ML20055A276

Small-Break LOCA Recovery in B&W Plants, Draft

ML20055A276
Person / Time
Site:	Rancho Seco
Issue date:	07/09/1982
From:	Demuth N BABCOCK & WILCOX CO.
To:
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ML20055A272	List: ML20055A271 ML20055A276
References
NUDOCS 8207160053
Download: ML20055A276 (46)

Text

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ENCLOSURE 1

~

SMALL-BREAK LOCA RECOVERY IN BABCOCK & WILCOX PLANTS by N. S. DeMuth l

I.

INTRODUCTION Analyses of loss-of-coolant transients sma cold-leg breaks in Babcock and Wilcox (B&W) plants indicate tha r

circulation in the primary loops may be interrupted by steam accumu t g in the " candy canes" (hot-leg U-bend).

The purpose of these analyses was evaluate, alternative strategies for recovery from small-ak loss-of-coolant accidents (LOCA) and I

for reestablishing natural-circulatio Calculations of a small-break f

t accident were perfomed with the TRAC computer code, version F,

for a 0.0445-m (1.75-in.)-diam.

cold-leg break.

The break size for s study was selected to provide conditions for loss o circulatio ooling, diile the primary pressure f

remained relatively gh (

ve the setpoint for accumulator injection).

i Figure 1 shows a TRAC

- diaoran for the B&W lowered-loop model based on owuee 1

the M design.

Loop presents the loop with the cold-leg break and includes t leg with t ressurizer (labeled PRIZER in the figure)y th steam

ator, two cold legs--one intact and one with the 0.00155-m 2

(0.01 t ) break.

Each loop-A cold leg includes a loop seal, a pump, and.

a conne for N

high-pressure injection system (HPIS).

The reactor t

coolant pum r

deled using the LOFT pump characteristics built into TRAC m e e.

but scaled wit pump data. The break is located in one cold leg between the HPIS connection and the vessel.

Loop B represents the unbroken loop and is similar to.loopIexcept that there is no break or pressurizer and the two cold legs are combined to increase calculational efficiency.

The small break in the cold leg was modeled using a TEE component i

(No. 35 in Fig. 1). A BREAK component at atmospheric pressure was attached to the end of the side tube, and the flow out the break was computed using the critical flow model in the released version (Mod 0) of TRAC-PF1.

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,The secondary side of each steam generator is attached to the main feedwater inlet, the auxiliary feedwater inlet, and a long pipe to the steam outlet with a side connection to a safety valve that vents to the atmosphere.

We represented the safety valve with a FILL component in Witch the mass flow out is controlled by the steam pressure.

Geometry and other t data wre obtaineit from the Oconee Final Safety Analysis Report 3 (FS d other data sources for the Oconee-1 plant.

The model for the auxilia fee er system allows flow to enter near the top of the steam generators, and t low of auxiliary feedwater is controlled based on the water level in the secon es.

The vessel us modeled with four azi al

segments, two radial segments, and seven levels.

The seven level clude lowr plenum, three active core levels, two levels in the upper p1 permit the vent valves to be above the hot-and cold-leg connections, an upper head.

The model includes connections from the upper head to each leg to simulate the upper-head circulation observed by in their flow tes s.

These connections are needed because TRAC, with only adial ring in the upper plenum, models poorly the flow up out of th h the center of the upper li plenum, into the upper head with turni tward, and back down into the upper plenurn.

Vent valves connections r accumulator injection into the downcomer are modele 6 of the v 1.

The response he p t following a small-break LOCA was simultted to evaluate the effectd sd the following operator actions in r storing natural circulation cool (1) cooling and depressurizing by venting steam from the enerator daries, (2) " bumping" the main coolant pumps, (3) ve g ste om the upper elevations of the hot legs (candy canes) and (4) ting a po ion of the flow from the HPIS into the candy cones.

The l

effects differf I

cooldown rates on recovery of natural circulation flows l

were asses i

veral calculations.

In the following section, we will analyze the t response for three scenarios:

(1) no operator-initiated cooldown; (2) operator-initiated cooldown without injection from the core flood tanks (accumulators);

and (3) operator-initiated cooldown with accumulator injection.

l 9 a

g.

II.

TRAC CALCULATIONS OF SMALL-BREAK LOCA RECOVERY A.

' Small-Break LOCA Mithout Operator-Initiated Gooldown "We modeled this transient starting from a steady-state power level of 2568 MW with the break assumed to occur at the beginning of the transient.

At 60.6 s the reactor and main feedwater pumps are

)ed on a

low-primary-pressure signal of 13.1 MPa (1900 psia). The r was M3 by an insertion of negative reactivity (-0.0536 hk/k),

d subsequent powr level was computed from the coupled reactor kinetics and y heat formulations in TRAC.

The reactor kinetics equ tions included re ivity feedback from fuel and moderator tamperature ch Injection of auxiliary feedwater into the upper regions of the ste nerat began 30 s after the reactor and main feedwater pumps wre tripped.

The emergency core cooling system (ECCS) ctivated at 81.5 s by a low primary pressure of 11.135 MPa (1615 psta), and HPIS began delivering flow 35 s later"(at 116.5 s).

T reactor coolant ps wre tripped at i

131.5 s (50 s after activation of th As shown in Fig. 2, the primar r

reased rapidly until 150 s, at thich time the HPIS began deliveri out 50 kg/s of cold (305.4 K) water to the cold legs.

Thereafter, the pr ry. pressure decreased slowly as the flow of heated wete

.e break we removing more heat than mes being rpduced by fission product decay was decreasing.

generated, and th wer For this calculation, tram the HPIS was divided equally between loops A and B and the loop-A' snown'in Fig. 3, was divided equally betwen the split cold The auxil feedwater refilled the steam generators to 50%

of the a

ting range in about 10 min. and was controlled to maintain this 1 el for th emainder of the transient.

The' auxiliary feedwater flow I

to both am genesauors is shown in Fig. 4.

The r

tions in the top of the hot legs are shown in Fig. 5.

At about 500 s the transient, the loop-A can# cane began to empty of liquid, and by 800 s, it was filled with vapor.

By 2300 s the upper elevations of the loop-B hot leg wre filled with saturated vapor, and natural circulation flow in this loop stopped. 'The flows in the loop-A and -B cold legs are shown in Fig. 6.

After natural circulation stops ir the loop-A cold leg (at 500 s), the flow in the split cold legs (Al and A2) is in opposite directions, even though the loop seals wre modeled individually.

Natural

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Fig. 3.

High-Pressure-Injection flow for Loop-A split cold legs. e

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(50ue) LOOP-A Aus r/w (oAsu) LOOP-e Aux r/w 3b-2 n-t o

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Auxiliary feedwater f1 rolled on the 50% lovel.

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Fig. 5.

Vapor fractions in the upper elevations of the hot legs. t~

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Cold-leg flo xits.

circulation flows in the loop-B cold ceased (at 2100 s) den the can@

cane on that loop fill stesa.

Cessation of ural onvection accompanied by the loss of pressure differential between comer and the upper plenum allowd the vent valves in the vessel m -+ M50 s, as shown in Fig. 7.

With the vent j

valves open, the vessel a our elevations of the hot legs were virtually j

isolate tern gener tors, and the core was being cooled by flow from the dich ed with steam flowing through the vent valves into the downc I

The gra flow out the break is compared in Fig. 8 with the total inflow from.

IS, and Fig. 9 shows the net mass loss (the difference between the break flow and HPIS inflow).

For the first 75 min of the transient, the primary was slowly emptying, as the break flow exceeded the inflow from the HPIS.C After about 75 min, the HPIS flow nearly equals the break flow -- W111 cutt # attributes this to the increase in " void fraction of i

the f,1uid reaching the break".

Throughout the transient the fluid entering the break contained a mixture of vapor and subcooled liquid.

The vapor fraction of this mixture varied greatly depending upon the direction of flow in the split cold leg (A2), and this markedly influenced the mass flow out the I M

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Vapor velocity through vent flow from upper plenum to downcomer is positive).

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Fig. 8.

I Comparison of break and HPIS flows.,

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Net mass loss from primary (

k flo minus HPIS flow).

I break.

When flow we inately fr. the vessel (downromer) toward the l

break, the vapor tion s relative y large and the liquid subcooling small, so that flo t

break was reduced.

When the flow was largely l

subcooled liquid from e--a uma HPI inlet) toward the vessel, the fluid entering the break had a vapor fraction, and the mass flow out the break increas this tr sient the core remained covered with liquid (Fig.

and fuel-rod temperatures remained below their steady-state value

g. 11).

I The ults this analysis with version PF1 of TRAC c pared favorably with results d with version PD2 and reported in Ref. 4.

The transients di ffered in t initial and boundary conditions and in the setpoints for actuating safety systems.

We modeled a small brgak of 0.0343-m (1.75-in) equivalent diameter, dile the transient of Ref. A had a break of 0.0343-m (1.35-in) equivalent diameter. An investigation of the critical flow model in the released version (Mod 0) of TRAC-PF1 has shown that this model under-predicts the mass flux at the break by about 30%, so that the break flow in the TRAC-PF1 calculations corresponds to an effective diameter of 0.039 m (1.54 in).

Other differences in primary response characteristics may be I

_g.

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Fig. 11.

Maximis:: average fuel rod temperature.

VO attributed to differences in trip setpoints and boundary conditions, such as the distribution of HPIS flow among the cold legs.

To test the effectiveness of operator actions to cool and depressurize the plant, the calculations were restarted with the steam ge tors being cooled slowly by venting steam from the relief or dump valves m scenarios wre considered.

In the first, the operator isolates the ors before starting cooldown, and in the second scenario the accumulators beg jecting den the primary pressure drops below the accumulator pressure.

B.

Small-Break LOCA Recovery Without Accumulator /Gjection The small-break LOCA calculations descr in e preceding section wre restarted at 3480 s and now included o tor ons to increase the steam generator levels from 50 to 95% of the operating range.

This level increase required about 500 s, during dich the steam-generator secondaries wre cooled by the inflow of auxiliary feed r.

After the steam generators filled to 95% of the no rating range, venting of steam from the atmospheric relief valves began.

h se flow was controlled by the pressure in the steam generator second M, such that at 6 MPa the steam flow was about 1% (8 kg/s) of the full-p r flow (778 kg/s).

As the steam generator secondaries the steam w was reduced to 4 kg/s at 2 MPa and 1 kg/s at 1 MP The xiliery feedwater flow, shown in Fig.12, was controlled to maintai e

level until 3480 s and the 95% level thereafter.

When the calculat r,- c -=<arted at 3480 s, the upper elevations of the primary, including the ssurizer, the upper plenum, the hot-leg candy canes a rmost cel s in the steam generators, wre filled with stea igures 1 1d 14 show the vapor fractions in cells 4, 5 and 6 of the loop-A loop-B oteam generators, respectively.

At 3480 s, cell 6 in the loop-A s gene or has a vapor fraction of 0.2, whereas in the loop-B l

steam genera 11 6 is filled with vapor.

As shown in Fig.1, cell 6 is l

the highest cel in the steam generators from dich heat can be transferred to the fluid in the secondary.

During filling to the 95% level, the vapor fractions in the upper cells of the primary increased.

However, after secondary cooling by discharging steam started, the primary-liquid level rose to fill cell 6 in both steam generators.

Thereafter, the transient increases in vapor fraction resulted in intermittent fluctuations in the primary liquid level.

These fluctuations could be caused by the weak coupling between l

subcooled primary liquid in the steam generators and the saturated steam / water ~-

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Auxiliary feedwater flow con the 95% level after 3480 s.

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Vapor fractions in the loop-A steam generator primary. l 3

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Vapor fractions in 1 generator primary.

mixture in the vessel.

As the steam erators cooled, the density of the primary liquid increased, and the heav e liquid intermittently settled into the cold legs thereb saturated id frec; the hot legs.

The vapor fr ions 1 the candy canes are shown in Fig.15.

During filling and cooling steam generators, slugs of hot fluid were drawn through the hot legs in e sw[n generators.

As further evidence of this intermitte rculation, t ss flows in the cold legs at the pump exits in Fig. 1 ca hat these flow transients are in the proper (positive) di n.

After i tural circulation ceased in loop A (at 800 s) and before it cea in loop E (at 2100 s) flow in the split cold legs of loop A was predominat a

from the vessel in loop A2 (containing the break) and toward the (positive) in loop A1.

Between the time that natural circulation ceased (2100 s) and the cooling of the steam generators began (3480 s), the direction of the flows in the split cold legs of loop A was changing, such that when flow in loop Al was toward the vessel, the flow in loop A2 was away from the vessel.

, After secondary cooling was initiated, flow in loop A2 was toward the vessel and break, Wille the flow in loop Al was toward the loop seals.

The direction of flow and amount of subcooling in the split cold leg containing

~ 3

s.

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($0 LID) LOOP-A TOP Or CAseY CAaK

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Vapor fractions in the up of the hot legs.

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m I

I p-S tsASS rLow RAM i

LOOP-Al esAS$ FLOW AATE asse i

==-o asAss rLow aan

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Fig. 16.

Loop flows in the cold legs at the pump exits.

, ~

the break had a marted effect on the mass flow out the break.

The fluid flowing from the vessel toward the break contained a tw-phase steam / water mixture, so that the mass flux through the break us lower than for the predominately subcooled liquid ' flowing from the loop seals toward the break and vessel. The liquid and saturation temperatures in the col containing the break (A2) are shown in Fig.17.

After secondary coo is initiated, the liquid in the cold leg containing the break is about 5 su led, W;ile the liquid in the other cold leg (A1) is nearly at saturation condi s.

The pressure, flow and temperature oscillations that occur after cessa n of natural circulation flow in loop-B are discussed her in Appendix A.

The small-break transient simulating o r or i rvention to cool and depressurize the plant was carried out to 2.7 g this time, the total flow supplied by the HPIS reached its maximum val about 56 kg/s, as shown in Fig.18.

After cooldown began, the liquid ma pplied by the HPIS j

exceeded the mass lost through the enk, so that by th end of the transient the primary contained nearly the s f fluid as Wien the small break was initiated; this is evident from 20.

However, owing to the liquid shrinkage during cooldown, the s am volme in the upper plene and candy canes remains relatively unchan Thus, the flow from the HPIS is able to make up the

)st through e break, but not the decrease in t

if quid volume due he i eased density.

The vapor fractions are shown in i

Fig. 21 for the up m and in Fig. 22 for the upper cells in the dcwncomer.

(Cell 6 cont tneInt valves and cell 5 the hot-and cold-leg connection In the upper num and downcomer, the vapor fractions reached their ium es while the steam generators were filling and did not decr significar1t y thereafter.

.rfmary i d secondary pressures in the loop-A steam generator are shown in 23 d the pressurizer level is shown in Fig. 24.

Even though the thermal ing betwen the fluid in the vessel and the liquid in the steam generators was weak, the primary temperature and pressure followed those of the secondary, Wifle most of the decay powr us removed by heating and discharging through the break the water supplied by the HPIS.

The primary cooldown and the wak thermal coupling between the vessel (and ' hot legs) and the steam generators (and cold legs) is illustrated in Fig. 25, dich shows the loop-B hot-leg and cold-leg temperatures.

The primary terrperature decreased from 550 K Wien the steam generators began filling to the 955 level to 460 K at the end of the transient.

The average _

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(DASH) LIQUID TDettATM ene.

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Fig. 17.

Liquid and saturation tures adjacent to the break.

s (souo) was n0W mATc (DASH) LOOP-Was ROW RATE e

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Fig. 18.

High-pressure injection flows in loop A. P

emesse (SOLID) MIGN-*IE554AE INJECTecal rLOW messes see..

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Comparison of reak and HPIS flo seene

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Fig. 20.

Primary begins gaining mass after operator-initiated i

filling and cooling of steam generators.

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Time (.)

Fig. 22.

Vapor fractions in the downcomer.

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(SOLID) LOOP-A 37001 Pelts 44Y (OA5H) LOOP-A S7GDI SECacARY

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Comparison of Loop-A pr r

ndary pressures.

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Fig. 24.

Pressurizer ster leve1. >

W

.g (SOLID) Loop-G te0T LIG (nasen toop-e can LEs ese-n n

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Comparison of hot-leg and ratures in loop B.

[

rate of primary cooling was 52 X/hr ( * /hr), Witch compares favorably wf th the technical specifica n limit of 1 F/hr.

Fuel-rod temperatures shown 0

f in Fig. 26 followed e

he liquid (

turation) temperature in the vessel and hot legs.

I l

Although a therm

• =

farce for natural circulation was established by cooling the steam ge ors our attempts to restore natural circulation

.by bumpin th =ein coolant ps, by venting steam from the can@ canes and by inj ng co ter into the can# canes failed because the liquid volume in t rimary be; een 1 h and 2 h would not support natural circulation i

flows.

r 2 h, o actions to restore natural circulation were simulated because w s

the operators could continue cooling the core and depressurizing the set point of the low pressure injection and residual heat removal systems.

C.

Small-Break LOCA Recovery with Accumulator Injection To further test the effectiveness of the various strategies for recovery of natural circulation flows, the small-break LOCA calculations described in Section II.A. were restarted at 2414 s, immediately after natural circulation flow in loop B ceased.

The initial operator action to increase the steam-generator level from 50 to 95% of the normal operating range was simulated, as,.

ens

=

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E w

eee w

g g

ses-no-

eso, l

ene-.

..m 1

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e use sees asse esos esse me

'en see.

j TM (e) ig. 26.

Maximum avera rod temperature.

f shown in Fig. 27, and ver. ting of ste r

the secondary side of both steam I

generators was begurs at 3000 s to cool a d depressurize the plant. The steam flow was controlled secondary p.

ures with the relief rates given in Sec. II.B. and th xil y-feedwater flow was throttled to maintain the level at 95% of no t1 operating range.

When the calculation was l

restarted at.2414 s,

_e_iev:tions of the primary wre filled with

-c l

steam as shown in Fig.

d the net fluid mass lost from the primary was i

43000 k In the cident sequence described in Sec. II.B, the net fluf ss was kg at 3480 s, den the operator began filling the steam gener to the 5 0 level.

Th imary id secondary pressures in the Loop-B swam generator are j

shown in F for this transient.

Primary-to-secondary heat transfer, I

established at 414 s den the steam generators begin filling, was maintained by venting steam at 3000 s.

The absence of effective thermal coupling between I

the steam generators and the saturated two-phase mixture in the upper plenum and hot legs resulted in the primary pressure remaining at 6 MPa for 3 min after primary-to-secondary heat transfer was established.

The sudden drop in pressure to 5 MPa (at 2600 s) was accompanied by slugs of hot fluid (Fig. 28) passing through the upper elevations of both hot-legs into the steam i

generators.

The pressure remained at 5 MPa for 8 min dile the steam

, 1

t

• 8 a e & W SadALL-SREAK LoCA (s G FILL. Cool. ACC. INJ.)

AUXILIARY FEEDwaTER rLow RATES N

i a

v W

v s

(souo) Loor-a Aux r/w 8',

tasa) Loor-e aux r/w -

1 m-m Q

m-4 8

=-

i g

1 e

e 2

R.

y,p

.m i

s-

'b-d

-e e

am a

==

a e

e.

a Ta w (.)

27.

Auxiliary feedwater flow Co t the 955 level after 2414 s.

1 ~

i s

u T

b (souo)LN.

-a voc or cuev cm

)J (casw) toop-e voc or caer cant

//

j' f

"I f

i i

g i

i-as-E i

i l

i i

i l

i e

j J

u.. _.

m i

TIE (.)

4

)

Fig. 28.

Vapor fractions in the upper elevations of the hot legs.

e +-

3 & W sWALL-SREAK LoCA (s o FILL. Cool. ACC. INJ.)

Loop-a sitAW oEMRAfoR (souo) Loop-e uwm peimany

~***

(casM) Loor-e storm sccomany ame's dM tt l

,,/

.i. * *.v e

Tied'(.)

. 29.

l Primary and secondary pre n loop-B steam generator.

I generators filled to the 95% level ooling of the secondary by venting I

steam was initiated.

The rapid drop pressure from 5 to 4.3 MPa that occurred at 3100 s

.panied by s of hot fluid passing through the

[

candy canes into ste lgenerators.

Continued cooling of the secondary produced a slow depr r

tion to 2.4 MPa (360 psia), den the calculation was terminated.

~

When the primary sure fell below 4.27 MPa at 3290 s, the accumu injecting liquid (Fig. 30) into the downcomer.

From this I

time il the ca lation was tenninated at 4800 s, the accumulators injected 3

about m

(12C bu kg) of cold water.

The average rate of injection j

(8 kg/s) ng time tould be equivalent to a 14% increase in the HPIS flow.

Figu shows the outflow through the break compared wf th the inflow from the HPIS alone and the total inflow from the HPIS and accumulators.

The difference between the outflow and total inflow is shown in Fig. 32.

More j

than half the mass lost out the break is recovered by inflow from the HPIS and accumulators in the last 2400 s of the transient.

.While the primary was cooling and depressurizing, the vapor fractions in the upper elevations tere not changing significantly.

This can be seen in Figs. 28, 33, and 34, dich show the vapor fractions in the candy canes, the --

e s.

S & W SMALL-SREAM LOCA (1 C FILL. COOL. ACC. INJ.)

,ACCUWULATOR INJECTION r,

e.

c c

J.

e.

e.

(.

I I

g g

s-

.se e

'.e.

e e

d

.' e

.' e mies ne'ee sees an's..e'ee g sees e

see e

s fl E (e) y Liquid volume disc r

ulators.

S&

AK LOCA ($

LL. COOL. ACC. INJ.)

oN OF SREAK ECC5 Flows seeeen 18eDOUGet BerAK i

=>cenow rtow seeses.

I 1 P "'

• SATOR INJECil081

/

sessa n s

n

/

t v

..eeneg e

sense.

e R

.aessesg

/'

8

.,/

neeenQ

.s g

-..e g

e.

.e

-eneen

,e.

,nes.een

.ee.

e see

.e.

,eee l

Fig. 31.

Comparison of break, high-pressure injection and accumulator flows.

B & W sMALL-BREAK LoCA (5 C FILL. Cool. ACC.1:3J.)

WA55 LoS5 Frow PaluAlty essee esmo.

j y

esses g

eses.

e Eg amm-E 9

g uses-men g I

I

.e

~

=#ses see ase asse sees asse asse mee aan e

ese

-. 32.

Primary begins gaining a

after operator-initiated filling and cooling of steam a a w sMALL-GREAK L 5 G FILL. Cool. ACO. INJ.)

VotD IN T R PLDeuw u

7

.~.

,)

I Y

k: ' v"*).

-~N': l i

1.:

r.

l Y*-- f%d f

scuoevn s l

s:

o m =Lrvtt s 2

a p j

i f

)

f N

W see see asse sees asse asse sees esse sees e

see Time (e)

Fig. 33.

Vapor fractions in upper plenum decrease following accumulator injection.

v

(

3 at W sWALL-SREAK LoCA (s c FILL. Cool. ACC. INJ.)

Vol0 IN TPC DoWNCoWER u

(-.

.-.,. 7 ~.

4 j

n i

E g

f scuo ttvtl s g

f pas

• ttvrL l

2 l

i f

}

p e

see n a' mee so o nee m'oe noe see Time (e) 34.

Yapor fractions in cells to cold legs decrease following accumulator inject o.

, { 'a upper plenum, and the downcomer, respe t vely.

The vent valves, dich opened l

2en natural circula lows cease t 2100 s, remained open allowing saturated fluid ta fr

he upper plenum to the downcomer.

The liquid an tu ton temperatures shown in Fig. 35 indicate that j

the primary temperature haawo40 K in the last 2400 s of the transient.

This corresponds to en av cooldown rate of 75 K/hr (135'F/hr), dich is somedat than the technical specification linits.

Fuel-rod tempe res clo followed the core-liquid (saturation) temperatures.

lihen the c lation wasj vrainated, the primary pressure (2.4 MPa or 348 psia) and tempera (500 or 440 F) were such that continued cooling and 0

depressuriz the set point of the low-pressure injection (LPI) pumps could be demons rated without recovering natural circulation.

To test operator strategies for recovering natural circulation flows, these calculations wre restarted at 3835 s, and the following actions were simulated:

1.

venting steam from the candy canes, 2.

bumping the reactor coolant pumps, and 3.

injecting a portion of the HPIS flow into the candy canes. -

r

e a w suALL-SREAM LoCA (s G Flu Cool. ACC. lu) isAT AMD.TLloulD IN top LEVEL of coltE es.

(SQLID) TSAT tw 10P lev [L or COE m-(DASH) TLIOulO ON TOP LEVEL OF C0K y

n C

-.y.

088' a.

g

~ ~ ~ ~ -..

i g

=

i g

b a.

I l

a i;-

Fig.

5.

Liquid temp res decrease X during 40 min cooldown.

The results of the imul ons are discussed individually in the remainder l

of this section.

y (1)

Venting Steam From leg Can@ Canes Val he hot-leg n@ canes are nomally used to ' expel air from the ary an acilitate reffiling after a refueling outage.

Since info on on th valve dimensions was not provided, a diameter of 1 cm (0.394 s as ed for the orifice.

The valves in both can@ canes were opened ful 5 s and remained open throughout the transient.

Critical flow through ese valves as computed using the model incorporated in TRAC-PF1/ Mod. O.

When the valves wre first opened, the pressure in the hot legs us 3.2 MPa, and the flow was 0.4 kg/s saturated vapor.

The pressure decreased steadily reaching 2 MPa (Fig. 36), den the calculation was terminated at 5400 s.

'From the time accumulator injection began (3290 s) until the calculation was terminated, inflow (Fig. 37) from both accumulators totaled about 20 m3 (20000 kg). The average mass flow supplied by the accumulators was 9.5 kg/s, "

3 & W SMALL-GREAK LOCA (sG FILL COOL, ACC VDITs)

PRIZER PRESSURE see' l

(souo) pecsmaizra ussuar ass 1

. ess i

i k

l

~

4 w.

\\

een y

w.

age l

}

M see neo asse aseo asse aa n m ee esse snee e

see e

j Ts w (e) l

. 36.

Primary pressure decreases accumulators inject and secondary is Cooled.

S & W sMALL-GREAK LOCA (

FILL. COOL. ACC, VDITs)

AccDWULATot ECTiose a

y aen 8'

p r-p 1

s m

aee e.

a l

n

.h ses y

a

_a y

g I

i) g

/

e B

B 3

3

e see mise see aies aies asies aies es'oe o'se ess' e esse TI E (e)

Fig. 37.

Liquid volume discharged from accumulators.

. O

compared with the flow of 56 kg/s provided by the HPIS.

Of the 43000 kg that had b,een lost from the primary den filling and cooling of the steam generators began, about 33000 kg was recovered by the end of the transient, as shown in Fig. 38.

Accumulator injection produced a sharp dec ease in the vapor fraction from 0.96 to 0.65 in level 6 of the upper ple as shown in Fig. 39.

Howver, the vapor fractions in the upper plen not changed significantly by opening the high-point vents.

The u er of the downcomer (level 6) remained filled with vapor, and the vapor fra ion in level 5 decreased from about 0.2 to 0.1, dile th igh-point vents wM open (Fig. 40).

From the time natural circulation cease i loop ntti the end of this transient, the vessel vent valves remained open g fluid (largely steam) to flow from ths upper plenum (level 6) into the omer; the average vapor velocity of *.his flow was approximately 1 m/s.

The imary cooldown rate, dich began at 2414s and continu ntil the transient was teminated at 5400 s, averaged 76 k/hr (137'F/hr) s wre cooled throughout the transient by liquid injected from the nd accumulators), and their temperatures closely followd the 11 d (saturation) temperatures in the aa AK LocA (s ILL. coot. Acc. vtwis) mass Loss rnow resuAny ese o Q, )

meene y

m ese.

'essee c

d senee- \\

"$ d R

• • • • N

)

1 a

.sease i

i I

"5

,meae.

mome e e e e e e e see see see one' TiuE(e) i Fig. 38.

Primary regains most of mass lost through break with accumulators injecting. !"

.c l

3 & w spALL-aREAK LocA (5 C FILL cool. ACC. VENTS) l votD IN THE UPPER PL[NUM 4

u t= - ~~\\;;#%p ]

\\

l!

u.

k4=.A !hll#

j k

8 scuom s y

I g

sAsm s 1;

/

i 2

u-ti f

l I

8

\\

u.

e.

^

^

x

- ~.

s s s,eee,see,e,e,see ane

.een

{

Time (e) l i

Vapor fractions in upper p1 r

ffected by opening

andy-cane vents at 3835 s.

i ea

-antAK LoCA 5 G FILL. Cool. ACC. VENTS) volo fu T wucown n

u l

),

. g.

s-I

=

I i

I j!

).

u.

i i

i.!

l scuom a y

l f

• • m a y

c l

M

. l F

I u-1

(%

e one m'ee mee,ine,ien,ies,ine den anee ee' e seee e

Time (e)

Fig. 40.

Vapor fractions in downcomer are not significantly affected by opening Candy-cane vents.

(2)

Bumping Reactor Coolant Pumps

' 'One of the strategies that reactor operators can employ in attempting to recover natural circulation involves starting up one or more reactor coo 16nt pumps then shutting them down.after about one minute.

The small-break transient was restarted (at 3835 s) during the cooldown per with the accumulators injecting to test this strategy.

We chose to the loop B (combined cold-leg) pump; this was equivalent to starting he pumps in loop B.

Based on information from engineers at B&W, w assumed t the pump could attain its rated speed 20 s after starting.

The pump was a d to operate for 30 s and coast to a stop in 70 s.

he loop flows fo this accident and recovery sequence are shown Fig. 41.

Since natural circulation was not recovered on the first att at s, a second try was made at 4400 s.

However, this also failed to e sh natural circulation.

During the times den the pumps operated, the loo flows were not greatly affected.

[

Bumping the pumps produced 3

-convection flow of the two-pnase l

mixture in the upper plenum and the y r

loop-B candy cane through the loop-B steam gene rator.

Condensatio coo ng in the steam generator produced the rapid decreases in pr ry pressure observed in Fig. 42, coinciding with bumpin pumps.

Dur operation,f the loop-B pumps, the temperature differe bet en the hot-and cold-legs, shown in Fig. 43, becomes very small.

.pf p the loop-B pumps also established a sufficient pressure gradient betwe

+" anwopr and the upper plenum to close the vent valves and to keep them c d until the pumps had stopped.

The rapid drops in prim stimulate large injections of liquid by the accumulators, j

as in Fig.

However, after the pumps stopped the primary pressure remain table for about 6 min causing the check valves in the accumulator-i injection ing N close, temporarily terminating flow into the downcomer.

(

As seen in

, the large injection flow (10000 kg) that occurred the l-second time che pumps are started nearly restored the primary coolant mass to its pre-accident level.

'shile the pumps are operating, the vent valves are closed and vapor from the upper plenum and vessel head is swept out of the hot legs.

The vapor fractions in the upper plenum and downcomer decreased markedly dile the pumps wre operating, as shown in Figs. 46 and 47.

About 6 min after the pumps stopped however, the vapor fractions returned to the levels that preceded bumping the pumps.

This is also observed in Fig. 48, dich shows the vapor..

~

s

~

B & W shtALL-BREAM LOCA (s C FILL, COOL. ACC. PladPs)

LOOP WLOw RATES seu asse-(007) L0or-G Mass FLOW RATE (DASW) Loor-Al MALI Flow aart x

. (souo) Loor-A2 mass now aArt '

esee- )

- n E "" I

!i

'E g e,seJ

!<d

~

g p

it

= asse.

1'l.

m 3

1 sese-1 ll see <

j

'i X

W::..:.;..:..:::...\\.QQ...$.i. '

Xf

.eee e

sie see s an'ee sees ame des see anos but (e) 41.

Loop flow rates show effe pumps.

umping reactor Coolant S &-W SuALL-SREAK LOCA s FILL. COOL. ACC, PundPS) ^

s LOOP-B sTE 6KRATOR nee'

~

(soup) Loor storn Pasu a r s

- ~)

(oAsu) toor-e statu secoseAar

. z_

w uet ss

_7

.x" w

E

,. /e-

, s s.,'-*.

"e sie== moe seine aies des des des see asse f aut (3) s Fig. 42.

Primary pressure drops rapidly Wien pumps start at 3835 and V

4400s.

s

. N

~. -

s s

k o

s

_ _ _ _ _ _ _ _ _ - - - - - - ~ -

-w

(

1 S & W sWALL-SREAK LoCA (s G FILL Cool. ACC. PUtes)

Loop-3 hot AND COLD LEG LloulD TEMPERATURES

=

(SoLIC) Loop-8 HOT us (casa) Looe-e coon us E

m.

h

..-- ~..~.~.,i is m-i f

f-

!: k:g il

't.i. il:!

u:

l i

!! iLi il e

.t i :..

s 8

Il 1.

i!

t.

1

-t

.I m-l 1.j

(

age see sem nee anse aes.

.ses esos e

ese see Ti w (s) 43.

Hot-and Cold-leg tempera rence is small during pump operation.

e a w smAu.-antAx LoCA ( G FILL Cool. ACC. Pub #s)

ACCuuuLAT INJECTION

^

u y.

4 1

j m.

5 1

e.

l

\\

l g

/

3 e

e s'o see an'se aso asse ase ee'en esse esos see s

l l

Ta w (s) i Fig. 44, Abrupt drop in primary pressure produces large accumulator i

injection.

1 l

l I -

, i'

)

3 & W SMALL-GREAK LOCA (s G FILL. COOL, ACC, PUWPS) i WAss Loss FRoM PRlWARY essee

'I

.esee-e y seeee-I

-' senes.

i l

-e 8.

ee.se,see,eee,eee

,s.e

.ee

. 9e 45.

Accumulator injections ne er mass lost during transient.

t 3 & W SMALL-SREAK LOC C FILL, COOL, ACC, PUWPS)

VolD IN Y R PLENUW u g y

y j

e r w - & :;fI %.

.i^..a 7

4

}

an-s l

'N Yn~ u a.? 1 ff i

I i;:

j jij -

g l

scuow.tvet s l

2 m yct s i;

s 3

• b i!

I l

r'.!

li!

l

\\

3)

M V:: -

l.

-as.

see see sees mese aSe asse sees.see see.

ese Time (e)

Fig. 46.

I Vapor fractions in upper plenum decrease during pump operation then return to previous levels.

. i k

s.

4 4

8 0 t1 suALL-tRCAx LoCA (s G FILL. cool. ACC. PuuPs)

VotD IN THC coWNCou[R u

s.

,. - ~~.-

i!

i i!

u i

]

sou u m

! ik -

t

,1 mm *

l l/

1 u.

?

f

..i:

f Y

==

=

^

1 s

J

\\

?

\\

1 u.

-u.

m.

m.

{

Tim. (.)

Fig. 47.

Vapor fractions in downe j

then return to previous lev 1.

crease during pump operation i

e a w suALL-eRCAx LocA (s r:LL. coot. Acc. Puurs)

MPoR FRACTaoNs 1 of CAM)Y CANES u

~

(50s.10) LOOP TOP OF CaseY CAE (DASN) LOOP 4 TOP or CAaeY CAM s.

.m V) i i

l ! 5..

i si

i. ii i u.

E

j:i :

.i l.

i i i

2.: : :

E I

.- ::n ii

(.P:! -

l u.

i

, v:-

Tiut (4)

Fig. 48.

Vapor fractions in Loop-B can@ canes decrease during pump operation then return to previous levels.

4 -

1-i stopped however, the vapor fractions returned to the levels that preceded bumping the pumps.

This is also observed in Fig. 48, which shows the vapor fractions in the candy canes.

The average vapor content of the two-phase mixture circulated by bumping the loop-B pumps was less than 20% of the total primary volume, however, natural circulation flows wre not m ained after being stimulated by pump operation.

The calculation was terminated at 4800 s after two e

to restore natural circulation by bumping the loop-B pumps had failed.

At t time the pressure was 2.1 MPa, and the cooldown continued.

l (3)

Liquid Injection Into Hot-leg Candy Canes The presence of valves at the top of the

-leg andy canes affords the

)

possibility of injecting cold liquid inte.

er ns.

To evaluate the effect of subcooled liquid introduced into the

-filled candy canes, the calculations were restarted at 3835 s with a FILL onent connected to the high-point venti valves.

Ten percent of the SIPIS flow

.7 kg/s) was injected

{

into the candy canes of each loop the cold-leg injection flows were reduced to 90% of their original deli y

t L

Dividing the HPIS cold legs b tMe the, and hot legs of each loop enhances the accumulator injection flo and hence refilling of the primary.

Figure 49 shows that cumulators a e discharged about 19 m of liquid 3

at 4800 s into th ans t with hot-eg injection.

From Fig. 30, only 3

12 m have been dis ge ft this time with all the HPIS flow supplied to the cold legs.

When e M"n n enz was terminated (at 5300 s), most of the i

mass lost through the br had been recovered by the combined injection of accumula+

PIS; the cumulators injected 26 m3 (26000' kg) in 2000 s (Fig.

giving average injection rate of 13 kg/s.

Thus, over the last j

2000 the trap ifent, flow from the accumulators was equivalent to an increase 3% in 'ie HPIS flow supplied to the plant.

The p pressure, shown in Fig. 50, continued to decrease as the plant cooled, eaching a value of 1.64 MPa (240 psia) when the calculation ended.

Injecting cold liquid into the candy canes caused flow surges in the hot legs similiar to those observed earlier in the transient before accumulator injection began.

These flow surges became more frequent and larger in amplitude, as the primary refflied.

This behavior can be observed l

l from variations in the candy cane vapor fractions shown in Fig. 51.

During the 1500 s in sich liquid was being injected into the hot-leg candy canes, the vapor fractions in the upper plenum (Fig. 52) and downcomer (Fig. 53) wre l -

I n *w

i o,

4 e a w suALL-aREAK LOCA (5 C FILL. COOL. ACC. SPRAY 3)

ACCUMULATOR INJECTION

=

m.

c E

P m.

Is.

m i

a I

I g

4 o

3 B

\\

a e

A I

l

e eso sino neo ae'ee ae'ee asies seios sees p

i

.)

'l '

Fig l

Accumulator injection en ed by hot-leg sprays.

3 AK LOCA (

FILL, COOL. ACC, SPRAYS)

LOOP-A STE GD(RATOR es.

j j

I (SOLID) LOOP-A STODe PRIMMtY t

'y) LOOP-A STGEN SEC0eSARY g3gt Ta, I

l f,

{

~.

,N-4,3g8 v.,

I 4

e es.

moe asas m e asas anse.see see sees esse TihE (e)

Fig. 50.

Primary pressure for transient involving

cooling, accumulator injection and hot-leg infection.,

1.

J s a w suAtt-aREAx LoCA (s C FILL. COOL. ACC. SPRAYh)

WPot FRACTIONS IN top OF CANDY CAES j

u (50Llo) LOOP-A TOP W CAsey Cast (DASH) LOOP-8 TOP OF CAsef Cast j-rn r.li1 f

o E

8 I lii I

p g;

i e:

1 I

i i.

i l

w j

l AJ A

.e ss

,ese s.ies e.see

)

l It Fig 1.

Vapor fractions in Canc(y Ca show effects of sporatic p'-

flow surges hot-leg i tion.

?

-aRCAK LoCA ( G FILL. COOL. ACC SPRAYS)

) 1 VotD IN THE y

. UPPER PLENUM u

7 1

I b

.E.

i 4'

f

\\l

\\

b

/

V

! % b.A:;} l' i

i

• +

g r

soolo Lrvet s

.2 f

8'N 8

!! i5l.

.I 1

i j'.

1 1'

e ese s s aies ass ase sees aies eies aies.see r i g. W(. )

Time e Vapor fractions in upper plenum reflect flow surges produced by hot-leg injection.

l..-

c r

t.

l a 4 W sWALL-alttAK LoCA (s C FILL Cool. ACC. sMtAYs)

Volo IN THE DoWNCoWR

~-..:......,.,. 7 s-I

!!! :5f u.

3 f.! !-

]

f scuo-LEvEt s it l

SAMVEL s lj!!.

f i

2 u-9?

i u-i

/

i.

?'"

I e

i

}

e ese so use ase as' n asha aske. sin e w asse e

(s)

Fi b

Vapor fractions in downcomer crease significantly with p

hot-leg injection.

l' decreasing.

The reastr$ liquid fractions in the upper plenum and downcomer, accompanie e more frequent flow surges through the loops, caused the radially out i s o., 14(ough the vent valves to decrease markedly, j

so that these valves bega losing.

When the calculation was teruinated i

recover circulation flows appeared imminent.

1 i

III.

USIONS Al l REC 0tEENDATIONS In bse of operator action to mitigate the plant response to a small-break L the void volume in the primary will increase until the outflow through the break equals the inflow from the HPIS.

This occurred at about 6000 s in these calculations, for sich the critical flow out the break corresponded to that for a break with an equivalent diameter of 0.039 m (1.54 in).

, Operator actions to cool the plant are effective in depressurizing to the setpoint of the LPI pumps even though the accumulators are isolated, and natural circulation flow is not reestablished.

Cooling of the core is by HPIS flows dich mix in the downcomer with steam flowing out through the vessel,

I e,

te l

vent valves.

A very wak themal coupling exists betwen the saturated fluid in the vessel and hot legs and the subcooled liquid in the steam generators, loop seals, pumps and cold legs.

This coupling is characterized by sporadic flow surges from the hot legs into the steam generators. As the liquid in the steam generators is cooled, it becomes heavier and intemittent ttles into the cold legs. This draws saturated fluid from the hot legs ugh the candy l

canes and into the steam generators.

Betwen one and tw hours into the small-break transien perator actions (opening vents and injecting sprys into the hot-leg candy c and bumping the coolant ptsups) wre found to be inef ual in restoring natural circulation flows.

The inability to restore a aint natural circulation is attributed to the steam volume present in y and the behavior of the vent valves in the vessel.

We concluded tha y or volumes 5 20% of the primary volume. (excluding the pressurizer) would necessary to support natural circulation flows.

Although the mass lost thr h the break can be replaced by inflow from the HPIS ooldown, the void volume remains r

relatively unchanged as the specific 1

liquid decreases.

After I

two hours, the plant had depressurize a condition such that recovery of natural circulation flow was not require o continue cooling the core.

An Mditional c (of plant c oldown beginning issnediately after natural circulation w ce d and including accumulator injection /Troduced similar results.

Th tor injection flows added 14 - 23% to the HPIS flow, thus increasing

~.^.,. :t Mitch the primary regained the mass lost through the break.

Howver is was offset to a large extent by more rapid cooling surization, so that natural-circulation flow was not reest shed befor

he plant had cooled nearly to the LPI setpoint.

the coa ldown transient with accumulator injection, both the loop-B p.

re arted and allowd to run for 30 s before being tripped.

This was twice in attempting to artificially stimulate natural-circulat on flows.

Although this technique proved ineffective, our evaluation of the plant behavior leads to the conclusion (nat starting one pump in each loop with operation continuing for 60 s may be more effective in establishing natural circulation.

' Venting steam from the hot-leg candy canes should be effective in recovering natural circulation because the venting reduces pressure in the upper plenum, thereby permitting the vent valves to close.

This did not occur l

v.

v.

t o =

l in *our calculations probably because the high-point vents (1-cm diam) were too small to relieve all the steam vaporizing from the surface of the fluid in the vertical section of the hot leg.

The use of a larger valve could be effective in establishing a

closed and liquid-filled flow path.

wever, the possibility of the valves failing open and causing or aggra g an accident would raise additional safety concerns.

The injection of 10% of the HPIS flow into the candy ca

• eems to provide the most promising means for recovering natural cir.

tion.

Condensation of vapor in the hot-leg candy ca reduces the pressure and stimulates ficw from the upper plenum towar e ste generators, thereby closing the vessel vent valves.

The cbserv h

r makes this strategy promising for establishing natural convectio Obviously, the

valves, controls, and piping necessary to accomodate the pratys tould introduce ri additional safety concerns and increase the probabili of small-break LOCAs from the hot-legs.

We recomend that similar ca c la o performed to evaluate the observed behavior for a range of brea es.

dditional calculations should use lower rates of cooldown to allow e time for evaluating strategies for recovery of natural c fon before plant is depressurized to the LPI setpoint.

The siz hig -point vents, the hot-leg injection flow, and the strategy for startin, ol pumps should be treated as parameters in further calculations...The goa

' thr-Malyses should be to define the conditions l-necessary for reestablish 1 nd maintaining natural circulation flows during SB-LOCA 1

W'

,.{..

RE(ERENCES 1.

NRC Memorandum from R. J. Mattson, NRR/DSI, to 0. E. Bassett, RES/DAE,

" Request for TRAC Calculational Assistance," dated October 23, 1981.

i 2.

" TRAC-PF1, An Advanced Best-Estimate Computer Program Pressurized Water Reactor Analy si s,"

Los Alamos National Labor report (in s

preparation).

3.

" Final Safety Analysis Report, Oconee Nuclear Station, Units 1, and 3,"

Duke Power Co. (May 1969).

4.

G. J. E. W111 cutt, " TRAC-PD2 Calculation of Small Cold-Leg Break in a i

Babcock and Wilcox Lowred Loop Plant,"

Al National Laboratory i

infomal report LA-SBTA-TN-81-3 (October 8

l 4,

p..

n.

Y i

. - 1

ENCLOSURE 2 PRELIMINARY EVALUATION OF LANL CALCULATIONS OF B&W SMALL BREAK BEHAVIOR Although we have not completed our detailed review of the LANL calculations in this matter, we have reviewed their draft report. We have determined that the LANL results do not indicate an imediate safety problem for the reasons stated below.

The LANL calculational results are not inconsistent with staff testimony at the TMI-l restart hearing. Licensee and NRC staff witnesses testified that natural circulation could exist at B&W Plants in two basic modes.

1) Single phase natural circulation in which water would flow through the coolant loops driven by density differences and 2) Boiler-condenser in the steam generators.

It was further stated 4

that natural circulation in mode 1 could be blocked by steam formation at the 3

top of the hot legs. This volume of steam was stated to be about 360ft per loop or approximately 7% of the primary system volume.

It was further stated that for natural circulation to occur in the boiler-condenser mode, a steam condensing surface must exist. Post TMI-2 accident emergency operating procedures require that the secondary system steam generator water level be raised to 95% of the operating range following a small break LOCA.

If the primary system water level were above the 95% water level on the secondary side, the condensation surface would be lost and natural circulation would not occur. The core would be protected however, since the 95% water level is well above the elevation of the top of the F-core and continued water loss from the break would establish natural circulation in the boiler-condenser mode before the core could be uncovered. The 95% level Therefore, corresponds to a primary system void fraction of approximately 20%.

i for void fractions of between approximately 7 and 20%, single phase natural circu-lation is not expected to occur in B&W designed reactors. The 20% void fraction value is consistent with that report by LANL.

No core uncovery occured in the LANL calculations. The LANL calculations indicate that although very little natural circulation occurred, the core remained covered and cooled. The calculations indicate.that the reactor system lost mass for approximately 75 minutes and then began to fill. At the end of the calculations (2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />) the reactor system was calculated to be gaining coolant inventory.

The calculations were not continued far enough in time to determine if natural circulation would be established. As previously stated, the reactor system was still filling at the end of the LANL calculations.

In a previously submitted TRAC calculation for which the break was isolated, the analysis was carried out suffi-ciently long that the reactor system completely filled and natural circulation was calculated to be reestablished in the single phase mode. The NRC staff is currently evaluating the validity of the TRAC code regarding these results.

The break size assumed by LANL was too large for natural circulation heat removal to be required.

Restoration of single phase natural circulation is not required at B&W Plants for break sizes large enough to remove decay heat. Analysis performed by B&W* and presented to the TMI hearing board indicated that no secondary cooling 2

was required for breaks of 0.02 ft and larger.

For break sizes of.01 ft2 and smaller natural circulation and steam generator decay heat removal was shown to be required by B&W if only one'ECCS train was operational. The LANL calculations assumed two ECCS~ trains were operational. The additional ECCS flow would condense i

more decay heat steam than in the B&W calculation so that a break size of less than 2

the.01 ft would remove decay heat. This is because the break flow would be increased as steam flow was decreased.

The draft LANL report indicated that the l

l i

• " Evaluation of Transient Behavior and Small Reactor Coolant System Breaks in the 1/7-FA-Plant" May 7, 1979 y

break size of their analysis was equivalent to an effective diameter of 1.54 inches or 0.0129 ft.

The draft report indicated that the break was sufficient to remove all decay heat. This result is consistent with the B&W calculations.

Plant operating procedures do not require that natural circulation be restored for the reactor system conditions calculated by LANL.

The draft LANL report incorrectly indicated that the operator would attempt to restore single phase natural circulation b/ bumping reactor coolant pumps and would be unsuccessful.

In fact restoration of natural circulation would not be required by the procedures since the core is expected to be cooled and the primary system depressurized.

The available ICC instrumentation (core exit thermocouples) would be used by the operator to confirm adequate core cooling. The operator is required to raise the secondary system water level to 95% of the operating range to establish a condensing surface well above the top of the core and to cool the primary system by controlling secondary system steam flow.

The TRAC analyses indicated that the operator would be successful in performing these operations required by the procedures.

A limited amount of two-phase natural circulation occurred in the LANL calculation.

This is evident from comparing the calculations of primary system depressurization response when operator action was assumed to cool the secondary system and the base case when no operator action was assumed.

For the depressurization case the primary system tended to follow the secondary system pressure decrease indicating that the boiler-condenser mode of natural circulation was removing some heat from the primary system.

The TRAC Code which was used in these calculations is still in the development stage and neither the code nor the input for these calculations has been subject to detailed evaluation by NRR, and there has been no QA by the contractor as yet.

l l

We have not yet had time for a detailed review.

However, even if the analyses show l

unexpected SBLOCA recovery behavior, it could be due to modelling considerations t

, rather than real phenomena.

We have requested RES to expeditiously examine these

-analyses and confirm their validity.

Comparison between the direct TRAC results and those by B&W will be difficult to-make however, because of differences in break size, assumed number of ECCS. pumps and-the inability of the TRAC code to calculate finite coolant levels within the reactor system.

r

,'W 6

\\

n MAY 7 1532

~.

Dr. He..ry :'yers 1Sub:0 c.ittee on Energy and the Environment Committee on Interior and Insular Affairs United States Ecuse of Espresentatives 1.'ashington, D.C.

20515

Dear Dr. l'yers:

.In ectly April you requested the NRC staff to cuent en a statement in

. paragraph 619 of the TMI Restart Pa,rtial. Initial Decision.

The statement read as follcus:

"If, however, the voids are steam, as would be expected in a small-break LOCA, the bubble in the hot leg should.be compressed and con-darsed as the primary system pressure is increased by operation of the H?I system."

The context of this statement in paragraph 619 is a discussion of the recovery frca a'small-break LOCA,when ECCS injection ficw exceeds break flow so that the reactor system is filling.

The discussion. addressed a Union of Concerned Scientists (UCS) concern that for this condition a steam bubble in the top of the hot leg U-bends might prevent the reestablisFqent of single-phase natural circulation.

Mr. Kam.erer of NRC replied to your request in a letter of April 8.,-1932.

The response included the following coments:

"If a steam' bubble exists and primary systea pressure is raised, the bubble will be compressed and there will be condensation.. The con-densation o: curs because as you raise system pressure sys' tem temoera-ture drons below satur.atioi~,

Condensation must occur to reach satura-tion conditions again.

The bubble'will not necessarily be completely condensed. This depends on bubble size and pressure change."

(Sentence was underlined in your April 19, 1982 letter.)

In a letter. of April 19, 1982, you asked if the quote uriderlined above from our April 8,19S2, response was correct.

Specifically, you questioned."... 'whether, in the case cf a small break LOCA, the heat transfer would be sufficient to keen the steam space from expanding into the core." The underlined quote is not correct because there cust be heat transfer to a heat sink to absorb the heat of vaDorization associated with the ccndensation of steam, and the heat transfer' rate is imhortant.

. Also, the system temperature does not actually drop; rather, the. saturation te perature increases with increasing pressure.

However, while the cond'ensation

~

rt.e of the hot leg bubble will affect the cverall recovery behavior, it does not, by itself, 5:vern the cverall adequacy of decay heat remeval during a small break LOCA in a plant designed by BL'. To be certain that there is no siailar nisunderstanding reprding this catter, we are providing a ccpy of this letter to the Licensino-J.:n.' Sod _sd]1 :-h -be. 4.pp a.l. EW nuds_hm.dur4 : h oc-A g - M M -

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he Mt l'..; '.'-hends under off-r.cn al c:ndi-icns is cc Plex.

The detailed hha.ict of the reactor undar these c:nditiens is still being actively studied by 5., tbs 50.' plant t':ners, and the staff.

Although uncertair. ties rer.ain, W

r..:hding heat trat:sfer rates for steam c r.dansaticn, We ha'.:e reache'd s;.me

. ;er.ar.a1 c:a:1esions about the effect of'these en:ertainties and the potential

-for ;.roducir,g unacceptable 'ccre heatup.

These conclusions are based en calculati:ns by EoM and/or by a staff centractor of a rur.ter of postulated snzil break LO:As.

Ue have concludc-d that the un:artainties can be chysically boundad tnd that these bounding assur.ptier.s do not produce ur. acceptable core heatup.

In the enciesure, the staff addresses your cuesticas re-garding the ir. pact of steam cor. der.sati:n rates and supporting analyses and o'escribes h:w the use of bounding assa r;tions :culd not result in macceptable core heatup.

Calculations related to these' conclusions are identified.

There are still uncertainties in the therr.al-hydraulic response of the B&W design undar seme of these conditions. We are pursuing with B&W and B&W plant cuners the details of the thenr.11 and hydraulic phenomena involved, including the possible r, sed for additional sensitivity analyses and a soall scele, inte-gral systems test.

0;r objective is to confirm the ar.a.lytical results described in.the unclosure and to aid in our further analysis of r. ore cernplex, cultiple failure events being studied in the context of the new symptem-criented, eargency procedure guidelines.

SinMcrel r d11 o

H.R. h.. ten i

harold R. Denton, Director Office of Suclear React'or Regulation

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Enclosure DYNAMIC RESPONSE OF B&W REACTORS TO SMALL 2REAK LOCAS Detailed analyse,s have been performed either by B&W'or by staff contractors for the two classes of small break LOCAs:

(1) those that can be subsequently isolated (e.g., letdown lines, PORVs), and (2) those that are not isolatable.

They are discussed in that order, below.

Small' Breaks Which Are Subseouently Isolated We have had our contractor, the Los Alamos National Laboratory (LANL) perform an analysis of a small break in the cold leg of the B&% reactor coolant system

~

that is subsequently isolated.

The calculations were perfomed with the advanced TRAC computer code, and we have some preliminary results. We are not aware of any calculations that have been performed by B&W in which a small break LOCA was subsequently isolated.

in our analysis, the system was assumed to lose primary coolant from the break until the upper vessel head region, pressurizer, and hot leg U-bends were filled with steam.- At that time, the break was assumed to be isolated by the operator.

Approximately.1,000 seconds later, it was assumed the operator began a controlled secondary system depressurization.

The analyses showed that the flow of cold water from the two high pressure injection pumps, coupled with controlled secondary. system depressurization, would condense the hot leg bubbles and restore natural circulation. "We are still evaluatino the ability of the TRAC code to model the heat co,nduction in the liquid near the steam-liquid ~ interface which is required to accur'ately cal-culate the primary system refilling process. However, steam condensation rates are not expected to influence the overall conclusion that no unacceptable core heatup would occur,.as explained in the following paragraphs.

if the steam was condensed at 100% efficiency by the cold HPI water, the top of the hot leg U-bends would refill.with liquid and single phase natural circulation would be restored.

If, however, the steam condensation rate is very low, and, in fact, in the limit the steam is assumed not to con-dense at all, two possibilities would result.

usz m on v Certified 3y V) b

-1

2 The first possibility is that the HPI pumps would repress'urize the system suf#iciently to compress the steam to a small enough volume,to allow liquid on te e upstream side of the hot leg U-bend to spill over into the downstream side and resume natura1 circulation.

If this did not' occur, the HPI would continue to inject ECC water and repressurize the reactor coolant. system until.the pressure re' aches either the PORY or the safety valve set pressure.

We would expect that core cooling would be maintained by a " feed and bleed" process until the steam bubble at the top of the U-bend eventually condensed by heat transfer to the pipes and.across the liquid vapor interface. Once the bubble condenses, single phase natural circulation throughout the steam generators would be resumed.

The other possible behavior if the steam condensation rate is low is associated

~

with a design difference in B&W reactors.

One plant designed by B&W, the Davis-Besse plant, does not have a "high head HPI pump (one that can pump water into the primary system at or above the safety valv'e. set point).

The shuto.ff head of this pump is about 1700 psi.

In the event the cold water'from'the HPI pumps does not condense the steam in the hot leg U-bend, the system.may repressurize to above the shutoff head of the HPI pump, and eventually ' reach the PORV or the safety valve setpoint.

The system will begin to lose primary coolant through the PORV or the safety valves and drain down.

Once the. primary coolant level

'on the downstream side of the hot leg U-bend extends into the steam generator tube region below the condensing surface of the secondary coolant, steam in the primary system will begin to condense, lowering the primary s'ystem pressure and closing the PORV or the safety valve.

As the system pressure decreases below 1700 psi, the HPI will actuate and begin to fill the primary systerh.

This would result in covering the condensing sur-face in the steam generator, and producing another repres'surization, which~,.

in turn, could stop HPI flow and cause the PORV or the safety valve to open and release enough coolant to reestablish a condensing surface.

A number of these cycles may occur before the charging system completely refills the system or before the steam bubble is condensed by heat transfer to pipes and across the liquid vapor interface.

Since the condensing surface in the steam genera-tors is above the elevation of the top of the core, natural circulation should be established before the hot leg steam bubble extends into the core.

DESIGNA$30?.Icia' Co?tified Bw W /

rw Small Breaks Which Are !!ct Isolated Small break LOCAs which are not subsequently isolated have been calculated by both B&W and by the staff's contractor, LAliL.

The analysis performed by 2

B&W was for a 0.01 ft cold leg break. They used a, computer code that conforms to the requirements of Appendix K to 10 CFR 50.

With the exception of the assumed number of HPI pumps availabl.e,'the modeling assumptions required by Appendix K do not affec't the thermal-hydraulic models of interest for,the, small break LOCA.

Thus, the results of the hydraulic analysis should be realistic.

The 0.01 square foot break was selected since this size is insufficient to remove decay heat via break flow (thus requiring the steam generators for decay heat removal).

It was also,predic.ted to result i,n t'he repressurization phenomenon for the reactor coolant system.

From these analyses, B&W concluded that a range of small break sizes could be postulated in which steam generated in the core would accumulate at the top of the hot leg U-bends and cause an interruption of natural circulation flow.

The interruption of natural circulation flow would isolate the steam being produced in the reactor core from the steam generator heat sink. The-net steam accumulation in the system was calculated to cause the primary system to repressurize.

Th'is Tepressurization was calculated to continue until the primary system coolant' loss through the break was sufficient to uncover a steam cond'nsing surface in the steam generators.

It is expected e

that some steam generated in the core would flow into,the upper elevations of the downcomer annulus via the vent valves and conde.nse in the colder water in that region.

However, cold water from one HPI pump was not calculated to be sufficient to condense all of the steam generated in the core.,

Similar to the isolated break case previously discussed, the repressurization of the reactor coolant system caused by interruption of natural circulation would lead to a boiler-condensei mode of two-phase natural circulation, and subsequently reduce system pressure.

Once the HPI flow is calculated to' exceed the break flow, the system coolant inventory will stop decreasing and begin to increase.

In figures 1, 2, and 3, the temporal behavior of systein pressure, liquid level in the hot leg piping, and liquid level in the reactor vessel are shown for this case as calculated by B&W.

The B&W^ analyses submitted to. the staff terminate at about the time system inventory begins to increase.

However, the continued recovery of the event is considered relevant to your concern, and is described further below.

DESI N TED 0.U J.G Certified By t(n,

p~

, As the system refills, the steam condensing surface in tile steam generato d at will again be recovered by liquid, and a steam bubble will.be trappe The scenario is now-expected to proceed

~

the top of the hot leg U-bend.

That is, if the similar to the isolated break case.previously described.

steam is rapidly condensed during the refilling process, singl.e phas circulation will be reestablished and primary system pressure will rema If the steam trapped at the top of the with no significant repressurization.

hot leg U-bend is not rapidly condensed, the. system would repress (1) the break flow exceeded the HP,I. flow and a condensing (2) the system repressurized and compressed the steam to a sufficie small enough volume so that water upstream of the hot leg U-be'nd coul spill over into the downstream side of the hot leg U-bend and reest natural circulation, or (3) the PORV/ safety valve setpoint was reached.

For the Davis-Besse plant, we believe only option I would occur since t HPI pumps are not sufficient to pump water into the system at or abo safety valve setpoiot.

The staff has also been calcul'ating and analyzing the response of d to be inter-reactors to small break LOCAs in which natural circulation is predicte Our con-rupted by steam accumulation at the top of the hot. leg U-bends.

tractor at LANL has recently completed a few.smal,1 break analyses a looked at four recovery enhancement actions presently either preposed These four options are:

(1) high B&W or being considered by the staff.

point vent operation, (2) momentary pump restart, (3) secondary s All pressurization, and (4) ECC spray at the top of the hot leg'U-bends.

of these options are being investigated to. determine their ability to en the reestablishment of single phase natural circulation during the recovery Initial results of our contractor's calculations portion of the accident.

show that for a realistically calculated small break (i.~e., nominal d two HPI pumps available, etc.) in a B&W plant, with a break size'in th d

of that for which B&W predicts repressurization would occur, the sy Although our not repressurize once the hot leg U-bends filled with steam.

l i

evaluation is not yet complete, we believe that the rea' son the LAN did not show a repressurization is because steam generated in th d the flow to the upper reaches of the vessel annulus via the vent valves, an from two HPI pumps was sufficient to condense all of the stea DESICATED ORI.G AL 6

h_

certined By

. ~

The results of the TRAC analyses are shown in figures 4 through 7.

In figures 4 and 5, the B&W results are overlayed to show the differences.

We believe they can be attributed to two versus one HPI pump being available. The results of the

. analyses to investigate natural circulation recovery enhancement methods were only recently presented to the staff at a meeting with LANL. These ana-lyses have not been documented by LANL in a formal report, and we have not re-viewed them in any detail. However, based on information received at the meet-ing with the contractor, the results show that the hot leg U-bend was not refilled following recovery from the small break-LOCA (1.75 inch diameter), and that none of the natural circulation recovery er.hancement methods previously listed were effective.

However, LANL reported that the core remained cover *ed and decay

. heat was continuously removed from the core. They attributed the heat removal to internal recirculation (steam exiting the core is vented to the vessel upper annulus and condensed by the cold HPI water entering the downcomer). This situa-tion physically could only persist until the decay heat was eventually removed entirely by the break flow or the system eventuall'y Gas refilled and natural circulation reestablished.

Before widely disseminating the results of the LANL calculations, we have asked our Office of Regulatory Research to carefully document and evaluate them and assist us in confirming their val'idity. We believe this careful approach is justified because the analyses showed that core decay heat was continuously removed and that no core uncovery or heatup was predicted.

In summary, although we are continuing our evaluation of the rate of. hot leg steam bubble condensation in.the recovery from both isolated and unisolated reactor coolant system sma.ll-break LOCAs in reactors like TMI-1, we do not believe that steam bubbles present in the reactor coolant. system resulting from small break LOCAs (either isolated or unisolated) in either the cold or hot legs of the primary system will result in unacceptable heatup of the core.

ESI D0 hAL

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rooe Ttut (s)

Figure 5 Co=parison of IEAC a52 MW primary pressures.

a

~

i B&W LowEltED-LOOP 0.01-F72 COLD-LEG #1 TEAK *

- L2 4

-g G

<g u.

W u-o 52 u.

Wo u

u.

o o

too 2eco 3 coo seco booo Sooo 7eco

.Tiut (s)

Figure 6 TF.AC-calculated core liqul~d volu=e fraction B&W LOWC3tED-LOOP 0.01-rT2 COLD-LEC BREAK 4

ew 0

ge 600 p

e d

5 p

4 1

g soo.

e l~

k i

see.

Q,

=

g sto.

k l

m s+o.

y m.

2 I

Sao o

eco

ooo sooo

. coo sooo sooo roco Tiwt (s)

I l

Figure 7 j

TF.AC-ca):ulated maxi =u= average rod cladding te=perature.

l i