ML20237A741

Forwards Conclusions & Recommendations Re pressure- Suppression Containments for Review & Comments

ML20237A741
Person / Time
Site:	Pilgrim
Issue date:	09/20/1972
From:	Hanauer S US ATOMIC ENERGY COMMISSION (AEC)
To:	Kruesi F, Oleary J, Rogers L US ATOMIC ENERGY COMMISSION (AEC)
References
CON-#487-4995 2.206, NUDOCS 8712150213
Download: ML20237A741 (8)

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• ressure-Suporess{on Centsinments

1. Conclusions and Rece-sendations Recent even' cs have highlighted the safety disadvantages of pressure-sup-pression containments. While they also have some safety advantages, on balance I believe the disadvantages are preponderant. I reco=me=d that the AIC adopc a pc41cf'oT discouraging ju gher'TsT~of pressure-suppression containment,s, and that such desig'ns not be acce* pled Er constructi6 ruer-mira filed af ter a date to be decided (say two years af ter the policy is adopted). .

2. Discussion A pressure-suppression containment system has so=e ceans of absorbing the heat of vaporization of the steam in the fluid released to the contain=ent volume. In all three r -

• 1s, the s' team is forced to bubble el., ugh a pool of water and is condensed. In t hge,s t in_gh e,u_s_e_ d e s ixs , th e s t e a s i s condensed by flowing it over ice cubes. The objective is to reduce the

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pressure in the containment through " suppressing" the partial pressure of the steac by condensing it. To be' effective, pressure suppression =ust take place concurrent with the flow of steam into the contain=ent, and its ef festiveness is therefore dependent en the rate at which steam is generated or released. If some unexpected event should result in steam generation or flev greater than the suppression capability, then the steam that is not condensed would add an increment of centairment pressure. Sinc e the objective of pressure suppression is to per=it use of a s= aller con-tain=ent, rated at lower pressure than would be required without suppres-sion, thea incomplete suppression:vould lead to overpressuriting a pressure-suppress u s containment so designed.

It may be noted that the Stone and Webster "subatmospheric" design has little effect on the initial containment pressure rise due to an accident, and is therefore not a " pressure-suppression containment" for the present discussion. In this design, chilled water sprays are used to reduce the containment pre ssure, and therefore the containment leakage, quickly af ter a postulated I.CCA.

Tne pressure' capability and volu=e are dasigned to take the full accident, without credit for condensation.

1.ike all containments, the pressure-suppression designs are required to include margins in capability. Experiments have been conducted by CE and Westinghouse to establish the! rate of steam generation that can be accoc=odated. The pressure-suppression pools, ice condenser, etc. , are then sized for the double-ended bieak stea.s flow, with margins for un-equal distribution of steam to the many modular units of which the cos-denser quate.

is composed. The rate and: distribution margins are probably ade-f Merr. djfficult to assess is the margin needed when' applying the expert- '

cental data to the reactor designi Recently we have reevaluated the 10-year-old CE test results, and decided on a more conservative interpre-tation than has been used all these years by CI (and accepted by us). We s

1 now believe that the for=er f_r.cerpretation was incorrect, using data frem tests not applicabtr. to accident conditions.

We are requiring an fudependent evaluation of the ice condenser design and its bases to make les's probable any comparable misinterpretation of this design.

, Since the pressure-suppression containments are s= aller than conventional l " dry" containments, the same amount of hydrogen, formed in a postulated

) accident, would constitute a higher volume or weight percentage of the I

containment a tmosphere. Therefore, such hydrogen generation tends to be a more serious problem in pressure-suppression containments. The sr.all CI designs (both the light-bulb-and-doughnut and the over-under configura-tions) have to be inerted becadse the hydrogen assumed (per safety Guide "

would inrsediately form an explosive mixture. The ct MmL 3 and the Ve,s_tf e house ice condenser designs _(they have equal volumes) require high-flow circulation and mixing systems to ensure even dilutio'n of the hydrogen to avoid fla=mable mixtures in one or = ore compartments (see following for an additional serious disadvan.;sge of this needed recirculation' and its valves)

By contrast, the dry containments only require reco=bination or purging starting weeks af ter the accident. e All pressure-suppression containments are divided into two (or more) cajor volumes, the steam fleving from one to the other through the condensing water or ice. Any steam that flows from one of these volumes to the other.

vichout being cendensed is a potential source of unsuppressed pressure.

Neither the strength nor the leakage rate of the divider (between the volumes) is tested in the currently approved progra=s for initial or peried-ic inservice testing. So=e effort is now underway to devise a leakage test, but none has so far been acce=plished.

Because of limited strength against collapse, the " receiving" volume ha's to be provided with vacuum relief. In all designs except CI Med 111, this

' function is performed by a group of valves. Such a valve stuck open is a large bypass of the condensation scheme; the amount of steam that thus i escapes condensation can overpressurize the contairunent. l 1 i Valves do not have a very good reliability record. Recently, five of the ,

vacuum relTef"v'inives for 't.he pressure ' suppression cor.t.ainment of Quad I Cities 2 were fq,und_ stuck _g,ar_tjy,, ogen. Moreover, these valves had been modified to include redundant " valve-closed" position indicators and test-8 ing devices, because of recenti Reg concerns. The redundant position in-dicators were found not to indicate correctly the particular partly open situation that obtained on the' five failed valves. We have o:17 recently begun to pay serious attentiori to these valves, so previous surveillance programs have not generally in'cluded them. The CE Mod 11*. design has an elegant water-les seal that ob'viates the need for vacuum relief valves.

3 ee The high-capacity atmosphere $ecirculstion systems provided for hydrogen mixing involve additional valves stich, if open at the wrong time, would I

constitute a serious staam by ass and thus a potential source of containment 9

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. - 3-over-pressurization. These valves are large, and must open quickly and reliably when recirculation is needed. In other engi= eared safety features, no single volve is relied on for such service, yet redundancy has not been provided even for single failures, open and closed, of these valves. This is a serious mission, since opening at the wrong time leads to over-pressur-ization, while failure to open when needed inhibits recirculation.

4 The smalle: size of the p;sssure-suppression contaissent, plus the require-cent for the primary system to be contained in one of the evo volumes, has led to overcrowding and limitation of access to reactor and pri=sry system components for surveillance 'and in-service testing. Separate shielding of cempenents has tended to subdivide into co=partnents the volume occupied by the primary system. (Some compartmentation of dry containments also eccurs.) A pipe break in one of these ce=part=ents creates a pressure differential; each compartment must be designed to withstand this pressure. {

A method of testir.g such designs has not been developed. I What are the safety advantages of Lpressure suppression, apart f:cm the cost saving. CI people talk about a deconta=ination factor of 30,000 from scrubbing of iodine out of the steam by the water. This is hard to swallow, but some decontaminationtundoubtedly occurs. One wonders why-CE doesn't do an enperiment to measure it, and get credit for it. The ice condenser decostasimation is measuraole but =ot sig=ificant. )

Recircula-ica of the contain=ent atmosphere through the ice has the potential <

for Eut rapidly reducing the centsin=ent pressure by cooli=g its at=esphere.  :

in the present design there's not e=cugh ice for that, so contair. ment sprays are furnished (in both volumes), just as in dry conta1 =ents. Re-circulation through the water in the CE designs seems not to have been '

tried, but may be necessary in Mod 111 for hydrogen control. .k's have no a alysis whether any significant cooling sill result.

t It is by no means clear that the pressure-suppression contai==ents are, over-all, significantly cheaper than dry contai = ants when all costs are included.

Infor=ation es this point vould be useful in evaluating costs and benefits, a:d should be obtained.

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a.c Wenclusions and Recommendations '

M.g; '6' Rotent avants have highlighted the safety disadvantages of pressure-sup-i@ .,

pression containments. While they also have some safety advantages, on l[ j l' balance I believe the disadvantages are preponderant. I recommend that i I d'% the AEC adopt a policy of discouraging further use of pressure suppression ks ' containments, and that such designs not be accepted for construction per.

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mits filed after a date to be decided (say two years af ter the policy is GQ % .-

iN ,2. Discussion

?$  ! ~c 9 Nv. A pressure-suppression containment system has some means of absorbing the d .M' heat of vaporization of the steam in the fluid released to the containment I,C volume. In all three CE models, the , steam is forced to bubble through a d@ dY pool of water and is condensed. In tihe Westinghouse design, the sesam is

$ S fA The objective is to reduce the 5 kk'condensedbyflowingitovericecubes.

prsssure in the containment through " suppressing" the partist pressure of 4 the steam by condensing it. To be' effective, pressure suppression must lN $fW),'.

6i take place concurrent with the flow of stesm into the containment, and '

M V 41 its effectiveness is therefore dependent on the rate at which steam is 9)/;

generated or released. If some unexpected event should result in steam

%. k; generation or flow greater than the suppression capability, then the steam i

Q/,y' that is not condensed would add an increment of containment pressure. Since

, j. the objective of pressure suppression is to permit use of a smaller con-

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1. uj W(, tainment, rated at lower pressure than would be required without suppres-sion than incomplete suppression would lead to overpressurizing a pressure-

%l 5'%,.suppr,essioncontainmentsodesigned.

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WY , and is therefore not a " pressure-suppression containment"little

b. discussion.

for the presenteffect on the initial c i, d'j, In this design, chilled water sprays are used to reduce the f;g containment pressure, and therefore the containment leakage, quickly after

?,g.g{apostulatedLOCA.

c The pressure espability and volume are designed to

' ^ ' , - tak.a the full accident, without credit for condensation, yc 4 Like all containments, the pressure-suppression designs are required to 3 '

include margins in capability. Experiments haw been conducted by CE q '

and Westinghouse to establish the rate of steam generation that can be iS j accommodated. The pressure-suppression pools, ice condenser, etc., are M then sized for the double-ended break steam flow, with r.:stgins for un-

, , , ' equal distribution of steam to the many modular units of which the con-g@j denser is composed. The rate and distribution margins are probably ade-s c6 ,

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mental data to the reactor design. Recently we have reevaluated the l

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% We are requiring an independent evaluation of the ice condenser design /j

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i .W . gince the pressure-suppression containments are smaller than conventional c . 1 ffty eedry" containments, the same amount of hydrogen, formed in a postulated hh b ; accident, would constitute a higher volume or weight parcentage of the hE {

h containment atmosphere. Therefore, such hydrogen generation tends to be 4~

3 M a more serious problem in pressure-suppression containments. The small Q

% k CE designs (both the light-bulb-and-doughnut and the over-under configurs-W, 1

M .Vj tions) have to be inerced because the hydrogen assumed The C3 Hod (per safety 3 and the Guide 7)

Westing- dj d Q would insaadiately form an explosive dixture.:/ house ice condenser  % de d@ p%$ circulation and mixing systems to ensure even dilution of the hydrogen to @ ,$

id flammable mixtures in one or more compartments (see following for an 4% ,

,,,h,%avoh additional serious disadvantage of this needed recirculation and b d its vl d@

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Qg By contrast, the dry containments only require recombination or purging

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% Whd.volumes, All pressure suppression the steam flowing fromcontainments one to the otherare divided through into two (or more) major the condensing N O water or ice. Any steam that flows from one of these volumes to the other O N without being condensed is a potential source of unsuppressed pressure.

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1 ic inservice testing. Some effort is now underway to devise a leakage h@%

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^ i, Valves do not have a very good reliability record. Recently, five of the H h i ti t of Quad [E 3 6 3,>Cities vacuum relief 2 were valves found stuck partly for t eopen Moreover, these valves pressure-suppress on con hadabeen nmen k j  ; .k modified to include redundant "vab w ,:losed" position indicators and test. N g -

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3; - dicators were found not to indic me correctly the particular partly open S situation that obtained on the f; a failed valves. We have only recently t-j p$ *l/.

r, $ begun to pay serious attention ta these valves, so previous surveillance k f) 'J,i . programs have not generally incli ded them. The CE Mod 11' design has an h 9 $

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,3, h ovsr pressurization. These valves are large, and must open quickly and j reliably when recirculation is needed. In other engineered asfety features, ,

no single valve is relied on for such service, yet redundancy has not been provided even for single failures, open and closed, of these valves. This is a serious mission, since opening at the wrong time leads to over-pressur-  !

.,w ination, while f ailure to open when needed inhibits recirculation. l

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The asaller size of the pressure-suppression containment, plus the require-ment for the primary system to be contained in one of the two volumes, has led to overcrowding and limitation of access to reactor and primary system components for surveillance and in service testing. Separate shielding of t

'5 components has tended to subdivide into compartments the volume occupied by the primary system. (Some compartmentation of dry containments also V

occurs.) A pipe break in one of chase compartments creates a pressure j

.h differential; each compartment must Be designed to withstand this pressure.

A method of testing such designs has not been developed. -

What are the safety advantages of pressure suppression, apart from the y cost saving. CE people talk about a decontamination factor of 30,000 from 2 scrubbing of io.itne out of the steam by the water. This is hard to swsilov, but some decontamination undoubtedly occurs. One wonders why Cf. doesn't do an experiment to measure it, and get credit for it. The ice '

condsaser decontamination is measurable but not significant.

Recirculation of the containment atmosphere through the ice has the potential

for rapidly reducing the containment pressure by cooling its atmosphere.

V,,' But in the present design there's not enough ice for that, so containment

'T sprays are furnished (in both volumes), just as in dry containments. Re-circulation through the water in the GE designs seems not to have been tried, but may be necessary in Mod 111 for hydrogen control. We have no ,

analysis whether any significant cooling will result.

M *, It is by no means clear that the pressure-suppression containments are, over.

all, significantly cheaper than dry containments when all costs are included.

I Information on this point would be useful in evaluating costs and benefits, and should be obtained.

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