The BNL staff have performed an extensive assessment of potential failure modes for core meltdown accidents in a Boiling Water Reactor (BWR) with a Mark III containment. The particular Mark III containment under consideration did not initially include provision for any system to control H/sub 2/ burning, so that H/sub 2/ phenomena were found to be dominant contributors to containment failure. However, all Mark III containments will now have provisions for the control of H/sub 2/ burning, which will influence our current assessment of H/sub 2/ phenomena

Jaung, R.

Pratt, W.T.

UNT Digital Library

-.r BNL-NOREG—37675TI86 007128 c BNL-MJRBG-37675AN INVESTIGATION OF HYDROGEN STRATIFICATION AND ITSAPPLICATION FOR THE ASSESSMENT OF CONTAINMENT FAILURE MOOESFOR A BUR MARK III CONTAINMENT DURING CORE MELTDOWN ACCIDENTS*R, Jaung and W. T. PrattBrookhaven National LaboratoryDepartment of Nuclear EnergyUpton, New York U973INTRODUCTIONThe BNL staff have performed an extensiveassessment of potential failure modes for coremeltdown accidents in a Boiling Water Reactor(BWR) with a Mark H I containment (refer to Fig-ure I). The particular Mark III containment un-der consideration did not initially include pro-vision for any system to control H2 burning, sothat H2 phenomena were found to be dominant con-tributors to containment failure. However, allMark III containments will now have provisionsfor the control of H2 burning, which will in-fluence our current assessment of H 2 phenomena.During core meltdown accidents, hydrogenmay accumulate inside the reactor containmentbuilding as a result of z1realoy-steam andsteel-steam reactions, radiolytic decompositionof water, corrosion of zinc-based paints andcoatings, and the interaction of molten mate-rials (from the damaged reactor core) with con-crete in the region below the reactor vessel.For such accidents, it is essential to determinepotential containment failure modes in order topredict the flow paths of fission products re-leased from the damaged core. The suppressionpool of a Nark III containment will effectivelyscrub any aerosol fission products passingthrough the pool and thus significantly reducethe off-site consequences of core meltdown acci-dents. Hence, the integrity of the drywell isimportant to ensuring that all of the fissionproducts released from the damaged fuel actuallypass through the pool. Hydrogen detonation ini-tiated within the wetwell region may fail thedrywell wall integrity. Drywell wall failurewill cause the fission products to bypass thesuppression pool with a corresponding increasedrelease to the environment. Therefore, it isessential to assess the probability of drywellwall failure as well as that of the containmentitself.Tha flammability and detonability limits ofa hydrogen-air mixture are mainly determined byits volumetric concentration. The local concen-tration of hydrogen within the Mark III contain-ment building varies with the position of thesource, the generation rate, generation period,geometric configurations, and mixing and trans-port processes. This paper examines th» hydro-gen concentration distribution within the MarkIII containment and determines the probabilityof the various hydrogen burning phenomena thatmay occur during a postulated core meltdownaccident.*Work performed under the auspices of the U.S.Nuclear Regulatory Commission. jHtlclt*(InchiitlFigure I Typical Mark III ContainmentBui ldings .HYDROGEN DISTRIBUTION IN REACTOR CONTAINMENTBUILDINGS /During degraded core accidents , the mixingand transport processes involving a i r , steam andhydrogen within containment are influenced bybuoyant force , pressure gradient force , conta in-ment geometry and forced convection induced byvent i la t ion and spray systems. Generally, todeal with these mixing and transport phenomena,the following factors should be considered:1. advective momentum2. turbulent momentum3. buoyant force4. shear forceDISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDDISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any spe-cific commercial product, process, or service by trade name, trademark, manufac-turer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.c5. pressure gradient force6. thermal diffusion7. turbulent diffusion8. atfvective heat transfer9. condensation rates and latent heatrelease10. heat transfer to the walls.Few experimental investigations of hydrogenmixing and transport have been conducted thatare directly applicable to degraded core acci-dents. Nevertheless, analytic methods may beemployed to estimate the evolution of local hy-drogen concentrations. These hydrogen concen-trations have heen estimated analytically asdescribed in the following section.Theoretical AnalysisWithin the BUR Hark m containment, thewetwell region combined with the dome region canbe analytically considered to consist of severalcells or subcompartments [shown in Figure 1(Cumm1ng)(i)]. The total height of the con-tainment building is about 150 ft, which corres-ponds to a characteristic Rayleigh Number of1 0 u to 10 l s. This extremely high Rayleigh Num-ber implies a strong turbulent mixing within thecontainment atmosphere. Hence, in each of theanalytically coupled cells, the hydrogen is as-sumed to be completely mixed with air. Thehorizontal distance between the walls for eachcell is always much larger than the boundarylayer thickness, outside of which the shearforce can be neglected. For the core meltdownaccidents under consideration there will be noventilation and spray systems available to in-duce forced convective flows. Therefore, thehydrogen mixing and transport processes are dom-inated by buoyant forces and pressure gradientforces. An adiabatic containment building isassumed.The theoretical approach Is less com-plicated- than other existing methods (suchas RALOC (Cunning),U) COBRA (Buxton),U)etc.). Nevertheless, the results derivedthrough this method agree well with those ob-tained from more complicated and costly com-n<jtational schemes.Code DescriptionAt this stage of development only hydrogenmixed with dry air is considered in the code.In each subdivided compartment, complete mixingof hydrogen and air is assumed. The horizontaldistance between walls is always much largerthan the boundary layer thickness. The shearingforce along the walls Is neglected. The heatloss to the walls is also neglected. Althoughthe magnitude of conductive heat transfer isalso negligible, It Is included in this code.When the hydrogen is released into a sub-compartment of the containment building, thehydrogen is assumed to mix with the air com-pletely in any given time step. This added hy-drogen will change the concentration, pressure,and enthalpy of the subcompartment. The pres-sure difference and density difference willdrive the hydrogen-air mixture into other com-partments. Horizontal flows are ignored due tothe immediate mixing assumption. Upward anddownward flows across each interface betweensubcompartments are considered. At the end ofeach time step, the transporting process willstop and the pressure of a compartment are bothlarger than the compartment above it, only onedirection flow (I.e., expansion) will be con-sidered. For each subcompartment, the time steprequired for the entering gases travelingthrough the whole subcompartment may be differ-ent. The computational time step is determinedas the minimum of these time steps. This timestep is also checked and adjusted to assure thatthe pressure of the receiver compartment is notlarger than the donor compartment unless the re-ceiver compartment is the hydrogen source com-partment (the hydrogen source compartment is as-signed to the lowest compartment).At the interface between two compartments,except for the expansion situation, the upwardacceleration 1s assumed to be equal to the down-ward acceleration. Downward expansion only oc-curs when the bottom boundary temperature islower than the atmospheric temperature withinthe containment and before the hydrogen is re-leased. Upward expansion only occurs whcs thedonor compartment temperature is lower than thereceiver compartment, and the pressure and den-sity of thp donor compartment are both largerthan the pressure and the density of the re-ceiver compartment. ^Calculation ResultsThe nine-cell nodalization model shown inFigure Z was used to represent the wetwell com-partment. This nine-c.-.n model Is similar tothe RALOC five-zone rrodel for the Grand Gulfcontainment building used in Reference (3).Compartments 1, 2, 3, and 4 represent the wet-well volume, which is located above the suppres-sion pool and below the level of the floor ofthe upper water pool. Compartments 5 and 6 rep-resent the annular region below the level of thetop of the upper pool. Compartments 7, 8, and 9represent the dome region. Two-directional flowsare considered when they pass through the inter-faces. The difference between these two modelsIs that this code assumed instantaneous hydro-gen-air mixing within each cell. Thus, no hori-zontal transport of hydrogen is considered. Thetotal volumes for both models is identical. Thepostulated hydrogen injection rate is SO Ib/minand the air Injection rate is 350 Ib/min over aperiod of 1800 seconds at 140°F. The results ofthe present calculations can be compared withRALOC calculation (Thurgood)(3) as shown' InFigure 3. The codes predict similar hydrogendistribution patterns during the hydrogenrelease.APPLICATIONHydrogen combustion phenomena are deter-mined by the volumetric concentration of H.-airmixture and the availability of ignitionsources. For a given H2-air gaseous, mixture,when the hydrogen volumetric concentration ex-ceeds 4% and the oxygen volumetric concentrationexceeds 5%, a deflagration will occur if an ig-nition source 1s available. The detonabUitylimits of a hydrogen-air mixture is not ascCompartmentHunSer123*567S99a7&S4321CongarimencVolume ft156653765831523191598335327649483338208249675249675ComoarwentHeight ft12.46712.46724.93*24.934U.97511.15527.23120.66920.669H2- SQ..RIR-3S0. LB/MIN,FOR 30.0 MNFigure 2well-defined as the flammability l imits. Basedon the "induction time" criterion applied inReference [43, this paper conservatively assumed |that when the hydrogen volumetric concentration Iexceeds 17% and the oxygen concentration exceeds j8%, a detonation is considered possible provided 'a strong ignition source is available. The mostl ikely ignition sources are electric equipmentwithin the various subcompartments of the con-tainment building. The lower portion of wetwellcontains electrically driven equipment, whichcould init iate combustion phenomena. WUen thedetonation is initiated, thermal pressure loads 'may reach 250 psia and last for tens of m i l l i - 'seconds. In addition, detonation waves can beenhanced by reflecting between the containmentand dryweil walls or focused during propaga-tion. By comparing the peak loads against the iultimate capacity of the drywell structures, i thas been determined that a detonation could fai lthe drywell wall.'A transient event initiated by a loss ofoff-site power and diesel failure was selectedfor analysis using this H2 concentration model.Operating procedures require that safety reliefvalves be partially opened in the time frame be-yond 26 minutes and then fully opened beyond 32minutes. Hydrogen and steam generated in thereactor vessel will be released via the safetyrel ief lines into the suppression pool. Noncon-densible gases such as hydrogen will leave theI - f i rs t cell9 - ninth cellA - SNL/RALOC results of ninth cellFigure 3suppression pool and enter the wetwell atmos- Iphere. The computer code described in Section2.2 was employed to predict the distribution andconcentration of hydrogen within the wetweilcompartment for this accident sequence. Thewetwell compartment; was subdivided into ninecells. The volumetric hydrogen concentration ofeach cell was used to determine the probabilityof a hydrogen deflagration or detonation.For this accident sequence an ignitionsource was assumed to be available only afterrestoration of power. Before the reactor vesselfailure, the reaction of 39S of the zirconiumcladding in the reactor vessel would result inapproximately 1330 pounds of hydrogen being gen-erated and eventually released into the wetwellcompartment. Hydrogen distribution correspon-ding to this H;, generation is shown in Figure 4using the nine-cell nodalization model. Thecalculation results show that for the first 60minutes, no global detonations are possible.Sixty-six percent of the time a global deflagra-tion was possible, 26% of the time a local def-lagration was possible, while 8% of the time alocal detonation was possible. If power is notrestored prior to reactor vessel failure core/concrete interactions could result in additionalhydrogen being generated and released into thewetwell compartment. It wis determined thatr cH2- 50.,BIR- 0. LB/MIN,F0R 2B.7 MIN*e3h°§1asi!1_.. _A•••••-m^r-njr; ~.ll/l/la11 -T~~-—: - -iff : : ___,ill • sT l r t - nlNUTES. 3 ZCIJCSFigure 4H2 - 10. L3/ttlN,FQR 3Q.0D0 MINTltC - nltwTCS. 9 ZDNC5adding hydroqer. tn the wetweli atmosphere at 10lb/min for 30 minutes (which is typ ica l of H2generation during core/concrete i n t e r a c t i o n s ) ,the hydrogen d is t r ibut ion within each cel l ishigher than 17 volume percent as shown in Figure5. Restoration of power during t h i s period maythus resul t in a global detonation.SUMMARYThe computer code described in Section 2,2has been used to evaluate H2 d i s t r i b u t i o n in aMark H I containment as a function of t ime.This can be used together with an assessment ofpeak loads during various H2 phenomena and aknowledge of the structural capab i l i t y of theMark I I I containment to determine containmentbuilding f a i l u r e modes.REFERENCES1. J. C. Cumming, et a l . , "Review of theGrand Gulf Hydrogen Ignition System,"NUREG/CR-2530, SAND 82-2, Sand1a NationalLaboratories, March 1983.2. L. D. Buxton, "Assessment of RAL0C-H0D1with 1980 Updates," Proceedings of the Sec-ond International Conference on the Impactof Hydrogen on Water Reactor Safety, Albu-querque, New Mexico, October 3-7, 1982, pp.• 469-483.3. M. J . Thurgood, "Application of COBRA-NCto Hydrogen Transport," Proceedings of theSecond International Conference on the Im-pact of Hydrogen on Water Reactor Safety,Albuquerque, New Mexico, October 3-7, 1982,pp. 509-530.4. R. Jaung, A. L. Berlad, and VI. T. Pratt,"Detonability of Containment Building Atmos-pheres During Core-Meltdo-wn Accidents," Pro-ceedings on Nuclear Reactor Thermal-Hydrau-l ics , Vol. I I , p. 1171, Santa Barbara,California, January 11-14, 1982.Figure 5

Investigation of hydrogen stratification and its application for the assessment of containment failure modes for a BWR Mark III containment during core meltdown accidents

https://digital.library.unt.edu/ark:/67531/metadc1105572/m2/1/high_res_d/6250309.pdf

Investigation of hydrogen stratification and its application for the assessment of containment failure modes for a BWR Mark III containment during core meltdown accidents

Abstract

Similar works

Full text

Available Versions

UNT Digital Library