Regulatory Guide 1.7
| ML13350A213 | |
| Person / Time | |
|---|---|
| Issue date: | 09/30/1976 |
| From: | NRC/OSD |
| To: | |
| References | |
| RG-1.007, Rev. 1 | |
| Download: ML13350A213 (10) | |
7U.S. NUCLEAR. REGULATORY COMMISSION Revision 1REGULATORY GUIDE September 1976OFFICE OF STANDARDS DEVELOPMENTREGULATORY GUIDE 1.7CONTROL or COMIBUSTIBLE GAS CONCENTRATIO6NS INCONTAMN14ENT FOLLOWING A LOSS-OF-COOLAINT ACCIDENT*
A. INTRODUCTION
Criterion 35, "Emergency Code Cooling," of Appendix A, "General DesignCriteria for Nuclear Power Plants," to 10 CFR Part 50, "Licensing of Pro-duction and Utilization Facilities," presently requires that a system beprovided to provide abundant emergency core cooling. Criterion 50, "Contain-ment Design Basis," presently requires that the reactor containment structurebe designed to accommodate, without exceeding the design leakage rate, condi-tions that may result from degraded emergency core cooling functionlng...Criterion 41, "Containment Atmospl-2re Cleanup," presently.requires that sys-tems to control hydrogen, oxygen, and other substances that may be releasedinto the reactor containment be provided as necessary control the concen-trations of such substances following postulated accidents and ensure thatcontainment intuegrity is maintained.In addition, the Commission has published progposed amendments to Part 50containing standards for combustible gas control systems. This guide describesmethods that would be acceptable to th e.NRC staff for implementing t.he pro-posed regulation, assuming it is promul gated.'as an effective rule by theCommission after consideration of.'ipublic comments, for l..ight-water reactorplants with cylindrical, zircaloy-clad', uxide fuel. Light-water reactorplants with stainless steel cladding:and those with noncylindrical claddingwill continue to be consideied on an individual basis.Bi DISCUSSIONFollowing'U loss-of-coolant accident (LOCA), hydrogen gas may accumulatewithin the containment as a result of1. Metal-water reaction involving the zirconium fuel cladding and the-.reactr r.*..coolant,This guide replaces Safety Guide 7, dated 3/10/71, and Supplement to SafetyGuide 7, dated 10/27/71.USNRC REGULATORY GUIDES Co.... is Mot be sent to 'he SetiJavy 0 ite Conission. U.S. Nuclei.Nlguvistory Gulde ate istst d to describe eind makhe availabte to the Public Rlegulaloiro Wai0"ingcn. D C Attention Oc.clitiong endmet hodaI accopti ble to the NFIC stalf of Implementing ssociftic part of the Seritice Secti,*igulttlois. to delineite tethniQuoi used by the itaff in eovie ihe guides ase issus d In thefollowwg tinbtow d divisionsiting specific problems at postulated accidents. or providi guidanc, to ipplicents. Regulatory Guides em. not substitiute& lo, regulations. and compliance $. Power Reactors 4 P-odcctswith ihlem isi olt quttied. MerhOds ind solutions different from those set out in 2. Reiteech end teal Rtactort, 7 the g uIdes wi-l be ifecepteble sIt atd bi o the indings etquils to 3 fuel, and Materitsals, acdtiev Occupational thitissualceao l c ofmlicont: bythie Commision 4A f riOn ninentotendSiling 9 Antitrust Cohmirncnis and fo improvements in these guides ite encowteged S Materatls and Pleant Protection tO Genetal*I alt times. end guides will he revised. mi appropriati. to ¢ccommodalt r.ommints and to rltIct new inlotmnetin or e.ptorince. However. ccrnmniett on Cope* of pubtished guides mey be obtained bn -.Iltlen idcatimg thetht tuide. t receivied within about two momthsl ofter its 4suance witt h p. divisions to the U S Nuclear tegualtoty Comn -sson Washington 0 C.liculaill useful'tn evaluating it o I.tt. eIy--on 2MM. Attention Ottecto'. Otlice ot Standards Oeiaoopmnit'i"A, ,.,,.,. 4,.
..............2. Radiolytic decomposition of the postaccident emergency cooling solutions(oxygen will also evolve in this process),3. Corrosion of metals by solutions used for emergency cooling or con-taenment spray.If a sufficient amount of hydrogen is generated, it may react with theoxygen present in the containment atmosphere or, in the case of inertedcontainments, with the oxygen generated following the accident. The reac-tion would take place at rates rapid enough to lead to high temperaturesand significant overpressurization of the containment, which could resultin a leakage rate above that specified as a limiting-condition for operationin the Technical Specifications of the license. Damage to systems andcomponents essential to the continued control of the post-LOCA conditionscould also occur.The extent of metal-water reaction and associated hydrogen productiondepends strongly on the course of events assumed for the accident and onthe effectiveness of emergency cooling systems. Evaluations of the per-formance of emergency core cooling systems (ECCS) included as engineeredsafety features on current light-water-cooled reactor plants have been madeby reactor designers using analytical models described in the "InterimAcceptance Criteria for Emergency Core Cooling Systems for Light-Water PowerReactors" published in the Federal Register on June 29, 1971, and as amendedon December 18, 1971.* These calculations are further discussed in thestaff's concluding statement in the rule making hearing on the AcceptanceCriteria, Docket RM-50-1.** The result of such evaluations is that, forplants of current design operated in confdrmance with the Interim AcceptanceCriteria, the calculated metal-water reaction amounts to only a fraction ofone percent of the fuel cladding mass. As a result of the rule makinghearing (Docket RM-50-1), the Commission adopted regulations dealing withthe effectiveness of ECCS (10 CFR Part 50, § 50.46).The staff believes it is appropriate to consider the experience obtainedfrom the various ECCS--related analytical studies and test programs, such ascode developmental efforts, fuel densification, blowdown and core heatupstudies, and the PWR and BWR FLECHT tests, and to take account of the increasedconservatism for plants with ECCS evaluated under § 50.46 in setting theamount of initial metal-water reaction to be assumed for the purpose ofestablishing design requirements for combustible gas control systems. Thestaff has always separated the design bases for ECCS and for containmentsystems and has required such containment systems as the combustible gas*436 FR 12248 and 36 FR 24082.A copy of the docket file may be examined in the NRC public dociment room.1.7-2 4 4control system to be designed to withstand a more degraded condition of thereactor than the ECCS design basis permits. The approach is consistentwith the provisions of General Design Criterion 50 in which the need :oprovide safety margins to account for the effects of degraded ECCS ['fictionis noted. Althoutgh the level of degradation considered might lead to anassumed extent of metal-water reaction in excess of that calculated foracceptable ECCS performance, it does not lead to a situation involving atotal failure of the ECCS.The staff fuels that this "overlap" In protection requirements providesain appropriate ind prudent safety margin against unpredicted events duringthe course of accidents.Accordingly, the amount of hydrogen assumed to be generated by metal-water reaction in establishing combustible gas control system performuncerequirements should be based on the amount calculated in demonstratingcompliance with 5 50.46, but the amount of hydrogen required to be assumedshould include a margin above that calculated. To obtain this margin, theassumed amount of hydrogen should be no less than five times that calculatedin accordance with 5 50.46.Since the amounts of hydrogen thus determined may be quite small formany plants (as a result of the other more stringent .equirements for ECCSperformance in the criteria of 9 50.46), it is consistent with tihe considera-tion of the potential for degraded ECCS performance discussed above toestablish also a lower limit on the assumed ameunt of hydrogen generated bymetal-twater reactions in establishing combustible gas control system require-menrts. In establishing this lower limit, the staff has considered the factthat tile maximum metal-water reaction permitted by the ECCS performancecriteria is one percent of the cladding mass. Use of this "one percent oftile mass" value as a lower limit for assumed hydrogen production, however,would unnecessarily penalize reactors with thicker cladding, since for thesame thermal conditions in the core in a postulated LOCA, the thickercladding would not, in fact, lead to increased hydrogen generation. Thisis because the hydrogen generation from metal-water reaction is a surfacephenomenon. A more appropriate basis for setting the lower limit would bean amount of hydrogen assumed to be generated per unit cladding area. Itis convenient to specify for this purpose a hypothetical uniform depth ofcladding surfacce reaction. The lower limit of metal-water reaction hydrogento be assumed is then the hypothetical amount that would be generated ifall the metal to a specified depth in the outside surfaces of the claddingcylinders surrounding the fuel (excluding the cladding surrounding theplenum volume) were to react.In selecting a specified depth to be assumed as a lower limit for allreactor designs, the staff has calculated the depth that could correspondto the "one percent of the mass" value for the current core design with the1.7-3... ....:...
..... .... ... ... .thinnest cladding. This depth (0.01 times; the thickness of the thinnestfuel cladding is used) is 0.00023 inch.In summary, the amount of hydrogen to be generated by metal-waterreaction in determining the performance requirements for combustiblegas control systems should be five time:; the maximum amount calculated inaccordance with 1 50.46, but no less than the amount that would result fromreaction of all the metal in the outside surfaces of the cladding cylinderssurrounding the fuel (excluding the cladding surrounding tie plenum volume)to a depth of 0.00023 inch.It should be noted that the extent of initi.al metal-water reactioncalculated for the first core of a plant and used as a design basis for thehydrogen control system becomes a limiting condition for all reload coresin that plant unless the hydrogen control system is subsequently modifiedand reevaluated.The staff believes that hydrogen control systems in plants receivingoperating licenses on the basis of ECCS evaluationn under the "InterimAcceptance Criteria" should continue to be designed for the 5 percentinitial metal-water reaction specified In the original issuance of thisguide (Safety Guide 7). As operating plants are reevaluated as to ECCSperformance under S 50.46, a change to the new hydrogen control basisenumerated in Table 1 may be made by appropriate amendments to the TechnicalSpecifications of the license. For plants receiving construction permitson the basis of ECCS evaluations under tile Interim Acceptance Criteria, theapplicant would have the option of using either a 5 percent initial metal-water reaction or five times the maximum amount calculated in accordancewith S 50.46, but no less than the amount that could result from reactionof all the metal in the outside surfaces of cite cladding cylinders surround-ing the fuel (excluding the cladding surrounding tite plenum volume) to adepth of 0.00023 inch.No assumption as to rate of evolution was as:;ociated with the magnitudeof the assumed metal-water reaction originally given in Safety Cuide 7.The metal-water reaction is of significance when establishing system per-formance requirements for containment designs that employ time-dependenthydrogen control features. The staff recognizes that it would be unrealisticto assume a' instantaneous release of hydrogen from an assumed metal-waterreaction. For the design of a hydrogen control system, therefore, itshould be assumed that the initial metal-water reaction would occur over ashort period of time early in the LOCA transient, i.e., near the end of theblowdown and core refill phases of the LOCA transient. Any hydrogen thusevolved would mix with steanu and would be rapidly distributed throughoutthe containmoent compartments enclosing the reactor primary coolant systemby steam flowing from the postulated pipe break. These compartmentsinclude the "drywell" in typical boiling water reactor containments, the1. 7-4 7. ."lower volume" of ice condenser containments, and the full volume of "dry"containments. The duration of the blowdown and refill phase is generallyseveral minutes, and the assumptions oi a two-minute evolution time, whichrepresents the period of time during which the maximum full heatup occurs,with a constant reaction rate and with the resulting hydrogen uniformlydistributed in the containment compartments enclosing the primary coolant*ystem, are appropriately conservative for the design of hydrogen controlsystems. The effects of steam within the subcompartments and containmentshould be considered in the evaluation of the mixture c.omposition.The rate of production of gasrs from radiolysis of coolant solutionsdepends on (1) the amount and quality of radiation energy absorbed in thespecific coolant solutions used and (2) the net yield of gases generatedfrom the solutions due to the absorbed radiation energy. Factors such ascoolant flow rate.. and turbulence, chemical additives in the coolant,impurities, and coolant temperature can all exert an influence on the gasyields from radiolysis. The hydrogen production rate from corrosion ofmaterials within the containment, such as aluminum, depends on the corro-sioii rate, which in turn depends on such factors as the coolant chemistry,thie coolant. pul, the metal and coolant temperatures, and the surfbce areaexposed to attack by the coolant. Accurate values of these parameters aredifficult to establish with certainty for the conditions expected to pre-vail following a LOCA.Table I defines conser, tive values and assumptions that may be used toCv.Iluate the production of cu:.-,)ustible gases following a LOCA.If thes;e ansumptions are used to calculate the concentration of hydrn-g-cn (and oxygen) within the containment structures of reactor plantsfollowing a LOCA, the hydrogen concentration is calculated to reach thelimit within periods of less than a day after the accident forthe smalle.st containments and up to more than a month for the largest ones.The hydrogen concentration could be maintained below its lower flammablelimit by purging the containment atmosphere to the environs at a controlledrate after the I.OCA, however, radioactive materials in the containmentwould also be released. Therefore, purging should not be the primary meansfor controlling combustible gases following a LOCA. It Is advisable,however, that the capability for controlled purging be provided to aid incontainment atmosphere cleanup.The Bureau of Mines has conducted experiments at its facilities withinitial hydrogen volume concentrations on the order of 4 to 12 volume percent.On the basis of these experiments and a review of other reports, the NRCstaff concludes that a lower flammability limit of 4 volume percent hydrogenin air or steam-air atmospheres is well established and is adequatelyconservative. For initial concentrations of hydrogen greater than about 6volume percent, it is possible in the presence of sufficient ignition1.7-5
- -.. .*~..i'*sources that the total accumulated hydrogen could burn in the containment.For hydrogen concentrations in the range of 4 to 6 volume percent, partialburning of the excess hydrogen above 4 volume percent may occur. The staffbelieves that a limit of 6 volume percent would not result in effects thatwould be adverse to containment systems. Applicants or licensees proposinga design limit in the range of 4 to 6 volume percent hydrogen shoulddemonstrate through supporting analyses and experimental data that contain-ment features and safety equipment required to operate after a LOCA wouldnot be made inoperative by the burning of the excess hydrogen.In small containments, the rduount of metal-water reaction postulatedin Table 1 may result in hydrogen concentrations above acceptable limits.The evolution rate of hydrogen from the mrtal-water reaction would begreater than that from either radiolysis or corrosion, and since it isdifficult for a hydrogen control system to process large volumes of hydrogenvery rapidly, an alternative approach is to operate some of the smallercontainments with inert (oxygen-deficient) atmospheres. This measure, the"inerting" of a containment, provides sufficient time for ccmbustible gascontrol systems to become effective following a LOCA before a flammablemixture is reached in the containment. Hydrogen recombiners can processthe containment atmosphere at a rate of only 100 scfm per recombiner.Therefore, for a 300,000 cubic foot containment with a 13 volume percenthydrogen concentration that was generated in the first two minutes of theLOCA, an inordinately larger number of recombiners would be required.There are presently no other methods of combustible gas control except forpurge systems, which release radioactive materials.For all containments, it is advisable to provide means by whichcombustible gases resulting from the postulated .metal-water reaction,radiolysis, and corrosion following a LOCA can be mixed, sampled, andcontrolled without releasing radioactive materials to the environment.Since any system for combustible gas control is designated for theprotection of the public in the event of an accident, the system shouldmeet the design and construction standards of engineered safety features.Care should be taken in its design to ensure that the system itself doesnot introduce safety problems that may affect containment integrity; forexample, if a flame recombiner is used, propagation of flame into thecontainment should be prevented.In most reactor plants, the hydrogen control system would not berequired to be operated for seven days or more following a postulateddesign basis LOCA. Thus, it is reasonable that hydrogen control systemsneed not necessarily be installed at each reactor. Provision for eitheronsite or offsite storage or a shared arrangement between licensees ofplants in close proximity to each other may be developed. An example of anacceptable arrangement would be to provide at least one hydrogen control1.7-6 system per site with the provision that a redundant unit would be availablefrom a nearby site.
C. REGULATORY POSITION
1. Each boiling or pressurized light-water nuclear power xeactorfueled with uranium oxide pellets within cylindrical zircaloy claddingshould have the capability to measure the hydrogen concentration in thecontainment, mix the atmosphere in the containment, and control combusti-ble gas concentrations without relying on purging of the containmentatmosphere following a LOCA.2. The continuous presence of redundant combustible gas controlequipment at the site may not be necessary provided it is available onan appropriate time scale. However, appropriate design and proceduralprovisions should be made for its use. In addition, centralized storagefacilities that would serve multiple sites may be used, provided thesefacilities include provisions such as maintenance, protective features,testing, and transportation for redundant units to a particular site.3. Combustible gas control systems and the provisions for mixing,measuring, and sampling should meet the design, quality assurance,redundancy, energy source, and instrumentation requirements for anengineered safety feature. In addition, the system icself should notintroduce safety problem! that may affect containment integrity. Thecombustible gas control system should be designated Seismic Category I(see Regulatory Guide 1.29, "Seismic Design Classification"), and theGroup B quality standards of Regulatory Guide 1.26, "Quality GroupClassifications and Standards for Water-, Steam-, and Radioactive-Waste-Contanaing Components of Nuclear Power Plants," should be applied.4. All water-cooled power reactors should also have the installedcapability for a controlled purge of the containment atmosphere to aidIn cleanup. The purge or ventilation system may be a separate system orpart of an existing system. It need not be redundant or be designatedSeismic Category I (see Regulatory Guide 1.29), except insofar as portionsof the system constitute part of the primary containment boundary orcontain filters.5. The parameter values listed in Table I should be used incalculating hydrogen and oxygen gas concentrations in containments andevaluating designs provided to control and to purge combustible gasesevolved in the course of loss-of-coolant accidents. These values may bechanged on the basis of additional experimental evidence and analyses.6. Materials within the containment that would yield hydrogen gasdue to corrosion from the emergency cooling or containment spray solutionsshould be identified, and their use should be limited as much as practical.1.7-7... .... .......................... ..............................
-pV~T'~~Nfl. .. .. -.,.D. IMPLEXENTATIONThis guide will be used by the staff in the evaluation of l.icensut.s'and applicants' compliance with the specified portions of the Commisston's,regulations, including the proposed § 50.44 when it is adopted as an effec-tive regulation. If the proposed regulations are not ndopted or are adoptedin substantially different form, this guide will be withdrawn or appro-priately revised.1.7-8 Table I. Acceptable Assumptions for Evaluating the Productionof Combustible Gases Following a Loss-of-Coolant Accident(LOCA)ParameterFraction of fission productradiation energy absorbed bythe coolant*Hydrogen yield rate G(H2)Oxygen yield rate G(O2)Acceptable Value(a) Beta(1) Betas from fission productsin the fuel rods: 0(2) Betas from fission productsintimately mixed with coolant:1.0(b) Gamma(1) Gammas from fission productsin the fuel rods, coolant incore region: 0..i**(2) Gammas from fission productsintimately mixed with coolant,all coolant: 1.00.5 molecule/l0OEv0.25 molecule/100EvHydrogen production is 5 times theextent of the maximum calculatedreaction under 10 CFR Part 50,§50.46, or that amount that wouldbe evolved from a core-wideaverage depth of reaction intothe original cladding of 0.00023inch, whichever is greater, in2 minutes.200 mils/yr (This value should beadjusted upward for highertemperatures early in the accidentsequence.)50% of the halogens and 1% ofthe solids present in the coreare intimately mixed with thecoolant water.1.7-9Extent and evolution time ofinitial core metal-water reactionhydrogen production from thecladding surrounding the fuelAluminum corrosion rate foraluminum exposed to alkalinesolutionsFission product distribution model., *. "-
"M ": ..- -I "POMWý-. IWPTMWý .Table 1 (Continued)ParameterAcceptable VolueAll noble gases are released to thecontainment.All other fission products remainlit fucl rods.Hydrogen concentration limit4 v/o***P.AO-d~id It IMAIiUj Limit (I V/0 ttt-geii Is present. )For water, borated water, and borated alkaline solutions; for othersolutions, data should be presented.This fraction is thought to be conservative; further analysis may showthat it should be revised.The 4 v/o hydrogen concentration limit should not be exceeded if ourningis to be avoided and if more than 5 v/o oxygen is present in the containment.This amount may be increased to 6 v/o, with the assumption that the 2 v/oexcess hydrogen would burn in the containment (if more than 5 v/o oxygenis present). The effects of the resultant energy and burning should notcreate conditions exceeding the design conditions of either the containmentor the safety equipment necessary to mitigate the consequences of a LOCA.Applicants and licensees should demonstrate such capability by suitableanalyses and qualification test results.1.7-10J.. ........S- ~ ~.., ~