Effect of water injection on hydrogen generation during severe accident in PWR

Nuclear Science and Technigues 20 (2009) 312 316Effect of water injection on hydrogen generation duringsevere accident in PWRTAO Jun° CAO XuewuSchool of Mecharical and Power Engineering, Shanghai Jiao Tong University, Shanghai 200240, ChinaAbstract Effect of water injection on hydrogen generation during severe accident in a 1000 MWe pressurized waterreactor was studied. The analyses were carried out with different water injection rates at different core damage stages.The core can be quenched and accident progression can be terminated by water injection at the time before cohesivecore debris is formed at lower core region. Hydrogen generation rate decreases with water injection into the core at thepeak core temperature of 1700 K, because the core is quenched and reflooded quickly. The water injecion at the peakcore temperature of 1900 K, the hydrogen generation rate increases at low injection rates of the water, as the core isquenched slowly and the core remains in uncovered condition at high temperatures for a longer time than the situationof high injection rate. At peak core temperature of 2100 -2300 K, the Hydrogen generation rate increases by waterinjection because of the steam serving to the high temperature steam-starved core. Hydrogen generation rate increasessignificantly after water injection into the core at peak core temperature of 2500 K because of the steam serving to therelocating Zr-U-O mixture. Almost no hydrogen gencration can be seen in base case after formation of the molten poolat the lower core region. However, hydrogen is generated if water is injected into the molten pool, because stcamserves to the crust supporting the molten pool. Reactor coolant system (RCS) depressurization by opening poweroperated relief valves has important efect on hydrogen generation. Special attention should be paid to hydrogengeneration enhancement caused by RCS depressurization.Key words Pressurized water reactor (PWR), Severe accident, Core damage state, Depressurization, Water injection,Hydrogen generation, Steam starvationof zircaloy and other structural materials. In some core1 Introductiondamage stages, hydrogen generation is constrained byIn a severe accident in a pressurized water reactorsteam insufficiency, hence the need of water injection(PWR), water is systematically injected into the coreinto the steam-starved core to ease the situation withto remove the decay heat and cool the core. However,the water evaporation. Thermal stress generated byrapiccooling of the injected water, however, mayconsequences of such an injection at various coredamage stages is yet to fully understood, on possibilitycause cracking and fragmentation in the ZrO2 layer.The loss of this protective layer may expose theof in-vessel corium retention, kinetics of hydrogenunderlying zircaloy to the high temperature steam,generation, reactor coolant system re-pressurization,giving rise to increased rate of the hydrogenimpact on fission products release, etc'". Answers forgeneration'23), along with increased heat generationthese uncertainties are required for accidentfrom oxidation. Thus, water injection at different coremanagement in the existing PWRs, and for safetydamage stagesmay affect local hydrogenanalysis in designing a novel PWR.concentration in containment and power level of theHydrogen generation during the in-vessel phasecore, and may have an impact on the hydrogenof a PWR severe accident is mainly due to oxidationSuppored by National Basic Research Progam of China (No.2009CB724301)中国煤化工●Corresponding author. E-mail adress: taojun007@sju.cdu.cnMHCNMHGRecived date: 209-0-03TAO Jun a al. /Nucler Science and Techniques 20 (009)312-31613management and effectiveness of core cooling strategy,3 Accident progression analysis (Base case)consequently.For developing and implementing core coolingThe core degradation progression, in-vessel severestrategy, and evaluating the negative impacts properly,accident and H2 generation was analyzed without anythe effect of water injection on hydrogen generation inmitigation measures. But a comparison was madea hypothesized severe accident in a 1000 MWe PWRbetween the situations with and without waterinjection as a mitigation measure. The results arewas studied.shown in Fig.1 for pressure of the reactor coolant2 Plantdescriptionandanalysissystem, water level of the reactor pressure vessel, coreassumptionstemperature and hydrogen generation rate.The plant is a 1000 MWe 3-1oop PWR with a large dry18「Pressure pulse caused by slumping(acontainment. Assumptions for the accident analysis areof molten core to the lower head、as follows.星7Hydrogen generation during water injection ispertinent to core damage stage and water injection rate.Various accident sequences, such as station blackout,Core uncovered entirelycoolant loss, steam generator tube rupture, feed water---- Steam generator dyoutloss, etc， will result in similar core degradation5000000015000 20000Time/sprogression if proper core cooling measures were not14 [taken. Water is injected into the reactor coolant system(2上by the emergency core cooling system that includesthe high pressure injection system, accumulator and0上low pressure injection system. Actual water injectionTop of the coreMolten core slumpingrate using the emergency core cooling system isto lower headdepended on pressure of the reactor coolant system ifsufficient water is available. It is also assumed that the2 t Bottom of the corereactor is running at its full power when the accident&5000 100000 150000 20000initiates. Referring to the accident sequence selectionin the research of water injection in five US nuclearpower plant', this scenario causes station blackoutplus loss of auxiliary feed water, a failure that is40000| Molen core slumping tolowerhead + ()Molten pool growing at lower core redioncommonly referred as TMLB'.3000 lDebris bed formed atHypothetically, the following events are tolower core redion|0happen to the fuel elements:2000 Rapid zrcaloyo. .(a) Fuel rod cladding ballooning and rupture atoxidationt0,1100-1500 K;(b) Rapid zircaloy oxidation at 1500-2100 K;10000(c) The cladding breaks at eutectic point of2470K;5000 100000 15000 20000(d) The ZrO2 protective layer failed at 2500 K;Time 15(e) Cohesive debris bed and molten pool growingFig.1 Simulation results for a hypothetical PWR accident. (a)at ~2800 K;pressure中国煤化工ES); (b) water level of(f) UO2 melts at 3120Kthe reacgeneration rates andcore temYHCNMHG314TAO Jun et al. 1 Nuclear Science and Techniques 20 (2009) 312 -316As auxiliary feed water is not available in apumps at high flow rate to quench the highTMLB' accident, the core decay heat cannot betemperature core as quickly as possible; (3) ensureremoved by the secondary circulation, and the steaminjection of water in accumulators into the reactorgenerators will dry out at about 3200 s. when pressurecoolant system if available; (4) remove core decayof the reactor coolant system begins to rise, ultimatelyheat by the reactor coolant system feed and bleed; andchallenging power operated relief valves of th(5) reduce the mechanical loads imposed on lowerpressurizer. The reactor coolant system is then havinghead of the reactor pressure vessel.a pressure at which the power operated valves cyclingThus, the reactor coolant system depressurizationopen and close between its open and close set points,by opening the relief valves is assumed.hence causing a coolant loss from the relief valves.There are three pumps for high pressure injectionThe cladding temperature starts to rise sharply due toand three pumps for low pressure injection in thiscore uncovery at about 6400 s. Before the formation ofplant. Two pumps in each group can be useddebris bed at lower core region under much lowersimultaneously, and we assume that the two hightemperatures, rapid zircaloy oxidation occurs, with thepressure pumps and two low pressure pumps workgeneration of large amount of heat and hydrogen.simultaneously to inject water into the core,Formation of cohesive debris bed and molten pool cancorresponding to high injection rate; and that one highbe seen at about 8800 s, when zircaloy oxidation ratepressure pump and one low pressure pump are used todecreased due to a large reduction in surface area, withinjected water into the core, corresponding to thereduced heat and hydrogen generation rates.lowest water injection rate.Eventually molten core slumps into the lower head ofThe analysis results are shown in Fig.2. Thethe pressure vessel, which is damaged due to highaccident progression can be terminated by watermechanical and thermal loads. The accident generatesinjection before the core temperature reaches 2500 K,a total amount of 374 kg hydrogen. Most of thewhich is designated as fuel cladding failurehydrogen is generated during the period from the coretemperature. After failure of the fuel cladding,temperature reaching about 1500 K to molten poolrelocation of Zr-U-O eutectic mixture occurrs, with aformation at the lower core region.gradual formation of the debris bed at the lower coreregion. Then the core cannot be cooled by injected4 Efect of water injection on hydrogenwater due to limited interface area between the debrisgenerationand water. The hydrogen generation rate and other keyEffect of the water injection on hydrogen generationparameters are summarized in Table 1. The hydrogenwas studied with water injections at the time when thegeneration rate in base case was compared with thosecore temperature reaches 1500， 1700， 1900, 2100,at the same core temperatures after the water injection. .Special attention should pay to the time period when2500, and 2800 KThe reactor coolant system remains at a pressurethe hydrogen generation rate remains at high level,when the power-operated relief valves are selbecause the hydrogen concentration in thesomewhere in between their open and closet points, ifcompartment of the pressurizer is decided by this timedepressurization measures are not taken. The conditionperiod if the power-operated relief valves are open.At a high water injection rate, the core iscorresponds to very low injection rate, as water canonly be injected into the reactor coolant system byquenched quickly and no noticeable hydrogen isgenerated; while at a low water injection rate, the corehigh pressure injection system. However, opening therelief valves is usually taken as depressurizationtemperature increases at beginning of the watermeasures of the reactor coolant system in highinjection, with a peak core temperature of 2215 K.How中国煤化工rate decreases aspressure sequences in PWR so as tol51: (1) avoid highpressure melt ejection and containment failure in earlythe|YHCN M H Gondition at highertime caused by direct containment heating; (2) ensuretemperature for a short time.water injection into the reactor coolant system byTAO Jun et al. /Nuclear Science and Techniques 20 (2009) 312 -316153000”300,0)PCT=1500K. D)PCT:1700K20 色2000+000400 t1000-Weu.noo村”?6000 70000 8000 9000 10000000 70000“ 80000 9000 100011000卓Tme1sTime1s。 C)PCT=1900K3000厂d)PCT-2100Ko-0800WearlipctonWeerinec.on3inbaed7000 80009000 100071000 80009000100003000. @)PCT-2300K112空30000PCT=250K→0三￥2000-2000 t1.0， +0.intated100自100Wiater injection0:intuoed700080007000 8000 9000 0000Time/st o) PCT=2800Kx 3000里2500Water inpcscon 06Base case圣1500-inbated--- High njecton ratee 100--- Low injection rate8 5002000o 8000 " 00001200001000Fig.2 Initiation of water injection at diferent core temperatures.When water injection rate is high, hydrogenTable 1 Hydrogen generation rate and other key parametersgeneration rate drops as the core is quenched andInitiation ofAccidentPCT after water H2 generationreflooded quickly. However, at low injection rates ofwater injection termination injection/K ratethe water, the hydrogen generation rate increases as(Corresponding H L H LHLthe core is quenched slowly and remains in uncoveredPCT/K)1500yes yes 15S0 1607condition at high temperatures for a longer time.1700yes yes 1770 2215The hydrogen generation rate increases with the1900yes yes 2300 2310 -steam volume, by the high temperature steam-starved2100es yes 2268 2300core. The rates are nearly the same at any water2300yes yes 2321 24032500yes yes 2724 2813injection rate, due to Zr oxidation constrained by28001o no_ 3200 3200availab中国煤化Ilective layer. TheNotes: PCT, peak core temperature; H denotes high watersituaticpens after waterinjection rate; L denotes low water injection rate; + denotesinjectidYHCN M HGe tmperure ofincreased hydrogen generation rate; “+ +" denotes even moreincreased hydrogen generation rate; - denotes reduced2100 K.hydrogen generation rate.316TAO Jun et al.1 Nuclear Science and Techniqucs 20 (009)312 -316The hydrogen generationrateincreasestemperature of 1900 K, the hydrogen generation ratesignificantly with the steam volume serving to theincreases at low injection rates of the water, as the corerelocating Zr-U-O mixture. The core can be quenchedis quenched slowly and the core remains in uncoveredeventually, but molten pool formation can be seen atcondition at high temperatures for a longer time thanthe lower core region. Increased hydrogen generationthe situation of high injection rate. At peak corerate can be seen even before water injection into thetemperature of 2100 -2300 K, the Hydrogen generationcore as steam flow rate through the core increases duerate increases by water injection because of the steamto reactor coolant system depressurization by openingserving to the high temperature steam starved core.the power-operated relief valves.Hydrogen generation rate increases significantly afterThe core cannot be cooled by injected water,water injection into the core at peak core temperaturebecause cooling of the core is constrained by the crustof 2500 K because of the steam serving to thesupporting the molten pool, which eventually slumpsrelocating Zr-U-O mixture. Almost no hydrogeninto the lower head of the reactor pressurized vessel,generation can be seen in base case after formation ofhence the hydrogen generation by the steam serving tothe molten pool at the lower core region. However,the steam-starved crust. However, almost no hydrogenhydrogen is generated if water is injected into thegeneration can be observed in base case after themolten pool, because steam serves to the crustmolten pool formation at the lower core region. Beforesupporting the molten pool.the water is injected into the core, bhydrogen generationDuring high pressure severe accident sequencesrate increases, too, for the same reason mentioned above.n PWR,reactor coolant system is usuallySteam starvation conditions occur on fuel roddepressurized by opening the power-operated reliefsurfaces during the accident progression due to dry-outvalves. However, this is accompanied with increasedof the reactor core and blockage formation. Recenthydrogen generation rate even without the waterexperiments at FZKO revealed the formation ofinjection, with increased steam flow rate through thea-Zr(O) precipitates distributed quasi homogeneouslycore due to the depressurization. Special attentioninside the oxide layer and the development of metallicshouldbe paid to this hydrogen generationscale on the outer surface of the oxide layer. Thisenhancement.resulted in thickness reduction of outside ZrO2Referencesprotective layer and exposed more metallic Zr to steamwith the water injection into the core, hence enhanced1Dorsselaere J V, Fichot F Nucl Eng Design, 2006, 236:1976-1990.oxidation and hydrogen generation. Such a situationwas not considered in this analysis. Thus, oxidation2 Haste T J, Trambauer K. Degraded core quench: Summaryand hydrogen generation during core quenching wasof progress 1996-1999. Report OECD/NEA/CSNIR(99)23. 2000.under-estimated to some extent.3 Duriez C, Adroguer B. Minutes of the COLOSS 205 Conclusionprogress meeing. Examination of VVER fuel behaviourunder severe accident conditions. Quench Stage, MeetingEffect of water injection on hydrogen generationReport SAM-COLOSS-M005, Clamart. 2001.during TMLB' accident was analyzed in a 1000 MWe4 U.S.NRC, Severe accident risks: an asessment of fivePWR, and the following conclusions could be drawn:U.S. nuclear power plant, NUREG-1150, 1989.The core can be quenched and accident progression5 Knudson D L, Rempe J L, Nucl Eng Design. 2004, 230:can be terminated by water injection at the time before133-150. .the formation of cohesive core debris at lower core6 Stuckert U, Stegmaier J. Behavior of oxide layer onregion.中国煤化工iarationn conditions.Hydrogen generation rate decreases with waterc and 14509C. 10thinjection into the core at the peak core temperature ofHCNMHGKarlsruhe, October,1700 K, because the core is quenched and reflooded2004.quickly. The water injection at the peak core