Hydrogen Hazaards in Npps

8/13/2019 Hydrogen Hazaards in Npps

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HYDROGEN: A KEY CONSTITUENT IN NUCLEAR POWER PLANTDISASTER

The basic goal of nuclear safety is to mantain the intrigity of containment and

containment to prevent the release of fission product into environments . Hencenuclear power plant is designed , constructed and operated in conformity of strignentsafety standards for the protection of plant personnel, public and environment.

In case of severe accident in a nuclear power plant,the heated zirconium metal reactswith steam produces hyrogen. The combustion of this enormous amount of hydrogencan generate short term pressure or detonation forces that may exceed the ultimatestrength of containment and lead to early containment failure. Defence in depth innuclear power plant is achieved primarily by means of four successive barriers which

prevent the release of radioactive material (fuel matrix, cladding, primary coolantboundary and containment, and these barriers are primarily protected by three levelsof design measures! prevention of abnormal operation and failures (level ", controlof abnormal operation and detection of failures (level # and control of accidentswithin the design basis (level $. If these first three levels fail to ensure the structural

integrity of the core, e.g. due to beyond the design basis multiple failures, or due toextremely unli%ely initiating events, additional efforts are made at level & to furtherreduce the ris%s. 'or most ))s, severe accidents lead to hydrogen release rates thatexceed the capacity of hydrogen control measures at conventional design basisaccident (D*+.ocal high hydrogen concentrations can be reached in a short time,leading to combustible gas mixtures in the containment. -oreover, a long termpressure buildup may occur due to steam generation through decay heat and/orthrough the generation of noncondensable gas from the interaction of the moltencore with the containment basemat concrete.

The production, distribution and combustion of hydrogen in case of postaccident scenario are very complx and highly plant and condition specificphenomena. Hydrogen combustion may ta%e place in variety of ways such as! milddeflagration fast or accelareted flames,deflagration to detonation transition. +nunderstanding of all these phenomena is crucial for planning and implementing

effective hydrogen management measures. These measures include enhancement ofmixing, deliberate combustion through igniters, use of recombiners, and postaccident inerting.

In an accident situation such as at 'u%ushima where the fuel became very hot, alot of hydrogen is formed by the oxidation of zirconium fuel cladding in steam atabout "$0012. This is beyond the capability of the normal hydrogen recombiners todeal with, and operators must rely on venting to atmosphere or inerting thecontainment with nitrogen.


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MAJOR SOURCES OF HYDROGEN WHILE PROGRESSION TOWARDSSEVERE ACCIDENT:

In case of severe accident when core is uncovered, the heat transfer from the fuel tothe steam is small compared with decay heat produced and, hence, the fuel

temperature increases. The high temperature leads to oxidation of the 3r fuel claddingand hydrogen generation and can also lead to clad ballooning and rupture. 2ladrupture is the cause of the first fission product release. The heatup rate can increaseto well above " 4/s as the local temperature increases above 5"$00 4, due to rapidoxidation of 3r and the strongly exothermic nature of the reaction. 6rders of magnitude of the H# mass assuming a complete oxidation of "007 ofthe 3r mass with steam following the complete chemical reaction!

3r 8 #H#6 9 3r6# 8 # H# 8 :H:H ; .> %?/mole 3r

@here :H is an energy released during the chemical reaction and 0.0& %g H# per4g 3r was oxidized. + very rough order of magnitude of hydrogen created by full 3roxidation could be up to "000 %g of H# for a typical )@A compared to at least $ to &time more for a *@A with the same power (around "000 -@(e, and around ""00%g of H# for a "000 -@(e @@BA.

During the loss of geometry of the core, experts presently consider that the mainsource of released H# comes from (C3r6 melt oxidation. 2orrect modelling of the(C3r6 melt oxidation is still under investigation, based on newly dedicatedexperimental programmes.

2rust failure and melt relocation to the lower plenum are latephaseE core damageprogression phenomena for which the uncertainties are greater than for the earlyphase phenomena .In the late core degradation phase, hot melt can drop into thelower plenum of the vessel, which may be filled with water. Fuch inGection of themelt into the water, for instance in the form of a Get, and fragmentation of the melt,would lead to an increase of the oxidation reaction surface (fragmented particles andto a strong production of steam, which can oxidize the available metal.Invessel hydrogen production coming from steel oxidation is around "07 to "


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containment cooling(D2H can happen .The 3r still present in the corium at the timeof the vessel breach undergoes a very fast oxidation with the available oxygen andsteam. This availability depends on the design of the cavity pit and surroundingcompartments. Bxperts generally assume that "007 of the remaining 3r is oxidizedduring the D2H event (or very shortly after in the containment or the cavity pit .

In case of a reactor vessel bottom breach when the reactor coolant system isdepressurized, a gravitational corium drop occurs and in case of a dry cavity pit, amolten core coolant interaction (-22I starts. The 3r and 2r masses contained in thecorium, coming from the remaining 3r and 2r masses in the corium at the time of thevessel lower head failure, undergo a fast oxidation in the steam and 26#environment, where the 26# is coming from the thermal decomposition of thebasemat concrete. Due to the violent gas release from the concrete into the corium atthe beginning of -22I, the masses of H#6 and 26# are well in excess of those of 3rand 2r and are in close contact with these. Bxperts generally assume that "007 of

these remaining 3r and 2r masses will be oxidized by steam to give H# and 26within the first hour (even less following the beginning of -22I. 6ther possible exvessel H# production ma occur by radiolysiss of water,corrosionreactions etc.Aadiolysis of water occurs both during normal operation andunder accident conditions. It may ta%e place in the core and in the sump. It involves

the decomposition of water molecules by radiation, which produces various radicals.The net result is the production of hydrogen and oxygen molecules in essentially astoichiometric ratio. The phenomena are reasonably understood for pure water atroom temperature. -ore uncertainties exist for elevated temperatures and thepresence of solved matter and impurities. )rovisions are usually already ta%en withrespect to water radiolysis for post design basis accidents (e.g. 62+.

In a containment, the only significant sources of hydrogen from corrosion arereactions of zinc and aluminium. 3inc is present in some types of paint and ingalvanized steel. These reactions are of importance for high and low pH values.Bvaluations have shown that the amount of hydrogen that could be produced bycorrosion reactions is in the order of "00 %g in some hours, i.e. far less than fromother sources.RISKS FROM HYDROGEN COMBUSTIONThe maGor ris%s due to combustion of hydrogen produced during sever accident are

the loss of containment integrity and failure of safety systems. These ris%s may causeincrease in pressure and termal loads manifold which can be reason for structural andcomponents damage.The various mode of hydrogen combustion may be -ild deflagrations, which occur in lean hydrogen mixturesJ 'ast deflagrations, which occur at higher hydrogen concentrations, or are caused byflame acceleration from flames travelling initially at lower speed, and/or Get ignitionJ Deflagrations that accelerate and transit to detonation (deflagration to detonationtransitionE, DDT, caused by the mechanisms described in the previous sectionsJ ocal detonation, at high hydrogen concentration, or as the conseuence of a DDT

processJ Klobal detonation.


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Thermal loads can be intermittent (free flames or continuous(diffusion/standing flames. 'ree flames occur after ignition in a gas cloudJ theytypically last seconds. Ftanding flames may occur at a location where the hydrogenfrom the source mixes with air. + typical example is a *@A suppression poolsurface, where standing flames can occur when hydrogen is driven from the drywell

through the water of the suppression pool. +t low H# concentrations, typically &L=7H#, deflagrations produce only slow flames, and loads are uasistatic. They arebasically calculated from the energy balance, the %ey parameter being the number ofhydrogen moles that react with oxygen. 2ombustion is often not complete, i.e. not allhydrogen will burn.'or structural response, it can be assumed that the loads are static, i.e. no dynamic

response needs to be ta%en into account. +bove about =7 H# concentration, flamesmay accelerate and larger loads may result. In addition, combustion is morecomplete, so that loads also increase due to the fact that more hydrogen is burned.

ote that flame acceleration is a complex process, and does not depend Gust on thehydrogen concentration,but also on the amount of bloc%age, the degree ofconfinement, the presence of diluent gases (steam, 26#, etc. +ccelerated flamesproduce pressure spi%es, characterized by a high pressure which lasts a very shorttime. @here flames accelerate in a confined volume L typically a reactor containment

or its subcompartments L the pressure developed depends on the size of the H# gasregion, the H# concentration, the size of the enclosure and the configuration ofobstacles. Detonations produce shoc% waves, resulting in high pressures, with a veryrapid decay after the pea% value. If the detonation results from a transition fromdeflagration to detonation, these loads can even be higher. )ea% pressures of #


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metallic materials. If there is practical no oxygen in the containment (M


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Klow plug igniters are simple electrical resistance heaters that produce a surfacetemperature of =00 to O0012, which is a positive ignition source for flammablemixtures of hydrogen air steam. Klow plug igniters are reliable, robust and are themost energetic of candidate ignition sources for containment, producing ignition atvery near the absolute limits of flammability.

0 S"a$1 i.nit$!Hydrogen is particularly suited to spar% ignition, having the lowest spar% ignitionenergy of any combustible fuel. Fpar% igniters are effective ignition sources. It isimportant that the freuency of spar%ing is commensurate with the time scale of localcombustible mixture formation (hydrogen release, mixing and steam condensationrates.0 Cata)+ti' i.nit$!2atalytic igniters employ the heat of H# L 6# reactions at a spatial catalytic elementto produce surface ignition temperatures high enough to cause ignition. 2atalytic

igniters are selfactuating, self powered and continuously available. The practicaluestion regarding catalytic igniter performance are related to the range of mixturethat can be ignited, the response time and their availability in terms of poisoning,fouling or mechanical damage. The range of operation is a particular concern in richlimit mixtures where the margin between flammability and detonability is narrower

than in leanlimit mixtures.Cata)+ti' $'om-ination

2atalytic recombiners use catalysts to oxidize (recombine the hydrogen and areoperable outside the limits of flammability. )+As are simple devices, consisting ofcatalyst surfaces arranged in an openended enclosure. In the presence of hydrogen(with available oxygen, a catalytic reaction occurs spontaneously at the catalystsurface and the heat of reaction produces natural convection flow through theenclosure, exhausting the warm, humid hydrogen depleted air and drawing fresh gasfrom below. Thus, )+As do not need external power or operator action. Installationreuires only to place )+A units at appropriate locations within the containment toobtain the desired coverage.Cata)+ti' $'om-ination an( "o!t&CO2 In3'tion

2arbon dioxide is inGected in such an amount that deflagrationdetonation transitionand detonation onset are prevented while catalytic recombiners remove the hydrogen

over time. Hence, the containment structures and euipment have only to withstandthe static loading caused by (accidental deflagration. However, the inGection ofincondensable gases provides a higher initial pressure for deflagration. Thus, it isimportant that the inGection of carbon dioxide be limited to such an extent that thecombined pressure load from carbon dioxide and hydrogen combustion does notexceed the containment pressure capability.

The nature of the hydrogen threat to containment and the choice of measures tomitigate the hydrogen threat depend strongly on the containment design. There is not

one single strategy or techniue that is universally appropriate for all designs andaccident scenarios, or even, for all phases of an accident in a particular design.Different measures may be more appropriate at different locations and at different


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times during an accident. + completed safety assessment for the particular plant is theonly valid context for Gudging the adeuacy of safety systems and accidentmanagement measures, including hydrogen countermeasures.

Aeferences". I+B+TB2D62">>"-itigation of Hydrogen Hazards in Fevere +ccidents in uclear )ower )lantsI+B+TB2D62">>"#.Tritium Hazard Aeport! )ollution and Aadiation Ais% from 2anadian uclear'acilities *y Dr. Ian 'airlie?une #00N$.I+B+TB2D62"

Hydrogen Hazaards in Npps

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