(billion tons/yr)India alone would need around 40% of present global electricity generation to be added to reach average 5000 kWh per capita electricity generationDr. Kakodkar “Atoms for Prosperity
14Global climate change is an immediate threat Just ten years from now, greenhouse emissions from developing nations will equal the emissions from the countries we now call developed. After that, emissions from the developing world will be the major driver of global climate change.While energy conservation, windmills, and solar panels may help, we cannot hope to rely on such measures alone to meet our world’s expanding appetite for more energy.John Ritch, Director General of the World Nuclear Association, 15th Pacific Basin Nuclear Conference, Sydney, Oct. 2006Comparison of sea-ice from 1979 and 2003.19792003Source:
16CNS Extraordinary Meeting Summary Report The displacement of people and the land contamination after the Fukushima Daiichi accident calls for all national regulators to identify provisions to prevent and mitigate the potential for severe accidents with off-site consequences.Nuclear power plants should be designed, constructed and operated with the objectives of preventing accidents and, should an accident occur, mitigating its effects and avoiding off-site contamination.The Contracting Parties also noted that regulatory authorities should ensure that these objectives are applied in order to identify and implement appropriate safety improvements at existing plants.
17Dr. KakodkarAn essential goal for nuclear safety is “Never Again” should there be any significant off site emergencyDual level design basisDesign BasisRisk Lowered to an acceptable levelNo impact in public domainExtreme EventMaximum potentialNo significant off-site emergencyExtra margin between design and ultimate load capacity should be sufficient to cope with this
18Can the nuclear community set for itself an ambitious goal to meet the challenge of the numbers? “Four decades from now, in any country of the world, it should be possible to start replacing fossil fuelled power plants, at the same urban or semi-urban site where these are located, with advanced NPPs that would, more economically, deliver at least twice the power that was being produced by the replaced plants”R.K. Sinha, “The IAEA’s Contribution to the Peaceful Use of Nuclear Power”, Nuclear Power Newsletter, Vol. 3, No. 3, Special Issue, Sept. 2006
19Number of reactors in operation Level of safety goals increases with multi-fold increase in deployment of nuclear reactorsSpecial Siting Criteria, Risk approachSpecial Siting Criteria (may/may not); CDF, LERFSiting criteriaDose CriteriaSafetyGoalsAdvanced future Reactor SystemsAdvanced reactors under constructionReactors under operation (existing technology)Number of reactors in operation19
20Monitored Process Parameter Achievement of safety goals through enhanced levels of Defence-In-DepthStrategy for safety measures and features of nuclear installations is two-fold:To prevent accidentsPreventing the degradation of plant status and performanceIf prevention fails, limit their potential consequences and prevent any evolution to further serious conditionsMonitored Process Parameter
21Passive and Inherent Safety Features are Instrumental in Meeting New Safety Criteria The conventional reactors or so called “Traditional ones” have seen an extensive use of “active” engineering safety systems for reactor control and protection in the past.These systems have certain potential concerning termination of events or accidents that are effectively coped with by a protective system limited by the reliability of the active safety systems or prompt operator actions.Since the reliability of active systems can not be improved above a threshold and that of the operator’s action is debatable, there is growing concern about the safety of such plants due to the large uncertainty involved in Probabilistic Safety Analysis (PSA) particularly in analyzing human faults.In view of this, a desirable goal for the safety characteristics of an innovative reactor is that its primary defence against any serious accidents is achieved through its design features preventing the occurrence of such accidents without depending either on the operator’s action or the active systems.That means, the plant can be designed with adequate passive and inherent safety features to provide protection for any event that may lead to a serious accident.Such robustness in design contributes to a significant reduction in the conditional probability of severe accident scenarios arising out of initiating events of internal and external origin.
22Example of Applications Passive Systems and Inherent Safety Features in Defence-In-Depth in AHWR
23The Indian Advanced Heavy Water Reactor (AHWR-Pu) AHWR is a 300 MWe vertical pressure tube type, boiling light water cooled and heavy water moderated reactor using 233U-Th MOX and Pu-Th MOX fuel.Bottom Tie PlateTop Tie PlateWater TubeDisplacerRodFuel PinMajor design objectives65% of power from ThSeveral passive features7 days grace periodNo radiological impactPassive shutdown system to address insider threat scenarios.Design life of 100 years.Easily replaceable coolant channels.Design validation through extensive experimental programme.Pre-licensing safety appraisal by AERBSite selection in progress.Detailed engineering consultancy in progressAHWR-Pu is a Technology demonstrator for the closed thorium fuel cycleAHWR-LEU extends the AHWR technologies with LEU-Th MOX Fuel for the global marketAHWR Fuel assembly23
24No unacceptable radiological impact outside the plant boundary with AHWR incorporates several technolological solutions to a higher level of safety and security against both internal and external threatsControl room and auxiliary systemsPneumatic supplyInstrumentation & control signalsElectrical power(Class 1 to 4)TurbinePumpCondenserControl and S/D systemsCoreExternal eventsMalevolent actUltimate heat sink (Cooling tower or sea)No unacceptable radiological impact outside the plant boundary withFailure of all active systems, andFailure of external infrastructure to provide coolant, power and other services, andMalevolent acts by an insider, one of the consequences of which is the failure of instrumentation signal initiated shutdown actions, andInability of plant operators to manage the events and their consequences, for a significantly long time.24
25Some important passive safety features of AHWR –1/4 Heat removal from core under both normal full power operating condition as well as shutdown condition is by natural circulation of coolant.
26Some important passive safety features of AHWR –2/4 Passive Containment Cooling(Th-Pu) MOX Fuel pins(Th-233U) MOX Fuel pinsCentral Tube for ECCS waterAHWR FUEL CLUSTERPassive Containment isolationPassive injection of cooling water, initially from accumulator and later from the overhead GDWP, directly into fuel cluster.
27Some important passive safety features of AHWR –3/4 Passive Poison Injection in moderator during overpressure transientPassive Poison Injection System actuates during very low probability event of failure of wired shutdown systems (SDS#1 & SDS#2) and non-availability of Main condenser
28Some important passive safety features of AHWR –4/4 Use of moderator as heat sinkWater in calandria vaultFlooding of reactor cavity following LOCA
29Fukushima and AHWRAHWR has been assessed for TMI as well as Chernobyl type of accidentsCritics comments: It is easy to become wise after the event (TMI, Chernobyl)Fukushima type event (Extended SBO) was anticipated even before it happenedPractically no change required in AHWR design to meet Fukushima eventGDWP and passive systems adequate to cater to the extended SBONo impact in public domain, No need of evacuationNo need of exclusion zone, sterilized zone
30Prolonged Station Black Out in AHWR Decay heat removal by Isolation Condensers A strong earthquake with/without Tsunami causing prolonged SBO for several days. Reactor tripped on seismic signal.Gravity Driven Water Pool is intact.Heat is removed by Isolation CondensersGDWP water removes decay heat for ~110 days with periodic containment venting allowed after 10 days.
31Passive Systems in Defense-In-Depth of AHWR Level 1 DID:Elimination of the hazard of loss of coolant flow:Heat removal from the core under both normal full power operating condition as well as shutdown condition is by natural circulation of coolant.Reduction of the extent of overpower transient:Slightly negative void co-efficient of reactivity.Low core power density.Negative fuel temperature coefficient of reactivity.Low excess reactivity
32Passive Systems in Defense-In-Depth of AHWR (Contd.) Level 2: Control of abnormal operation and detection of failureAn increased reliability of the control system achieved with the use of high reliability digital control using advanced information technology.Increased operator reliability achieved with the use of advanced displays and diagnostics using artificial intelligence and expert systems.Large coolant inventory in the main coolant system.Level 3: Control of accidents within the design basisIncreased reliability of the ECC system, achieved through passive injection of cooling water directly into a fuel cluster through four independent parallel trains.Increased reliability of a shutdown, achieved by providing two independent shutdown systems. Further enhanced reliability of the shutdown, achieved by providing a passive shutdown deviceIncreased reliability of decay heat removal, achieved through a passive decay heat removal system, which transfers the decay heat to GDWP by natural circulation.Large inventory of water inside the containment (about 8000 m3 of water in the GDWP) provides a prolonged core cooling meeting the requirement of grace period.
33Passive Systems in Defense-In-Depth of AHWR (Contd.) Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of consequences of severe accidentsUse of moderator as heat sink.Presence of water in the calandria vaultFlooding of reactor cavity following a LOCA.Level 5: Mitigation of radiological consequences of significant release of radioactive materialsThe following features help in passively bringing down the containment pressure and eliminates any releases from the containment following a large break LOCA:Double containment;Passive containment isolationCore catcherFiltered vent
34Peak Clad Temp v/s frequency of occurrence – a quantitative probabilistic safety criteria
35Core Damage Frequency Per Year Ref: Lecture on Near Term Advanced Nuclear Reactors and Related MIT Research, by Prof. Jacopo Buongiorno, MIT, USA, June 16, 2006.AHWR~ 1x10-8
37Incorporation of Hard vent To StackFrom ContainmentHard Vent system is designed to prevent the over pressurization of the containment beyond design pressure occurring due to failure of multiple safety systems because of an extreme event such as prolonged SBO with non-availability of GDWP water or large seismic event causing cracks in GDWP along with LOCA.Also retains the radio-activity in the scrubber and minimize activity release beyond the containment boundary.Scrubber tank contains water + NaOH solution (ph = 8.5).NaOH combines with Iodine whereas Cs which is in form of CsI, CsOH, CsO2, Cs2CO3 is soluble in water.A 4 inch Dia pipe is provided at the top of primary containment for venting, which will be connected to scrubber tank.3 I NaOH = 3 H2O +5 NaI + NaIO337
38Passive Autocatalytic ReCombiner System (PARCS) Postulated AccidentsDBA : Single failure (LB LOCA): No hydrogen generationBDBA : Multiple failure (LBLOCA and non-availability of Wired Shutdown System) ~ 30 kg in 300 s.Prolonged SBO + non-availability of GDWP ~ 450 Kg in 2 hr starting after 40hrs of transient (~5000 m3 at ambient)Peak H2 generation rate ~ 0.3 kg/sThe released hydrogen will be combined by Passive Autocatalytic Recombiners (PARCS) located at several locations in the containment designed in such a way to reduce the hydrogen concentration in the containment below the flammability limits.Experiments are being carried out for demonstration of hydrogen removal using PARCSRecombination rate ~ kg/hr/m2 (for 2 - 4% H2 conc.)Overall box size : x 400 x (L X B X H)(8.29 m2 of Catalyst Deposited area)Estimated Conversion rate : 0.83 kg/hrNo. of Recombiners for one Plant ~ (Total Conversion Rate = 83 kg/hr)
39Design objective of the core catcher Design of Core CatcherSacrificial Concrete(300 mm depth)High porosity concreteWater pool(500 mm depth)Riser Tubes( 100mm)Structure of core catcher7.4 mWater from GDWPSacrificial concrete layer mixes with the melt, reduces its temperature, solidus temperature (typically from 2800oC to 1500oC) and helps in spreading the melt over large surface areaPoison added in sacrificial concrete prevents recriticalityHigh porosity concrete layer below the sacrificial concrete helps in flooding water from belowRiser tubes inject water within the melt-concrete mixtureThe downcomers supply water to the water pool from GDWP passivelySacrificial concretecompositionDesign objective of the core catcherRetention of the melt in the cavityQuenching it within 30 minutesStabilize it for substantial period of time (several days)39
41Indian High Temperature Reactor Programme Compact High Temperature Reactor (CHTR)- Technology Demonstrator100 kWth, 1000 °C, TRISO coated particle fuelSeveral passive systems for reactor heat removalProlonged operation without refuellingStatus: Design of most of the systems worked out. Fuel and materials under development. Experimental facilities for thermal hydraulics setup. Facilities for design validation are under design.Status: Optimisation of reactor physics and thermal hydraulics design, selection of salt and structural materials in progress. Experimental facilities for molten salt based thermal hydraulics and material compatibility studies set-up.Innovative High Temperature Reactor for Hydrogen Production (IHTR)600 MWth , 1000 °C, TRISO coated particle fuelSmall power version for demonstration of technologiesActive & passive systems for control & coolingOn-line refuellingIndian Molten Salt Breeder Reactor (MSBR)Large power, moderate temperature, and based on 233U-Th fuel cycleSmall power version for demonstration of technologiesEmphasis on passive systems for reactor heat removal under all scenarios and reactor conditionsStatus: Initial studies being carried out for conceptual design41
42Technology for fuel kernel by sol-gel technique is well established – Focus is on technologies for TRISO coating and fuel compactInitial trials with zirconia kernels completedFabrication trials of TRISO fuel using natural UO2 kernel carried outFuel compact prepared by two different techniquesHigh packing density (45-50%) achievedOPyCSiCZirconiaIPyCBuffer PyCX-ray radiographic image of TRISO particle with Zirconia kernelRadiograph and tomograph of fuel compact made by different techniqueSEM images of particle with Nat. UO2 kernelFuel Compacts
43Fabrication of C/C composite tubes and coating with SiC High Temperature Fluidized bed Coater(Inset shows fluidized beddistributor assembly)Cooling towerInduction heating systemSample with graphite fixtures and graphite susceptorFluidized Bed DistributorHeated graphite being dipped in fluidized bedAr rotameterFluidised bed based SiC coating method developedHigh density C-C composite fuel tube samples fabricated in collaboration with National Physical Laboratory, New DelhiPre-form was made using high strength carbon fibersPre-form subjected to multiple cycles of resin impregnation and hot iso-static pressing with intermediate machining cyclesMachining trials of graphite components (AFD)
44Thermal hydraulic studies for liquid metal (Pb-Bi) Liquid Metal Loop (2009)Major areas of developmentAnalytical studies and development of computer codesLiquid metal loop for experimental studiesLoop at 550 °C in operation since 2009Loop at 1000 °C under commissioningSteady state and transient experiments carried outIn-house developed code validatedExperimental and analytical studies for freezing and de-freezing of coolantTest bed for development of instrumentation –level probes, oxygen sensor, EM pump and flowmetersYSZ based oxygen sensorComparison of steady state correlation [Vijayan, 2002] with experimental data44
45Sufficient time margin before shutdown or passive alternate heat removal system needs to act Case-1250% step increase in powerLOCANo heat sinkCase-2Similar to case-1, but with a 300% “spike” in power before stabilizing at 250%~40 min~58 minSufficient time available to activate primary and/or secondary shutdown system, or passive gas-gap filling system
46Negligible rise in peak temperatures after shutdown due to decay heat Minimum temperatures well above freezing point of coolant even after 1 hour
47Innovative High Temperature Reactor (IHTR) for commercial hydrogen production 600 MWth, 1000 °C, TRISO coated particle fuelPebble bed reactor concept with molten salt coolantNatural circulation of coolant for reactor heat removal under normal operationCurrent focus on development:Reactor physics and thermal hydraulic designs – OptimisationThermal and stress analysisCode development for simulating pebble motionExperimental set-up for tracing path of pebbles using radio-tracer technologyPebble feeding and removal systemsTRISO coated particle fuelPebbleHydrogen: 80,000 Nm3 /hrElectricity: 18 MWe, Water: 375 m3/hrNo. of pebbles in the annular core ~150000Packing fraction of pebbles ~60%Packing fraction of TRISO particles ~ 8.6 %233U Requirement 7.3 %
48Molten salt corrosion test facility Thermal hydraulic studies and material compatibility studies for molten salt coolantMolten salt loopMajor areas of developmentAnalytical studies and development of computer codesMolten salt natural circulation loop for experimental studiesMolten fluoride salt corrosion facility using FLiNaKExperiments being carried out upto 750 °C mainly on Inconel materialsMolten salt corrosion test facility48
49Design features of Indian HTRs leading to inherent safety TRISO coated fuel particles: Retention of fission products up to 1600 °CHigh thermal inertia of ceramic core and low power densitySufficient margin between reactor operation and boiling point of the coolantNegative temperature coefficient of the core and coolantNatural circulation of liquid metal / molten salt coolant in single phaseLow pressure of the systemPassive removal of heat under normal operation and postulated accident scenariosHigh temperature heat pipe for CHTRChemical inertness of the lead based coolant with air/water
50Molten Salt Breeder Reactor (MSBR) This concept is attractive to India because of large thorium reserves and possibility of breeding 233U in thermal spectrum – For the third stage of Indian Nuclear Power Programme
51Schematic of Indian MSBR Design guidelinesHeat removal by natural circulation of molten saltsAvoid moderator to reduce solid high level waste generationAbility to tolerate outage of reprocessing plantEnhanced safety as compared to current reactors for possible deployment near population centresTurbineIHXCondenserRedox control(Fertile Salt)Helium bubbling and Redox control(Fuel Salt)PumpFuel SaltSelection of salts, materials and conceptual design in progressFertile SaltFertile salt drain tankCoolant salt drain tankFissile salt drain tanks
52Inherent safety features of MSBR (1/2) Continuous addition of fuel to maintain criticalityLess initial reactivityFission products, including xenon and krypton, are continuously taken out of the system,No excess reactivity reaquired for xenon overrideNo danger of their release under accident conditionEntire fuel salt inventory can be dumped into smaller subcritical dump tanks, through freeze valves,Reducing the chances of any untoward incidents.The molten salt has a high boiling point (~1400°C), hence there is a very low vapor pressureNormal operating temperatures ~ 700 to 800 C52
53Inherent safety features of MSBR (2/2) The density of fuel salts decreases with increase in temperature,With increase in temperature fuel salt is pushed out of the core leading to reduction of reactivityNo scenario for ‘fuel melt down’Modification of existing safety codes required for defining CDFMolten fluorides are simple ionic liquidsStable to the irradiationDo not undergo any violent chemical reactions with air or waterFuel has no burnup limitsLife is dictate by life of moderator and structural materials53
55Accelerator-driven Sub-critical reactor system BARC is developing technologies for Accelerator Driven System (ADS) mainly for Thorium utilization and waste transmutationMajor Role:Accelerator-driven Sub-critical reactor systemHigh conversion sub-critical blanket with thorium for producing 233UIncineration of minor actinides and some fission productsSub-critical reactor coreSteam generator plantTurbo-electrical generation plantAccelerator-driven Sub-critical reactor system
56Generation of fissile materials from thorium by spallation reaction using high energy proton acceleratorsInherently safe, flexible fuel cycleHigher burn-upReduced doubling time for ADS-breedersIntense, low-energy-cost neutron sourceFissile factory for U-233 from Th-232Suitability for transmutation & burning nuclear waste
58SummaryIn the Indian context, large scale deployment of nuclear reactors is required, with possible deployment near population centresEnhanced level of safety is one of the primary goals for advanced reactors under design in BARCDefence-in-depthPassive safety devicesPSA studiesMargin assessmentAdvanced materialsAdvanced Reactor concepts
60Modification to Strengthen “Severe Accident Prevention Features” Improving availability of onsite power supply- Providing back up emergency DG (air cooled) at a higher location- Providing a smaller/mobile DG to power essential loads and charge station batteriesImproving steam generator heat sink- Securing FFW diesel engines pumps from external flood and margins w.r. t earthquake evaluatedAdditional diesel engine operated pumps to transfer deaerator storage tank inventory to steam generatorProvision of hook up connections outside reactor building, qualified for maximum anticipated earthquake and floodProvision for Passive Decay Heat Removal (PDHR) system for 700 MWeImproving onsite water storage for one month SBO periodAugmentation of water inventorySources of water near stations are identified for fire tendersHook upto Primary Heat Transport System /ECCS- Injection into PHT system for making up leakage during SBO- Injection into PHT for unsuccessful long term ECCS operation
61Other measuresIntroduction of Seismic Trip (already exists in NAPS & KAPS)Strengthening provision for monitoring of critical parameters under prolonged loss of powerCreation of an emergency response facility capable of withstanding severe flood, cyclones & earthquakeProvision for Tsunami early warning system