ML20027A767

Sys Response to Total Loss of Steam Generator Heat Sink.

ML20027A767
Person / Time
Site:	Three Mile Island
Issue date:	08/07/1979
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML20027A678	List: ML20027A767 ML20027D175
References
FOIA-82-543 86-1103585, 86-1103585-00, NUDOCS 8204290445
Download: ML20027A767 (9)

Text

Docket ::c. 50-289' (Restart)

Licensee's Exhibit No.

B&W Docur.ent 86-1103585-00, " System Response to Total Loss of SG Heat Sink,"

(August 7, 1979)

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a j EYSTEM RESPONSE TO TOTAL LCSS CT E REAT SINK. -l

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1. Introduction s

An a=alysis of a cenplete loss of feedwater transient accident for the 177-FA lowered-loop plants has been conducted. .The analysis was perfor=ed utilizing a " realistic" decay heat curve, and assu=ed that offsite power was lost and that the operator actuated one of the EPI syste=s at 1200 seconds.

1

2. Si--srv and Cenclusions 2.

An analysis of a co=plete loss of feedwater transient for the 177-FA, lowered-loop plants has been performed utilizing a " realistic" decay heat curve. .Cen-

'sistent viph the operating procedures, it was assu=ed that the operator would

• initiate the HPI syste= by 20 =inutes. A . single failure in the E?I syste= vas included in the evaluation. -

The analysis demonstrated that 1 EPI pu=p provided sufficient makeup to pre-vent core uncovery. The ulti= ate heat sink for this transient is the contain- ,

=ent via energy release through the pressurizer safety valves. Since no cere u=covery occurs, cladding te=peratures coul-d re=ria within a few degrees of the satuated fluid te=perature and no cladding rupture nor =etal-water reacticn occurs. Thus, the criteria of 10 CFR 50.46 is satisfied for this transient.

3. Re'sults of Analvsis -

3.1 Method -

Since the systen response'for this transient is r.elatively quiescient, detailed L noding of the pri=ary syste= is not required, thus, the analysis in this re- ..

port was perforced using a six-node CRAFT =odel to develop the history of the reactor coolant systen hydrodyna=ics. Figure 1 shows a sche =atic diagram of ,

I the model. Node 1 comprises the cold leg pu=p discharge piping, the reactor ,

vessel (RV) dcwncocer, and the 1cuer plenu= of the RV. Node.2 represents the seca= generator, prt=ary side and the cold legs suction piping, while Node 3 represents the core, RV upper plenu=, and the hot leg piping. ~ Nodes 4, 5, and 6 of the model are used to si=ulate the pressurizer, contain=ent, and the secondary side of the steam generators, respectively.

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1. The reactor is cperating at 102% of the steat -state power level of 2772

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2. Loss of =ain feedwater ficw to the stea: generator occurs at ti=e :ero.

The auxiliary feedwater syste=s are assuned net to operate. ,

3. Offsite pcwer is not available. 1 1

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4. The reactor trips on high pressure at 2300 psig.

5. No credit is taken for operation of the PORV.

6. The pressu'ri er safety valves start to open at the set pressure of 2500 psig. They are assu=ed to be full open at 103% of the set pre'ssure.

7. The discharge rate through the code safety valves is calculated using the Bernoulli equation, for subcooled fluid discharging through the valve, and the Moody correlation, for two-phase or stea= flow through the valve.

The flow area utilized for the safety valves was chosen such that the Moody calculated discharge rate, for stes: flow through the valve at the valve rated pressure, is equivalent to the design capacity of the. valve.

8. Actuatics of one HPI train, via operator actica at 20 minutes, is as'suced.

A single failure is assened which renders the other EPI train inoperable.

Operator guidelines specify that, upon Icss of SG heat sink, he should manually actuate all EPI trains.

j 9. In order to si=ulate a realistic decay heat curve, 1.0 ti=es the 1971 ANS standard was utilized.

I 3.2 Results

• l

-Figures 2 throush 5 shcw the transient syste: response for this accident. The .

folicwing table presents key results of the analysis: -

I Secuence of events ', - . .Ti=e, s ,

Loss of =ain feedwater, turbine trip, O.

loss of offsite power (RC pumps coastdown)

Reactor trips on high pressure' 8.

SG side inventory boiled-cff 100.0 Pressuri:cr goes solid 350.

Two pressuri:cr code safeties open 400.

Long ter: cooling estab. (based on 1 HPI) 8900.'

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..7 Figure 2 shows the core pressure transient. Following the si=ultaneous loss .- j

. 1 of =nin feedwater and offsite power, the fluid in the RCS expands due to de- .;t creasing heat transfer via the steam Sencrater, ird the RCS pressure increases.

At 8 seccnds, the high pressure trip setpcint (2300 psig) is reached, thus " -.

causing the re. actor to scram. Pressure then starts to decrease due to cen- ,  !

i traction of the fluid in the RCS caused by the decrease in core power. At l 1

100 seconds, the steam generator inventory has been boiled-off, which results l in a less of heat sink and a heatup of the RCS fluid, and repressurization of l the system. At 350 seconds, the pressurizer becenes " solid," and the system pressure rapidly increases to the code safety valve set pressure of 2500 psig.

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The system pressure re=ains at this value for the rensinder of the transient with the safety valves acting as a path to the ultimate Feat sink of the RCS .%

for this accident, i.e., the containment. -

Figure 3 shows the pressurizer =ixture level response for this accident. The 5 initial pressurizer response follows the sa=e behavior as the syste pressure transient. Due to the loss of heat sink at 100 seconds, the pressurizer - 7.-

I starts to refill and beccmes_ solid.at.350 secends At 2180 seconds, the liquid Icvel in the pri=ary system falls below the surgeline entrance and steam passes into the pressurizer. Shortly thereafter, the pressurizer =ixture level dreps

' slightly and steam exits through the code safety valves. The pressurizer re-mains in this conditien for the re=ainder of the transient. .

The RCS liquid inventory, with .the ~hxclusion of the pressuricer, during this transient, is given in Figure 5. Makeup to the RCS was initiated at 1200 seconds via operater action to start one HPI train. The RCS recched saturated l conditiens a,t 1725 seconds, and rapidly starts decreasing in inventory.. With

, the liquid level in the prinary system falling below the pressurizer surge-l line no: le at 21S0 seconds, the loss rate in system inventory sicws. At- -

1 l 8900 seconds, the EPI flow excceds the core bcil-off and the systen starts to refill. At no ti=e does the core uncover. Thus, the cladding tenperatures

. will be raintained within a few degrees of the saturated fluid te.aperature and no cladding ruptures nor metal-water reaction will occur.

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