ML19322A418

Responds to 790723 Request for NRC Opinion Re Amount of Media Attention Given to Question of Fuel Melting & Core Meltdown.Forwards NRC 790330 Press Release & TMI-2 Event Tree Analysis for Core Meltdown & Evacuation Procedures

ML19322A418
Person / Time
Site:	Crane
Issue date:	09/10/1979
From:	Harold Denton Office of Nuclear Reactor Regulation
To:	Johnson V PRESIDENT'S COMMISSION ON THE ACCIDENT AT THREE MILE
Shared Package
ML19322A419	List: ML19322A418 ML19322A420
References
NUDOCS 7910240496
Download: ML19322A418 (25)

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USNRC 'Offi'ce.of'Public Affairs. Press Release ' dated Marcn? 30;?197

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.ay Reference 3: TMI-2 Event Tree Analysis for Core Melt Doyn and RecommendedJ

,.M C Evacuation Procedures dated April 1,.1979. _

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Your letter of July 23,,1979,. requests the NRC's understanding of the public. 2;m%

hypothetical conseque'nces"of;such' a development, how news media attention to tne. question of' tuel melting and core meltdown, thelP

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persisted'so long~ following 'the' accident at Three Mile Island, Unit 2..

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y It is difficult to pinpoint the primary mecnanism(s) that caused this concern since the press was connunicating with NRC personnel and industry personnel through many channels.

It is possiDie that tne initial puulic reaction to tne TMI-2 accident results from an impression of assumed nuclear catastrophes involving the melt-down China syndrome sequence of postulated events. Althougn -

little credence is ascriaed to this rapid, imminent, and unaltered scenario, it is a comonly held misconception which quite naturally coulo result in intense and persistent interest from members of the public.

s Consideration' of fuel melt. started during the first days after the accident whers evidence of severe core damage at the Three Mile Island, Unit 2, facility was inferred from high in-core thermocouple temperature readings and primary coolant sample analysis ' results. Also, a bubble of nonconcensable gases had collected in the. reactor vessel upper-head which led to anxiety over'the outcome of'the:

. ['

' event-lOne important operatiodal_ objective was to reduce and finally el.imi~nater,.

the 'bubbl.e.

0.'n.e. met _hoclexplored ' for%e.liminat. i,ngl these noncondensab..in

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v i nclndev, rap i.d: depressurTratione ofithevreactor cool'a.nt system which-- - ol edithe.p%;9 h

Q cbns!i de ra~ti on of. poss.i dl e3As':f( tiub615f expan si$

'NEinj4ction.?Evsn th'o6gh3tihisicouse?6f acti'o'n sisMdt isken,. we-thinkTthsb H %

' ' discussions. of possible c'onsequen'c'es"of this sequence helped Dring on the' melt @MW q i

. &down concern becausefexpansi'en of' the DuoDie in 'the' rTactor vessel 'cou' ld.'deterMMD

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- penetration) of the water [: flow re' quired to cool the core and-results iii s fue.TimelWW

.The potential for hydrogen ewlosion in the reactor vessel was also of'~ concern

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. until i t was conc.luded tnat1tnere vas no signifi cant. oxygen O(ing generateo in'

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-. alarm ~of a core ' meli.down.h F0llowing 'the 'Ma'rch 30,;1979,. Commissionimeetiin % M M $

an NRC; press 1 release (~ Reference' 2) ~was. forwarded toi EMM m

c e;me sed:Referenc s

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k w.;Withyregard. to?therprobabfli:tyjofia:: core melt and i~tsthypotheticalP r

y some staff work ~was 'doile 'on March 30-31, 1979'(sed.encibsed Re'ference 3) E 0nly relative probabilities (high, low,, medium) were used to determine' core meltdown'

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assuming trip of the operating reactor coolant pump as the initiating event.

4 The event tree focused more on the sequences which could lead to a core melt than the actual proDabilities involved. Core cooling from natural circulationt

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was not assumed in this event tree. The overall probability of a core melt appeared to be relatively low. Reference'3 also provides the sequence of major. h M '

events following a postulated core meltdown. The fission product releases would i

~be similar-to those of the Reactor Safety Study (WASH-1400). ~ Evacuation scenar'ios

<+l relating to other postulated events following'the accicent are< also. presented-inh

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. We have discussed this? with Mr.. William Stratton andl uncerstand th'at this. infori Sg.

'mation 'is respons'ive to.your. request.1 ~ '

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' Evacu'ation A ocedures dated ApFil 1,. l'9 n s.

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Your letter of July 23,.197 requests.the'NRC's understanding of the'public y g d.

hypotheticar consequences 1of~ skhs a deveTopment,, and. why it; pers news media attent. ion to'the q stfon_of fuel mel ng and core meltdown,: t leng3 J

A 41 jf ggggl pgg, y 99,3 y y { g Al A WMMMkggf.N.ygMpqgg,h fol, lowing the. dident.atnThree. treUrsiandt it;2L w It is difficult to'detemine the e act me nism(s)' that caused this concern' " Y + '

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since the press was communicating wfth NR/c personnel and industry personnel through many channels. Consideration \\o fuel melt started during the first days after the accident where evidence [of severe core darnage at the Three M Island, Unit 2, f acility was inferre/fArn high in-core then=ocouple tempera-ture readings and primary coolant s, ample analysis results. Also, a bubble of noncondensable gases had collected /in the reactor vessel upper head. One impor-tant operational objective was te reduce and\\ finally eliminate the bubble. One method explored for eliminatin<y these noncondeqsable gases included rapid depres-surization of the reactor cogTant system which 'ipvolved the consideration of

possible gas (bubble) expanpion and initiation of\\high-pressure in.jection. Even though this course of action was not taken, we thihk that ' discussions of possible;l'

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. consequences of this seqpence brought,on the meltdok concern because expansion-4 l

of the bubble in the r9 actor vessel could deter penetration of ~ the water flow-I required to cool the rare and result-in a fuel melt.. The potential for hydrc-gen.expros. ion in th. react. or-ve. s,se..l; wai a,lso-of". co. ncern hntil' it.w

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Friday, March 30, '1979. ;Thestranscript 'of' this1 meeting (Referbnce 1) shows: '

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that the possibility of core meltdown related to Nhble exnansion had hoon relayed to the news p edia try a pres s briefing. Subsequent mis interpretation efat4ts..in.fo m ation*nd-related-press-refesses-appecr"to-heve menset-be-pel ie-

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'g UNITED STATES l$,'{te S

NUCLEAR REGULATORY COMMISSION o

I

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OFFICE OF PUBLIC AFFAIRS 4

WASHINGTON, D.C. 20555 i

No.

79-67 Tel.

492-7715 FOR IMMEDIATE RELEASE (Friday, March 30, 1979)

NOTE TO EDITORS:

6:30 p.m. EST on Friday, March 30.The following was telephoned to the me at The Chairman of the Nuclear Regulatory Commission Joseph M. Hendrie said this afternoon that there is no imminent danger of a core melt at Nuclea.r Plant..

the Three Mile Island Additional technical experts headed by the Director of the Office of Nuclear Reactorfrom the Commission Regulation Mr. Harold Denton reached the site early this af ternoon.

At the direction of the President, provided with augmented communication facilities.they have been team at The NRC the site is working closely with the utility per-sonnel and experts from other federal agencies and the State of Pennsylvania.

Governor Thornburgh.Close contact is being maintained with Efforts to reduce the temperatures of the reactor fuel are continuing.

These temperatures have been coming down slowly and the final depressurization of the reactor vessel has been delayed.

There is evidence of severe damage to the nuclear fuel.

Samples of primary coolant containing high-levels of radiciodine and instruments in the core indicate high fuel temperatures in some of the fuel bundles, and the presence of a large bubble of non-condensible gases in the top of the reactor vessel.

r:[

Because of these non-condensible gases, exists of interupting primary coolant flow within the reactor the possibility should the pressure be further decreased and the contained gases allowed to expand.

In the unlikely event that this were to occur some of the fuel would fail to cool and further damage to that fuel could occur.

final safe state for the fuel are under consideration.Several options to ' reach a the meantime, In condition.

the reactor is being maintained in a stable

.s y

• 2-J There have been inter ittent releases of radioactivity

~

into the atmosphere from the primary coolant system.

licensee is attempting to stop the intermittent gaseousThe releases by transferring the radioactive coolant water into the primary containment building.

hour in the immediate vicinity of the site atbeing measure Off-site levels were a few millirem per hour. ground level.

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NRC PROCEDURES FOR DECISTON TO RECOMMEND EVACUAT10N Who Decides 1.

Combination of consequences and times require immediate initiation of evacuation:

Senior,tRC Official on site recor:Inends to Governor.

2.

Unplanned svent with substantial risk takes place or is imminent or situation judged excessively risky but there is time for con-sultation.

Senior NRC Official notifies Governor and NRC HQ.

Chairman makes recommendation to Governor after consulting with Commissioners if possible.

3.

Planned event involving significant additicnal risk.

Chairman

(

and Commissioners makes recommendation.

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h eg Unplahned Events EXPECTEll PLANT RELEASE TARNIllG EVACUATION

' EVENT

RESPONSE

Atl0 TIME TIME

_SEBiaRIO

1. Loss of vital Restore function withis No significant Possible pre-function or un-I hour change cautionary planned leaks.

evac 2 mi; staj inside 5 mi Switch to Alternate small leak less possible pre-Examples Function involving than 1 gal / hour cautionary i

Primary Coolant in evac 2 mi; Reactor Coolant Auxiliary Building stay inside Pump Trip; 5 mi Loss of offsite large leak 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> Evac 2 miles power; 50 gal / min Stay Inside S miles Loss of feed-water; Serious possibility of failure to restore a Depressurization vital function 1

to 90 on RllR; i-See 2

[

Leak in Auxi-liary Iluilding coe %N%

4 These tables incl sde a number of ass imptions about activity and

[

weather, chosen calistically4 In in actual relea se, the release

)

[

rate and weather

hould be evaluated as they are at the time, and I

the decision base 1 on those values, y

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EXPECTED PLAfiT RELEASE -

HARNIllG EVACUATION' il EVEllT

' RESP').'ISE Atl0 TIHE Title SCEl{ARIO d

2. Sequence lead-liaintain Containment Design Contain-4 hour Precautionary p

ing to Core Integrity (likely) with nient Leak Rate Melt Containment Cooling Evac 2 mi all I

around and 5 mi', 90* sector.

stay inside 10 mi l

Containment expected to Significant

-24 hour Evac 5 mi all i

Dreach release of core (time for con-around and 10 fission products tainment. failure) mile, 90*.

sector, stay-1 inside 15 mi

3. Ilydrogen flame Mixture in flannable Precautionary 9o*

2 mi (?) -f f %4 5Mr or explosion range a

l possible inside i

reactor vessel Explosion; major

'# b I

damage Core fiel t See 2

.**1(./s

(

l ikccw4snu, G) 1 s

4. Evacuate or Lose Loss of Control Treat Evac 5 mi all Control Room like major release around and 10 L

mi 90* sector, stay inside 15 miles l

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4 EXPECTED PLANT RELEASE HARNIllG EVACUATION EVENT RESP 0flSE AND TIME TIME SCENARIO Planned Probability of losing Timing of maneuver Precautionary Manuever vital function can be set to evacuation 2 provide as much l nil es.s tay time as necessary inside 5 miles PLUS

~

See releases under See outcomes loss of vital under loss of function vital function.

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Action Guidelines J

Notify evacuation authorit'ies two hours in advance (if possible) a.

to standby for a possible evacuation.

b.

Projected doses of 1 rem-whole body or 5 rems thyroid. stay inside.

c.

Projected doses of 5 rems whole body or 25 rems thyroid mandatory evacuation of all persons.

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Assumes general warning already that some form of evacuation may

(

become necessary.

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Weather The table is based on a realistic prediction of. the weather for the next few days, based on the April l forecast which would result in high doses at a given distance. At the approach to decision time for evacuation, the appropriate meterol.ogical condition will be factored into the dose estimates to determine ~ the evacuation time, sectors, and distances for the evacuation.

NRC is' predicting the dispersion characteristics of the region for the currently measured meteorology as the incident progresses.

Rain could lead to higher local radioactivity levels.

J Heat Generation The reactor core is now quite cool compared to the conventional design-basis calculations, r

a.p u.~

1.

The reactor is new, so no fuel has more than 3 months equivalent operation, compared to 1-2 years average for other plants.

2.

The neutron chain re~ action has been shut down for over 4 days.

It should also be noted that the concrete basemat of this plant is unusually thick.

As a result of the above differences, calculations for this plant at this time predict that the core will not melt its way through the containment.

9

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Event 1

- Sprays and Coo.lers Operative Time =0 Flo.w stops, core and water start heat-up Time =100' min Cor'e starts to unco'ver Time =150 min. Core begins to melt

,ime=200 min Molten core is in lower head of reactor vessel, pressure T

~

is 2500 psia Time =210 min Reactor vessel fails, containment pressure goes to 25 psia Time =210 min Hydrogen burns, containment pressure goes to 67 psia Steam explosion possibility - minor consequence CONTAINMENT SURVIVES (Failure assumed 130 psia) -

.Tima=10 hours Molten core has melted about i meter into basamat Tita= days-Major pr' blem - handle hydrogen, oxygen - maintain contain-o ment integrity CAUTION:

Keep sprays. running

..(

Keep water many feet over molten debris WITHOUT RECOM5IllERS Hydrogen continues to' build up EASE.MT SURVIVES Event 1

Conclusion:

This event should not produce major releases Event 2

- Sprays'and Coolers Failed Before Flow Stops Time =0 to Time =210 min'.

Same as Event 1 - containment pressure is 25 psia Time =810 min Containment pressure is 70 psia Time =1 day

'. containment fails due to steam (mostly) overpressure -

,about 135 psia C0!!TAltlMENT FAILS Event 2 Cenclusion:

This event leads to major releases.

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---o i

APR 1 1979

~

This table includes a number of assumptions ab ut activity and weather.

These assumptions have been chose'n conservativ-v.

In an actual release, the release rate and weather should be evaluated as they are at the time, and the decision based on, those values.

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f Event - Spontaneous failure or decision to perform.a potentially

-S.k risky maneuver Find out what actually happened and 'what is functioning.

In tables Predict what could result - different likelihoods Predict release rate Determine present weather and forecast Assumed constant-in table Dose Prediction In table Action Guidelines Per Appendix 7

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I

PAGE i __

u EXPECTED PLANT RELEASE MARllITIG EVACUATION Wil0

~

EVENT

_ RESP 0?lSE (RANGE?)

AND TIFE TIi1E SCENARIO DECIDES

1. Loss of vital Restore Function Within No significant Hone

• function or 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />

' change decision to per-form a poten-tially risky Switch to Alternate Small leak less

'None*

maneuver.

Function involving Pri than 1 gal /heur j

Coolant in Aux Building Examples Large leak in 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> Evac 2 miles

1. Reactor Aux Building

$tay Inside Coolant Pump 50 gal / min 5 min i

Trip.

Failure to restore Core melt; see 2.

Leak in Aux vital function item 2 below &

Building.

Appendix 1 3.

Loss of of f-1 site power 4.

Lass of feed-

• for sufficiently risky maneuver, water do precautionary evac 2 mi and stay inside 5 mi; whether to do S.

Depressuriza-this or not d! pends on details tion to go of naneuver a1d plant situation.

on RilR.

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PAGE 2

EXPECTED PLANT RELEASE llAR?lIllG EVACUATION Wil0 EVENT

_ RESPONSE (RANGE?)

AND TIME TIliE scrNARIO DECIDES 2,

Core Melt itintain Containment Tech Spec Con-4 hour Precautionary Integrity (likely) with tainment Leak Containment Cooling Rate Evac 2 mi all around and 5 mi sector; sta;r inside 10 mi l

?.

I Containment Beached Reactor Safety 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Evac 5 m1~all l

Study Categories' around and 10 PWR 4 - See mi sector, st'a.r Appendix 1 inside 15 mi.

~

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3.

Ilydrogen Ex-No significant change No significant None plosion Inside in reactor or primary change Reactor Vessel system Core Crushed (unlikely)

Core melt See Item 2 &

Appendlx 1 l

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I APPENDIX 1 MAJOR SEQUENCES OF EVENTS 1

liajor sequences evaluated here hre tied to the loss of forced circu-lation in the RCS.

The loss of flow from the reactor coolant pump (RCP) is the generalized initiating event from which other initiating

' events such as loss of offsite power can develop.

APPENDIX 1.a SEQUENCES OF POSSIBLE SYSTEMS FAILURES Figure 1.b-1 shows the loss of RCP event tree.

This tree shows the various options available given the loss of the RCP, and ir.dicates which combinatians of events or failures would lead to core meltd:wn (CM).

The sequences denoted with an asterisk are those whi.ch would be ex-5I pected to follow the core n11tdown progression discussed below, leading to the variety of atmospheric radioactive releases and consequences discussed later.

Some ' core meltdowns could be expected to be delayec

. for roughly a week becaus'e of the availability of ECC injection over fthatperiod.

This method of core cooling, however, is not expected to i

be adequate to prevent core melt; as such a core meltdown is assessed

, to occur at roughly a week. - A rough measure of relative probabilities of the various outcomes is indicated by the notation of L, M, H (low, medium,high)'.. The column on the right-hand side of the page indicates the relative probabilities of the sequences, with "LM" as the highest probability and L M as the lowest.

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- Sprays and Coolers Operative

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Time =0 Flow stops, core and water start heat-up Time =100 min Cor'e' starts to uncover Time =150 min. Core begins to melt Time =200 min Molten core is in lower head of reactor vessel, pressure is 2500 psia Time =210 min Reactor vessel fail.s, containment pressure goes to 25 psia Time =210 min Hydrogen burns, containment pressure goes to 67 psia Steam explosion possibility - minor cons.equence CONTAINMENT SURVIVES (Failure assumed 130 psia)

~

Time =10 hours Molten core'has melted about 1 meter into basemat Time = days Major problem - handle hydrogen, oxygen - maintain contain-ment integrity

2....

CAUTION:

- Keep sprays running

- Keep water many feet over molten debris

- WITHOUT RECOMBINERS Hydrogen continues to build up BASEMAT SURVIVES Event 1

Conclusion:

This event should not produce major releases Event 2

- Sprays and Coolers Failed Before Flow Stops Time =0 to ' Time =210 min.

Same as Event 1 - contain. ment pressure is 25 psia Time =810 min Containment pressure is 70 psia Time =1 day

. Containment fails due to steam (mostly) overpressure -

about 135 psia CONTAINMENT FAILS Event 2

Conclusion:

This event leads to major releases.

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, The following are essential in the event of core melt.

1.

Sprays and coolers are required to prevent major releases.

2.

Hydrogen must be recombined or otherwise removed from containment.

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Large Leak in Auxiliary Building'-(AB)

The activity level in the reactor coolant is so hi0h that substantial releases can come from small amounts spilled in the AB which requires once through ventilation.

A leak of 5 gpm to the AB atmosphere is assumed for the expected level of leakage.

A leak of 50-gpm is taken as a large leak to consider a major leak in pump shaft sealing or some similar mishap.

Based on the leakage experienced already only the noble gases and no iodine are assumed to evolve.

The AB ventilation exhaust is assumed to flow through the charcoal filters.

id.

Hydrogen Exolosion in Reactor Pressure Vessel A detonation of the hydrogen oxygen bubble in the reactor vessel could rupture the vessel and/or crush the core.

Rough analysis indicates that the pressure vessel would not rupture.

Postulation of the core response is difficult.

If the core is crushed, it could effectively prevent core cooling leading directly to the core melt sequence described earlier.

It is unlikely that compression would lead to criticality.

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Action Alternatives Evacuation Stay Inside 1.

2 miles 2.

2 miles 5 miles 2 miles all around 3.

5 miles 90' sector 10 miles a miles all around 4.

10 miles 90' sector 15 miles a.

All sector choices governed by wind direction.

If shifting, more than one quadrant may be affected'.

b.

These are initial values; as the release continues measurements may indicate the need for reconsideration of action up, to 20 miles.

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Weather The table it' based on F stability and 1 m/sec wind speed, in view of the April 1-3 forecast.

At the approach to decision time for evacuation, the appropriate' met. condition will be factored into th'e. dose equations to determine the evacuation time, sectors, and distances for the evacuation.

NRC is predicting X/Q for current meteorology as the incident progresses.

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Notify evacuation autho'rities two hours in advance to standby for a possible evacuation.

b.

Predicted doses of 1R whole body or SR thyroid in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> -

mandatory evacuation of children and pregnant women.

c.. Predicted doses of SR whole body or 25R thyroid in 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> -

mandatory evacuation of all persons.

Assumes general warning already that some form of evacuation may become necessary.

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