ML19220C542
| ML19220C542 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/10/1979 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUDOCS 7905110193 | |
| Download: ML19220C542 (9) | |
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TESTIMONY OF DARRELL G. EISENHUT BEFORE THE SUBCOMMITTEE ON NUCLEAR REGULATION OF THE C
SENATE COMMITTEE ON ENVIRONMENT AND PUSLIC WCRKS
~ [g ' )y TUESDAY APRIL 10,1979 9:00 A.M.
The following is the preliminary summary of the significant events that occurred at the Three Mile Island No. 2 nuclear facility on March 28, 1979, and in the days that folicwed.
Attached is a detailed chronology of these events listed with the times they each occurred.
At '. bout 4:00 am on March 28, 1979, the secondary (nonnuclear) cooling This system of the Three Mile Island facility suffered a malfunction.
system normally pumps water through the plant's steam generators where the water turns to steam which then flows to turn a turbine generator.
The steam is then condensed back to water, which is pumped by a condensate pump through a clean up system, through a feedwater pump, and finally back to the steam generatcrs, and flow continues around this loop.
A malfunction in the main feedwater system caused the feedwater pumps to turn off (trip), which in turri caused the turbine-generator to turn off and stop generating electricity.
Since the steam generators were not removing heat due to the stoppage of feedwater flow, the reactor coolant system pressure increased and the pressurizer relief valve opened to reduce reactor pressure. The reactor turned off by the rapid insertion of the plant's control rods (scrammed) as designed and the nuclear chain T
e reaction stopped leaving behind principally residual, or decay, heat.
events all occurrea within the first 30 seconds following the event.
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., Up to this point, this sequence is norrial and plant response was as expected, and the auxiliary feedwater system should startup and deliver secondary ccolant to the plant's two steam generators to remou Eeat.
In addition, the pressurizer relief valve should close as reactor pressure decreases.
All three of the auxiliary feedwater pumps started but were unable to deliver flow because their flow paths were blocked by closed valves.
Auxiliary feedwater flow was established through the opened valves about eight minutes later.
In addition, the pressurizer relief valve failed to closed and therefore allowed the reactor coolant system pressure to continse to de-crease.
As the reactor pressure reached a preset value (1600 psi), the plant's Emerrancy Core Cooling System (ECCS) started as designed and began to inject cold water into the reactor.
It is at this point that an indication of a rapidly rising pressurizer level apparently led the plant ope.rators to tenninate the ECCS flow. At this point the Three Mile Island incident had been underaay for 11-12 minutes.
Between about 1 and 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> into the transient, the operators turned off the four large pumps which circulate the reactor coolant through the reactor.
It is following this action that we believe the severe damage to the nuclear fuel began.
For the next several hours there was a very large temperatura difference across the nuclear core indicating little flow of coolant through the core.
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. During this several hour period, when fuel damage was occurring, primary coolant from the reactor primary coolant system was being discharged to the reactor containment ficor from flow out of the pressurizer relief valve and through the drain tank.
Part of this coolant, which contained radioactivity, was automatica'ly pumped frem the reactor containment building floor to tanks in the auxiliary building. The tanks overflowed permitting radioactivity to be vented from the auxiliary building.
This discharge was secured in about 40 minutes. The reactor containment was sealed (isolated) at about 9:00 sm.
.Through the afternoon and early evening of March 28, 1979, the licensee isolated the stuck open PORV and tried to depressurize the reactor coolant system sufficiently to be able to turn on the resdiual heat removal system.
Since his attempt:: failed, it was decided to repressurize the system.
After repressurization (about 8:00 pm), one of the main reactor coolant pumps in loop A was restarted and flow through the reactor core was established.
Heat was being transferred out of the reactor through the steam generator while using the condenser. The primary system was maintained at a pressure of 1000 psi and a temperature of 280*F.
Reactor cooling has essentially been in this mode since ihat time.
Efforts have mainly been devoted to maintaining this condition while a series of analyses have been conducted and while measurements have been taken to confirm a variety of parameters.
These efforts have been directed toward preparing for 1 9 3
the next steps in the cooldown process.
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PxELIMINARY CHRONOLOGY OF THE MARCH 2A, 1979 ACCIDENT AT THREE MILE ISLAND Time (acoroximate)
Discussion 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 pumps.
The turbine tripped.
3-6 sec later An electromatic relief valve opened to relieve pressure in the RCS* (2255 psi).
9-12 sec later The Reactor tripped on high RCS pressure (2355 psi) to terminate the nuclear reaction and reduce pcwer generation to decay heat alone.
12-15 sec later The RCS pressure decayed to the point (2205 psi) where the relief valve should have reclosed.
The RCS continued to depressurize for about the next two hours.
15 sec later The temperature in the RCS hot leg peaks at about 6100F with a pressure of about 2150 psi.
30 sec latar The auxiliary feedwater pumps in both safety trains (1 turbine driven pum, ano 2 electrically driven pumps) were started and were running at pressure ready to inject water into the steam generators and remove the residual heat produced in the reactor core.
No water was injected since the discharge valves were closed.
- Throughout, RCS denotes " reactor coolant system."
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, _ Time (aoproximate)
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.
4:04-4:11 am The pressurizer level indication went offscale high and the operator manually tripped the first HPI pumps at about 4:04:30 and the second at about 4: 10:30.
4:06 am Water in the RCS flashed to steam as the pressure bottoms out at 1350 psi. The hot leg temperature wa: about c.850F.
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 steam generators.
4:11-4:12 am The operator restarted the ECCS to inject water into the RCS to control pressurizer level.
4:11 am The pressu'rizer level indication comes back on scale.
4:15 ?.m The RC Drain (Quench) tank rupture disk blew at 190 psig due to continued discharge of the relief valve that had failed to
- close, 4:20-5:00 am The RCS parameters stabilized at a satur-ated condition c ab,ut 1015 psi and 5500F.
5:15 am The oparator tripped both RC pumps in Loop b.
5:40 am The operator tripped both RC pumps in loop A.
b
. Time (aoproximate)
Discussion of Events 5:45-6 am The reactor core began a heatup transient.
The RCS hot 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 operator by closing a block valve.
The operator also isolated steam generator B to prevent leakage of radioactive secondary water from leaking S. G. tubes.
7:00 am The RCc pressure had increased to 2150 psi and the relief valve was opened to relieve RCS pressure.
7:15 am A pressure spike of 5 psig occurred in the RC drain tank dua to steam from the relief valve.
i:45 am 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 contair. ment peaked at 4.5 psig.
9:00-11:00 am The RCS pressure i'ncreased frcm 1250 psi to 2100 psi.
11:30 am The operator opaned the pressurizer relief valve to depressurize the RCS in an attempt to initiate RHR cooling at 400 psi.
12:00 am - 1:00 pm The RCS pressure decreased to about 500 psi and the core ficoding tanks partially discharged.
The relief capacity was -
not sufficient to vent enough to reach 400 psi.
2:00 pm The pressure in the containment spikes at 28 psig causing containment sprays to be initiated.
The operator stopped the spray pumps af ter about 2 mippjps of operation.
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Time (aoproximate)
Discussion of Events 5:30 pm The pressurizer relief valve was closed in order to repressurize the reactor coolant system.
The RCS pressure increased from 650 psi 5:30 - 8 pm 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 generator.
Thereafter, the reactor was being cooled by reestablishing ~ condenser vacuum and steaming to the condenser by steam generctor A with the RCS cooled to about 280 degrees F and 1000 psi.
March 29 The RCS temperature and pressure was stablized at about 280 degrees F and 840 to 1020 psi. The maximum reading on the incore thermocouples was 6120F, but several were not within range _for computer readouts (print _ing "?")
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which was subsequently found'Yo in'dicate greater than 700 degrees F.
March 30 The RCS temperature and pressure wac vcable at nearly 280 degrees F and between about 1000 to 1060 psi.
Several incore thermocouples were beyond the range for compt.er readout, the maximum indicated reading was 659 degrees F.
The NRR staff estimated the bubble size in RCS to be about 1200 ft3 and requested the licensee to refine their calculation of the bubble size.
March 31 0
The RCS temperature and pressure remained stable at about 280 F and 1000 psi.
Slight drop in pressurizer level 251-191". Temperatures in the core as measured from the incore ther, scouples were gradually decreasing (maximum indicated about 5000F). The hydrogen recombiner was in an operable status but additional shielding was needed anc was being obtained.
Two samples of containment atmosphere were analyzed which showed a hydrogen concentration of 1.7% and 1.0%.
Licensee calculated bubble size to be about 620 ft3 @ 875 psig.
.. Acril 1 No substantial change in RCS temperature and pressure.
Incore thermocouples continue to show decreased trend.
Licensee continued hookup of hydrogen recombiners and addition of shielding.
Licensee calculated values of bubble size varied.
Containment air samples indicate 2.3% hydrogen.
April 2 Reictor pressure stable at about 1000 psi.
Incore thermocouples continued to show a decrease with all measurements below 4750F.
Inlet and outlet temperatures were still about 280 F.
One. hydrogen recombiner was put in 0
aperation.
Analysis indicated that the oxygen generation rate inthe reactor 7as lus than originally estimated. Measurements indicated that the bubble was being significantly reduced.
Aaril 3 0
Eaactur pressure and temperature stable at 1000 psi and 280 F, respectively.
Thermoccugle readings analyzed-maximum 477"F, only 3 thermocouples were above 400 F.
Gas bubble size much reduced.
Cantainment about 1.9%
One pressurizer level indicator failed.
April 4 0
Reactor pressure and temperature stable ct 1000 psi and 280 F, respectively.
Thermocouple maximum tc...:perature was 4660F.
Gas bubble si7e 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. readina is 4620F.
Pressurizer level respondinq normaliy to pressure changes i dicating the gas bubble was greatly redu.ed.
Ceatainment atmosphere indicates 2% hycrogen.
One recembiner operating, cne in standby.
Pressurizer was vented to containment about 15 minutes every 6-8 hours to degas the primary system.
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. April 6 Reactor presscre 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.
Shi f t in thermo-couple readings.
The three thermocouples previously reading about 400 F are presently reading between 2850F and 3150F.
Central thermo-0 cougle increased from 3750F to 425 F and is the only one reading 0
400 F.
Containment measurements indicate about 2% hydrogen.
Pump-back system for pumping waste gas decay tank volume to containment began.
April 7 0
Reactor press te and temperature stable at about 1000 psi and 280 F, respectively.
At about 8 pm, the licensee began to slowly lower reactor system pressure.
The slow decrease ended when reactor pressure reaches near 400 psi.
This step will be repeated and is a step toward cold shutdown and includes degasification to prevent bubble formation as pressure and temperature decreases.
Hydrogen conceiration in the containment is abcut 1.9%.
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