ML20085A474

Response to Questions Re Effect of double-ended Rupture of Up to Three Tubes During LOCA on Capability to Reflood & Cool Core Per 10CFR50,App K Requirements

ML20085A474
Person / Time
Site:	Crane
Issue date:	11/30/1977
From:	BABCOCK & WILCOX CO.
To:
References
TASK-06, TASK-07, TASK-11, TASK-6, TASK-7, TASK-GB GPU-0484, GPU-484, NUDOCS 8307060520
Download: ML20085A474 (11)

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A preliminary assessment has indicatsd that the double-ended l

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rupture of up to 3 tubes during a LOCA would not seriously impair the capaoility to reflood and cool the core in accordance with the conservative requirements of Appendix K to 10 CTR Part 30.

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t QUISTION 5 3 /t N. A. Cylfil What is the==wi=== secondary system pressure developed after turbine trip with first subsequent random failure being loss of main feedwater flow control leading to flooding of super-

' heat section of steam generators. Assume turbine trip without byp' ass (loss of condenser vacuum).

t Resnonse to Question 5 i

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The aszimum secondary side pressure developed, assuming turbine l(

trip without bypass and a subsegnant loss of main feedwater flow control, is equal to the setpoint of the main steam safety valves.

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Ihere are two banks of safety valves. Tha '*high" bank setpoint is j#

about 1313 psia which includes 3% accuanlation. The==w4===

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allowable steam generator pressure is 1375 psia.

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QtTESTION 6 0

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.Does applicant know that time-dapendent levels will occur in E

pressurizer, steam generator and r,eactor vessel afte a rela-7 tively small primary coolant ~ break which caussa coolant u v

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g approach or even partly uncover fuel pins? What does operator g g do in respect to interpreting level in pressurizer!

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During primary system refill from high pressure' injection pumps.

i there is some period when neither condensation nor natural u

convection is present to effect beat transport to secondary

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side. How is transition to natural convection without assistanes

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f on irl=s:v coolane cumns obtained.

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Resconse to Question 6 O..

There are two overriding concerns with any LOCA:

(1) Initial removal of fuel-stored heat.

(2) Continuous removal of core fission product decay heat.

For small breaks, fuel-stored heat is removed during the first few seconds of blowdown. The B&W ECC3 system, using internal vent valves, precludes the interruption of decay heat removal for all accidents within the range of relatively small breaks (break size

<0.01 ft ).

Break location. ECCS injection, coolant phase separation, 2

teactor Coolant System (RCS) mixture levels and steam generator conden-sation have"been considered in arriving at this conclusion.

As we understand the question, the concern is related to possible

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interruption of steam condensatios within a steam generator,due

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to refilling.of the primary system. In general, such a situation can.

3.g occur only,at extended times during the Mnal recovery stage of a I,0CA when steam condensation is no longer regnized. Eowever, even if this situation occurred earlier in time, the performance of the vent valves would be to equalize water levels between the hot and cold regions of the primary syntem, thereby assuring continuous fluid coverage of the core with no adverse consequences.

This I.s substantiated by a more detailed examination of the fluid conditions during a relatively small L3CA. Such an accident can be viewed as a very slow transianc during which, at any particular time, the system is not meaningfully. different from steady-stata conditions. The ICS can then be properly described as a sealed nano-i For the B&'d system, because of the vent valves, this manometer noter.

is double looped as illustrated in Figure 6-1 with important volumes identified by letters.

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Reseense te QueItion 6_

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4 Dare.are two,everriding concerns with any I.CCA:

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ar Init al removal of fuel-stored heat.

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(2) Continuous removal of core fission product decay heat.

-<s Tor esall breaks, fuel-stored heat is removed durist the.*1rst few seconds of blowdeern. De B&W ECCS systen, sains internal vens valves, pr4cigdes. the interruption of decay heat removal for all accide agwishin the reage of relatively small breaks (break size

<0.01 fg2).. preek location ECCS injection, coolant phase separation.

Raaesor Coolast! System (RC3) mixture levels and staae generator condon-sation have 'bges considered in arriving at this conclusion.

e As we understand the guestion, the concern is, related to possible

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interruption of stams condensation within a steam generator due s,,.

to refilling.of the primary systen. In general, such a situation can.

occur only,atrest. ended times during the final recovery stage of a I.0CA N

wnen steam condensation is no longer required. Iowever, even if this situs:ioa oesurred earlier in time, the performance of the vent valves would be to equalize vecer levels between the hot and cold regious of the primary,seses, thereby assuring continuous fluid coverage of the

. ore.ith m,edrerse o nsequen.es.

21s h substantiated by a more detailed examination of the fluid conditions.daring a relatively small LOCA. Such an accident can be i

viewed as 4.yery slow transient during sfaich, at any particular t

time, the system is not meaningfally, different from steady-state conditions.inne ICS can then be properly described as a sealed nano-Tor:5.he R&V system, because of the vent valves, this manometer meter.

is double looped as illustratad in Figure 6-l'vich important volumes identified.bytletters.

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(3) n e open manometer paths D, y, C, E, aN g assure

'that hydrostatic belances exist between regions E and A, and between regions K and A.

If these balances do not exist, fluid movement will occur t.o produce them.

Af ter e= cess mass injection is achieved, the EC3 starts to refill.

During refill, a rising water level in region E may eliminate condenslag heat tressfer. Note that a rise of level is,E aise l

means a rising level in K and A. ' Das, no immediate core concern

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exists. Stesa pockets will be formed at J and C.

If the level concia.nes to rise, a'two phase mixture will be orced into D and F.

his will occur through the, necessity of maintaa..aing a hydro-statis balsase with E.

Eowever, if sondensation ceases, the

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energy balance is no longer asistained. As energy is not being l

I adeguately removed from the system, the system must repressurise.

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(1) n e break flow increases until it removes enough

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energy, or the break allows renoval of enough asas

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(2) Repressurization contianas until energy renoval I

is brought about through the pressuriser relief

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valve path E.

Most likely, nochanism (b will repeat for several cycles prior to anchanism (2) occurring. In any case, uncovering of the core saa-not take place. Again, if the core fluid level is lowered, then the fluid level in E must be low and condensation is a credible phenomenon.

De flow pattera in D, the horizontal section of the hot leg, is of interest during repressurizatios. his is illustrated in Figure 6 along with the pressures within the system. De following hierarchy of pressures exists:

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mainly because of static balances and the two-ph'ase nature of the j

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fluid in the core and hot legs.

i QUESTION 7

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~ Waat is the particular ' design of the start-up piping and pumping t

system for Febble Springs! Does it involve operating with a I

liquid-solid secondary systas! Bas the Staff performed a safety analysis of this systaaf

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Resoonse to Question 7 De current Febble Springs design does not involve operating with A vacuum heacup liquid-solid secondary system during heatup.

I schase recently selected as an Ng3 option is now used for starrup.,

De system is' illustrated in the attached sketch (ylgure 7-1).

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'. i.w' During heatup and operation of the integral economiser oece through I

Y steam generator (IEDTSG), two possible overstress sooditions require i

particular attantion. Cae is ascessive compressive loading of the f-steam generator tubes cansed by too large a temperature difference between the steam generator tubes and the steam generator shell.

Se secoed is possibly overstressing the shell-to-lover-cube, sheet In order to avoid these

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veld by adding feedvater that is too cold.

1 sse' functions are conditions, two functions mast be available.

(1) the ability to draw a vacuum in the secondary s. e of the sesam t

generator to permit boiling at low temperatures and' consequent shell candlansation, and (2) the ability to heat and maintain the temperature of the feedveter before it estars the steam generator so that the maximum allowable temperature differencg is not exceeded.

Drawing a vacuum and the subsequent low temperature boiling will allow heacup of the planc without exceeding the==N=

allowable cube-to-j Maintenance of the required temperature shell differential tosperature.

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in the main feed line is accomplished by a circulation loop for each r,

C...y his loop will draw fluid from the four sesam generator steam generator.

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t OUESU ON 11 Are any'special precautions taken for storage and handling of hydrazine?

Reseense to Question 11' special precautions are taken for personnel protection whos handling hydrazine. De hydrasine purchased for the plant will be a 35 by weight aqueous solution, which can cause irritacios to i

che eyes and skin. D ose handling it will be required to wear goggles, rubber gloves and protective clothing. -

Eydrazine is stored in two places within the plant. One is the 300 gallon hydrazine storage tank, which is in the "W=ry

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Building and contains a 35% aqueens solution for the co-eM a==nc Spray System. It has a nitrogen blanket and is not vented to che

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roon, so no vapors in the roon are expected enaept during filling and draining operati;ns which will occur only on a very inQ. esc basis. De normal ventilation in the area provides five air changes per hour,.

ne' 35% aqueous solution has no flash or fire point and does not represent a fire hasard. D e other storage area is in the Turbine Building where the feedwater chemical injection hycrazine storage tank is located. This tank contains about 1000 gallons of diluted hydrazine solution, with a 12 to 5% hydrazine content. De. tank is vented to the room, but t$a ventilation systen, which provides two air changes an hour, will prevent migration of any hydrazine vapors.

OUIsncN 12 Wat is status of investigation of a<rits of a primary vessel coolant level indi:ation system for use in post LOCA cooling I

for small breaks!

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i Reseense to Question 12 B&*J is no longer considering the use of primary vessel coolant level indication systems. Present analyses show that adequate system protection is provided by existing equipment and sensor design. For the specific case of small breaks in the primary system, please note the response to Question 6.

CUESTION 13 t

':he fire protection systen aa'y be sharacterized as 's "hard" or "sof t" system in respect to independence or depecdence on fire detection and extinguishing systems.

In a local sens', in what particular locations is this plant dependent on d=4af=trative protection and early detecting-

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extinguishing techniques to protect vital' shutdown system from

• g fire damage? Is complace barwat amoumed for local plant space or area such as one spreading room?

Resoonse to Cuestion 13 i

Generally, Pebble Springs design features include separation of redundant safety-related components by three-hour fire barriers, t

l in addition to appropriate provisions for fire detection and suppression. In certain cases, it is not feasible to separate redundant components by fire barriers. Dese areas have been analysad to verify that snitabla detection and suppression systems are provided and other design provisions incorporated to assure

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safe plant shutdown. Dese areas are limited to the following i

described below:

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t Separation well in excess of Regulatory Guide 1.75 j.

guidelines is provided throughout the control roca, R- /2 -zu 1

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Many experiments.have been run whir h show that as long as a fluid (quality less than, say, 701) covers the core, no

• adverse core

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Thus, tenperatsre excursion ten occur at decay heat power levels.

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the design probism associated with small LOCAs is to achieve 1

steady mass and energy balances which assure that the core remains This means that mass injection equal t.o mass loss, sed covered.

energy removal equal to decay heat is achieved. For a spectrum of break, si.tes approprihts for relatively small LOCAs, conservative t

analheia assures that no uncovering of the core occurs prior to achieving excess mass injection. Thus, any concerns with very I

small-break LocAs deal with the energy balance once excess j

injection has been achieved.

For certain small breaks, the steam generator would ac. as an energy removing device. Energy removal occurs through a three-step initially, a solid flow-forced convection process would sequences control heat renoval, later a two-phase natural circulaton process f-involving both convection and condensation heat transfer would In

,f[f-control, and finally a pure condensation mode would result.

t this latter mode, fluid has fallen to approximately level 3 on As steam is produced in the. core through boiling, it yigure 6-1.

es travels through D, y, and C and is condensed in the lower regions

.L Concerns over the impact of noncondensible gases have been of 1.

--had for this phase and the following points apply:

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(1) Insufficient noncondensibles are available in the i;

initial 1C3 finid to block the flow of steam at C

-(this is a 3-ft dissacar pipe).

(2) Best transfar coefficients with noncondensibles

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present are sufficiently large to condense steam in the lower regions of E.

Even if the heat transfer were momentarily inadequata, this would merely cause a pressure increase and resultant tenparature increase until the temperature difference compensated for the lower heat transfer coefficient.

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CUISTION 26 3

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Considering such matters as (1) aff-site power failure, (2) con-denser vacuum failure, (3) spurious main feedvater valve closure

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(see item 21 preceding) and recent incidents of failures in auxiliary feedvater systems it appears that, single failure criteria netvichstanding, at least short term failures of the auxiliary feedvater system umst be considered to estimate the j

needed reliability of such system.

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i Uhat, for instance, uould be the peak primary system pressure.

consequences to primary coolant system safety and relief valves and race of primary coolant loss fo11oving failure of the Auxiliary Feedwater pumps when needed?

1 Resoonse to Question 26

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The feedvater systems are designed to current NEC regulations.

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since these regulations include criteria for design and analysis assuming one single failure, and the safety grade Auxiliary b

Teodwater system contains multiple redundant trains (four 502-size capacity pumps are installed with independent power sources), the l

Pebble springs design complies with the latest requirements.

Postulatics of an event whereby all feedvatar is lost requires multiple failures in the main and auxilizry fe dvater systems.

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Nonetheless, a preliminary analysis has been made to decezzine the event sequence, assuming that all feedvacer is lost-instantaneously without regard for a realistic mechanism. The following is an f

I estimate of the sequence of events expected:

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.f-O sec All feedvater is lost and the RCS begins to increase in pressure.

e 7 see Reactor trips on 'high ES' pressure.

1. 10 sec Pressurizar begins to relieve decay heat via steam to the RC drain tank at the pressuriser safety valve seapoint of 2500 peig. (RC3 pressure about 2750 psig).

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1. 2 min Reactor coolant expansion causes the pressuriaar to become water solid, and water relief to the RC drain tank begins (ICS i

' pressure about 2500 peig).

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<10 min Containment pressure increases to the ISTAS l

l setpoint (4 pais), and high pressure ICCS coolant injection to the core starts automatically.

45 min High pressure ECCS injection flow heat removal rate is about equal to the decay heat generation rate. Prior to this time, boiling has occurred in the core; and after this time,,it will diminish.

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A coolable geometry is maintained at all times.

4 Long tera ECCS high pressure injection vill continue l

to provide coolant from the borated water storage tank (3WST). When 'he 393T low-level signal is reached, the opetator an switch the ECOS high pressure coolant injection to the recirculation mode, if auxiliary or main feedwater has not been restored (see 7ebble Springs section 6.3.1.4.1 for a discussion on this mode)..

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