ML19260C890

Potential Reactor Sys Voiding During Anticipated Transients

ML19260C890
Person / Time
Site:	Rancho Seco
Issue date:	01/31/1980
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML19260C889	List: ML19260C888 ML19260C890
References
NUDOCS 8002060361
Download: ML19260C890 (32)

Text

f, iN POTENTIAL REACTOR SYSTEM VOIDING DURING ANTICIPATED TRANSIENTS PREPARED BY BABCOCK & WILCOX NUCLEAR POWER GENERATION DIVISION LYNCHBURG, VIRGINIA JANUARY, 1980

~

1933 002 80020603b!

J

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 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 9/10/79 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 2.3.3.S Oconee II 1/4/74 Trip 3.0 Conclusion 4.0 References i933 003

List of Figures 2.2.1 Column W61dment Flow 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 Trik 2.3.3.4 Oconee 1 10/8/79 Trip 2.3.3.5 Oconee 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" pressurizer)

Attachments Pressurizer Level Error 1933 004

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 ninder natural circulation after loss of offsite power. The source of this steam is postulated to be from stored heat in inetal, 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 Surmiary Based on review of several past B&W reactor trips along with several essumptions and calculations discussed in Section 2, the following statements can be made:

(1) Minimum RC pressure after trips about 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.

1933 005

• Specific concern per reference 1

(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.

P.0 Analysis, Assumotions and Event Descriptions 2.1 Analysis of Steam in the Uccer Vessel Plenum 2.1.1 Staanant " Hot" Water Typically, the hottest bundle in a 2772 MWT core 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) wou'J have to be blocked.

If this blockage could occur, then the following statement can be made: Any B&W reactor trip resulting in minimum RC tap pre.ssures less than 1740 psis (the tap pressure i.s s 40 psid lower than core plenum pressure) voidino ml ! 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 stagrating (or having an extremely low flow rate) in the upper plenum (per Fig. 2.1.2) and acquiring the steady state enviremental 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 100% power outlets themocouple data.

1933 006

The most realistic assumption is the desian condition of s 5% core flow through the guide tubes into the upper plenum. Following a reactor trip, the coolant temperature in t%is region will decrease rapidly to below 600F as core outlet temoerature-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 "clow" mixing. The available stored energy in the metal will be:

Stored Energy = Cp x Mass x aT m

m Cp =.11 BTU /lb, F m

ai = 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 1933 007

• This is water densi y at 575 F,1600 psia.

4

• • l.925x10 BTU's/6.08x101b = 32 BTU /lb or s 26 F temperature increase.

steam (to cause spillaver into the hot legs) from just residual heat, 20 'imes more mass of hot metal would be required.

(s 10 BTU /'b are needed to reach Tsat and another 533 BTU /lb to vaporize the water. This equals 543 BTU required to convert a pound of water to lb a pound of steam).

2.1.3 Conclusion The previous calculations and discussion basically show that it is impossible to prcduce steam in the upper head above a measured pressure of 1740 psig, highly improbable above 1620 psig, and highly unlikely above 1500 psig. (Very few transients have ever gone below 1600 psig.)

This statement assumes no primary system breaks.

2.2 Analysis of Steam in the Candy Cane 2.2.1 Pressurizer outsurge water mixing with hot lec flow At the tire period of minimum fressure after a trip the hot leg coolant temperature is s 550 F during 4-pump operation and s 5700F durin9 natural circulation. The respective hot leg flow rates are s 70x10 and 2.5x10 lb/hr during this period. The pressurizer is outsurging s 650 F water at s 40000 lb/hr into the hot leg.

Conservatively assuming only10% of the hot leg flow mixes with the pressurizer outsurge, the resulting maximum fluid temperature in the candy cane will be s 551oF for four pump operation and 5810F for natural circulation.

1933 003

The saturatior, pressures for these temperatures is well below 1500 psig.

2.2.2 Residual Heat Stored in Metal The residual teat stored in the hot leg metal is conservatively calculated to be '.1.89x10 BTU's. This calculation is based on (1) hot leg piping temperature remains at s 605 F for 60 seconds 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 3

4 Hot leg metal mass = 70.45 ft x 490 lb = 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 BTV's Ib"F The volume of water in the hot leg is s 1.97x10 lb Therefore:

1.89x10 BTU $10 BTU /lb 4

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

Therefore, the saturation pressures for 551oF* + 80F = 559 F and 6

5810F + 80F = 589 F are below 1500 psig.

Per previous calculation 1933 009

I1 21I 9

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-- q weldment t

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l' Top view of upper plenum cover for 177 FA Plants I

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1933 011

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.1500 psig. This low pressure snould not De reacned during normal transients.

2.3 Event Description For Mass / Volume Balance 2.3.1 Descriotion 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 abnomally low pressuri:or 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) Determine 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".

(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" 1933 012

in the RCS.

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

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

1 1

I 1

l P

+V P

+VP E

+

E

+

33 44 55 V p) + V P22+VP33+VP44+VP55 Il 22 j

Where p = Coolant density at "

eO p = Coolant density at Time 1 5 = Volume of water in pressurizer at Time 0 V

• v 5 = Volume of water 'in pressurizer at Time 1 1

Solving for V 5 V

1(P1 - P 1) + Y (P2-p2)+V(P3-P 3) + 4(p4 - p, ) + V5 5 5

2 3

1 E 5 For raised loop plants (TECO)

(

V

= 3739(ap)) + 887(42 3

4 1 + 3.207 (level)] p5 5

1 p

Eq.1 5

For lowered loop plants

= 3800(ap)) + 887(ap ) + 3197 (ap3) + 2774 (ap4) + [A) + 3.207 (level)) p V

2 5

1 p5 Eq.2 1933 013

RC VOLUME BREAFDOWN

-m Fig. 2.3.1

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O Raised Looo Lowered Lo 177 NSS (ft6.oD 3

177 KSS (ft )

Cold Water Volume

3739 3800 V)

=

Core Water Volume

887 887 V

=

2

Hot Water Volume

3062 3197 V

=

3

', 3., e

Steam Generator Water Volume

2774 2774 V

=

4

,_.a Total External to Pressurizer 10462 10,658 A + 3.207 X A + 3.207 X

Pressurizer Water Volume

Ve

=

Level (inches)

Level (inches)

A = 263 (Davis Besse)'

A = 13,4 (0conee) 1 = f(P) Tcold)

P) =P 8

tap

= f(P, T,y,)

8 P = F. g + 50 2

2 2

m 3

• II 3, Thot)

P3*Iti 8 = f(P, Tave)

P

=P 4

4 4

3p

( 5, Tsat)

=

5 P5=Ptap 4

2.3.3.1 Davis Besse 9/18/79 Trio 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 minimum of about 1710 psig at 1 minute after reactor trip and recovered. RC T reached a mirimum of 546 F following the trip and stabilized at 550 F.

The initial conditions prior to the trip are listed below.

Reactor "ower s 100% Rated Power RC Temperature (T,y,):

582 F R.C.S. Pressure:

2200 psig R.C.F. 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.

1933 015

Davis Besse 9 '

'eactor Trig.

Time (0) Initial Conditions T

=5589 T

p

= 2215 psia

=

hat cold ap Therefore p) = 46.24 44.64 p

=

42.57 p

=

p = 44.60 4

p5*

Time (1) = 42 sec

=

550 p

= 1740 psia T

= 557 T

=

hot cold tap Therefore p#

46.40

=

p/ '

46.27

=

2 1

p 3

3 46.23 pi

=

4 P'

41.18

=

5 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 incher(level 5). The estimated pressurizer level temperature compensation error (per Attachment 1) is 14" inches (level 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 s 10" 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 ccid leg RTO response time delay is incorporated.

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

Tt.a graphs arc representations of this data.

1933 016

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3 017

2.3.2.2 Davis Besse 9/iS/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.

1933 018

TECO 9/15/79 TRIP 21.840 -

21.420 w[

21.000 a

1 3

20.580 S

E 20.160 g

50 a:

19.740 a-M 19.320 2'

P 18.900 U

18.480 8

8 8

8 8

8 u

49.410 49.815 50.220 50.025 51.030 51.435 51.040 52.245 52.650 2

3 Time (seconds)(X10) m e

2.3.3.3 Daivs Besse 9/26/79 Trio At 20:56:33 hours on September 26, 1979, a high turbine throttle pressure limit alam 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 React'or 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 (Tave):

582 F 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 dats analysis is as follows.

1933 020

Davis Besse 9/26/79 Reactor Trip Time (0) Initial Conditions T

T

=

1 psia hot cold "

tap

=

Therefore p) 46.18

=

P 44.59

=

2 P3= 42.67 44.56 p

=

p5*

Time (1) = 50 sec 552 548 p

= 1700 psia T

=

T

=

hot cold tap Therefore p/

46.62

=

p/ '

46.53

=

2 1

P 31, p'

46.35

=

4.

P' 41.00

=

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 The initial mass of steam in the pressurizer is 648 'ft x 6.21 Iv/f 3 or 4030 lb. The final mass of steam is $1234 ft3 x 4.23 lb/ft or 5219 lb.

This additicnal 1190 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 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: !!here possible, data used in these analyses is based on computer print out.

The graphs are representations of this data.

1933 021

9 e

Fig. 2.3.3.3 1

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=

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m E

A O=

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(D cz:

=

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f i

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(HI) 13A31 'SS3Hd 1933 022

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 condi': ions 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.

1933 023

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

= 558 p

= 2150 psia T

=

hot cold tap 3

46.26 lb/ft Therefore p) =

3 44.75 lb/ft p

=

2 3

42.87 lb/ft p

R70 WM p

=

4 p5*

Time (1) = 60 seconds 553 550 p

,1750 psia T

=

T

=

hot cold tap 3

Therefore p I

46.55 lb/ft

=

3 I'

46.47 lb/ft p

=

2 1

p' 46.40 lb/ft

=

3 46.51 lb/ft pI

=

4.

P' 40.81 lb/ft

=

5 Solving Equation 1 V 315 ft

=

5 This corresponds to a calculated pressurizer level of 96.2Minches.

(level 1)

The indicated pressurizer level was 31.0 inches (level 5). The estimated pressuri:er level temperature compensation error (per Attachment 1) is 15.0 inches (level 3).

The initial mass of steam in the pressurizer is 709 ft x 5.97 lb/ft or 3

4236 lb. The final mass of steam is 1283 ft 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".

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

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

Note: 1lhere possibic, data used in these analyses is based on computer print out.

The graphs are representations of this data.

1933 024

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2.3.3.5 Oconee II 1/4/74 Trio 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 sequence 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.

1933 026

Fig. 2.3.3.5

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3.0 Conclusion During any B&W reactor trip where a primary system break does not oc'ur 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.

4.0 References 1.

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

Potential Voiding in-the RCS During-Anticipated Transients 1933 023

t ATTACHMENT 1 Pressurizer Level Error The pressurizer level ind' ation 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 temperature) pressure is assumed from the temperature measurement being at Tsat' During fast depressurizations the RTD in the pressurizer does not respond efficiently, especially in cases where the RTD is exposed to steam enviroment. The heat transfer time constant can vary from 30 to 180 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 /2 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 Psa,) 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 alarms 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).

1933 029

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