ML19220C667

Discusses Events within Primary Sys Re Core Assessment

ML19220C667
Person / Time
Site:	Crane
Issue date:	04/06/1979
From:	Kaufman N NRC COMMISSION (OCM)
To:	NRC COMMISSION (OCM)
References
NUDOCS 7905110417
Download: ML19220C667 (7)

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N. Ka u fr.a n 4/6/79 IHI' CORE ASSESSMENT

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Detemine the events within the IHI primary system ir. sofar as they might indicate core configuration.

METHOD:

Core density changes were inferred from reduced radiation transmission to intermediate and source range detectors.

Reactor coolant system pressure, reactor inlet and outlet temperatures, and,

steam generator pressure and levels were obtained from recorder plots.

Operetor actions were obtained from computer events monitor.

System behavior was theorized by a process of hypothesis and test for data consistene.y.

When possible, simple calculations were performed relatise to heat up, boiloff, and volume changes to confim reasonableness of process understanding.

Time inferred from graphs and/or recorder charts.

Estimated +3 minute accuracy for event specification.

RESULTS:

The sequence of inferred events one hour af ter turbine trip is attached.

This sequence states as fact many items that are conclusicns and hypotheses that seem to fit the available data, and it should be used accordingly. Where facts are unable to be tested, a (?) is shcwn.

Al'so attached is a diagram sumarizing core density behavior in the period from 70 to 220 minutes as deduced frem icn chamber traces.

In these traces, changes in transmissivity are interpretted as fluid /dcre density changes.

Changes in slope are considered important as indication of phase chrnges or elevation change. Also included in the diagram are plant and process information considered relevant and calculaticnal notes.

98 297 CCNCLUSIONS:

1.

Significant core damage occurs in the ene hour period from 6 a.m. to 7 a.m. -

Ion chamber signals suggest significant voiding at this tire.

Significant superheat occurs.

Relatively little inficw to lower plenum is noted during early portion of period when significant heating of voided core occurs.

Spike in icn cha-ters at 146 minutes suggests structure change.

Radiation noted in contafi ent. More than enough time and heat exists tn cause metal reactions if ctoling restricted.

7905110 %

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f TMI CORE ASSESSMENT Page 2 8

CONCLUSIONS (Cont.):

2.

Greater damage on B-leg side of core than A-leg.

Significant ficw noted on A-leg throughout period between 6 a.m. and 7 a.m.

as signaled by preferential heatup of cutlet RTD and more r.apid temperature decrease or A-leg inlet RTD during latter part of period.

3.

Core a P greater than combined a P of pumo and steam cenerator.

Whenever pressure is reduced folicwing 7 a.m., cold HPI and seal water ficw from pump and pump discharge toward outlet temperature detector in the pump suction.

This conclusion could be erroneous if the A-Generator cutlet tap were cooler than mixture of lower plenum and HPI water.

Addi tional ly, magnitude of temperature surges indicate A-leg in highly vaporous state.

7 Reverse ficw occurs in A-leg starting about 2 p.m. and lasting until EI p.m.

During this period, the plant is at low (500 psi) pressure and blowing down with flood tanks providing cold water to lower plenum.

4 Additional damage during period followino 7 a.m. cossible, but not certain.

Outlet temperatures at superheat or saturation and cold water from HPI and/or core ficod tanks in inlet plenum suggests care flew between 7 a.m.

and 2 p.m.

Icn detectors suggest some density change, but nothing approach-ing the response noted at 6 a.m.

Frcm 7 a.m..(204 minutes) on, HPI,. core flood, A-Generator and relief valve suggest ample heat removal capability.

Cn the other hand, highdP and reverse ficw suggests low flow given lack of pump.

5.

Metal / Water Reaction Rate might be bcunded by Hydraulic Data From 130 to 176 minutes, the primary system pressure rises following valve closure.

This rise might well include contributicn frca H associated with metal /viter reaction.

During this pressure rise, the in c$re density decreased by a factor of 1.5 to 2.5.

With available pressures and temperatures, it should be possible to estimate H, fraction in steam and thus, reaction fraction.

It might also be possible to bcund the energy added in addition to decay heat which caused pressure rise, thus scoping exothermic reaction.

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THI SEQUENCE OF EVENTS AFTER THE-Page 2 FIRST HOUR FOLLOWING TURSINE TRIP..s 150 min.

- - Indication of steam on both generators since li ttle or no heat removal.

Lower flow rate in 8-leg finally raises outlet temperature detector to superheat indication 30 minutes later than A-leg. Containment radiation spike reported. Adiabatic calculation (characteristic of dry out) would permit metal /

water reaction fr. 6 min. al. hot spot and 10-15 minutes for fuel rod of ave age heat. Simple calculation would permit core volume ano downcocer volume at core height to be boiled out a t 152 mir.utes.

150 - 176 min. - Pressure continues to increase with exponential appearance frca 660 ps; at 130 minutes to 2200 at 176 minutes. Ou tlet temperatures in both legs indicate superheated steam.

Increased ficw at inlet indicated by decreasing temperatures, with greater flow from the A-leg. Little or no cooling indicated at either genera tor. Apparent exponential rise suggests autocatalytic H7 generation. Core density indicated by ion chambers decreased less than expected from change in pressure, suggesting H2 presence.

176 - 180 min. - Quench of core and collapse of voids indicated.

Pressure discharge from relief valve. Lower plenum water forced into cold legs, with greater flow to A than 8 (A contains less water) and some cooling by the A-Generator. Main steam isolation valve and turbine bypass valve on 8-Generator closed at 180 minutes. Outlet temperature indicates superheat.

Quench suspected to be from water or steam from A-leg.

180 - 204 min. - Quench water / steam reheats and, in 7 to 11 minutes, voids agaio *;egin forming in the core, although not es extensively as cefore (about one-half). Little flow (possible backficw) ir the A-leg as indicated by hot water at inlet temperatore detector. Mcre ficw on B-leg and toward inlet plenum.

Outlet temperatures at saturation. Relief block valve opened at 204 min.

204 - 4a0 min. - HPI turned on.

Pressure drops to 1200 and is maintairM between 1200 and 1700 psi until 5.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, when the relief block v alve was closed. When valve closed, pressure rose to 2100 psi and remained there through period.

Throughout period, outlet temperatures were near saturation and inlet temperature generally decreased as the result of HPI flow.

Periodic surges of lower plenum water move into cold legs whenever system pressure decreases, more so in the A-leg than B-leg, indicating greater vapor volume in A-leg.

Core voids or significant density decrease noted at 223 minutes, associated with pressure surge and backflow from lower plenum to inlet legs. Slow correcticn or collapse noted in this case.

98 299

U1I SEGUENCE OF EVENTS AFTER THE

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FIRST HOUR FOLLOWING TURBINE TRIR:

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7.5 - 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> - Relief block valve opened at 7.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and pressure drops to 500 psi in 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. At 8.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, the core flood actuates.

Outlet temperatures shcw superheated steam.

Generators cease to remove heat for' 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> starting at 8.0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.

During this 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> period, inlet temperature decreases and outlet temperature is at superheat condition.

Small change in core density occurs a t 9.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

10 - 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> -

Reverse ficw in A-leg begins at 10.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.

B-leg may have tried to reverse at 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br />. Unexplained level change in B-Generator at 11.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> that appears to be frem charging, although logs and dita sheets centain no reference to such action.

Primary side contraction culls lower plenum water into 8-leg.

Even though Hpl flow is increased at 12.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, reverse flow continues in A-leg.

Relief valve again closed at 13.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and pressure rise begins.

Increasing pressure

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pushes HPI water toward A-leg generator and pushes lower plenum water into inlet of B-leg. Pressure increase and its effects extend frca 13.5 to 14.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.

During the episode, cooling occurs in A-Generator.

Some density change noted in core as pressure incceased, then a general density decrease occurs.

Suspect hot fluid from upper plenum pushed through core.

At 16 houre, A-lcop Primary pump turned on and significant events end.

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Kaufman 4/6/79 TMI SEQUENCE OF EVENTS AFTER TFE FIRST HOURLFOLLOWING TURBINE TRIP s

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- B-loop flow stops, B-Genera tor pressure begins to decrease, primary pressure quasi-stable at 1100 psi, with relief valve lif ted and choke ficw, I cut near saturation, T inlet subcooled, HPI off (?), A-Generator level slowly dropping, B-Generator level increasing.

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90 min.

- Increase in B-Generator level occurs.

90 - 100 min.

- Cold fluid on B-Generator causes depressuri.zation of primary system with density changes in core region.

Density decreases nearly linearly for 8 minutes as presssure decreases.

Magnitude is 1/3 of subsequent density change.

Temperature at saturation in both inlet and outlet legs. A-Generator pressure begins to fall coincident with flashing, suggesting reduced ~ heat removal due to reduced fluid density.

100 mfn.

- A-loop flow stops. A quench occurs from fall back of cold water from head or B-loop or possibly from core barrel check valve leakage, although source is uncertain. Cuench inferred from nearly step wise density increase.

B-Generator pressure drop stops. A-Generator level begins to rise and pressure drop continues, suggesting continued icw density in primary side of A-Genera tor.

100 - 116 min. - Pressure continues decredse, water now in core reheats Rough calculation estimates 12 minutes required to reheat twu times core volume up to saturation (factor of 2 is alicwance for inflow).

Inflow indicated by inlet temperature decrease.

Core outlet at or near saturation.

116 min.

- Voids begin to form in core. Voids (rather than water density change) marked by sicpe change in source range detector. Cutiet begins to show superhea t.

Pressure continues to f all. A-Gene r a to r level stabilizes, indicating some heat rejection.

Rate of pressure de.rease in A-Generator decreases.

116 - 124 min. - Upper half of core voids, superheated steam in outlet. Hot spot probably steam cooled or voided at 121 minutes.

Signifi-cant s team flow in A-leg and B-leg core inlet a t 124 minutes.

Significant steam ficw in A-leg cutlet, less in B-leg.

124 - 132 min. - Rest of core voids at 128 to 132 minutes. At 132 minutes, gate valve on pressurizer relief valve closed by operatcr.

Pressure begins to rise.

Inlet temperature indicates little, if any, ficw fran A-leg to inlet, but some ficw from S-leg to inlet.

A-leg superheat rate of at leas t 10 F/ min.

132 - 150 min. - Core appears voided with some density reduction as pressure rises. Outlet plenum contains superheated steam.

Some ficw from B-leg to lower plenum, little or no flow from A-leg.

At 146 minutes, the neutron detectcrs indicate one or more changes

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