ML17202V098

Responds to 890717 Request for NRC Staff Position on 10CFR50.44 Hydrogen Mitigation Requirements.Use of ACAD Sys Could Pose Threat to Plant for Many Possible Beyond Design Basis Events.Sys Should Be Rendered Inoperable

ML17202V098
Person / Time
Site:	Oyster Creek
Issue date:	11/06/1990
From:	Varga S Office of Nuclear Reactor Regulation
To:	Long R GENERAL PUBLIC UTILITIES CORP.
Shared Package
ML17202V099	List: ML17202V098 ML17202V100
References
GL-89-09, GL-89-9, NUDOCS 9011090432
Download: ML17202V098 (13)

Text

... ':

.e UNli'EOSTATt::S e

NUCLEAR REGULATORY COMMISSION WASHINGTON. D. C. 20555 November.6, 1999-ENCLOSURE 1 Docket No. 50-219 Dr. Robert L. Long Director, Corporate Services/

. Di rector, TMI-2 Planning and Nuclear Safety

• GPU Nuclear Corporation One Upper Pond Road Parsippany, New Jersey ()70S4

SUBJECT:

CLARIFICATION OF NRC STAFF POSITION ON HYDROGEN MITIGATION

'REQUIREMENTS-:.10 CFR so.44 - OYSTER.CREEK,NUCLEAR* GENERATING

• STATION This letter is in res'po'rlse to your July 17. 198.9 letter and your request

~ '.

followf ng the._June 27,_ 1989 meeting with the NRC staff to Mr. Ashok Thadanf,..

Direct.or of* the Divisfon of Systeru Technology, for a staff statement on compliance.by BWR Mark I plants with the hydrogen 11itigation requirements fn.*

• 10 CFR 50.44.'* The staff'*s*'position* on compliance by BWR Mark I plants 1s. *:.

'discussed in detail in.Enclosure 1~

11 NRC Staff Position on BWR Mark I Compl 1anc' with. 10 CFR' SO. 44:

11 The *staff*' s-position with respect to Oyster Creek'.s compliance with 10 CF~. so:44 Js as.follows:..

~;

4 I.

Oyster Creek 11ust* have capabi Hty to meaS.Ul"e hydroge"n concentration.

• in the.contahwent, (i.e., drywell and*wetwell) as**required by Section S0.44(b)(l).

• 2~..,.. Oyster *creek must be *able to insure a mixed-atmosphere in**

• con~ainment,.as required by Section 50.44(b)(2).

3.

• Oyster Creek:,rnust have an initially-inerted containment, as required by Section _S0.44(c)(3). * *

4.

Oyster Creek must be able to control co111bustf.ble gas concentrations in the containment. following a postulated 1,.0CA, as required by *

• . s*ections 50.44(b)(3) and S0.44(g). *.

(a) The abOve sections. of 10 CFR 50.44 do not directly require*

• • • a purging/repressurizati.on *system.

However, Se.ct ion

• so. 44(b)(3) d6es cal 1 for the control of combustible concentratio.ns following a LOCA and Section S0.44(g) sped fies the added requirements a purging/repressurization

• system -must. meet if the supportf ng *analysis shows that such a system iS needed.

Therefore, if the supporting analysis*

for Oyster Creek shows such a system f s needed, the system.

should satisfy the requfrements identified 1n S0.44(g).

.1.::

Dr. ~obert November 6, 1990 (b) Section 50.44(a) identifies those sources to be evaluated with respect to hydrogen/oxygen generation following a LOCA.

Section 50.44(d)(l) further defines the amount of hydrogen generated by metal-water reaction to be considered for the OBA LOCA.

To this extent, the rule addr.esses the hydrogen/oxygen source terms.

However, no specific models are identified within the rule for either radiolytic ~ecomposition or metal corrosion.

Regulatory (juide (RG) 1.7 has been used for this purpose.

Alternative radiolytic models, as described in GE report NED0-22155 are not acceptable for calculating the amounts of hydrogen/oxygen generated to show compliance with Sections 50.44(b)(3) and S0.44(g).

The basis for this position is provided as a Safety Evaluation in Enclosure 2.

NED0-22155 was used by the staff ONLY for determining whether additional hydrogen control capability in the form of recombiners had to be provided in accordance with Section 50.44(c)(ii). *

5.

At the pre.sent time, the staff ts* unable to determine if Oyster Creek*

must rely on a purge/repressurization system as the primary means for combustible gas control fol~owing a LOCA as discussed in Sections

• 50.44(b)(3) and 50.44(g). Without an Oyster Creek plant unique analysis using RG 1.7; models and accompanying assumptions, such a

. determination* is impossible.. However, the staff believes it 1s highly probable that a purge/repressurizatio.n system will be shown to be needed.

(a)..If Oyster Creek relies upori a purge/repressurization system as the primary*rri*eans for.comb4stible gas control following a LOCA,.

then the p 1 ant must hav*e either 'an internal recombi ner or the capability to install an external recombiher following the start of an accident, as. required by Section 50.44(c)(3){ii).

(i.)

NED0-22155 may be used for the purposes of determining whether recombiner capability must be provided.

Within.this 1 imited context, purg~/repressurization.may *be shown as not the primary means of hydrogen control.

However, the consequences of use of the purge/repressurization system considering RG 1.7 assumptions must be considered when evaluating.potential oxygen sources *

(ii)- The *following prerequisites must be met if elimination of the need for recombiner capability 1.s to be considered as

• coritained in Section 50.44(c)(3)(fi). These prerequisities were first set forth in Generic letter (GL) 84-09 (May 8,,

1984).

1

. :1.~

~-*or. Robert November 6, 1990 (A)

Oyster Creek technical specifications and limiting c.onditions for operation (LCOs) must require that the containment atmosphere be less than four percent oxygen when the containment is. required to be inerted; (B)

Oyster Cr.eek uses only nitrogen or recycled c*ontainment

'ir in all pneumatic control systems within containment; and (C) There are NO potential sources of oxygen in containment

• other than those* resulting from reactor coolant radfolysf s. *Reliance on a system which uses a*ir to repressur1ze the contaf rvnent to comply with other.

requirements of 10 CFR 50.44, such -as an ACAD system, is

• f nconsf stent" with ~his prerequisite..

If Oyster Creek uses *an ACAD system*, ft does not fulfill*

the thir.d prerequisf te of GL 84-09, and recombf ner cap-

.abf Hty must* be provided for fn accordance with 10 CFR 50.~4(c)(3}(11). However, use of an NCAD system, which meets the reqof rements of GDC' 41, 42* and 43, wf 11 not be

.fnconsistent wfth the.third P,rer~qufsfte of GL 84-09. *

(b}

If recombfners are. required at Oyster Creek such recombfners

_-must meet the. requirements* of Sectfon**so.~4(d). lU, Section 50.44{:c).(3)(ff).

. {~) If external recombiners are relied-upon at*Oyster Creek, the penetrations used for the externa 1 recombf riers must meet*.the

• crf terf a_. *1 n Sectf.on 50.44(cJ (3)( ff ){A) and (B).

~' *.

6.

Oyster Creek. mus.t have hf gh pofnt, vents for. the reactor coo 1 ant' system, the reactor vessel head, and any other-systems required to.

• maintain adequate ~tiolfng.ff the acc~mulatfon of noncondensfble gases would cause loss ~f these. systems,. as _required by 50.44(c)(3)(iH). **.

. In sun:mary please respond to this letter whether you agree,wfth the staff's*

posf Uon on comp 1 fance by Oyster Creek with 10 CFR 50.44. If you disagree wfth

• the staff'.s positfo'n on what fs required by Oyster. Creek for compliance* wfth 10

• CFR 50.44, identify wfth specificity the areas_.'of.the staff position wi.th which you disagree, and.-the basis for your disagreement, including all necessary*

• technical.and legal references. Jor those areas: where you agree with the

• staff's position on the* requirements. of 10 CFR 50.44 *. d~monstrate 'how _Oyster Creek is in compliance with those requirements. Foreach requirement of 10 CFR 50.44 where Oyster-Creek fs not. in *compliance, provide a list of corrective actions necessary to achieve compliance and a schedule for implementation of those actions. Such corrective actions should be fully implemented within 270

• • j

,.. Dr. *Robert November 6, 1990 days of the date o.f this letter. Your should indicate*whether you intend to meet this schedule. If you do not intend to meet this schedule, provide an alternative schedule together with the basis for not fully implementing corrective actions within 270 days.

Your response shall be provided within 60 days of the date of this letter.

The requirements contained in this letter affect fewer than 10 respondents and, therefore, is not subject to Office of Management and Budget review under P.L~ 96-511.

,N'

Enclosures:

As stated cc w/enclosures:

See next page i

c Division of Reactor Pro c Office o.f Nuc*1 ear Reactor

.e

• Dr. Robert L. Long GPU Nuclear Corporation cc:

Ernest L. Blake, Jr.

Shaw, Pf ttman, Potts and Trowbridge

• 2300 N Street, NW-Washingtoni DC 20037

• J.B. Libennan, Esquire Bishop, Libennan, Cook, et al.

• 1155 Avenue of the Americas New York, New York 10036 Region Administrator, Regf on I U.S. Nu.clear Regulatory Corrmission 475 Allendale Road King 6f Prussia, P~nnsylva~fa 19406 BWR Licensing Manager GPU Nuclear Corporation 1 Upper Pond. Road

• Pars fppany, New Jersey 07054 Mayor Lacey Township ***. **.

• 818 West Lacey Roa~

Forked River,*New Jersey. 08731

. Licensing Manager Oyster C.reek Nuclear Generating Station Ma f 1 Stop:

• S-i.te Emergency Bldg.

P. 0.

• Box 388 Forked River, N~w Je~sey 087~1 Oyster Creek Nuclear Generating Station Resident Inspector

• c/o U.S. NRC Post Office Box 445 Forked *River, New Jersey" 08731 Corrm1ss i oner New Jersey*Depart"1ent of Energy 101 Conmerce Street Newark, New Jersey 07102 Ken Tosch, Chief New Jersey Department of Environmental Protection CN 415

. Trenton, New Jersey 08625 Mr. E. E. Fitzpatrick Vice President and Director Oyster Creek Nuclear*Generat1ng Station Post Office-Box* 388 Fqrked River,_ New *Jersey. 08731 j*

.* I NRC STAFF POSITION ON BWR MARK I COMPLIANCE WITH 10 CFR 50.44

. STANDARDS FOR COMBUSTIBLE GAS CONTROL SYSTEM IN LiqHT WATER COOLED POWER REACTORS ENCLOSURE 1 The staff has been conc~rned with the potential generation and control of hydrogen within the contai nmen.t following a LOCA si nee the first power plant was constructed.

However,* it was not until 1971, that the staff.documented its acceptance criteria in this regard.

On March 10,*1971, Safety Guide 7, "Control of Combustible Gas Concentrat.ions in Contain~nt Following a Loss-of-*

Coolan~-Accident" was issued.

One of the criteria stated in the guide was the amount of zirconium metal-water reaction that was to.be considered as part of the hydrogen production analysis.

The staff selected five percent by weight of the zirconium within the reactor core as the upper limit. It was felt at the time that this value represented a reasonab.le limit of core degradation while maintaining a coolable geometry.. The staff believed that much beyond five percent core damage, the subsequent core reloc;ation could lead to a complete core melt down.

The guide was silent *on how the designer should cope with the hydrogen that

.resulted from the metal-water reaction.. However, the technology, in a manner of speaking, eliminated most of the conventional control methods..

It was known that the r:-eaction was*a strong function 9.f the zirconium temperature. *This coupled with an understanding of the temperature response of the reactor core during an event showed that the hydrogen generated from the metal-reaction

. process would:occur within a matter of minutes.

As* a result, there were no systems that could respond to such high rates.of hydrogen production. This left only.two viable options.

One ~was to provide sufficient. c:ontainment volume.

so as*.not to yield a flammable mixture when the metal-water produced hydroge_n.

• entered the co_ntajrime.nt.. The other was to inert the containment during normal *

• operation.

By' Hmitirig the amount of oxyge~.within*'the containment, it

• transferred* the flammable control parameter from. hydrogen to oxygen.

This

• change e.nabled the: containment to accommodate any amount of hydrogen without affecting the.flammabl~ limit of the containment atmosphere..

. F6~' the case ~f ~he Mark I desi~n, the ~o~i~inment vol~me proved to be too*

• sma.11 to. preclude a* flammable mi x~ure upon the i ntl'.'oduct ion of the hydrogen prod~ce~.by the metal-water:r~action. Therefore, the use of ~he criteria set forth in Safety Guide 7 resulted in the iner~ing of.all. Mark I power plants.*

• except Vermont Yankee.

The.owners* of Vermont Yankee.obtained a legal ruling

_allowtng ihe*plant to operate without ine~tfng.

There w~re no* changes in the staff.criteria until 1976.

On October 21, 1976,

• ~.... the HR~ published its intent to create a new regulation cbncerning combustible.

gas control for.* nuclear power plants. This was done by noticing a proposed*

.. rule 10 CFR 50:44, "Standards for Combustible *Gas Control Systems", for public -

comment (41 FR 46467);-

The proposed rule maintained the criteria established*.

• in Safety Guide 7. _ In particular, the metal-water reaction was maintained at the five percent specified in the guide.

However, the.accompanying other sources of hy~rogen/oxygen production such as radiolysis and corrosion were not transferred into the rule.

They remained within the guide since it was felt that. putting such detail into the rule would overly complicate the rule.

The staff, however, continued to use the production models specified in the guide.

. as the basis for reviewing the adequacy of the hydrogen analyses.used to support co~pliance of the rule.

- The industry responded during the comment period with only minor comments**

except for one area.

BWR owners' argued that the use of five percent by weight

~as a poor.method io establish -the amount.of metal-water reaction..

• They argued that this approach wotild penalize a desig~with a thicker clad while maintaining all other design paramete~s *the same.

The staff took this major criticis~ under review and coricluded that ~here was merit in the argument.

As a result,. the N.RC published a revised version

• of 10. CFR 50. 44 on November* 27, 1978 ( 43 FR 50162) -which es tab 1 i shed the amount of metal-water reaction based on *an average depth-of-fuel-c~addfog involve*ment rather than a percentage of cladding mat_erial.

The staff chose a ~epth of 0.00023 inches of involvement as the new ~etal-water criterion; This value was*

selected to yield the equivalent of five percent by weight for the reactor design containing the thinnest clad.

For all B~R desigris,,this change rep~esented

~relaxation in the *previous staff criteria.

Beca~se *of-the thick BWR clad design, the new rule reduced the total *amount of metal-water reaction to about one half o.f the amount calculated using the five percent by weight criteria.for a.typical Mark I design.

As a result of ttiis -relaxation, it was now possi~le for some* Mar~. I plants

• to show that the hydrogen produced by metal-water reaction would not* yield

• an almost immediate fl~mmable mixture.. Dependent upon the.other sources of hydrogen production.. coupled with.the. containment volume, it was now possible to show 'tor some plants that the ~ombustible control system _(i.e~ Contajnment Atmospher1c Dilution (CAD) or recombiner) was sufficient without the 'need for inerting.

In fact, Hatch Unit 2 o~ June 13, 1978, became the first Mark I.*

containment design to be licens~c:f under.the new rule.* The_ liqmsee was able.

to show through analysis* that "the capacity of its installed hydrogen

• rec.ombiners, as the only means of coinbustib.le*gas control, was sufficient to demonstrate compliance with 50.44. It should be. noted that the supporting analyses used -the hydrogen/oxygen squrce terms.and models described in Safety Guide 7..

Following issuance'. of the Hatch *2 licens.e, appr_oxiinately half of ;ihe. BWR _.

lice~sees with Mark I contaihm~nts had s~bmitted or were pl~nning to submit requests to el'iminate t.he need for operating the* plal)t inerted.

The_ bases'

.for iuch ~ re~ues~were plant specific _analyses using.the criteria and model$

specified in the-rule*and Safety Guide 7 *. Before the staff could act on.these requests, the Three Mi le Island, Unit 2 (TMI-2) accident occur,red.on March 29,

• 1979. -The staff stopped all review on. these r~quests until a thorough evalua-tion of the accident could be made.

Up until this point in time, the staff and the majority of the industry believed that the Design Basis Accident (OBA) for which 10 CFR 50.44 was based represented the worst accident that needed design consideration.

Any event beyond the OBA would lead to complete core meltdowns, which no design could accommodate.

The.TM! accident proved this thinking to be wrong.

First of all, the accident went well beyond the design basis of 50.44.

The ":best estimate" metal-water reaction at TMI was about 56 percent rather than the approximate 3 percent written into the rule.

This amount of hydrogen was easily handled by the large dry containment..

However, had the* same amount occurred within a Mark I contain-ment, the hydrogen concentrations would have been well into the detQnable range.

Secondly, and most important, was the* observation that a r.eactor core could be severely damaged and still retain a coolable geometry.

Therefore, there was merit to consider how various design~ would respond to. similar accidents!

Thus, based on the evidence obtained from this.accic;tent, the staff revised 50.44 effective January 4, 1982 (46 FR 58484).. For the most part, there were no deletions from the rule~

Rather, there were new items added to reflect the

need to consider TMI-like accidents.

In other wbrds, tho~e sections that existed prior to *this latest revision are still based on.-DBA consider~tions.

However, for syste_ms required under these sections, their use in TMI-1 ike accidents would obviously.need to be considered f.rom a safety viewpoint.

~o address TMI-lik~-accidents the rule r~quired Mark III designs and i~e condenser containments to provide systems and* components necessary to _establjsh and maintain safe shutdown and to maintain.. containment integrity, assumi.ng hydrogen equivalent to a,75 percent.metal-water reaction.. For Mark I and II containment inerting and hydrogen recombiner capability were sufficient to accommodate hydrogen.from a"75 pe*rcent metal-water reaction withou*t resulting in a bur~able mixt~re.

For those Mark I or Mark II containment designs that ~ely on a purge/

repressurization system'fo~:control of combustibl~gas within trie OBA envelope; they are also required to provide hydrogen recombiner capabil;ty.

The *need for *such a system was again based.upon the evaluation of the TMI accident.

Extreme public reaction was encountered at the mention of venting at TMI even 'though the radiation levels were e_xtremely low by any. st.andard.

As a r'esult, it was believed prudent for those designs whjch rely on venting as the primary means of combustible gas control to provide an alternative wh~ch was not based on venting; notably the recombiner.

The addition ~f recombi ne.r capability provided an ad.di t iona l option~ For OBA and beyond,.

the recombiner could be used in lieu.of venting.

The recombiner would also be used in long term' recovery.

Recombination would eliminate *the need to release large amounts of hydrogen into the plant environment.

The new* *

• r,equirement did ncit require the recombiner on-site, but the licensee must show that a penetration and the necessary power and instrumentation channels are available.

Additionally, the licensee must show that the recombiner could be

• hooked~up and-made *operational in a timely manner.*

'~. The final requfrement was to install a high point reactor vent(s).

The intent was to preclude the potential of a steam bubble which would prevent natural

. circulation as observed.during the TMI accident.

1 Each* one of these three new requirements were introduced to better cope with accidents beyond the OBA.. Previously accepted licensee's _FSAR analyses had shown that these added requirements would not be needed for the OBA.

Therefore, it is clear that the staff's intention was to eliminate the weaknesses found in the.Mark I and II desfgns"to cope with TMI type accidents.

T~~oughout the evolution of staff requirements, as described above,. there were no specific reporting requ.irements written into *the original rule or any.of the revisions.

There were, however, imJ)lementatfon dates provided within t~e r4le.

As a result, the responsibility of compliance was given to the license~ ~ithout the need to provide the supporting justification to the staff.

Therefore.~ the staff. has* not reviewed the pl.ant specific analyses supporting the rule beyond the licensing stage, except for those isolated cases where the licensees have requested design or TS changes affecting these systems.

.. F~r those limited cases, the.staff has used the. models and equations provided in Safety Guide 7 to confirm the licensee's analyses :which dempnst.rate compliance with th' provisio_ns o.f 10 CFR S0.44(e), (f), and (g).

In addition, the licensees ha~e also used Safety Guid_e 7 assumptions.

The staff, using these assump~iol')s, have* cone 11,Jded in every case that the licensee needed an active combustible* gas con_trol 'system to prevent flanvnable conditions.

The OBA metal-water reaction generated hydrogen was insufficient; in about half the cases to achieve *

.flammable conditions for a non-inerted case.

However, when added to*the

• oxygen/hydrogen~produced by events such as~radioJysis arid corrosion, the

combination is more than sufficient* to yield a flammable condition witMn

. 30. days whether. the containment is i nit i a_l ly i nerted or not.

- 'With the iss'uance of the latest rule ch~nge,' the BWR Owners Group (BWROG) which

. *represents plants that use a*purge* repressuriz~tion system took exception to the requirement for recombiner capability in 10 CFR 50.44(c)(3)(ii).

A -.

BWROG wa*s* formed*to provide the staff with the necessary generic justification to support the indi~idual *1icen~ee's position.

During these discussion~, the

. NRC has granted relief from the implementation date stated within the rule.

A brief summary of the BWROG. position is necessary to foll.ow the staff actions..

It *believed that the staff should. re~_ognize the benefits gained by the.foerted containment when judging the.ber)efits*of the recombiner capability requ-irement..

This is particularly' important since the.B\\rt'ROG has said that the individual plant costs were estimated, to be well over one million dollars to provide*

recombine~ capability.. The demonst.ratfon that recombiner capability *was not warranted, in light of the addfti~nal cost,. relied on th~ acceptance of radiolysts.modeis ihat are signifttantly less conservative th*n those recommende.d by Safety Guide 7,.

These mode.'1 s were documented 1n NED0-22155, dat.ed June :1982..

-s-This was the first time anyone from industry proposed the use of anything other than Safety Guide 7 assumptions for the calculat;on of radiolysis generation

. rates..,. But, it was done in a very liini.ted fashion.

The models were initially presented only to support the exemption from the recombiner capability*requirement.

The differences between the.models in Safety Guide 7 and NED0-22155 result in hydrogen/oxygen generation rates* due to radiolysis which are several orders of

. niagni tudes *1ower.

Using the.models presented in NE00-22155, the BWROG was able to show for a typical Mark I design that the initial inerted containment would be al_l *that

• -is needed for the first 30 days of ~n acci.denf.

This was key to*the argument since it showed that neither a recombiner nor venting would be needed for at 1 east one month fo 11 owing an accident. *

• Jhe staff *i:Jid look at thes'e new models in *view of the request.. From the*

staff 1 s review, the req'uest was in two parts.

First of all, w~r~ the models for hydrogen/oxygen generation rates *reasonable for the purpose of evaluating.

the* need for providing a recombiner or the capability to.install an e~ternal

• recombiner following the start *of an accident as required.by. 50.44(c)(3)(ii)?

In this context, the staff believed that the model's could be a more "best estimate" approach than those required for the sup.porting analysis of *50.44(g). *.

If the staff accepted these. new.models for the purpose of dete~ining reco.mbiner

• capability, the analysis. using these revised models wou.ld determine if the-* plant relied upon a purge/repressurization system.

This was the second part of the request.

N~te that th~*staff would ~llow two analyses, Th~ analysis used to show compliance with 50.44(g) would still.utilize Sa.fety Guide 7 a.~~umptions while the analyses supportfng t.he need to have recombiner capabil hy could use

• the more reali.stic assumptions of NED0-22155, if found accepta~le:

this was the ~cop~ of the staff's review.

Within this li~ited* ~ontext, the staff did pro\\ii de qua 1 if i ed endorsement of the mode 1 s and therefore* the overa 11 conclusions of the BWROG position.

The staff indicated that the models were

• • appropriate for the major segment of. accidents under consideration.

For degraded core accidents where sfgnificant amounts of metal-water*re~:ction hydrogen are produced, the hydrogen acts as an inerting component with respect

.to oxygen.

As a result, either set of modelS would show no need for an active combustible control.system.

However, *the staff found that there was a small number of accidents, both within the OBA envelope and slightly beyond, where the assumpt1ons*used in NED0.-22155 were_ at least questionable.

The staff weighed the be.nefits to. be gained for thfs sector of*accidents to the costs.of providing recombiner.

capability.

On balance, the staff concluded that the costs outweighed the*

~enefits for this limited situation.

To reflect t.h1s position, the staff i~sued Generic Letter 84-09.

However, at no time did the staff consider.

changing the 'acceptance criteria for the OBA.

Safety Guide 7. requirements.

have 'been and *continue to be the basis of acceptance. ror the OBA events..

In tact, as a result of_this limited review, the staf~believed that it had.

confirmed the appropri~teness of the Safety Guide 7 models.

~*

• .. When granting relief for the hydrogen recombiner capability in GL 84-09 the staff also recognized the increased importance in controlling possible oxygen suppJies other than those recognized in Safet~ Guide 7 s~nce the recombiners would not be available.

As a result, the thrust of the generic letter was to accept the BWROG position of not requiring recombiner capability, but to require added assurance that there would be no significant oxygen sources available ~ithin the individual plants that could cause the atmosphere to become deinerted during the course of the accident.

For those limited supplies of. oxygen such as instrument air, the licensee was.expected to show that the amount Of oxygen that could be expected to be released into the containment would riot ca.use the containment to become deinerted within the first 30 days after an accident.

• Since issuance of the ge~eric letter, several licensees have plan~s for which resolution is needed. *Some of these plants have-what is referred to as an.

ACAD system to satisfy 50.44(e), (f), and (g).

• This system uses standard air to repressurize the containment and thereby reduce the hydrogen concentration.

All of these plants were reviewed at the licensing stage prior to the THI

.accident'.

At-that time, the staff.found this type of *combustible gas control system acceptable for meeting the OBA event.

However, with the results from the TMI accident, the staff has focused on the, role of the ACAO system for the entire spectrum of accidents including those that are referred to as degraded core events.

• rh*e staff consideration included the Emergency Procedure Guidelfnes ~(EPGs) and the evolution of these guidelines into the plant specific Emergency Operating Procedures (EOPs)~ These plants have chosen not tp recogniie' the ACAD system

• as a*possible system for use in ~he mitigation of an accident.

However, in many cases the system is functional and could.be operated from the :control room.. It is the potential of misuse that most concerns the staff~ *11 operated

• durjng -an accident which is beyond the OBA,* the ACAD would actually deinert the containm~nt.

T,he neglect of a supposedly safety sys.tem equally concerns t.h.e staff. _If

.one were to assume that the operators would not activate the ACAO during the accident,* then there is total reliance on the 'initial inerting process to maintain an inerted containment during the entire 30 day period.

However, based on past *reviews it appears to the staff that using Safety Guide. 7 assumptions would show a need for an a*ctive combustible gas control syst_em.

Under *these* restrictions, the** staff believes that using Safety Guide 7 assumptions, an active combustible gas control system will.be required.

• Therefore, the staff has requested these* analyses from each of the -licensees to confirm compJiarice with 10 CFR 50.44(e), (f), and (g).

If the analyses show that the *active ACAD system is needed witMn the 30 day period, the ACAD must be considered as an oxygen source with respect to meeting*

the requirements of Generic Letter 84-09.

Therefore, if the ACAD system is*

intended to be used, it would both represent an oxygen source contrary to the guideline~*of Generic Letter 84~09 and represent a threat in the dealing of accidents beyond the OBA.

~.

• . It was in this atmosphere, that the.staff has discussed the re so 1 ut ion of

.Generic Letter 84-09 with the five.licensees.

The licensees wish to obtain relief from the. requirement to have recombiner capability.

However, the

.combustible gas control system which each affected plant relies on to meet the OBA requirements of the rule has* been shown to be a threat to safety for all

• but OBA events.

This is further aggravated by the fact that the staff has found the models contained in NED0-22155 which the licensees have generically*

used to support the position of not requiring an act1~e combustible gas control system unacceptable for OBA applications.

Details of the staff evaluation are provided in'.Enclosure 2 of the staff's letter.

The staff recognizing this apparent difficultly, understands the position of the licensee.

They have in good faith put into operationACAO systems.

At the time, these systems were more.than adequate to accommodate all credible events:.However, after TMI,.the staff and the industry have recognized the*

rieed to consider the *degraded core evenfi. It* is unfortunate that the use

• of these orce recognized safety sy'stems inust now be considered a safety threat.

But, the f~~t *remains that the. systems can no longer* be considered as An effective safety system for all acc*idents.

Theretore, ihe li~ensee mu~t.also recognize the potential threa~ of this "OBA

• safety system" for beyond OBAs.

.Otherwise, the 1 i censee must provide recombfner capabi 1 i tY s i nee the use* of the system represents an. oxygen source as defined *

~ithin GL 84-09 and it cannot be demonstrated* that the containment will remain

• i nerted for 30 ~ays..

-In an att~pt to resolve this iss~e~ the-staff has suggested th' possibHity of 'usi11g the normal 'riitr:ogen inerting*system as the 11 necessary 11 active combustible gas contro*l system and declare: the ACAO sy~tem non-operational.

However.; the

'.*staff needs to understand the design and availabiHty of the.nitrogen inerting

-.. *system.* To this* end, the st:att *issued a* request to each license*e to show how.. *:

'it.believed it* met.the OBA requi.rements of 50.44.

I~ is felt that the supporting.

analyses inust use Safety Guide.]_*assumptions since the models contained in NED0-22155 have not.been approved by' the staff for the ent*i re* spectrum of'

., *accidents. *.

The s~aff.is confident that the analyses will-show that an activ~ system is

.needed to show c*omp l.i ance with* 50. 44.

To this end, the staff al so requested design information of the normal nitrogen inerting system and an evaluation.

of how the* system meets GOC 41,.42, and 43.

• ThiS respons*e was important since the staff *has indi~ated that the system used to satisfy. the provisions of 10 CFR* 50.. 44(g) need only ~atisfy these GOCs.

This position was offered fn light of the fact *.that the licensees had previously provided an ACAO system to meet this same.requirement.

• Summary L

It is the staff's opinion that thf!! use of the ACAO system could p~se a threat to the plant for many of the possible beyond design basis events:

Since the operators w.ill have a difficult task in determining whether or

• not the event is within the OBA envel-0pe, it is the staff's opinion that

• the ACAO syste~ should be rendered inoperable.

.... ~

e* -

• 2..

The staff has evaluated the methodology for determining the generation rates of oxygen by radiolytic decomposition of water in the inerted containment as documented in GE Report NED0-22155 dated June 1982.

The staff finds this method unacceptable for analyses* used to demonstrate compliance.with 50.44(e), (f) and (g).

• The basis.for this denial 1s provideg in the related Safety Evaluation enclosed with the staff's litter.

3.

The licensee. should demonstrate how the plant compltes with the requirements of Sections 50.44(e), 50.44(f), and 50.44(g) for design basis"loss-of~cooJan.t accidents (LOCA~. The ljcensee should include analyses using Regulatory Guide* l. 7 "Control of Combustible Gas Concentration in Containment Following a LOCA,.assumptions and methodology -for rates of generations of oxygen

. and hydrogen described-in Standard Review Plar, (SRP) 6. 2. S* Appendix.*: A.

~-

4.

If an active combustible gas control i.s _shown. to be required based on the analyses, a description pf that ~ystem should be provided to show.

how it ~eets the General Design Criteria (GDC) 41~ 42, and 43 as required by 10 *CFR 50.44(g).

5.... Generic Letter. 84-()9 grants relief from the -hydrogen recombi ner capab11ity for TMI 1 i ke accidents,_ for those i nerted Mark I contai n111ents *that do not

• rely upon ~afety grade purge/repressurization systems.as the primary means of hydrogen control, wh.en all sources of oxygen except radiolysis* are

~.

. removed from the containment~ Accordingly, any H censee for Mark I BWR which has concluded that a recom!;>iner capability*is not required for it~

facility should address-the three criteria.for oxygen*souri:e requirements.

provided ln Generic ~e~ter 84-09.

It is.the stat.f's position that if a faci.lity has an *ACAO system operational, then the licens~e does not comply with the requirement~ of Generic Letter* 84-09~.

6.

The. licensee should develop procedures for *.post-accident *combustible gas..

control. compatible with the guidelines of Revisio*n 4 to the BWR Owners -

• Group Emergency_ Procedure g'uidelines_.

Ii i's 'the* sfaff's. inte.nt* to expedite closu.re of this long s.tand-ing issue.

This

• requ!'St for foformation concerning the normal inerting system is intended to al.low the 1,icensee the opportunity to show. how this system can meet the. rule '. -

using the st_aff guidance provided above.

.;.