ML19322D979

Potential Reactor Sys Voiding During Anticipated Transients

ML19322D979
Person / Time
Site:	Crane
Issue date:	01/31/1980
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML19322D978	List: ML19322D977 ML19322D979
References
NUDOCS 8003130534
Download: ML19322D979 (32)

Text

_ ___.

. __ _.._ _ _ J. _.. i. __._

__i

~

e POTENTIAL REACTOR SYSTEM VOIDING DURING ANTICIPATED TRANSIENTS f

PREPARED BY BABC0CK & WILC0X NUCLEAR POWER GENERATION DIVISION LYNCHBURG, VIRGINIA JANUARY, 1980 8003180 4

CONTENTS 1.0 Problem Introduction and Summary 1.1 Introduction 1.2 Summary 2.0 Analysis, Assumptions and Event Descriptions 2.1 Analysis of Steam in the Upper Vessel Plenum 2.1.1 Stagnant " Hot" Water 2.1.2 Residual Heat Stored in Metal 2.1.3 Conclusion 2.2 Analysis of Steam in the Candy Cane 2.2.1 Pressurizer Outsurge Water Mixing in Hot Leg Flow l

2.2.2 Residual Heat Stored in Metal 2.2.3 Conclusion 2.3 Event Description for Mass / Volume Balance 2.3.1 Description of Analysis 2.3.2 Derivation of Equations Used 2.3.3 Mass / Volume Analysis 2.3.3.1 Davis Besse 9/18/79 Trip 2.3.3.2 Davis Besse 10/15/79 Trip 2.3.3.3 Davis Besse 9/26/79 Trip 2.3.3.4 Oconee I 10/8/79 Trip 2.2.3.5 Oconee II 1/4/74 Trip 3.0 Conclusion 4.0 References

List of Figures 2.2.1 Column W41dment Flev Paths 2.2.2 Upper Plenum Flow Paths 2.3.1 RC Volume Breakdown 2.3.3.1 TECO 9/18/79 Trip 2.3.3.2 TECO 10/15/79 Trip 2.3.3.3 TECO 9/26/79 Trip 2.3.3.4 Oconee 1 10/8/79 Trip 2.3.3.5 Ocones II 1/4/74 Trip A-1 Pressurizer level vs. corrected pressure tap measurements (320" pressurizer)

A-2 Pressurizer level vs. corrected pressure tap measurements (400" pressurizer)

A-3 Error in level indication vs. indicated level (400" pressurizer)

A-4 Error in level indication vs. indicated level (320" pressuriz'er)

Attachnents Pressurizer Level Error e

~

~

1.0 PROBLEM INTRODUCTION AND

SUMMARY

1.1 Introduction The NRC has expressed concern that steam pockets can and do form in B&W primary systems during some reactor trips and furthermore, this steam could collect in the candy cane and potentially hinder natural circulation after loss of offsite power. The source of this steam is postulated to be from stored heat in metal, stagnant pockets of " hot" water, or pressurizer outsurge water (650 F); any or all causing flashing during low pressure periods after a trip.

The purpose of this report is to show that the formation of a steam pocket in the primary system during most reactor trips is highly improbable and that the production of a volume of steam required to hinder natural circolation is virtually impossible.

1.2 Sumary Based on review of several past BEW reactor trips along J

vith several assumptions and calculations discussed in Section 2, the following statements can be made:

occates (1) Minimum RC pressure after trip /5about the same time as the minimum coolant temperature (as expected) and not because of a primary system " Steam Pocket" restricting a further pressure decrease.

These minimum temperatures correlatewith secondary side steam pressures.

(2) With the most conservative assumptions steam pockets are virtually impossible during transients where minimum measured pressure is greater than 1740 psig.

l

• Specific concern per reference 1 l

\\

\\

(3) Using more realistic assumptions regarding potential steam production, it is unlikely that any net steam production can occur at measured pressures greater than 1500 psig.

2. 0 Analysis, Assumptions and Event Descriptions 2.1 Analysis of Steam in the Uoper Vessel plenum 2.1.1 Stagnant " Hot" Water Typically, the hottest bundle in a 2772 MWT core l

is a 1.5 relative peaked bundle. This localized power could conservatively result in localized stagnant water temperatures of s 620*F in the upper control rod guide (column weldment).

For this situation to occur, at least 53 in of flow area in the column weldment (see flow path 2 on figure 2.1.1) would have to be blocked.

If this blockage could occur, then the following statement can be made: Ariy B&W reactor trip resulting in minimum RC tai pressures les.s than 1740 psis (the tap pressure 1s s 40 psid lower than core plenum pressure) voiding may occur.

Since this blockage is highly unlikely, a somewhat more realistic hot spot assumption would be localized maximum temperatures of 610*F (instead of 620') in the upper vessel plenum. This assumption is based on (1) water stagnating (or having an extremely low flow rate) in the upper plenum (per Fig. 2.1.2) and acquiring the steady state enviromental temperature of the 59 column weldment flow temperatures and (2) plugged return holes (per figure 2.1.2, Flow Path 3). This condition would not allow flashing until the pressure decreases to 1660 psia, i.e.,1605 psig at the pressure tap.

Typical outlet temperatures were verified from TMI-2 pre March,1979 power outlets thermocouple data.

The most realistic assumption is the desian condition of s 5" core flow through the guide tubes into the upper plenum. Folicwing a reactor trip, the coolant temperature in this region will decrease rapidly to below 600F as core outlet temperaturedere.ces eliminating the potentigt for " hot" water pockets at any pressures above HPI injection levels.

2.1.2 Residual Heat Stored in Metal After a trip, the metal in the upper vessel will be conservatively assumed to remain at 610 F until a minimum pressure is reached.

The upper plenum coolant temperature at this minimum pressure should be s 550 F (in this region) but 575 F will be conservatively assumed due to hypothetical " slow" mixing. The available stored energy in the metal will be:

Stored Energy = Cp x Mass x AT Cp, =.11 BTU /lb, F'

AT = 610 - 575 = 35 F Mass,s 5x10 lbm = total mass of metal above the outlet hot leg piping including reactor vessel Stored Energy = 1.925x10 BTU's The amount of water in this region is 1366 ft.

3 4

1366 ft.3 x 44.5 lbm/ft = 6.08x10 lb of water Conservatively assuming this energy is instantly released to the water, the water temperature will increase from 575 F to s 601 The pressure required for flashing at this temperature is s 1505 psig (at the pressure tap). Therefore, in order to fill this region with

• This is water density at 575 F,1600 psia.

6 'TU's/6.08x101b = 32 BTU /lb or s 26 F tamperature increase.

4

• • l.925x10 B

e

[:b steam (to cause spillover into the hot 1 cgs) from just residual

[L heat, 3 times core mass of hot metal would be required to provide

[

the 547 BTU /lb. necessary to vaporize enough water to produce h>

w 1366 cubic fcct of steam.

I(i

$1 2.1.3 Cc,ne,1usion d

The previcus calculations arid discussion basically show that (i

it is impossible to produce stcaat in the upper head above a

)

o measured pressure of 1740 psig, highly improbable abeve 1620 psig,

)

5 and highly unifkely above 1500 psig.

(Very few transients have j

f ever gone belcw 1600 psig.) This statement assu=es no primary i

system breaks.

j E

[

2.2 Ansivsis of Steam in the candy Cane 2.2.1 Pressurizer outsurce watcr mixiric with hotJg flow f

At the time period of minimum pressure after a trip the hot Icg coolant ta=i:arature is + 550'f during 4-pump operation and E

i s 570"F during ratural circulation. The respective hot leg ficw E

6 6

rates are s 70x10 and 2.Sx10 lb/hr during this period. The j

5 presturizer is outsurging s 650*F water at s 40000 lb/hreinto the l1 1

hot ieg.-

E Conservatively assuming only 10% of the hot Teg flow pq i;

mixes with the pressurizer outsurge, the resulting maxima fluid j

E tenperature in the candy cane will be s 551*F for four pump g

f operation and 581*F for natural circulation.

• i D) se

')

w kk

=_

e w

een $w sa

=

h--

, amen enen e ee 4-ee - a ee e+-4

e. eup we = m e

=eam-.,w alam.ee+=e-

The saturation pressures for these temperatures is well below 1500 psig.

2.2.2 Residual Heat Stored in Metal The residual heat stored in the hot leg metal is conservatively l

calculated to be s 1.89x10 BTV's. This calculation is based on (1) hot leg piping temperature remains at s 605 F for 60 seconds i

after the trip and (2) the residual heat is dumped instantaneously into the hot leg flow volume at the RTD indicated temperature of s 550 F and 1600 psia.

3 Hot leg metal volume = 70.45 ft Hot leg metal mass = 70.45 ft x 490,1b = 3.45 x 10 lbm ft" The AT driving function will be 605 - 555 or 50 AT.

Therefore stored energy released is

.11 BTU x 50 F x 3.45x10 = 1.89x10 BTU's WF The volume of water in the hot leg is s 1.97x10 lb Therefore:

1.89x10 BTU N10 BTU /lb 4

197x10 lb This is equivalent to s 8 F in temperature addition.

Therefore, the saturation pressures for 551oF* + BoF = 559 F and 6

5810F + 8oF = 589 F are below 1500 psig.

Per previous calculation e

. ~

Column Wh2 Flow Path, Fig. 2.1.1 s

s Y

i l

s

/

l PLENun COVER

+ +t t

+

+tt x

Flow Path d

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---u--

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U UPPER EN O FITTING f-

' w - #

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Upper Plenum Flow Path Fig. 2.1.2 l

. =

k, I v h olumn q

wel dn.ent y J I

C"3 C3 C"3 D C""3 Q

i wJ lj ll u

e k

Top view of upper plenum cover for 177 FA Plants I

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Flow Path Flow Path

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2.2.3 Conclusion Using the conservative assumptions on residual stored heat and hot leg - pressurizer outsurge mixing, the RC pressure which would allow flashing is below 3500 psig. This low pressure snould not ce reached curing normal transients.

2.3 Event Description For Mass / Volume Balance 2.3.1 Description of Analysis A mass volume balance on the RC system before and after trips was analysed for five cases. These cases were chosen because of their abnormally low pressurhor level, low RC pressure, or were referred to in reference 1.

The method of analysis was basically:

(1) To break the' RC system down into 4 different temperature /

' pressure volumes (per figure 2.3.1):

(2) Calculate the " shrinkage'.' of each volume at a lower temperature / pressure condition after a trip and (3) Determina how much water from the pressurizer is required to maintain a solid system conditions.

(4) Subtract this volume of water from the initial pressurizer volume to get " level 1".

i (5) Calculate the amount of water that would have flashed in the pressurizer due to the lower pressure condition.

This is " level 2".

(6) Finally, calculate the pressurizer level error per attachment 1.

This is " level 3".

(7) Subtract level 2 and 3 from level 1 to get level 4.

If

" level 4" is less than the plant measured level (" level 5")

then a viable explanation would be potential " steam pockets" l

in the RCS.

The results of this study show that the indicated pressurizer level (level 5) is less than or equal to the calculation of level 4 and therefore a steam pocket occurring in the RCS is highly unlikely.

2.3.2 Derivation of Equations Used The equation used for this analysis is:

1 1

I I

l P

+V P

+YE P

+

P

+V Vpjj+vP22+V933+YP44+YPS5" 11 22 33 44 55 Where p = Coolar.t density at Time O p = Coolant density at Time 1 V5 = Volume of water in pressurizer at Time O V 5 = Volume of water in pressuri:cr at Time 1 1

Solving for V 5

V 1(p1 - p 1) + V (P2 - P 2) + V (P3-P 3}+ 4(P4-P4 5 5

+

5 2

3 1

P 5 For raised loop plants (TECO)

V

=3739(ap)+887(4

'3)+2774(ap)+[Aj + 3.207 (level)] p 5

j 2

4 5

p Eq.1 5

For lowered loop plants V

= 3800(apj) + 287(ap ) + 3I97 (AP ) + 2774 (ap4) + [Aj + 3.207 (level)] p5 2

3 1

p5 Eq.2 1

RC. VOLUME BREAFDOWN f

'f3 ll

}_'.

V

/J i+,

/

II

.3

'O

?,

&(

g 0

f%

o-i de o

o e e e o

, e ~-

M '* ",i I

/, [

o*

o e Tube Sheet 0

e

/

1 P

g i

R&?

t

/

_/

j e~

i

,/

/

.//-

i.

i' s;

d

's ).," ;

I l/ /

.,/ l n P.;

• t. 's 'i l

l, /

j i

,, t.

l.,

I ;

=,",

N' e.

.s

\\

Raised Looo Lowered 3

C m,

177 NSS (ft )

177 Nss

Cold Water Volume

3739 3800 V

=

1 y a.

-N Ng = Core Water Volums =

887 887 V

' s 2

, -... +

Hot Water Volume

3062 3197 V

3 o

L'.

5 ry-,

i

) = Steam Generator Water Volume =

2774 2774 V

=

4

,. j f l Total External to Pressurizer 10462 10,658 Pressurizer Water Volume =

A + 3.207 X A + 3.207 X V5 l

=

=

-i Level (inches)

Level (inches)

A = 263 (Davis Besse)*

A = 134 (Oconee) 1 = f(P) Tcold)

P) = Ptap 8

= f(P, T,,)

P2=Ptap + 50

'2 2

f(P, Thot)

P3=Ptap 3

3 f(P, T,y,)

P4=Ptap 4

4 5 = f(P5, Tsa$)

PS=Ptap

a-_-

_____i

~

2.3.3.1 Davis Besse 9/18/79 Trip At 12:43 on September 18, 1979, while operating at s 100%

power a test was in progress on the main steam turbine Electro Hydraulic Control System (EHC) at Davis Besse 1.

While transferring EHC pumps a low pressure signal in the EHC initiated a turbine trip. The Anticipatory Reactor Trip on Turbine Trip tripped the reactor s 0.4 seconds after the turbine trip.

The reactor trip caused the RC pressure, RC T and average pressurizer level to decrease rapidly. The pressurizer level indication dropped off scale some 50 seconds after the reactor trip, remained down scale for some 50 seconds and then slowly increased. The RC pressure reached a minicum of about 1710 psig at 1 minute after reactor trip and recovered. RC T average a minimum cf 546 F following the trip and stabilized at 550 F.

The initial conditions prior to the trip are listed below.

Reactor Power s 100% Rated Power RCTemperature(T,y,b 5824 R.C.S. Pressure:

2200 psig R.C.S. Flow:

4 pumps operating Pressurizer Level:

202 inches Tests in Progress:

EHC Test The pertinent data during the transient is shown on figure 2.3.3.1.

The data analysis is as follows, i

i 1

Davis Bssse 9/18/79

~

Reactor Trip Time (0) Initial Conditions T

=5589 T

=

2215 psia p

=

hot cold tap Therefore p = 46.24 p = 44.64 p = 42.57 E 60 p

=

4 p5" Time (1) = 42 sec

=

T

= 557 T

= 550 p

1740 psia

=

hot cold.

tap Therefore p'

46.40

=

46.27 p/

=

2 3

'= 45.97 3

1 45.23 p/

=

4 p

41'18 5

3 Solving Equation 1 V # = 415 ft 5

This corresponds to a calculated pressurizer level of 47.5 inches.

(level 1)

The indicated pressurizer level was 21.6 inches (level 5), The estimatcd pressurizer level temperature compensation error (per Attachment 1) is 14" inches (level 3).

3 3

The initial mass of steam in the pressurizer is 650 'ft x 6.21 lb/ft or 3

3 4037 lb. The final mass of steam is 1214 ft x 4.37 lb/ft or 5304 lb. This additional 1267 lb (5304 - 4037) of steam will come from s10" of water. (level 2).

Therefore, level 1 - level 2 - level 3 = level 4 = 47.5-14-10 = 23.5".

Level 4 is slightly greater than level 5; therefore, the probability of steam pockets is low.

• A 4 second hot and cold leg RTD 'est;'nse time delay is incorporated.

Note: ilhere possible, data',tti J '7,ese analyses is based on computer print out.

The graphs are representation; of tn.is C:ta.

m s,

w m,

---eo

Fig. 2.3.3.1 l.

m as um ews c;

QJ 43 cD

- o c.c s an-ao UN M

=,l m%

eoo c *- ec as

-o 6

o a.a a a>

- s.

\\

. o <a r0 N 6& QJ I

w-i~

I h %===

M% 6

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c o

1

- o o a,.c e -,o s. m -

CJ Q 3 QJ m GJ

,m U 43

\\

3 QJ c to 6 wCJ%o6-c'y

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uns

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a

>-w w

l e

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1 cn,o u

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w c-mgo

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%-,o u u

ec=

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m-5

\\

o x

to V

,,a A

\\

m-o w

as o "

o e

y n,

\\

>UC w

g p

s- - e e

n =

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on-t

.a a-u v e.

a

+

ua

\\

m x

w v

a cn

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r-3.:

N m

e i

s A

i.

3

~

~

o

=

0 S

LaJ ti e

E

1. I I

I

1. ' /

.,.__......-.:-+'

/

-~~'*...

o r

>l m

w w

a o

me

~ w a

w o

x a.

e

=

=

I e

m I

1 1?

I o

o o

o o

es

,a o

o o

.e.

.m 40 o

e

=

n

3. '3HA1Yb3dVl31 1

I I

I I

I t

I o

o en e

o o

o*

g E

E o

a o

~

~

n N

SISd '380SS3Hd *3'8

.I I

t i

I I

I I

f f

t t

e o

cm es a

cm o

o en o

cm o

"o to C

o 40 v

N

.T.

.N.=

o.3 N

e (S3H3NI) 13A31 H321HASS3Hd e

,m r.

w.

5 s

i 2.3.3.2 Davis Besse 9/15/79 Trip Figure 2.3.3.2 shows the RC pressure during this trip.

}

Since RC minimum pressure was 1848 psig no steam production was possible per discussion in Section 2.2.

i 1

i e

l I

e i

1 j

i i

\\

}

e 1

I, i

~

TECO 9/15/79 TRIP t '

fi.840 tl' I

21.420 w[

21.000 l:

R 1'

v 3

20.580 v

ax 20.160 m

=50 ac 19.740 E

19.320 y

?

N 18.900 F

w b

1

{3,430 t

i i

i e

t i

!l 49.410 49.815 50.220 50.625 51.030 51.435 51.840 52.245 52.650 2

Time (seconds)(X10) l

_.m.-____

2.3.3.3 Daivs Besse 9/26/79 Trip At 20:56:33 hours on September 26, 1979, a high turbine throttle pressure limit alarm was received. The throttle pressure limiter is used to protect against an excessive decrease of steam pressure when steam generation of the NSS falls below the steam demand of the turbine.

It acts directly to close the high pressure turbine control valves in an effort to maintain steam header pressure. Rapid closure of the control valves caused a mismatch between heat generation and heat removal with a resultant increase in Reactor Coolant System (RCS) temperature and pressure. Seven seconds later (at 20:56:40) the reactor tripped on RCS high pressure and was followed by a turbine trip.

RCS temperature and pressure then dropped and the Integrated Control System (ICS) reduced the feedwater flow abruptly.

Reactor Coolant System Tave decreased to 548.5 F approximately 57 seconds after the trip. The pressurizer level indication dropped off scale at 21:57:21 and remained below the indication range for approximately 21 seconds.

The initial conditions prior to the trip are listed below.

Reactor Power:

s 100% Rated Power R. C. Temperature (T,y,):

582 F 4

R. C. S. Flow:

4 pumps operating Pressurizer Level:

200 inches The pertinent data during the transient for this analysis is shown on figure 2.3.3.3.

The data analysis is as follows.

__ Z

.Z

_.ZZ.E Z_ :1_... _

Davis Besse 9/26/79

~

Reactor Trip Time (0) Initial Conditions Ihot T

=

= 2215 psia cold tap Therefore p) = 46.18 P

44.59

=

2 p3= 42.67 44.56 p

=

p5" Time (1) = 50 sec 552 548 T

p

= 1700 pria

=

T

=

hot cold tap Therefore p/

46.62

=

l' 46.53 p

=

2 3

P' 46.22

=

3 1

p' 46.35

=

4.

I P

41.00

=

5 3

Solving Equation 1 V ' = 243 ft 5

This corresponds to a calculated pressurizer level of 26.4 inches.

(level 1)

The indicated pressurizer level was 0 inches (level 5), The estimated pressurizer level temperature compensation error (per Attachment 1) is 15" inches (level 3).

3 3

The initial mass of steam in the pressurizer is 648'ft x 6.21 lb/ft or 4030 lb. The final mass of steam is S1234 ft3 7, 4.23 lb/ft or 5219 lb. This additicna11190 lb ( 5219 - 4030) of steam will come from s 9" of water. (level 2)

Therefore, level 1 - level 2 - level 3 = level 4 = 26.4-15 9 = 2 4". Level 4 is slightly greater than level 5; therefore, the probability of steam pockets is low.

A 4 second hot and cold leg RTD response time delay is incorporated.

Note: Where possible, data used in these analyses is based on computer print out.

The graphs are representations of this data, i

l

~

i l

TECO 9/26/79 TRIP jl 2300 i

i 240 2200 i

ll 200 2l00 4

1 n

E 590 m

g n

u 5

160

- w 2000 580 C

s

=

570

^

1 o

u, a

RC TAVE N 120 1900 W 560 w

o

= 550 RC PRESS.

j 2' '

i 1800 540 80

?

P u

h' 1700 520 w

l 40 i

PZR LEVEL I

i I

8 0

1600 0.0 1.0 2.0 Time af ter trip (minutes)

I

.=-. - -

.= =: -==- - - - - - - - -

2.3.3.4 Oconee 1 10/8/79 Trio On October 8,1979, Oconee 1 was operating at 100% FP with on line Reactor Protection Tests in progress. At 13:27:35, Oconee 1 experienced a reactor trip attributed to RPS activation of pressure / temperature channels A, C, and D.

Control Rod Drive Breakers CB 3 and 4 were tripped for the test. An additional breaker for Group 5 was opened which caused Group 5 to drop. This resulted in a P/T Trip.

The initial conditions prior to the trip are listed below.

Reactor Power 100%

RCS TAVE 579 F Pressurizer level 221 inches The pertinent data during the transient for this analysis are shown on figure 2.3.3.4. ' The data analysis is as follows.

...e s

m+=.-

w m

p<-

~

~

Oconee I 10/8/79 Control Rod Drop Trip Time (0) Initial Conditions 602D T

= 558 T

p

= 2150 psia

=

hot cold tap 3

Therefore 46.26 lb/ft p) =

3 44.75 lb/ft p

=

2 42.87 lb/ft p

3 p = 44.70 lb/ft 4

p5" Time (1) = 60 seconds 553 550 p

1750 psia T

=

T hot cold tap 3

Therefore p'

46.55 lb/ft

=

I' 46.47 lb/ff3

[

p

=

1 p'=

46.40 lb/ft 3

p '[ = 46.51 lb/ft

'4 P

40.81 lb/ft

=

S

1. =

Solving Equation 1 V 315 ft 5

This corresponds to a calculated pressurizer level of S6.Z411nches.

(level 1)

The incicated pressurizer level was 31.0 inches (level 5). The estimated pressurizer level ten.?crature cocpensation error (per Attachment 1) is 15.0 inches (level 3).

The initial T. ass of steam in the pressurizer is 709 'ft x 5.97 lb/ft or 4236 lb. The final mass of steam is 1283 ft3 3

x 4.4 lb/ft or 5652 lb. This additional 1416 lb (5652 - 4236) of steam will come from s 10' of water (level 2).

Therefore, level 1 - level 2 - level 3 = level 4 = 56-15-10=31.".

L,evel 4 is equal to level 5; therefore, the probability of steam pockets is low.

A 4 second hot and cold leg RTD response time delay is incorporated.

Note: L'here possibic, data used in the:;e analyses is based on computer print out.

The graphs are representations of this data.

e

. ~. -.

l Fig. 2.3.3.4

.}

i l

l 1

h

.O a

1 g

~

/

S w

w m

s.

m n

g g

5 5

O M

g

~

=

w O

5

~

L

~

/ A=. _

~

5 u

/-

=

m

=

m m

w

=

/

/*

.$. /

• s' a

.N oo

%y Y

I I

I t

i I

l ti I

E E

R R

R E

2 2

S

=

=

SISd '3HOSS3Hd 3H I

t i

I t

I g

1 1

g 8

E E

=

=

=

3 2

=

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2.3.3.5 Oconee II 1/4/74 Trip Figure 2.3.3.6 shows the pertinent transient results of this trip.

During this transient the minimum Tap pressure attained was s 1900 psig. Therefore, a 1940 psia upper vessel pressure would require a 631 F temperature to produce net steam. This temperature was never attained at any position or time during this transient. While it is given that operator action (or plant responses) could have been better, the steam production possibility did not exist. Furthermore, a review of the sequance of events of this trip indicate that the severe level and pressure changes at the beginning and later in the transient were primarily due to a temporary loss of ICS power.

I l

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5 Oconee II C00 1-4-79 Trip 580 HOT LEG TEMPERATURE

.A/ %

i 560 T~~~#

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2500 540 g

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f SIGNAL WENT OUT OF RANGE

\\

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200 SYSTEN PRESSURE I

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Time after trip (minutes)

3.0 conclusion During any B&W reactor trip where a primary system break I

does not occur or the pressurizer does not empty and HPI is operable - net steam production in the primary system is highly improbable and generation of a volume of steam required to block natural circulation is virtually impossible.

i 4.0 References 1.

11/15/79 phone conversation between B&W and NRC,

Subject:

Potential Voiding in-the RCS During-Anticippted Transients l

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ATTACHMENT 1 Pressurizer Level Error The pressurizer level indication is based on the pounds of mass the pressure taps measure and then distributed to varying amounts of water and steam at the saturation conditions at the pressurizer pressure. Since pressure is not measured in the pressurizer (only tec:Derature) pre.ssure is assumed from the temperature measurement being at T sat'

' During fast depressurizations the RTD in the pressurizer does not respond infficiently, especially in cases where the RTD-is exposed to steam enviroment. The heat transfer time constant can vary from 30 to 180 6

seconds. Since most depressurization after trips occur in 30 to 50 seconds the RTD will not respond effectively at the low level point.

Figures Al and A2 show potential level errors for 400" and 320" pressurizers.

Figures A3 and A4 show what typical errors can be expected as a function of indicated level and system pressure. These curves can be used to correct the indicated level if system pressure (i.e. Tsat) and pressurizer RTD temperature are known. The difference between RTD temperature and calculated T (Based on Psat) will give the level error, sat In a typical trip @ Oconee 1/1/77 the system pressure went from 2150 to s 1800 psig in 27 seconds. The alanns printer monitored a 648 pressurizer temperature at 1800 psig instead of the expected 621 F.

At the indicated level of 70" the real level was s 78" (per figure A-3).

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