ML19220C749

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Forwards 780413 Memo Re TMI Core Damage Assessment,Updating 790406 Rept Based on Early Data.Memo Also Forwarded to ACRS Members Attending 790508 Meeting on Reactor Fuel
ML19220C749
Person / Time
Site: Crane Constellation icon.png
Issue date: 04/27/1979
From: Boehnert P
Advisory Committee on Reactor Safeguards
To: Bender M, Carbon M, Ebersole J
Advisory Committee on Reactor Safeguards
References
ACRS-SM-0091, ACRS-SM-91, NUDOCS 7905140154
Download: ML19220C749 (1)
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Text

{{#Wiki_filter:\\ l pa 7tc-s 0, I UNITED STATES ~O ^j* ,' 3 NUCLEAR REGULATORY COMMISSION (', ' y ADVISORY COMMITTEE ON REACTOR SAFEGUARDS [ JA? #/a g 7,, p WASHINGTON, D. C. 20555 ( April 27, 1979 I I L [ M. Bender D. Moeller M. Carbon M. Plessset J. Ebersole J. Ray W. Kerr C. Siess S. Lawreski NRC MEMO Qi CCRE EAMXJE ASSESSME!TI' KR T4I-2 Attached is a copy of an NRC memo on an assessment of T4I core damage. This memo is a follcw-up to an April 6,1979 NRC reprt that estimated fuel damage based on the earliest available information. Copies of the subject memo have been provided to ACRS members attending the May 8, 1979 Reactor Fuel meetinc, under separate cover. s prl Y

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Paul Boehnert l Reactor Engineer

Attachment:

as stated cc: ICRS Technical Staff I. e ,I / 7905.14015i 0 m 1 106 \\ v s ~s - ---.....~.-....w.,.._. s rw, i I

Eej ~ gaaso,I o / . J' UNITED STATES + [Y. < [ h NUCLEAR REGULATORY COMMISSION 7 ig',gr, [ j ADVISORY COMMITTEE ON REACTOR SAFEGUARDS $'s,I' 3'[ p8 WASHING ton, D. C. 20555 l ( +...a April 26, 1979 i P. Shewmon A. Bement H. Etherington J. Crocker C. Mark W. fiathiS D. Okrent NRC MEMO Gi CCRE CAMAGE ASSESSMFNT FCR W.I-2 Attached is a revised copy of an NRC memo on 'n4I core damage that was attached to my April 25, 1979 notice of the May 8 Reactor Fuel Meeting. Some pages were emitted frem the earlier copy. F Paul Boehnert Reactor Engineer i

Attachment:

As stated ? f e 9 e g 0R k. loo ( \\no 4. .n....-y. s - p.,

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%, C df APR 13 lo79 %s i t n MEMORNIED1 FOR: Roger J. Mattscn, Director, Division of Systems Safety, NRR i l FRCit: R. O. Meyer, Reactor Fuel Secticn Leader, Core Performance Branch, Division of Systems Safety, NRR

SUBJECT:

CORE DAFAGE ASSESSMENT FOR TMI-2 l Attached is cur assessment of the core damage at TMI-2 for use in the SER for natural circula:icn. It recresents cur inde::encent evaluation of the facts available and of the industry /vendcr/ licensee analysis, which we have heard in several briefings. An earlier estimate of fuel damage was made by Rubenstein et al, and a recent meeting was held at NRC with industry excerts. Memcranda describing those evaluations are attached to this document. A Q& 'M Ralph.0. " eyer,/Secticn Leader Reacter Fuels Core Perfarnance Branch Division or dysta=3 Safety I

Attachment:

l As stated i cc: R. Tedesco K. Knf el P. Check l C. Berlinger D. Crutchfield (FOIA File) l D. Houston M. Tokar V. Stello B. Grimes G. Knighton G [C[. J. Vagicwede / D. Pcwers POR O I , W' -a m, l L I UD, I4c ~ ' e 6

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o CORE D/MGE A. Introduction For the usual analysis of hypothetical accidents, initial core conditions are assuted and consequences are calcul; tad. This uculd involve complex thernal-hydraulic calculations and fuel behavior analyses. At Three Mile Island, however. some of the consequences are known (i.e., some information on fission product release, hydrogen generation, and instrument readings is available), so we will use reverse engineering" as our principal method of backing out an assessment of core damage. I i f We start with the assumption that the core was uncovered and allcwed to heat up for sijnificant periods of time. Figure 1 shows the system pressure history for March 28, which includes three periods of significant uncovery. The periods of uncovery correspond approximately with the major periods when system pressure was belcw the saturation I pressure. We will assu e that the first core uncovery began shortly after 92 minutes into the accident at which time excore ion chambers shcw a response spike corresponding to the loss of water shielding. Although the two later periods of uncovery may have produced additional core damage, we will focus on the first period because decay heat was larger then and because that period produced the large radiation instrument reading (at 150 minutes) in the containment indicating major fuel damage. Because the fuel damage to be discussed below is so extensive, we will 106 150 ee~* --e=,e m 1

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i conclude without demonstration that virtually all of the fuel rods in the core failed in the sense of experiencing defects large enough to release Furthermore, the rods probably failed by a LOCA-like ballooning-and-gas. rupture mechanism. Because of the massive oxidation that folloacd, the node of f ailure is probably immaterial As a point of reference, Table I lists melting temperatures of the varicus materials used in the fuel system. B. Fuel Rods Fission product and hydrogen measurements at TMI-2 give important clues about the condition of the fuel rods. We will deal with fission product releases first. Air and water samples containing fission products have been analyzed. While we have analyzed both for indicaticns of fuel conditicns, we have concentrated on the Xe-133 concentration in the air sample. This isotope was selected for analysis for several reasons: (a) it is a noble gas and will not react, plate out or condense, (b) it has a relatively long half life (5.29 days) and a high production rate (6.8 atoms per 100 fissions) and therefore will be abundant thus reducing measurement errors, and (c) fission product release correlations are much better established for noble gases than for other fission products. Bettis (BAPL) has evaluated the Xe-133 activity and concluded it is equivalent to 315 of the total core inventory. We have independently checked this calculation (but, of course, not the sample activity) and agree (31.53). 106 152

b TAELE 1. IELTiiG TEEUATUFES PATERIAL TEFEFAilRE, UC TEIFEFATUP2,OF (D 2805 5C60 j ZIRC-4 1850 3352 ZR0 2715 4919 2 INCO E 718 1.260-2256 2300-25':5 304 SS 1399-1421 2550-259] AL.;)-BC 2030 3685 q Ac-IN-CD 800 1472 to -G:2 30* 2750 4982 2

  • Two fuel assemblies contained gadolinia test rods.

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l Fission products including gases are normally retained by the U02 pellets. A normal pellet release to the fuel rod internal voidage is only 1 or 25 (even for a successfully teminated LOCA) so that a 30; release indicates additional release frcm fuel pc11ets not just a release of the gap activity. Fuel pellet releases are strongly Gependent on temperature, and Figure 2 shcws a correlation of release versus temcerature for Xe-133 (from a recent ANS-5.4 draft standard). The correlation, however, is for steady-state releases and we are dealing with e transier.t. Further errors are possible because of kinetics changes due to oxidation to U 0 or U 0. Nevertheless, it is a reasonable 49 38 approximation and is consistent with recent short-time annealing ex-periments (private communication 4-10-79, R. A. Lorenz, ORML) and earlier anneal.ng work (G. W. Parker et al., ORNL-3931 - See attachment A). Parker heated irradiated samples in a furnace for 5.5 hours. The samples had burnups ranging from trace to 40C0 M'.!d/t (about the same as TMI-2). Parker measured releases of about 55 at 1600 C,15% at 1800 C and 40% at 2000 C with an uncertainty of about a factor of 2 in release. These experimental releases for conditions roughly similar are 'n to TMI-2, but for different isotopes, fair agreement with Figure 2. Using Figure 2 we could conclude that (a) the fuel was unifor-nly heated 0 (uniform in axial and radial directions) to about 1750 C, or (b) 0 30", of the fuel melted while 70 remained belcw 1200 C, or (c) any intermediate condition existed. Becausc cf the core uncovery sequence, 106 154 T

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-c the fuel rods probably did not heat up unifonnly in the axial direction. It is reasonable, however, to treat the fuel rods as isothermal in the radial direction because of the low heat flux. Figure 3 illustrates this point with a comparison of a full-power radial temperature profile and a decay-heat-power temperature profile. There are physical limits or, how hot the fuel can get during the periods of core uncovery because the fuel rods * . a large heat capacity and a low heat generation rate. If cnc assumes zero heat removal (this would produce the most rapid heatup rate possible) during the first peciad of uncovery, the heatup rate is still fairly slow. Figure 4 shows the adiabatic temperature increase with time for the peak-power axial location, for the low-power ends of the rods, and for the average location. Since there must have been some heat removal thus further slowing down the temperature rise, pellet temperatures probably did not reach the melting point. Figure 5 shows the temperature changes with time for a surface heat transfer 2 coefficient of 0.5 BTU /hr-ft - F, which is a very small value. The results on temperature distribution are, therefore, not conclusive. It is unlikely that fuel temperatures were uniform and no lower than 1750 C, and it is also unlikely that any fuel (L'0 ) melting took place. 2 The fuel, however, did get very hot compared with its normal operating temperatures. Oxidation of Zircaloy by steam and the attendant deccmposition of water provided the major source of hydrogen in the TMI-2 vessel and containment. \\D6 i56

RRDIRL FUEL TEMPERATURES 8 l ow b-m G ik R-e* I;Ea I-EE 8-ta - "r la a = = = = = = = = + o o_ n~ LEGEND o - PRE-RCCIDENT 1.5 HOURS POST-RCCIDENT (CORE COVERED) o-i 0.0 0.t D.2 0.3 0.4 0.S i CD RADIUS (CM) FICJRE 3. L7 .a 1 0 8

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i The Contain.r.ent Systems Branch has estimated the amount of hydrogen present in the plant (Attachment B) after the periods of core uncovery that caused fuel damage. They included amounts (a) consumed by the hydrogen explosion (226 lb mole), (b) remaining in ccntainn.ent af ter the explosion (80 lb mole), and (c) in the primary systen bubble (76 lb mole), wn ech was corrected for radiolysis. Comparing the above amounts with the total cmount of hydrogen that could have been produced if all of the Zircaloy in the fueled region reacted with water, we get 41%. As with the temperatures, an ambiguity exists. This could mean that (a) about 40", of the cladding wall thickness is uniformly oxidized throughout the core, or (b) 40% of the fueled region. of the core has fully oxidized cladding, or (c) any interr.ediate condition exists. Figure 6 shows the time required for total wall thickness oxidation as a function of temperature (Cathcart-Pawel correlation). It is clear from Figure 6 that ccmplete oxidation is possible in cladding 0 segments that reached temperatures of around 2000 C during the period of core uncovery. It is also clear from Figure 6 that all of the cladding did not experience sustained temperatures of around 1750 C else it would all have oxidized. This is further evidence that fuel temperatures were not uniform throughout the core, and that temperatures locally were very high. Based on early estimates by the Analysis Branch of core uncovery, we will assume simplified uncovery histories shown in Figures 7 and 8 for the 106 160

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follcuing calculation. Fuel tint is covered will be considerert to be cold (i.e., no cladding oxidatien). Fuel that is uncevered will t;c allo'.:ed to heat up; fuel that heats up uill be given a heat transfer coefficient that is adjusted such that the total integrated oxidation is 4 0'.. Tnese calculations give the oxication distributions shcwn in Figures 9 and 10, and these distributions are insensitive to n'any of the assumptions that ware nade. Figures 9 and 10 thus are more probable distributions than 100% oxidation over 40", of the core or 403 oxidation over 100" of the core. Figure 11 is a recent best-estimate embrittlement correlation (Kassner et al., AriL) that shsws high-temperature fragmentation of quenched tubes at about 203 oxidaticn. Using this correlation, Figures 9 and 10 indicate that a frag ented region of about 5 ft. in height exists near the tcp of the core. It may well be right at the top of the core as a result of simplifications in our analysis. In any event, at least 4 to 6 ft. of intact (but partially oxdized) fuel rods remain standing at the bottom of the core. Figure 12 shows fragmented Zircaloy cladding after oxidation in a simulated-LOCA test. Kassner (Atil-78-25 and AtiL-73-49 reports that at high temperatures (> 1250 C) many fragmants are produced whereas at lower temperature the rod may simply break into two pieces. Inasmuch 0 as TttI-2 temperatures were higher than 12:0 C and oxidation was severe, small fragments of the size shown in Figure 12 should be expected along with larger tube-like pieces. )0 \\

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Fuel pellets nernally crach during operation and crack healing can occur at pc..er. Figure 13 is a typical example of a cracted pellet. Quenching during core flooding may also promote frasrenting of the pellets. Severely fragmented regions are commonly seen in fuel pellets cs a result of extreme tanperature conditicos in test reacters. Poudered regions in fuel pellets have also been seen in some CCF tests,- but these t2sts are characterized by very high powers (> 20 kw/ft) and very stecp tempercture gradients unlike the low-pe..cr uni form (radici) temperature TMI-2 fuel. Therefore..e wculd expect the TMI-2 funi to be in millimeter-size granules and larger pieces including whole pellets. C. Unfueled Ccmconents (Control rods, guide tubas, etc.) Figures 14 through 17 show the control rods, the burnable poison reds, the power shaping rods, and the central instrument tube. All of these reds and the instrum:nt tube are inserted into Zircaloy guide tubes in the fuel assembly. The materials of which those components are made are indicated en the figures. An important clue about the condition of unfueled ccmponents is provided from instrument readings. The fact that all 52 thermocouples worked .wcat throughout the accident and continue to give credible information 3 suggests that a central tubular structural acmber survivied. It is tempting to conclude that all Zircaloy gui'e (,bes also survived, but this may not be the case since the t".u-is well protected by multiple barriers. 106 169 w

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Fixed CPND accet iy croce section INCONEL OVER5 HEATH CALIBRATION TUBE y i C., BACKGROUN.D g / (. -?' g5<-~/ __v s\\l/'S DETECTOR O. ,m' ( t j/ V (N (/Nh ? .lo:).j{:- 0.cou N Ep-V'J } q?m ) @~,,, ) nsura:opowanso sau n osTaCroa (7) y ,s g \\)L@7 Ci \\' d 0 / / ~... - \\ 'THERil.OCOUPLE (1) Ficuae17. 106 \\70

ITEL has n:ade calculations of guide tube temperatures by parametrically varying heat transfer conditions (see Attac!rr.cnt C). Their resul ts show that guide tube tcmperatures lag the fuel rod tenperatures by only about 20 F. Babcock & Nilcox has performed similar calculations and concluded tnat there is a much larcer spread in temperatures. .:e believe the l'EL calculations are more nearly correct and that l j teraperatures of unfueled ccaponents were close to fuel rod temperature + t Since fuel rod temperatures are believed to have excecdad 1750cc l in the hot region of the core, then in that region (a) Ag-In-Cd and its stainless steel cladding nauld have melted, (b) Inconel spacer grids would have melted, (c) Zircaloy guide tubes would have oxidized, i and (d) Zircaloy cladding of the burnable poison rods would have oxidized. In the cooler parts of the core below abcut the 4 to 6 ft. elevation, we would expect all unfueled ccacenents to be intact, althcugh perhaps damaged, just as the fuel rods are expected to be intact. Control rod segments could have caly fallen about 3 inches if severed by melting in the hot region, and the Ag-In-Cd absorber should be in place because it is an insoluble metal. Although the burnable poison reds would also be expected to be in place, their poison is probably lost; boren is known to leach out of B C-Al O2 3 pellets when exposed 4 to water in a radiation envifonment. \\o6 \\lc'

D. SUFlif.P,Y Many or all fuel rods may have balloon 2d and ruptured, but this mode of initial defecting is probably irrelevant in light of later more extensive dcrage. In the hot upper central regica of the care, fuel te.gerctures probably exceeded 1750 C releasing larga quantities of fissica products; about 30% of the total core inventory of noble gases was rel eased. About 40" of the Zircaloy cladding reac:cd with water. This region of severe exidaticn was localized above the 4 to 6ft elevaticn and may not have included peripheraT tundles. The severely oxidized fuel probably fragr.ented into pieces ranging from millimeter size to whole sections of rods. The temperature of unfueled components lagged the temperature of fuel 0F rods by only abcut 20 so that they also experienced tcmperatures above abcut 17000C. Conse<1uantly, in the hot region of the core Zircaloy ccmpenents shculd have oxidized, and components with Inconel, stainless steel, and Ag-In-Cd should have melted. Because of many layers of protection, the thermocouple tubes have survived even in the damaged core region, although the outer sheath of the instrucent tube may be badly damc.ged. Nearly all of the broken and oxidized fuel debris should remain trapped in the upper core region because the ppper end fittings have a grillage that would act as a screen. Furthemore, the compaction of fuel de ris \\o6 \\l

is limited because it is fabricated with a packing fraction of about a61 and the theoretical maxirum packing fraction (for a bed of spherical particles) is only about 63^:. It is very likely that fuel debris are also trapped in sc:"a mixing cups (See Figure 13) contributing to non-uniforr: thernoccuole reedings. An earlier estincte of fuel da. mage in TMI-2 was made at ?!RC by Rubenstein, Peyer, Tokar, $nd Jotnston. That estimate is in general agreccent with the present estimate although cur current evaluation is more refined. A memorandum su=arizing the earlier estimate is attached as Attachment D. E. Recc =cndations Reactor fuel is rugged, and it is unlikely that limits for natural circulation conditions will be related to fuel behavior.The general criterien with regard to the fuel should be that additional Zircaloy oxidation and fission gas release should be avoidcd. Significant oxidation rates do not occur until 900 or 1000 C (See Figure 6) U Significant fission gas releases do not cccur until even higher temperatures (See Figure 2). These temper;tures should be avoided in the (relatively) undamagd regions of the TMI-2 core, but these temperatures are so high that other limits will prcbably pravail. By new the adiabatic heatup rate is low (See Figure 19) and ample time will be provided to detect fission gas or hydrogen releases. The re fore, on-line methods of such detection, if feasible, should give adequate ,7 \\0

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Attachment B See mer.orandum from ti. Butler, Chief of the Containment Systems 3 ranch, to R. TCdesco, Assistant Director for Reactor Safety, and entitled, "Three tiile Island, Unit 2: Ar.alysis cnd Evaluation of Selectcd Contcir. cent Related Issues," to be issued on or about April ic, 1979, 106 182

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, {t. (, 'l,.. r / g' s ~ Amo:c.T D. E. G. C?se, 0 ;uty Dirac'.or MEM351,0U:' FOR: Offica cf Nuclear Raactor Pcgulatica TMI Fuel Teara FRCM: ESTI'GTE OF FUEL C/".;GE IN THREE MILE ISLA? D (T I)

SUBJECT:

Enclosed is a brief report describing the prelici.ury conclusions of the team farmad to analyze the probable damage to the fuel system at TMI. t0c n g Q 's 4-JIF M u w L. S. Rubcnstein, PSS/NRR

  • ~A C/9

/ M: eyer,L:-W"3/NRR / / c .f DS R. e8 ?d. /& M. Tokar, DSS /NRR W. V. Johnston, RSR/RES W.v' sE Enciararc - As stated O 270dPC lO rs h'O k J r ;/e. s o-Jw 106 181

On Tuesd2y, April 3,1979 a team consisting of L. 5. Rubenstein, PSS/NR3 R. O. M2/er, DS5/NRR M. Tok a r, SU./ NkR W. Y. Jcic 'on, RSR/RES was forr.:ed to survey the fuel croe?s analyzing t'.2 dan ;? to the fuel systc cf TMI cr.1 dr;w scm2 prelimin ry c:.nclusien; feca their deliberaticas regard!rg that d aage. The follo-ir.: indivicer.ls and crpnizations s cre ca.itactd ca April 3 and 4,1979: E. L. Zebrcski (EPRI) - (representing the MetropolitEn Ediscn Croup) J. Taylor /J. Tuicnio, EJU R. Denning, BCL D. I'cCloskey, Sandi a J. Scott, LASL In addition to the informatica cbtained from conversaticas riith these org2ni:1tions and the NRC staff, the t22..1 cit:ined a " sequence ci cvents" frca So'.i (Eaclosure 1) a group of curves describing the pressura, temaercture change at TM:-2 during the first 15 heces fr:a 0. Eisenhut, 2nd a BAPL radicche:.i .1 :n: lysis of the prim;ry coalcr.t t icn at 1600 hours izrch 29, !!!i and decay corrected t: 0700 het::.42rch 30, 1979. The prinary inform 2 tion used in our analysis of fuel system damage was obtained from the Ca2 Ccmpar.y, the Matre;0litan Edison Industry Group, and frca calculcticns of the SRC staff (Reactor 'tel Section, CPB; Fuel Schavior Granch, RES). System Effects Using the chronology of events obtained from 35W and the control rnca strip chart tracing of systen pressure for the first 15 hours of operation, we were able to deterrina that th:r2 were three periods in 5;hich the primary systea pressure was bclce a saturation pressure correspondinq to a temperatur e of t 20*F. T h e sy '., t : changr s which caux.d these per:cis are descri!H in the se:,1ence of events provid:d by C?.W enclosed ui th this report. The details of what eccurred to cause the pressure chan.;2s in the primary system are not dit. cussed here as these are considered in other staf f repcrtr. (see e.g., IE Sulletin 79-05A, f;uclear Incident at Three Mile Island) and wi'.1 be evaluated by others. 106 135 W~

. Exanineticn of Figure i shcas that the first peri d The saccnd appraninately 1.75 to 3 hours t'ter start of the tr>nsient. i 4.5 to ceriod, v hich ucs relatively short in cu etica, cccurrad in t.e r system 5.5 hour time fraze cad rescited in ; s aall decr:ase in prinn,The final pressura belos 3:.aration pres;tre. after primary system pressure extt.:ded frea r.pproxicately 8-14 hou care start of tha transient. experien.2d was ex. posed to (xtsasive amounts of s:cra ccoling :ca The group was able to infer from excmination of che fuel damage. pressure historias, repcrts c therr. coup 12s, the beh:vior of the incera i;tu time cotair.ed frem the incer rhodium self-powered neutrea detectors ($?;.~D's), at.d 3'-1cng ntar:: Range Ex-Ccre Detectors, cnd the contai:~. ant radiati:n *:.cnitcrs s details of when the fuel pins lost their integrity, f that exposure, and the arount of d; ge ;o the fuel. As previously stated, the e"idance for the level of uncoverir.; was It can be shom1 obtained from a E5W analysis cf the incore SPN0's. that : Abcve about 700*F, incer2 SPHO's (Rh) act as thermionic elements ar.d generate currents which are corr 212. :ble to Thus, if a disccr.tinuity is obs.r.2d in current ceasurement, c transition in temperature c:y be temperature. It ucs assur.cd that this discontinuity rep-inferred. resents an clevatica at which voiding of the cociant has occurred. Similarly, the excore Intermediate P.ange Detectors tay be used to provide an indication of voiding. The information cStained fren these detectnrs was consistant with result; frca the Industry Grcup calculaticq1 hour s.ithout int to full uncovering. 106 186

. Fuel Se t~m Cc ndi '.i_nn_s_Du r i na Pe r_i_c_d_o f 1 s t Wc ov er i no u During the first ptriod of r.:for uncnverir.g of the care (ct least 5 feet of the core Jas unc;.cr ed for cb:u, cn hour, ad perhaps all bcen unce.ered for :a:" enc-hclf heur), the of the core ca hav: uncovered por. ion reiched te..peratures high occugn to i_il facl At this point, fissien prc acts were rel.:cred inic th2 rod cladding. sub:: ;2n t ti c mi r." o f the primry coolcnt as evidenced Uy :h: . nur:d ceciant activity contci r.:..cn'. activi ty t; niter;. Castd ;n ..e ~ and thr: c..: aunt of :. drcg2a r0ler:0 fr:n r cticn of the Zirc;loy cladding uith rater', cl1 of the fuci rods probal; d hcted and releastd fission p. '.Jucts. Fuel te,:eratures *:ere estic:ted from ca.1:0 ations b: sed en the fis:ico he:t product cnclysis of the s.aic of prica ; ccolant, cid :lso frc:Encc cn bcc transfer csnsidarc. ions. for tengeratures e.1d t:eperatin t.-da;2nden rale:se rat., that would be requireJ to proden th. ceasur:d level e.' ctivit/, fuel teateras of 1400 to greater th:n 1500'C ver: cbt P.ed. Estic.. : by 0 P.". b c : c-on their er. parit::nt, indicat:J tha'. the Cs and I reletscs naasure j s.cuid Tha hcve required feel t peratures cf at le:.t l':00*C f:r cn hcur. heat tr:nsfar calculaticas indic:ted, en th other h:r.d, that the fell In either caso cince temperature may h;n been enly about 1100*C. is 2,40'C, feel cf..ing r.s unlikely. Thee the melting point of UO2 rationalind by con:idaring that a sanll temperctura differences can b2 ecy have t en at th: hicher temper tures. There is portion of the cor: also a possibility of scae eu ca: tic formatica bet.,ean US., tr.d Zro, at ter? erat.;res ab:ve appro::fr.telj lE00'C, but no signiYicence r;s Later enalysis by marbers attached to the cc:arrance of s% a eut:ctic. of ANS-5.4 fission gas working group (i ::luding one cr e;--R0".) indicctas i fuel peliet tcaptraturcs as hi,h as 20:7C bcred on Xe " data cad tha h'hile noble gas assumptica that hal f of the core remained c cl. activitics icnd themselves to c.nller an;iytical unccrte.inties th:n iodine or cesium.:tivities, the uncertainty in the cora fraction that is responsible for the relea e still randers this result inconclusive. 3 of the Hydrogen balance calculations indicate that from 15 to 30t total Zircaloy inv:ntory hcs, caen cxidi. ed". Scme of the oxidati0n, T The however, undcebtedl3 cccurred drr ing the 1ctter uncoverings. varies as a function of height in the extent of the oxidnion prob 2bl; with the gren..st amount cf cxication having occu red in the Later celculations accounting cor?, uncovered (upper) partions of the fuel rc3. for hydrogen in the bubble, in the contain:ent, lost in the hydrogen explosion, and gained by radiolysis sugget.ts that 1 mast 40t of the Zircaloy in the feel region nay heve been oxidized. 2 CPB Staff Calculation iCPU sTii Calculation 3 B5V, Industry Group and NP.C Staf f s Industry Group Calculation

  • Industry Creup a MC Stat fs 106 in7 BAW Calculation m

As the pricary coclan level was restored during the la t'.cr portion of ina ti..:a ;2riod of the first unct' ing, th;rni in1 c;chenical shcci icading; cf the oxidi ::d cnd..;ritticd cl PE:ng are bclieved to have cccure:d nd to have resultad in cle ding u fragm:ntation. At the end of the period of first ar.:nvering, virtuill; til of th; fuel rods had 6efectcd :.rd relecte; fission prrducts. Ai th ough to h:ve temperatures had been high e_ncu:h ice a Icag en ugh tir; caused sevare cicdding c. rid:; ion, cc;tir.ued cpe 2:icn of incare instrument; strongly indic:tes th:', fuct ass::tiy c;ruct.rci 73mt:c: such as cuid: tus2s r 2in-d intact. Centrol rod catarial: cre believed to have remain;d in place, cc indics.ud by ths ab;ence of silver in the pricary coolant. of 2nd " ice Urco"eri-- Fuel Systen Cenditions 7.n' Effe-cts Curino Peri 4 At about 41/2 hcurs into th? event, the ccre irrel again de:r ascd tn expcs: the upper 5 f:st of the fual cssenblies. The duratica of this additional va:avering u s shorter tnan the firsi., tne sy;;em pressure was hig,cr, and tha cvorr.ll temper:ture effects were les: severe, as evidenced by the fcct th:t the ther c::cples in tn2 outer paript. cry of th ccre rec 2in;d on-scale. Eecau2 of the r;duced severity of the c;ce Conditions during th; :ecend unc:v: ring, cs cerr.arad with tr.2 first unccvering, less da 2c2 is believcd to have cccurred to th: fuel systcm. Fuel System Canditiens Durine Period of Ord 1:cccvering At about nine hcurs into the cvent, the core cociant level again d2 crc::ed, pessiblity cc,n to 7 to 71/2 f t. frca the top of tne act:ve fuel level.* The core re:nined uncovered ;t this level for :bcut one to three nour;,The af ter 1/hich U2 coolan'. level uas mia raised cad covered the core. low syste.n pr2ssure (s450 osi cinim6m), the rather lengthly period of uncevering, and the addi;icncl ler.gth of fuel surface uncc.mred, uncc';btedly resulted in ecditional fuel system c:amage due to Zircaloy c.sidation cuci

c. brittlement (fello. :d cccin by care fragment:tien due to thermal shnck mdurinj the recm=ering of coolant level), although the amount of cddi t:caal dracge is presently unquant1fiable.

Fuel System P m Su. crv The picture of the core that has emerged is that the core configuration currently consists of a basket-like shape of relatively intact asser.blies that surround a central region of severely oxidized, and The fuel probably fragmented, fuel rods in the upper central part. 106 183 , D a_w__J _o_n_.. - _ _.un er. nation received via telecomunication frcct BfN ( April 3)

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the ga de tubes, gri ds, anc c:,d ; i c tes cre,.;.i t u...cd '_. Le i nt s : tcing ;:rtions proviu r., a skelet;:1 structer ithicn se;;-^rts thc rtrPartial fiev i:]::'.27, c3ur ed the dc....:;e c a r s e.a i l e s. cor.tiquinc el3 ;tel of fucI deur1s is tncu<:ht to be respcasic:e ic. The c'; spar ey of the incar.- ttm.jccuri; re,'di-,-Vil ~ therrccoup.c readings. suggesus tn3t a rc3 cn of the core is rocre he: f i. 4 r k 4 I r i s i I 106 18'

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.u.. n c.s S umm r.v. The grce? ccn:luded tht alth:'!-S th3 c. ore is badly da-'ged, essenticily all cf th Te:1 has t... ired in ec coce cr.4 that the o.;rr.11 packing - ~.. ',. T h m. '. c "., d a...., i +u.v c. *. h.. s*, '. I r. ". ~*'.".a. '. - n o i. c..~r.. ~s.. + o 5' '......i n. a ~ s o.1 i. c". '.. v 'u 'n".-. ' s.,.1. .~ M 1...c. r +u.... L e s.v,.., L: r r.u*. w Sp11.. g, -...,, e.r p.. s. t t'... c ".v r. I t r.o a i u r 'e 5. '. -.~.'1 " c'.' d .4....e. '. i t - "... c-.. ~.- ^ ". '. I o' e (t '.) o. e t... .v .c. ^ 2 to +. w~,d i.r.'e.~ '^ - )ou.,. u Lm.T. c -U. - f i '

4. ~..

.. ~.. ' '. 5. ^.....

4.... n.. o ". ~. u... i n. "i _. r s, r-u ccre ccn it:ca during trcn,i jon i.o natural cor.ve: tion ccoling.

If feasible, th.' additice or a a.sp.:trcrut2r tu cenitor the v...ivity cf the.scap c - -,.cnt ior...' 1:ssica ;'re urts r:t e: ad.t, c tract i cian to a ne tural ct'.w tien ccc i..:9 utii pro tid: cn ir.;; pen' nt alcrt to possiale J. ~. dir11culte;3. d Tito quc';tiam were ccarid.: red: Ir. n pur.g tr b likely to Ic.d to cn unst fe cemition? ansi Uha*. signi.1: uill ir; din te ur.decir:' le cor.d iticns o in the care? Ol' 106 199 ' 6 -p ' e), ~)

3 D. im s W 1 <1 197 y r. .. ~ r, s e, q-,. .1. -,n.

r. p... f.

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c......

rq uiar c m t.y is c.n::tcd. Tha gcid tun:: crd p M sen re'.'. a re 6.rd s t.:il riy La r.' c cladding. S:.ce grid: s;ould tm iccated at or > i.., j t. u,,s..a. .. ) p..,..,..e..

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a. o i. ".' 't. %, r.

c 'nra '. c.m. > d 3 . u. y are that.'id n'.n 00n: f Miing t.7 ke t2! n clace, the c'.0-cil pcching d :nsity ef th: Ic t '.. r tica iT a:a ..y m..a to b2 gr:at r th2n 70" and th0: Z.~ ta ?'." of U.9 fuel N cic.:dir':,fr:a Gis rcycn it beliond +3 L, u...o_ a er u

c. 1 1 1

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_. i n'.. r a.' + u". ' i, l o " d... s~..' - ' ". n.-. 4 c o r. e i c'.... a' 1 ;" o," i F.."... d.. - :... a < i s The cricinct f! : p.mry is prn.... ly ret ir. ; al though tha rods ny ca (. g S. g. > p...,.. r una...,..,, jg, ...,$p a i,.g.

s.. u. v

,3 c,a ~,. g r i g.r c n.. a, j o se l,. s .t u.. s - ...., I c.,.,u.. s 0a. a..,. .t.. -..,._ TW: racve cent tic c d.'uld not ;'. cclu& cticfcctory achi.S.". ant of nr.tm ;i ce.vec'.ien floits. What shoult! tr enitnsd tc determina undesircble chanqcs in the core dering the transition to natural con /ection ccoling? The tcp erature distributico of the core exit thcr'r.0 couples are the tr.oct isc;'orant 106 200 T

s D. P.o s '. 3-AFR 13 1R.3 c ?r. 'i +. ic:; r,n:. i :c r i r '? signais. Ti. g ra u,n be. 4. .'s that mi' e:< i t Tc 's si.celd n conti.v."x..iy tra: N.1,. r cur :c. i '. I n D.- ._ '.he -1i sir.c c r i t e n. f r."- r .in.!r w.w '..?. : 1 ' c. : :. i s - C; l. o  : Ed t!. n Tw '.< c'.. ' _ * * ' ' '. - -. h

s..

'6.- (- 'l '. -a' '.,--l'-- '3 '- -.. - u a. i_ r r. '. ;. r '. ' ? ' ' - n.] U.: 7.- Q 6 l l., /. :. P. ') ' elny IC ' L to rdd ' ~ - ......': 3 u L ~y 1c. t, ,..,i., . j\\ u.i .) i.-.. g. ~' - u,..o. .j .a .s .w' 4. c a l O t C '.' 'l i L ::.'.ii Ol. - f.O c '. 7..* :: C-b* ~.. ! r L.. c.' I ' E2 ,_..2 _,.,.3, __rJ s i,- i ,7h....WI. .i - iw a.,. .....ii,, e u. -....c.. g. ': xC C O ' l.,.. '.. Th e fo' '. cc.O,c:.: t i c s r e ri z e d.: avcil dl-in: t rur:...

. i,.i a nd i t

p.,,, i <,. c. u,..-,.i.r.,.,.. ._.i..._ ... u..._ c m........t... ,o D2tector Event Cor O'.orhatin:, Cri teria Basis

1. Exit Tc's Lir.it r:0. in film boiling rot to ::~ ~ J cr2. ct:,

Liait r.o. c',2 c E'G F ccre c. .ce rcverse precev ac.

2. RTD Vair.tain pa itive LT cr:rs I.'o ficw r'. /ersai Ho-1eg ccre.

Cold 11.; pe r::li t te.d.

3. Icn cha:.. bars Void for.n'.icn Ambi pcus sicr.:1 t'rce o cnd M If +, re:e d fcr futura som: 1,:::1 r.upari.e t int rrctatien,v::tch fc's.

may be p. u tcd.

4. Noise detecticn If + indice.tc; buSLics in Same as abcve.

core or 1ccp, ch..ck Tc';, 53.

5. Syste:a Pr:ssure If incr:asina cy:'c. cf facts Not direct indicaticn Branch thouid revi w thi;.

of core cc.uiticn, but for cas L.,Lole fcre.. icn detection.

6. Pr::suri cr Level S ca as abcVe.

Sam 2 as abc". Ad n t;ona! L)c.._ction - F ::ibility needs to bc estchlish;d. / 1 spectrocriry Increasir.g activity of Xe, I Overheated ccre alert of ccolarit via 2 sampiing 1ir.e for major error in procedure. II 0 analy :i: on Coron, 00, 11 > Core criticality and line monitor chcr:iis try con t.rol radiolys i,..ud !! content cont.rol. > W A lb r. ?. d., 1, W. V. 41ur.l ton, Ch i. f fue 1 r, h.e/ra r R.- .in h Br.in. h D j y j ,4,.o .it U.. i. t. w .8..*.

e w .e

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y. 1' i.,.

,u n.- u.-.. m. v ... j t. :. i.,,7 +3 p. ., p,. ] c :. r,.a4 ...i. '( j, a. u..., -.,.. +'.4. v..i..,..,.-. i ..c..r... r, r. J. 1,., a 1.r, ..u) 4. u. i..,n_

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n. j...., 3,'.u".t V,...1

'. n.*... c i r c '. <. 's 1' c. - '. +... --. n

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s. ii-Since futur-fic'.. trar. i.:: carnet b_ ruled cu t c..d si:.:' the traru. ti:n to riatural circa:ation c:u'd cccur in. ol un tari ly, '... rec c.:r :...J thct the contir.ucus reco.'dir.g of all t:;ar..;;;.ple re: dings t e initiitcd cs ccca 32 p v, 2 nv 1 a. /'3 -// /, i- / (n. '.!:((. L t. ~ ;.;-) Ralph 0. l' yer, Leader Reactor Fuels S..ctic-Core F.:rfor:..a::ce Dr.nch 106 202 0 -. ~ _. m}}

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