Regulatory Guide 1.7

Control of Combustible Gas Concentrations in Containment Following a Loss-Of-Coolant Accident

ML003739927
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Issue date:	11/30/1978
From:	Office of Nuclear Regulatory Research
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RG-1.7, Rev 2
Download: ML003739927 (6)

U.S. NUCLEAR REGULATORY COMMISSION

Revision 2 November 1978

" REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.7 CONTROL OF COMBUSTIBLE GAS CONCENTRATIONS IN

CONTAINMENT FOLLOWING A LOSS-OF-COOLANT ACCIDENT

A.

INTRODUCTION

Criterion 35,

"Emergency Core Cooling," of Appendix A, "General Design Criteria for Nu clear Power Plants," to 10 CFR Part 50, "Do mestic Licensing of Production and Utilization Facilities," requires that a system be provided to provide abundant emergency core cooling.

Criterion 50, "Containment Design Basis," as amended, requires that the reactor containment structure be designed to accommodate, without exceeding the design leakage rate, conditions that may result from degradation, but not total failure, of emergency core cooling functioning.

Criterion 41,

"Containment Atmosphere Clean up," requires that systems to control hydro gen, oxygen, and other substances that may be released into the reactor containment be provided as necessary to control the concen trations of such substances following postu lated accidents and ensure that containment integrity is maintained.

In addition, the Commission has published amendments to Part 50 in which a new j 50.44,

"Standards for Combustible Gas Control Sys tems in Light-Water-Cooled Power Reactors,"

was added. This guide describes methods that would be acceptable to the NRC staff for imple menting these regulations for light-water reac tor plants with cylindrical, zircaloy-clad oxide fuel. Light-water reactor plants with stainless steel cladding and those with noncylindrical cladding will continue to be considered on an individual basis.

B.

DISCUSSION

Following a loss-of-coolant accident (LOCA),

hydrogen gas may accumulate within the con tainment as a result of:

• Lines indicate aubstantive changes from previous issue.

1. Metal-water reaction involving the zirconi um fuel cladding and the reactor coolant,

2. Radiolytic decomposition of the postacci dent emergency cooling solutions (oxygen will also evolve in this process),

3. Corrosion of metals by solutions used for emergency cooling or containment spray.

If a sufficient amount of hydrogen is gener ated, it may react with the oxygen present in the containment atmosphere or, in the case of inerted containments, with the oxygen gener ated following the accident. The reaction could take place at rates rapid enough to lead to high temperatures and significant overpressurization of the containment, which could result in a breaching of containment or a leakage rate above that specified as. a limiting condition for operation in the Technical Specifications of the license. Damage to systems and components es sential to the continued control of the post LOCA conditions could also occur.

The extent of metal-water reaction and asso ciated hydrogen production depends strongly on the course of events assumed for the accident and on the effectiveness of emergency cooling systems. Evaluations of the perform ance of emergency core cooling systems (ECCS)

included as engineered safety features on current light-water-cooled reactor plants have been made by reactor designers using analytical models described in the "Interim Acceptance Criteria for Emergency Core Cool ing Systems for Light-Water Power Reactors"

published in the Federal Register on June 29,

1971, and as amended on December 18, l97.ul.

as* FIt 1228 and 6 FR 2402.

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These calculations are further discussed in the staff's concluding statement in the rulemaking hearing on the Acceptance Criteria, Docket RM-50-1.*

The result of such evaluations is that, for plants of current design operated in conformance with the Interim Acceptance Cri teria, the calculated metal-water reaction amounts to only a fraction of one percent of the fuel cladding mass.

As a result of the rule making hearing (Docket RM-50-1), the Commis sion adopted regulations dealing with the effectiveness of ECCS

(10 CFR

Part 50,

§ 50.46,

"Acceptance Criteria for Emergency Core Cooling Systems for Light-Water Nuclear Power Reactors").

The staff believes it is appropriate to con sider the experience obtained from the various ECCS-related analytical studies and test programs, such as code developmental efforts, fuel densification, blowdown and core heatup studies, and the PWR and BWR FLECHT tests, and to take account of the increased con servatism for plants with ECCS evaluated under § 50.46 in setting the amount of initial metal-water reaction to be assumed for the purpose of establishing design requirements for combustible gas control systems. The staff has always separated the design bases for ECCS and for containment systems and has required such containment systems as the com bustible gas control system to be designed to withstand a more degraded condition of the reactor than the ECCS design basis permits.

The approach is consistent with the provisions of General Design Criterion 50 in which the need to provide safety margins to account for the effects of degraded ECCS function is noted. Although the level of degradation con sidered might lead to an assumed extent of metal-water reaction in excess of that calcu lated for acceptable ECCS performance, it does not lead to a situation involving a total failure of the ECCS.

The staff feels that this "overlap" in protec tion requirements provides an appropriate and prudent safety margin against unpredicted events during the course of accidents.

Accordingly, the amount of hydrogen as sumed to be generated by metal-water reaction in establishing combustible gas control system performance requirements should be based on the amount calculated in demonstrating com I pliance with § 50.46, but should include a margin above that calculated. To obtain this margin, the assumed amount of hydrogen should be no less than five times that calcu lated in accordance with § 50.46.

Since the amounts of hydrogen thus deter mined may be quite small for many plants (as a result of the other more stringent requirements for ECCS

performance in the criteria of A copy of the docket file may be examined in the NRC pub lic document raom.

§ 50.46), it is consistent with the consideration of the potential for degraded ECCS perform ance discussed above to establish also a lower limit on the assumed amount of hydrogen gen erated by metal-water reactions in establishing combustible gas control system requirements.

In establishing this lower limit, the staff has considered the fact that the maximum metal water reaction permitted by the ECCS perform ance criteria is one percent of the cladding mass. Use of this "one percent of the mass"

value as a lower limit for assumed hydrogen production, however, would unnecessarily pe nalize reactors with thicker cladding, since for the same thermal conditions in the core in a postulated LOCA,

the thicker cladding would not, in fact, lead to increased hydrogen gen eration. This is because the hydrogen genera tion from metal-water reaction is a surface phenomenon.

A more appropriate basis for setting the lower limit would be an amount of hydrogen as sumed to be generated per unit cladding area.

It is convenient to specify for this purpose a hypothetical uniform depth of cladding surface reaction. The lower limit of metal-water reac tion hydrogen to be assumed is then the hypo thetical amount that would be generated if all the metal to a specified depth in the outside surfaces of the cladding cylinders surrounding the fuel (excluding the cladding surrounding the plenum volume) were to react.

In selecting a specified depth to be assumed as a lower limit for all reactor designs, the staff has calculated the depth that could cor respond to the "one percent of the mass" value for the current core design with the thinnest cladding. This depth (0.01 times the thickness of the thinnest fuel cladding is used)

is

0.00023 inch (0.0058 mm).

In summary, the amount of hydrogen to be generated by metal-water reaction in determin ing the performance requirements for combus tible gas control systems should be five times the maximum amount calculated in accordance with § 50.46, but no less than the amount that would result from reaction of all the metal in the outside surfaces of the cladding cylinders surrounding the fuel (excluding the cladding surrounding the plenum volume) to a depth of

0.00023 inch (0.0058 mm).

It should be noted that the extent of initial metal-water reaction calculated for the first core of a plant and used as a design basis for the hydrogen control system becomes a limiting condition for all reload cores in that plant unless the hydrogen control system is sub sequently modified and reevaluated.

The staff believes that hydrogen control sys tems in plants receiving operating licenses on the basis of ECCS

evaluations under the

"Interim Acceptance Criteria" should continue

1.7-2

to be designed for the 5 percent initial metal water reaction specified in the original issuance of this guide (Safety Guide 7).

As operating plants are reevaluated as to ECCS performance under § 50.46, a change to the new hydrogen control basis enumerated in Table 1 may be made by appropriate amendments to the Tech nical Specifications of the license. For plants receiving construction permits on the basis of ECCS evaluations under the Interim Acceptance Criteria, the applicant would have the option of using either a 5 percent initial metal-water reaction or five times the maximum amount calculated in accordance with § 50.46, but no less than the amount that could result from reaction of all the metal in the outside surfaces of the cladding cylinders surrounding the fuel (excluding the cladding surrounding the ple num volume)

to a depth of 0.00023 inch

(0.0058 mm).

No assumption as to rate of evolution was as sociated with the magnitude of the assumed metal-water reaction originally given in Safety Guide 7. The metal-water reaction is of signifi cance when establishing system performance requirements for containment designs that employ time-dependent hydrogen control fea tures. The staff recognizes that it would be unrealistic to assume an instantaneous release of hydrogen from an assumed metal-water reac tion. For the design of a hydrogen control sys tem, therefore, it should be assumed that the initial metal-water reaction would occur over a short period of time early in the LOCA tran sient, i.e., near the end of the blowdown and core refill phases of the LOCA transient. Any hydrogen thus evolved would mix with steam and would be rapidly distributed throughout the containment compartments enclosing the reactor primary coolant system by steam flow ing from the postulated pipe break. These com partments include the "drywell" in typical boil ing water reactor containments, the "lower volume" of ice condenser containments, and the full volume of "dry" containments. The dura tion of the blowdown and refill phase is gen erally several minutes. Therefore, the assump tion of a two-minute evolution time, which represents the period of time during which the maximum heatup occurs, with a constant reac tion rate is appropriately conservative for the design of hydrogen control systems, even with the additional assumption that the resulting hydrogen is uniformly distributed in the con tainment compartment enclosing the primary coolant system. The effects of partial pressure of steam within the subcompartments and con tainment should be considered in the evaluation of the mixture composition.

The rate of production of gases from radioly sis of coolant solutions depends on (1)

the amount and quality of radiation energy ab sorbed in the specific coolant solutions used and (2) the net yield of gases generated from the solutions due to the absorbed radiation energy. Factors such as coolant flow rates and turbulence, chemical additives in the coolant, impurities, and coolant temperature can all exert an influence on the gas yields from radi olysis.

The hydrogen production rate from corrosion of materials within the containment, e.g., aluminum depends on the corrosion rate, which in turn depends on such factors as the containment coolant chemistry, the coolant pH, I

the metal and coolant temperatures, and the surface area exposed to attack by the coolant.

Accurate values of these parameters are difficult to establish with certainty for the con ditions expected to prevail following a LOCA.

Table 1 defines conservative values and as sumptions that may be used to evaluate the production of combustible gases following a LOCA.

If these assumptions are used to calculate the concentration of hydrogen (and oxygen) within the containment structures of reactor plants following a LOCA, the hydrogen concentration is calculated to reach the flammable limit within periods of less than a day after the accident for the smallest containments and up to more than a month for the largest ones. The hydro gen concentration could be maintained below its lower flammable limit by purging the contain ment atmosphere to the environs at a controlled rate after the LOCA; however, radioactive materials in the containment would also be released. Therefore, purging should not be the primary means for controlling combustible gases following a LOCA.

It is advisable, however, that the capability for controlled purging be provided to aid in containment at mosphere cleanup.

The Bureau of Mines has conducted experi ments at its facilities with initial hydrogen volume concentrations on the order of 4 to 12 volume percent. On the basis of these experi ments and a review of other reports, the NRC

staff concludes that a lower flammability limit of

4 volume percent hydrogen in air or steam-air atmospheres is well established and is ade quately conservative. For initial concentrations of hydrogen greater than about 6 volume percent, it is possible in the presence of suffi cient ignition sources that the total accumu lated hydrogen could burn in the containment.

For hydrogen concentrations 'in the range of 4 to 6 volume percent, partial burning of the hydrogen above 4 volume percent may occur.

However, in this range of 4 to 6 percent, the rate of flame propagation is less than the rate of rise of the flammable mixture. Therefore, the flame can propagate upward, but not horizontally or downward. In this case, only a fraction of hydrogen will burn in the contain ment and complete combustion will not occur until the hydrogen concentration is increased above 6 volume percent. The staff believes that a limit of 6 volume percent would not result in effects that would be adverse to containment systems. Applicants or licensees proposing a design limit in the range of 4 to 6 volume

1.7-3

percent hydrogen should demonstrate through supporting analyses and experimental data that containment features and safety equipment required to operate after a LOCA would not be made inoperative by the partial burning of the hydrogen.

In small containments, the amount of metal water reaction postulated in Table 1 may result in hydrogen concentrations above acceptable limits. The evolution rate of hydrogen from the metal-water reaction would be greater than that from either radiolysis or corrosion. Since it is difficult for a hydrogen control system to pro cess large volumes of hydrogen very rapidly, an alternative approach is to operate some of the smaller containments with inert (oxygen deficient)

atmospheres.

This measure, the

"inerting" of a containment, provides sufficient time for combustible gas control systems to become effective following a LOCA before a flammable mixture is reached in the contain ment. Hydrogen recombiners can process the containment atmosphere at a limited rate of 100

150

scfm per recombiner.

Therefore, an inordinately large number of recombiners would be required to control the hydrogen concentra tion that is postulated to be generated in the first

2 minutes of the LOCA.

There are presently no other methods of combustible gas control except for purge systems that release radioactive materials.

For all containments, it is advisable to pro vide means by which combustible gases result ing from the postulated metal-water reaction, radiolysis, and corrosion following a LOCA can be mixed, sampled, and controlled without re leasing radioactive materials to the environ ment.

Since any system for combustible gas control is designated for the protection of the public in the event of an accident, the system should meet the design and construction standards of engineered safety features. Care should be taken in its design to ensure that the system itself does not introduce safety problems that may affect containment integrity. For example, if a flame recombiner is used, propagation of flame into the containment should be pre vented.

In most reactor plants, the hydrogen control system would not be required to be operated for 7 days or more following a postulated lte sign basis LOCA.

Thus it is reasonable that hydrogen control systems need not necessarily be installed at each reactor.

Provision for either onsite or off site storage or a shared arrangement between licensees of plants in reasonably close proximity to each other may be developed.

An example of an acceptable arrangement would be to provide at least one hydrogen control system per site with the provision that a redundant unit would be avail able from a nearby site.

C.

REGULATORY

POSITION

1. Each boiling or pressurized light-water nuclear power reactor fueled with uranium oxide pellets within cylindrical zircaloy clad ding should have the capability to (a) measure the hydrogen concentration in the containment, (b) mix the atmosphere in the containment, and (c)

control combustible gas concentrations without relying on purging and/or repressuri zation of the containment atmosphere followingI

a LOCA.

2. The continuous presence of redundant combustible gas control equipment at the site may not be necessary provided it is available on an appropriate time scale. However, appro priate design and procedural provisions should be made for its use. These provisions should include consideration of shielding requirements to permit (a)

access to the area where the mobile combustible gas control system will be coupled up and (b) the coupling operation to be executed. In addition, centralized storage facilities that would serve multiple sites may be used, provided these facilities include provi sions 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, re dundancy, energy source, and instrumentation requirements for an engineered safety feature.

In addition, the system itself should not intro duce safety problems that may affect contain ment integrity. The combustible gas control system should -be designated Seismic Catego ry I (see Regulatory Guide 1.29, "Seismic De sign Classification"), and the Group B quality standards of Regulatory Guide 1.26, "Quality Group Classifications and Standards for Water-,

Steam-, and Radioactive-Waste-Containing Com ponents of Nuclear Power Plants," should be applied.

4. All water-cooled power reactors should also have the installed capability for a con trolled purge of the containment atmosphere to aid in cleanup. The purge or ventilation system may be a separate system or part of an existing system. It need not be redundant or be desig nated Seismic Category I

(see Regulatory Guide 1.29), except insofar as portions of the system constitute part of the primary contain ment boundary or contain filters.

5. The parameter vplues listed in Table 1 should be used in (a) calculating hy4rogen and oxygen gas concentrations in containments and (b) evaluating designs provided to control and to purge combustible gases evolved in the course of loss-of-coolant accidents. These val-,

ues may be changed on the basis of additional experimental evidence and analyses.

6. Materials within the containment that would yield hydrogen gas due to corrosion from

1.7-4

the emergency cooling or containment spray solutions should be identified, and their use should be limited as much as practical.

D.

IMPLEMENTATION

The applicability of the standards for com bustible gas control systems to existing and future facilities is set forth in

§ 50.44.

Therefore, except in those cases in which the applicant or licensee proposes an acceptable alternative method of complying with these standards, the method described herein will be applied by the NRC staff in accordance with

§ 50.44. Where this may involve addition, elim ination, or modification of structures, systems, or components of the facility after the con struction permit, manufacturing license, or de sign approval has been issued, backfitting decisions will be determined by the staff on a case-by-case basis.

1.7-5

TABLE 1 ACCEPTABLE ASSUMPTIONS

FOR EVALUATING

THE PRODUCTION OF COMBUSTIBLE

GASES

FOLLOWING

A LOSS-OF-COOLANT

ACCIDENT (LOCA)

Parameter Fraction of fission product radiation energy absorbed by the coolant Hydrogen yield rate G(H 2 )*

Oxygen yield rate G(O2)*

Extent and evolution time of initial core metal-water reaction hydrogen production from the cladding surrounding the fuel Aluminum corrosion rate for aluminum exposed to alkaline solutions Fission product distribution model Hydrogen concentration limit Oxygen concentration limit Acceptable Value a.

Beta

1.

Betas from fission products in the fuel rods:

0

2.

Betas from fission products intimately mixed with coolant:

1.0

b.

Gamma

1.

Gammas from fission products in the fuel rods, coolant in core region:

0.1**

2.

Gammas from fission products intimately mixed with coolant, all coolant:

1.0

0.5 molecule/100 eV

0.25 molecule/100 eV

Hydrogen production is 5 times the extent of the maximum calculated reaction under

10 CFR Part 50, § 50.46, or that amount that would be evolved from a core-wide average depth of reaction into the original cladding of 0.00023 inch (0.0058 mm),

whichever is greater, in 2 minutes.

200 mils/yr.

(This value should be adjusted upward for higher temperatures early in the accident sequence.)

509o of the halogens and 1% of the solids present in the core are intimately mixed with the coolant water.

All noble gases are released to the containment.

All other fission products remain in fuel rods.

4 v/o***

5 v/o.

(This limit should not be exceeded if more than 6 v/o hydrogen is present.)

For water, borated water, and borated alkaline solutions; for other solutions, data should be presented.

This fraction is thought to be conservative; further analysis may show that it should be revised.

The 4 v/o hydrogen concentration limit should not be exceeded if burning is 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/o excess hydrogen would burn in the containment:

(if more than 5 v/o oxygen is present).

The effects of the resultant energy and burning should not create conditions exceeding the design conditions of either the containment or the safety equipment necessary to mitigate the consequences of a LOCA.

Applicants and licensees should demonstrate such capability by suitable analyses and qualification test results..

,

1.7-6