Effects gap heat transfer on LWR fuel behavior during an RIA transient: In-pile experimental results with helium and xenon fillet rods

Nortlh-Hollm~~

253

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EFFF_LTS O F G A P H E A T T R A N S F E R O N L W R F U E L B E H A V I O R D U R I N G AN RLA ., TRANSIENT: IN.PiI~- EXPERIMENTAL RF~ULTS WITH HELIUM AND XENON FILLEU

RODS T o s h i o ' F U J I S H I R O and Sadamitsu T A N Z A W A

Division of"Nuclear Safety, Japan Atomic Ener~ Research lm.;itme, T~kai.mm'a, Ibar.ki.ken. Japan Reeeivt~d 15 October 1982

Gap heat transfer characteristics and their effects on L~ l~. fuel bebavior during an RIA have been studied through t,'le in-pile e x t e n t with UO2 pellet fuel rods. The report describes the experimental results obtrined in the NSRR tests in which FWR type test fuel rods of helium and xenon filled as the gap gas have been irradiated in the pulse reactor, NSRR. to simulate ,he prompt heat up of RIAs. The relation between the cladding tempecature history and the gap heat transfer conditiortt, and the effects of leap gas composition on fuei behavior and on the fuel failure threshold are di~ussed b~sed on the in-pile experimental data.

I. Introduction In a postulated reactivity initiated accident (RIA) of light water reactors (LWRs), a prompt power excursion of the reactor occurs by the insertion of a large amount of excess reactivity due to an inadvertent control rod withdrawal or ejection or to other causes. This reactor power excursion causes a prompt overheating of fuel rods, and results in fuel rod failure. The LWR fuel behavior under this RIA condition has been studied in the in-pile experiments of SPERTCDC [!], TREAT [2] and PBF [3] in USA, and of NSRR (Nuclear Safety Research R.eac~.~y)[4] in Japan. In the older experiments of SPERT-CDC and TREAT, test fuel rods were irradiated in a simple capsule under atmospheric pressure and temperature, and the general characteristics of the RIA fuel behavior were studied. In the PBF experiments, fuel rod behaviors were stud!ed under high pressure and temperatu~ conditions which simulate a hot start up condition of a BWR. Although the PBF experimental conditions were mucl~ more realistic than the SPERT and TREAT experiments, the number of the experiments were very limited to understand the influences of various parameters. Extensive parametric studies have been conducted in the NSKR experiments to understand the effects Of various parameters in detail. This report wtll de,~'ibe the effects of ~[ap heat transfer on LWR fuel behavior during an RIA transient based on the data of the NSRR

experiments. PWR type test fuel rods were subjected to the pulse irradiation in the NSRR to realize a prompt heat up of an RIA. Gap heat transfer peculiar to the RIA conditions have been investigated through the comparison of the helium and xenon filled rods~ and the effects of gap 8as composition on fuel thermal behavior and on fuel failure have been discussed. The initiation of the fuel rod failure is largely influenced by the cladding ten~perature rise, because the major causes of the fuel failure in RIA are thought to be the cracking of the cladding which is embrittled by severe oxidation and local wall thinning at high temperature of near melting point, or the rupture of the softened claddings at high temperature if the internal pressure is large. Thus the precise estimation of the claddi.ng ~.emperature: rise is one of the most important tasks in the RIA rue: behavior study. The cladding temperature is affected by the gap heat transfer between the fuel pellet and the cladding ~s wt'., as the external cooling conditions. Es"pecialiy in the F.tA conditions, the cladding temperature will be sensitiv:-~ to the gap heat transfer, as the cladding is rapidly heated by the fuel pellet in which a large amount of eneq:y is deposited in a short period. The objectives are to understand the transie.,t gap heat transfer and its influences on LWR fuel b~.havior during RIA transient through the inpile expei:ment ~. PWR type test fuel rods were irradiated in a pulse reactor to r~!iz¢ a wompt heat up of an RIA/~ty

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order to get better information on. 8alP heat transfer, special fuel rods was filled as the gap gas were used. ~.,: ordinary helium filled rods.

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;o ~ t e major fac-lity called as the N0clear Safety Research R~tctor tNSRR) is a modified T R I G A Annular Core Pulse Reactor whose features are (!) the large pfiisin 8 power capab~/,ity to heat up the moderately enriched fbel over the melting temperature of UO 2 by nuclear fissh3n, and (2) the large (22 cm in diameter) dry irradiation space located in the center of the reactor ~:ore to accomodate a sizable experiment. The general arrangement of the NSRR is shown in fig. 1. The core strdcture is mounted r,n the bottom of 9 m clcep open-top water pool. The center of the core is

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Fig,. 2. Histories of NSRR reactor power and core eneqly release for the maximum pulse operation.

the experimental cavity to which the experimental capsule is inserted through the offset or ~ e vertical loading tubes. The pulse operation is made by quick withdrawal of Ua~.~ent control rods out of :.he c6re by the pneumatic d:tving system. The maximum puhin8 of 4.7 $ (3.41% d k ) brings peak reactor power of 21000 MW and core energy release of i 17 MWs with minimum reactor period of 1. ! 3 ms, as shown in fig. 2. The energy deposition in a test fuel rod in the capsule is controlled by the amount of the core energy and the enrichment of the test fuel. For the maximum

CONIR WATI

Table ! Test Fuel Design Summary ,CR T~CAL (~CFSET L

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GE PIT

UO 2 Pellets Diameter Length Density Enrichment Shape

9.29 mm 10 mm 95~$ TD 10% Chamferred

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Cladding Material Wall thickness Oute~- diameter Gap Pellet-cladding

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"LE G R I P P X N G

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Element Overall length Active fu©1~ngth Weight of fuel pellets Number of pellets Fill gas

Zircaioy-4 0.62 nun 10.72 tam

0.095 mm

265 mm 135 mm 95.5 g 14 He

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Sensor Tead Fuel pulse approximate~ ~ oal/g.UO2 can be deposiied i n a 10% enriched ~ type test fuel rod, which is far above the melting enthalpy of UO2.

2.2. Teat fu¢! rod

Thermocoqpt

Fuel Pressure Sensor Col~ule

The PWR" type fuel rods were used as the test samples. The major characteristics of the test rod are listed in table i, end a schematic drawing of the rod is shown in fig. 3. The test rod has nominal 9.29 mm diameter pellets of 10% enriched UO 2, contained in a Zircaloy4 cladding of 10.72 mm O.D. The surface roughness of the pellet and the cladding internal are around I ftm and 0.3 ~tm respectively. The total length of the fuel stack and the rod are 135 mm and 265 mm respectively. In the fuel rod, pure helium or xenon gas is filled as the p p gas at I atm at room temperature.

2.3. Test capsule arrangement and instrumentation A schematic of the test capsule arragement is shown in fig. 4. A single test rod was fixed vertically at the center of the test capsule of 120 mm I.D. and 800 mm in height. Demineral~d water was filled in the capsule as the coolant till 550 mm from the bottom, and the test rod was cooled by the stagnant water s: <:mbient temperature "and pr,,-ssure. As the thermal .',pacity of the water is quite large as compared with the energy generation in the fuel rod, bulk coolant temperature rise in the test is witlfin I°C at maximum. The test rod was instrumented with P t / P t - R h bare wire thermocouples for cladding surface temperature measurement. To get a good response and small fm effect in the measurement, thin thermocouple wires of 0.2 to 0.3 ram diameter were attachedby spot welding to the cladding ~rfac,¢. The temperature was measured at six pointsas m ~ t c d in fig. 4. The coohmt temperature was measured by C / A sheathed ~ p l e s at several points.

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( Oni!: ram)

Fig. 4. Schematic of test capsule arrangement.

3. Experimental resulm

3.1. Transient histories of cladding temperature Claddin.g temperature histories of helium and xenon filled rods were compared in ~ig. 5 at three different energy deposition levels. As ihe thermal conductivity of xenon is about 1/20 of that of helium, xenon gap gas will have an effect to decrease the gap heat transfer if the contact between the pellet and the cladding is not enough. Such-influence of gap gas composition was evident at the lower energy deposition range as seen in fig. 5b and c. In these cases, both the maximum cladding temperature and the fi!m boiling duration decreased for xenon filled rods. The influence oboe,wed was larger in the case of 220 cal/8 UO z than in 160 cal/g UO 2. At very high energy deposition level of around 260 cal/g UOz, however, the difference between xenon and helium filled rods became smaller as shown in fig. 5a. Fig. 6 shows the first se~.ond ~f the temperature transients of fig. 5b. It will be noted that the cladding temperature rise is very quick at the first 0.5 to I s into Iransient, and then becomes slower. The influence o f xenon gas is observed only after the temperature rise speed has decreased.

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F,g. 5. Compari~n of cladding temperature I'.istortes of helium and xenon filled ro~..gfor different energy depositions.

Fig. 7 shows a comparison of the cladding temperattire histories measured at different points of the same xenon filled test rod which was st~bjected to the energy deposition of 220 c a l / g UO:. A s indicated in this figure, temperatare variation depending on the measured position, which corresponds to the influence of local variation of pellet-cladding contact, was very small during the first rapid temperature rise, and became evident after the temperature rise speed decreased. The ob,~ervationg mentioned above sttggest that a good iher-

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mal contact is produced during the first 0.5 to I s in which the influence of both the gap gas composition and ~.ocal variation of the contact are very small, and that the major influences of the fglp conditions appear in the later part of the temperature rise.

3.2. Maximum cladding temperature and film boiling duration Th= maximum cladding temperatur~ of helium and xenon filled rods were plotted as J function of energy deposition in fig. 8. The lines indicatethe tendency of the data. As the departure from nucleate boiling (DNB) occurs at the energy deposition of above 140 c a l / S UO~, temperature jump is observed at this threshold energy deposition for DNB. At around 150 cal/8 U O z,

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FiB. g. Maximum cladding temperatures of helium and x~non filled rods as a func3iomof cner~ deposition.

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9. Film boiling duration vc-'~us energy deposition.

the temperature difference between helium and xenon filled cases is small. The difference increases with the energy deposition until 220 cal/g UO 2. However, beyond the energy of 240 cal/g UOz, the temperature difference becomes smaller with the energy, diminishing at around 270 cal/g UO z. At 270 cal/g UO 2, cladding is mostly, molten and failed at quenching. Film bolting duration was plotted as a function of energy depoJ,ition in fig. 9. As the duratlt, n is dependent on th~<~;wJal position, the data plotted are only those measur¢~ at thi~ midplane of fuel active |ength. Al. though the difference is not so large between nelium and ~enon filled rods, the simiLtr relation as observed in the comparison of maximum cladding temperatures in fig. 8 will be noticed, i.e. th¢ duration for xenon filled rod tends to be shorter than that for helium rods for the energy between 160 and 220 cal/g U():. the difference is largest at around 200' cal/g UO 2, and diminishes at around 270 cal/g UO2. 3.3. f u e l

failurethreshold

deposition is quite-similar to these photosxEphs ","he failure of the hefium filled rods is initiated at 260 c:,l/g UO 2 by generating small cracks at the thinner par! of the wavy cladding. Based on these observations, the damage of xenon filled rod at 258 cal/g UO 2 was judged to be that at slightly below tSe failure threshold, i.e. the threshold energy deposition for fuel failure was thought to be no~ influen__,~_ by gap gas compositior~ under the free convection cooling condition of the present experiment )

4. Dbcumlon

4.I. Gap closureand re-ogening Cladding temperature transient from DNB to RNB (Return to Nucleate Boiling) can be derided into three phases, i.e. rapid heat up, slow heat up, and cooling down phases as illustrated in fig. I !. As mentioned hJ

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Fig. 10 is" the post-test appearance of xenon :fdled test rods which were subjected to the eneqD, depositions of 258 cal/g UO 2 aad 272 cal/g UO 2. At 258 cal,/g UO2, cladding s~rface became wavy, indicatin$ an iQitiation o~ thiyming and thickning ef cladding by melting. Claddint4 failure, did not occur except at the spot-welded point of a thermocouple where eutectic formation of z~'~,y and platinum cairned a small lx~netrating hole on tl~e cladding. At 272 ¢al/g UO,, cladding was melted mid broken into two parts. The appearmu:e of helium f'~.~l rods at the same energy

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Fig. i l. Three phases of cladding thermal l~havi~r during ~lm boiling,

the i..revt.,:.ussection, rapid heat up phase conespomb to i period i,~.which good ~ con'.act is e~tablbhed by Sap ck,suic The, transit/on to the slow heat up phase w~.ll probgbly correspond to the reog'waing , of t~ie ~]p. Te understand the :elation between the cladcEc.& thermal behavior and t.he gap conductaw~ the mechanism of gap closure and ~ n 8 is discumed in the first place. Ut,der :b: ,~rev~t ex4perimefitat condition of RIA's, the e..~erp~y ~en~ation in the fuel pellet takes place v,oithtn !0 ms. So. the temlx-rnture ~ in t ~ pellet is almost adiabatic, and the rapid Ihermal expansion of ~he pellet -,,;.!l resu!t in qui:k g ~ closure. Then the ciaddin~ liil; ~ expanded by the pellet thelrmll: expins~,n ~:yond :,ts elastic limit while it ii still cold. As the v~ei,.' .,,'.tess.of the cladding i, ia~e for this Io~" .emperatree • high c~nt~ttt pressure and co,'tsequeut b a high ,:on~.~ c~ conductance will be real,-.za~dduring th~s qeriod. l'i.e dl.tachme-.t of the pellet and the clliddir-g will ~.,,.:cs:r t,y the thermal expansic, t~ of me cis.dding ttse!f wh,.,n me cladding gains temperatt_,re enough to Jeheve :.~e internal expandin~ Ior~-e of the pellet. As~Jming one dknenstcn~l r~diai expansion, the detachme6t for this ~a.,,e ,:zn .he estimated by the condlti,m tha, the sum Of tnerma! expansion of the cl~ddi-'li~and the d~r~is~ of ~lle: ~iameter exceeds the elastic co~nponent of ihe ~,',t~l stra,n of t'te cladding. The history of tl',e claddl.r.g ,teformz:lioil till the detachment will be e>:pi~ined on a stres,t strain curve shown in f.,~. 12 in the fc,llow!ng way A:'ter the ~a~ closure, :he cladding is exp~t~ded v~.rv cltwi,.ly ,dons the hn¢ ARC by pellet thermal e~.-,n~si()n, the,~ I,oth with the cladding teml~4rature increase and with the decrease of pellet lemperalure, the s:r:~s~ ts rehc,'ed along ihe line C to D. As the fir.;t order

tion oi ~ depo~tion by ~ the heat loss to and t~ u ~ d m ~ a l conu,-aint irom the cl~din~ The cladding therml expansion needed to re;~= the contact hi approximated by the eliiitic strain DC'.. In die cak'~tion. ~ t ~ rite ~ d thenml eatmuion oil the pc'let was =aimatodby tOllowing equatiom. Pellet iemperature: .~ [ r ' , , ,

c

(i)

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J,'o Linear thermal expansion" d/), D

-4.792 X l0 - i + 7 107 x 10-eTa;d2

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where T, jo:

- UO i temperature (K), • cp ,," specific heat capacity (cal/g Ic). / ' ( T u o , ) = integral of specific 5eat capacity given ~s a function .of UO l tempera:urn (cal/t; UO2), Qt,mu~ = prompt energy 6epetition (cal/g UOi). The condition for~he detachment will be described a~

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ap, Tp.

(3)

where A( --- strain needed for the deta~hngnt, IX:," in fi& 12, . a c. % -

thermal cgc.ansion coeffi-ient of cladding ~ind pellet ( g - i~,

A T~ - cladding temperatv.re increase at detachment (g;, A~"p .>~ pellet temperiturt~ decrease .,it detac~i.,nent (K) Approximate value of p~.llet temperature decrease ~ Tp is estimated as a mnction of ATc by ai~umi,ag that th, heat loss of the pellet is equal to the heat gain of" the cladding, i.e.

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where Op: Pc - spec~c weight of ~!let

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FI~. 12. History of cladding deformation on ~he stress-stiain CHive.

appro~,~ation, ~ total egpan~on AC' ,hi ellinmltd t~tly by the thermal ~panskm of the pellet as a fmw,-

and cladding "(S/c'~?), %, c~ - specific heat of iiellet and citdding (cal/gK), Dp - pellet diameter (cm), Oci, De,,. - t;mer and outer diameter ot c l a d d ~ (,~,). Combining eq's. (3) and (4), ATe "is expres.~ as a r-

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As tge second te~n of the d ~ a t o r of eq: (5) is about i l g of 0 .. f£~t term for the LWR type rod dmign, ~ n ~ x ' t of heat lots to the ¢oola~i in the approxiniation of eq. (4) will not re~. If'in a large e~or. Fm the ttre~-ttraiu curve Of zirealoy, the following • equations from MATRPO 15] wcre .u~l: In the elastic region: o-E.~,

(6)

•E-~ 1.148 x" 10n - 5.99 × 10' T.

(7) •

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Fi~_at4.~-~mpat~sonof turninl~ I~int temperature T~ wi,h the ~tlculated cladding temper~tuw at detazhment.

I n the plastic region:

- x,".

(s)

where K; P. - coeflicients given as the functions of. temperalUf~,

E - Yonn~'s modulus (N/m2). ~" -., temperature (K). The equation for cladding diametral thermal expansion ased are ~ D / D - - 2.373 × 10 "~ + 6.721 X 10 -6 T,

(9)

wh~e T - temperature (°C). Fig. 13 shows the caic,:lation results by thi~ simphfied model. Toil.; strain ~-r, th~mal expamion needed for the detachment ~ and the cladding temperature rise needed for the thermal expansion A T~ i.e. the cladding tempera:ure rise st

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detachment, are plotted as a function of energy deposinon. In fig. 14, the calculated cladding t~'mperatm'¢ at detachment are c-,mparud with measured turning point temperature 7~ which is defined as in, fic.ated in the figure. The solid line is the catlcUl-at'edAT, based on the above equation~ (I) through (9). The dashed line is the result of s!,nilar calcu:ation, but the ini*iai exp~,t:sion ~' the cladding wa~ ~s':mated by the measured residual expao.~ion, in st,'atd of by equat;o.'~s t l) anG ~2). The good agreement between the calculated and the measured tempera*ure Td suggests that the turning point temperat.are T~ will correspond to the start of re-oper~ ing uf the pellet-cladding ~;ap. The figure ~.lso ~or,tpares the T~ of xeno:~ and he|ram filled rods. This comparison e:~tibits t.hat temoerature rise 'ill! Td is not influenced by the filled gas compm~tion. 4.2. Speed of tempera.:ure riye corrcspcaJing to the gap :ondiflon

'Ct,e average ~pee~ of cladding temperature rise ~as a~. large ~ i04°C/s in the prompt, heat up peri "~, and it doe-teased to about 1/50 in the "siew ~eat up pet~d for helium filled rods, and to about 1/t, O0 for xenor, fdlfd.rods. To und~stand how this very prompt temperature rise wa; realized and flow the sl ~,wing down of the heat up ~ i~ c o n n ~ e d tc-~be .~ap coud;o,~Ons, measured tempe~'atur¢ rise speeds were compp.red ~ith the ~adculation go,~ the case of ccn:plcte o~tac.t at the p~lI-A ¢laddiag interface b s ~ d on a six~ple heat.~on,~action model. As dis~'tm~ in ~ e p r e v i ~ s s~tion, the'.q~31 pellet g e m i ~ m r ~ iuc.,~mes _almost adiabaficKily dur: n8 the pulse irradiation, and the major heat transfer

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begins ahei" th, I~0t fuel pellet comes in:o contact w~th ~he cord cladding in the later part of the pul~. "The

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teml, eramrc =n thepellet a ~ t c h d d i n g w=s -=timated by

o,'=e dimeasiona! conduction in the two .senti-infinite bod.~, of &fferent ini~i.',d temperature~ codt~cted at time.. zero T h e initial, temperature of U02 ~as estimated from the adiabatic tempe',att~re rise during tire pu:se ir;-,.b,ion b ) e q . (I) and ~ for the cladding was. a ~ u m e d as the initial t~npe..'ature in d~e experhnent, ' i.e ,n~.,n tempe=atur¢. Theinterracial temperat,J:e of the r,ellct ~/nd the cladding. T=, was estimated by •

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T,=(

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[ ( ~,~,'.),,, "" I + ~ (~,~ h,o~

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(i~).

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where 7~ q,.. T,,,, = imtial U O 2 and zir~aloy tcmpcr-.)ture (K), -; thermal conductivity ('W,,'m K). = density ( k g / m ~I. (" .- specific heal capacity (J)'kg K).

/, p

The temwr,~ture distribution .,?..feach side of the inter='ace was calc,Jla(ed by lhe profile method'by the following equ.mons and initial conditions.

0

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f 2 3 4 5 6 7 8 9 10

Oi~to'tce f,om the Inter'fore (l()tmm) • Fig. 15. An example of transient ~ p c r a t u r e profiles in UOa

p¢.'ler and zircaloy cladding calculated by one directional heat cpnductiou model.

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O,, -,T I T(x) T:) .~ " I nitial

i, il.

=- trmperaturc al x (K), = initial temperature (K). -= ;li:;tance from the interface (m). --- ti,r.=e (s).

c(,;,d,tions:

r,,--- TL,,~:

for x < 0,

T. = T,,,

tor ~ > 0.

r(O) -- 7",

for x -= 0.

r-=k. 15 si=ows an example o f . t h e temperature profiles

calcula,.ed by "this heat conduction mode.=.. Cladding surf~;ce tempe.rature was estimated from the zircaloy ~cmperata.rt. at the point of the cladding thickness fromtl~c interface. Fig 16 shows the caIculate,l cladding surface temperature histories [,;r different" ene..-gy depositions. The temi~ratur¢ rising ~peeds are obtained as the first differential~ of these curves. In fig. 17. mea-

S00 230

800 700

200

':'- 600 170 F. 500 140 400 300 -

200

100 .1

50

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°O0 T.':mt (ms !

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1,50

Fi& J6. C a l c u l ~ ¢ i a ~ . i ~ .~!a¢¢ t ~ , - r j t ~ d i f f c ~ t energy depositions.

him~ri~ for

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B a s ~ on '.he dh¢~auion in the previous secdons, :be ¢,:feeAs of ~ Sas ~ U c n on maximum cttdding temperature shown in fig. 8 can be explained i~. the foH6wing wa:.r. 3 Firstly, the temperature ~ up to around 700 g.~0eC i, attr_ined during the prompt heat up p e r o d ia wifich the temoer~ture rise is~not very se, sitivc to the | -& ~ fillN ~ . - , ~ , , - " t " 4 - ~ . . • I, | ..... g~p gas ¢ o m ~ i t i o n . Com~equ ntly, the influence ,~f gap gas is net evident at lower energy de~fosi.tiom, a~, the / I • moat o! the c/adding '.em~erature increase.is attained in the prompt beat up perio.J. Sccandly, sin~e the tempera. ture increase beyond this initial r;se is attait;ed after gap r, :.-opening, the effect of xenon gsp ga~ to ~he maximum temperature is enhanct~ wi,h the increase of ener.~y d',~aosition. .However, the results that the effect.< of gap gas decrease at the energy d .~3os.~tion of over 240 ~:al/g UO2 ,too 15o zuo 250 jo can not be exphinod by the abo:e discussions, since, if F.M~ I)e~dflon Icol,~lUOz} the gap is kep! open in the-dow heat up period, Ih¢ effects of lower ihern'.al cot.ductility gas must beo.~me Fi~ 27. Comparison of'nmumred data of average temperature increase rate for prompt .and dow he~t-ap l~hau~t .and the larger for th~ hig.h,'¢ energy de"~shions, too. calculation. .Then, we ht:ve tL think that the gap r-.,og~ening may become inco'~Jplete ,'or this higher energy deposition. Fig. 18 is a cross-set,tonal v~ew o! poll-test fuel rod irradiated.at 260 oal,/8 ,.v ; Oz. In the very rapid heat up surr,d data of averaged t~,perature increase ra'te for the condition of glAs, fur| i~ilet is easily creaked by prompt and slow heat up periods Were compared with thermal stres~ as show a ~he figure. So, once the luel the calculation. The data were plott.e~:, as the f,mction of sticks to the cladding, the .~racked fragment ol the fad evergy deposition, an~ the calcula~.ed speeds "for the pellet will ke,cp sticking to the cladding internal seJ'face. different time are indicated .by the solid liner. As the The experience that the pellets in the fuel -od which was prompt heat up period is around 30 "o 50 nU after the subjeoed to this higher .energy depositicn were n.oi transient, a good agreement can be noticed between the eadly taken out of the cladding during the disassem• measared and the calcul.ation, for VrOml~t heat up. Tiff,, bling a!so support:; th/s assumption of ~ellet siicldng. coninddea~ indicates &at temperature behavior in this "rhe formation of |JO2-zh'cRloy h-~teraetion layer ss period can be estintstes well by a m~ple her_~ conducshown in the photograph wa~ usually observed at the tion model from the fuel pellet ~o the cladding. Tbe~ata en~r D, deposition of over 240 c~l/g UO 2. Where this for the show heat Up peri.'od are 11~ avemge~ for the UO2-zirca]oy eutectic layer was formed, the pellet kept time fccm 200 to fg~.) rag, ConsequenOy, the datl of contaftwith the cladding fill the furl rod was cooled hefium filled rods tend to be a Lttle highez than the down~ calculation, trod those for the xenim Idled are lower In fig. 19, in terfac/ a! temperature at the pellet-cladthan the calcuhtticn. "ibis diff~ence indi~tes that in ding .boundary obtained by :q. (7) i~ plothd as a the slow" heat. up period, cladding temperature is in-" function of enerFy deposition. ".."hi~c~.e.~,~t'~on suggests fluenced both by, gap ~ o n d ~ an¢ the heat transfer at cladding surface, i.e., for helium filled rods, decrcate • that the temperatur*, at the cladding"~x/mcr wail will excc~__ 1200°C ir..standy after the contact with the UO 2 of heat mmsfer at clad¢~ng surface due to film boi!ing pellet a: the en.ergy depmidon of 240 ~ l / g UO2 or results ~, the lfi~er cladd~."g temperature rise .thaa in . higher. The in! :ttation of U O : - z i r ~ l o y eatectic will be the simple conduction ~ ~nd, fo.~ xenon filled rods,

~.

262

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{~) ~ o | ~ t i c m cloee

(2}

at o IOoet~ for from a a~Ock

.o o ¢rJck

F'g. 18. Cros~-secU:',~alview of a post.test, fuel tog irradit:ed ;It 260 cal/g UOz.

possible a,. this high tem~rat,.,re of around' 1200°C. Cons.'querttly, the phenomena at the energy deposition <,~ ov,,~ 249 cal/g O f 7 con be understood as th~ results of sttckang of the cracked pellet to the cladd;h8" .b~" euteL:,-c formation. Regardtae ,~e fuel faih:re threshold, the influence, of gap gas c,3 ,~positien will bevome negligible, s~w:e ihe eifects ~.f g.~p 37s re: fuel therm~! behavior diminishes at ~h,.' energy of near failure threshold by the euteclic fo; rnalion. 30¢0r-..---

"Etr~Jgh the kgpile RIA simulation tests with hefium and xenon fi~led LWR type fuel rods, {undamental kaowl,,j:lge on the transient variation of ~ p heat transfer and its influence ~o fuel thermal ,behavior, .rod oa the effects of gap Sa~ oompodtion of fuel beha~ors were obtained. The conclu~iom are a.,~follows. (1) The cladding temperatur~ riu: in RIA is compmed of ,'wo steps; i.e, rapid and slow h ~ t up phases, ~2) The rapid heat .up" phase .?:~r~csponds to the period in which'gap clo..ure by i:~ellet theymal expansion &re established. The. tr "~asitioa to the slow ::~eat up phase " ¢orres.pon.,h to the.gap re-opening d:te to thermal expansion of the" cladding. Major- influence of gap gas composition a.ppef,fb after tins transition to glow h~at up pha~. <.3) T h e eff~:s of gap gas composition oi cladding temperat,jre ~ increase ~ t h energy deposition till around 240 cal/g UO 2. The e~fects d ~ s e beyond 240 c~l/g UO 2 due to inperfect re.opening of the pellet-claOding gap by euK~:fic formation between UO 2 at,d z/rca?oy, The infl:~ence of gap gas cx,mposition on fuel failure threshold is negligible. o4

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F~g. i:,L Initial pel!e: ~¢mperattne sn6 interfacia! temper-~t.ure versus energy deposittor,.

A

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The aut/mrs extend their tlta~ks to Dr. T. Hoshi for l-,i, review of thic cepo~t, and to Mr. K. lwa~ for x~'telping with t~. experiments, a~d to the members o~ rd,e i

263

N S R R operation sect'i~ for ~ i r

~he e , ~ m e e u .

teclmic~l ~upport of •

itee~ It! T. F u ~ x

-XJ. Joan,. P.=. M,~Do,~d a M e.g.

McCardell, Lish{ water ruc,.or f~ag r ~ m ~ earing m c t i v i~¢ initiated.acciden: exit:mints, l~Uk':.~G/CR.0269, TILF.E.1237. (Auaum 1978~.

[21 L t i an i sin ¢ ,... Pl~.osmphic studies ot nmal-c~J, UO2.c,o,e fuel md ia TRF,a~T, ^ ~ - 7 3 2 5 {1966) 158-163.. [3} P:E. MacDo~ut~d ,.t aL; ~ I of Ull~-walez-m~ftor fuel d~.taae dpri~-~jia reactiv/'zy.inisiated acci.dent, Nudear Saf~_y ~1 (1980~ 552-.60Z t/on, ~lfianmtal ~ ; l~epe~ on The NSRR F..xperimeats !";2), JAERI-M 8"~-012(t982). [5] D.L. E l a f . a ~ and G.A. geymann, MATPRC~Venion i !. a handbc.ok of mate~-mbpr.:~m'ties for ~ in the anaiyms of litht wrier re~lor fael rod behav/or, NUREG/C~-O4itT, TREE 1280 (Febp.tat3, 1979).