ML19221A199

Revised Summary of Significant Events of 790328.W/chronology Encl

ML19221A199
Person / Time
Site:	Crane
Issue date:	04/09/1979
From:	Harold Denton Office of Nuclear Reactor Regulation
To:
References
NUDOCS 7905210006
Download: ML19221A199 (9)

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1 P^ELIMINARY 7

d' E5CRIPTION OF EVENTS g

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_AT T H_E 1liR E E_f1IL_E _1 SLf.ND 2 FACILITY ACCIDENT j

The folicwing is a summary of the significant events that occurred at the Three Mile Island No. 2 nuclear facility cn March 28, 1979, and thereafter.

Attached is a detailed chronology of these events listed with the times they each occurred.

At about 4:00 am on March '.,1979, the secondar; (nonnuclear) cooling sys tem of the Three Mile Island facility suffered a tral function.

This system normally pumps water through the plant's steam generators waere the water turns to steam..hich then ficws to turn a turbine generator, The </ater is then condensed back to water, is pumped by a condensate pump through a clean ^up sys tem, through a fee 6.a ' er pump, and finally back to the steam generators, and continually flows around this loop.

A,al function in the rnain feedr.ater system ccused the fee 6.ater pu:rps to turn of f (trip), which in turn caused the turbine-generator to turn off and s top generating electricity.

Since the steam generators were not rcmoving hcat due to the s tcppage of fee 6.ater ficw, the reactor coolant system pressure increased and the pressurizer relief valve opened to redi.ce reactor pressure.

I. mediately, the reactor

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These events all occui r ed,<i thin the first b

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.Up to this point, this sequence is nonnal and the auxiliary f','.iater system should startup and deliver secondary coolcr.c to the plant's two steam generators to remove heat.

In addition, the pressurizer relief

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valve should close as reactor pressure decreases.

All three of the auxiliary feedwater pumps started but were unable to deliver flow because their ficw paths were blocked by closed valves.

In addition, the pressurizer relief valve failed to close.nd therefore allo. sed the reactor coelant system pressure to continue to decrease.

As the reactor pressure reached a preset value (1600 psi), the plant's Emergency Core Cooling Sys tem (ECCS) started as designed and began to a. d Q 7,. g... p g 2 M er i h. c v d a b A l. A fo o inject cold water into t5e reactor;g -tas-at 'tnis point that an I

indication of a rapidly rising pressurizer level apparently led the plant operators to terminate the ECCS ficw.

At this point the Three Mile n.C c t.cl O IO -II Island 4*ident had been unden;ay for Jr)T minutes.

cuc M e.Cd E e tween abo u t 1 a nd P. h ou rs i n to th e (+e-i -.t, the operators turned of f the four large pumos which circulate the reactor coolant through the reac tor.

It is follcwing this action that we be,ieve the severe damage to the nuclear. fuel beg an, for the n?xt several hours there was a very large temperature dif ference across the nuclear core indicating little flow of coolant through the core.

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,,- i e.s a During this several hour period, when severe fuel damage,is ocmurring, primary coolant from the reactor primary coolant system was being dumped T

onto the reactor containment floor from flow out of the pressurizer e

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relief valve and through the drain tank.

This coolant, which contained radioactivity, was partially pumped from the reactor containment building floor to tanks in the auxiliary building.

The tanks overflowed permitting adioactivity to be ' vented from the auxiliary building.

This situation lasted until about 9:00 am when the reactor containment was sealed (isolated).

Smi+ mms-t-ie, fYom about 6:00 am until 8:00 pm, the licensee tried to depressurize the reactor coolant system sufficiently to be able to turn on the residual heat removal system.

Since his at tempts failed, it was decided to repressurize the system.

Af ter repressurization, one of the main reactor coolant pumps was restarted and t'lcw through the reactor core was re-established.

Since feedwater was being provided to the steam generator, heat

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Reactor cc'oling has essentially been in this mode since that time.

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e PRELIMIt'ARY CHRONOLOGY OF THE 1%RCH 28, 1979 ACCIDENT AT THREE MILE ISLAND Time (approximate)

Discussiun of Events Before 4:00 am TMI operator working on Feedwater System.

4:00 am The loss of all (main and auxiliary) feedwater flow occurred while the reactor was operating at 98% power.

The transient was initiated by a loss of condensate pu.mp s.

The turbine tripped.

3-6 sec later An electromatic relief valve opened to relieve pressure in the RCS* (2255 psi).

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9(r-[ secjla te'r The Reactor tripped on high RCS pressure -

h. c r (2355 psi) to terr,inate the nuclear

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" reactor and redore power generation to decay heat alone.

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12-:5 sec later 2

The RCS pressure decayec to the point J

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(2205 psi) where the relief valve should have reclosed.

The RCS continued to depressurize for about the next two -

hours.

15 ', c c l a t e'r' The temperature in the RCS hot leg Ifl= ;! v,1. / s. Y pead'at about 6100F with a pressure of about 2150 psi

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sec la tir' The auxiliary feed..<a ter pumps in both 1

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a safe ty trains (1 turbir,e driven pu.,p r.

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and 2 electrically drivsn pumas) were

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started and were running at pressure ready to inject water into the steam generators and rencve the residual h at produced in the reactor core.

No, cater was injected since the distharge valves

' sere closed.

Throughout, RCS denotes reactor coolant system."

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Discussion of Events 4:01 am The pressurizer level indication ~ began to rise rapidly.

The steam generators, A and B, had low levels of water and were drying out.

4:02 am The ECCS was initiated as the RCS pressure decreased to 1600 psi.

e pressurizerAel indication went) 4:04-4:11 am TA, e m y aj.y 4.o!'dfscale hijL rend the operator manually h

trip ed tFie first HPI pumps at about 30' and the second a t a bo u t 4 :)(C 4:0

os:ts t:of 4:06 am k'ater in the RCS flashed to steam as the pressure bottoms out at 1350 psi.

The Ad hog' leg temperature was about SP,$F.

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4:07-4:08 am The Reactor building sump pump came on.

4:08 am The operator opened the valves at the discharge of the auxiliary feecwater pump allowing water to be injected into the s team generators.

)[2.4:)(am h2 N S 4:/E h The operator restarted the ECCS to inject 1

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watar into the RCS to control pressurizer level.

4:11 am The pressurizer level indication comes back on scale.

4:15 am The RC Drain (hench) tank rupture disk blew at 190 psig due to continued discharge of the relief valve that had failsd to open.

4:20-5:00 cm The RCS carameters stabilized at a satur-ated condition of about 1015 psi and 5500F.

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,5 1 The operator tripped both RC pumps in 3

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_S : 40 n The ope,ator tripped petg RC pumpf in 7:'ll Loop A.

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Discussion of Events 5:45-6 am The reactor more began a heatup transient.

The RCS not leg temperature went offscale at 620 degrees F within 14 minutes and the cold leg temperature dropped to near the temperature of high pressure injec-tion water (150 degrees F).

6:20 am The failed open relief valve was isolated by the ~pgator by closingMo_ck valve.

[ThF 5p~ era tor also isoTated s team genera tor.

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B to prevent leakage of radioactive secondary water from leaking S,Er.6 tubes 7:00 am The RCS pressure had increased to 2150 psi and the relief valve was opened to relieve RCS pressure.

7:15 am A pressure spike of 5 ps.g occurred in

'the RC drain tank due to steam from the relief valve.

7: 45 3m A pressure spike of 11 psig occurred in the RC drain tank and the pressure in the RCS was at 1750 psi.

9:00 am The, pressure in containment peaked at 4.5 psig.

9:00-11:00 am The RCS pressure increased from 1250 psi to 2l00 psi.

11:30 am The operator opened the pressurizer relief valve to depressurize the:RCS in an attempt to ini tiate RHR cooling dt 400 psi.

12:00 am - 1:00 pm The RCS,'ressure decreased to about 500 psi and the core flooding tanks partially discharged.

The relief capacity v.es not sufficient to vent enough to reach 400 psi.

2:00 pm The pressure in the containment spikes at 23 psig causing containr.3nt sprays to be initiated.

The operator stopped the spray pumps a f ter abcut 2 minutes of opera tion.

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Discussion of Events 5:30 pm

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The pressurizer relief valve wa's closed in order to repressurize the reactor coolant sjs tem.

5:30 - 8 pm The Ar5 pressure increased from 650 psi to 2300 psi.

8 pm RC pump in loop A was started at which time the hot. leg temperature decreased to about 560 degrees F and the cold leg temperature increased to 400 degrees F, indicating flow through the steam genera tu..

Therea f ter, the reactor was being cooled by reestablishing' condenser vacuum and steaming to the condenser by

1. Yteam cenera tor A with the RCS cooled to about 280 degrees F and 1000 psi.

March 29 The RCS temperature :nd pressure was stablized at about 250 degrees F and S40 to 1020 psi.

The maximum reading on the incore thermocouples was 6120F, but several were not wit range for computer readouts i,7. ut % - ? 4 E.2.j 3

wM4 =: :" :7v.41y frM-L indieetmner than 700 degrees ~ F.

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vA a_t w The KCS temperature and pressure was AJLstable at nearry 280 degrees F and s+'

about 1000 to 1060 psi.

Several incore thccmoccuples were beyond the range for computer readout, the maximum indicated readina was 659 degrees F.

The T'. huff-e' ima'ed the '.Z; size b RCS to be about 1200 f t3 eM-epested ' ic M ee t reEine e i' W aM[ ]en k of-2he-bubble-s i e,.

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March 31 4

p The RCS l.emperature and pressure remained stable at about 2SO F and 1000 U

psi.

Slight drop in pressurizer level 251-191".

Tenper;tures in the core as measured from the incore thermocouples were gradually decreasing (maximum in'dicated about 5000F).

The hydrogen recombiner was in an operable status but edditional shieldine was needed and v.as bcing obtained.

Two samples of con'.ainment etnosphere were analyzed which i

showed a hydrogen concantra t.on of 1 1% and 1.0%.

L i ce n s e e 4:4 44.c-_o we N bubble size to be thout 620 ft3 g 375 psig.

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April 1 No substantial change in RCS temocrature and pressure. ~

Incore thermocouples continue to show decreased trend.

Licensee continued hookup of hydrogen recor.biners and addition of shielding.

Licensee calculated valves of bubble size. varied.

Containment air samples indicate 2.3% hydrogen.

April 2 Reactor pressure stable at about 1000 psi.

Incore thermocouples continued to show a decrease with all n.easurements belcw 4750F.

Inlet and outlet O

te:r.peratures were still about 28Q F.dugtw y ceMhb b N Cne hydrocen reccmbiper was put in AWW %l g %.

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Analysis indicated that the oxygen generation rate in reactor less than originally es tiir.a ted.

Measurements indicated that the bubble was being significantly redaced,

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0 Reactor pressure and tamperature stable at 1000 psi and 280 F, respectively.

Thermocouple readings analyzed-maximum 4770F, only 3 ther.LoCCJples were D

above 4CO F.

Gas bubble size much reduced.

Containment about 1.9%

hydrogen.

One pressurizer level indicator failed.

April 4 0

Reactor pressure and temperature stable at 1000 psi and 230 F, respectively.

Thenr,3 couple maximum. tempera ture was 4660F.

Gas bubble size decreasing.

Vent valve on pressurizer intermittently opened and degassing continues through letdown system.

April 5 0

Reactor pressure and temperature stable at 1000 psi and 280 F, respectively.

Maximum thermocouple reading is 4620F.

Pressurizer level responding normally to pressure changes indicating a completely full system.

Contair. ment atmosphere indicates 2% hydrog,n.

Cne recombir.er op eating, one in standby.

Pressurizer verited to ccntainment about 15 minutes every S-8 hours.

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April 6 Reactor pressure stable at about 1000 psi and temperature about 2850F.

At approximately 1: 25 pm, reactor coolant pump 1 A tripped and reactor coolant pump 2A was started within about 2 minutes.

Shift in thermo-couple readings.

The three themocouples previously readiag about 4000F are presently reading between 28SOF and 31SOF.

Central thermo-cougle increased from 3750F to 4250F and is the only one reading e M d nA 400 F.

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Containment measurements indicate about 2% hydrogen.

Pump-back system for pumping ' caste gas decay tank ' volume to containment began.

I April 7 Reactor pressure and temperature stable at about 1000 psi and 280 F, 0

respectively.

At cbout 8 pm, the licensee gegcn to-slowly lcwer reacter system press f

The sicw decrease m } endnen reactor pressure reached. 500 psi.

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p,cvAg Hydrogen cancentration in the containmen is abou t 1.. %.

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