PWR_TK-Fuketa-NT133-2001

FUEL CYCLE ANDMANAGEMENT

KEYWORDS: reactivity-initiatedaccident, high burnup, fuel failure

BEHAVIOR OF HIGH-BURNUPPWR FUELS WITH LOW-TINZIRCALOY-4 CLADDING UNDERREACTIVITY-INITIATED-ACCIDENTCONDITIONSTOYOSHI FUKETA,* HIDEO SASAJIMA, and TOMOYUKI SUGIYAMAJapan Atomic Energy Research Institute, Department of Reactor Safety ResearchTokai-mura, Ibaraki-ken, 319-1195 Japan

Received July 26, 1999Accepted for Publication August 1, 2000

Experimental programs on fuel behavior during sim-ulated reactivity-initiated-accident (RIA) conditions atthe Nuclear Safety Research Reactor (NSRR) in Japanand the CABRI test reactor in France appear to indi-cate that cladding failures may occur at enthalpy val-ues lower than would be expected. Results from twoexperiments designated as HBO-1 in NSRR and REPNa-1 in CABRI indicate that the occurrence of fuel fail-ure is strongly influenced by corrosion of cladding inthe tested fuels. However, data had been limited to fuelrods with conventional (1.5% Sn) Zircaloy-4 cladding.Results are described from newly conducted NSRR ex-periments, TK test series, for 38 to 50 MWd/kg U pres-surized water reactor fuels with low-tin (1.3% Sn)Zircaloy-4 cladding, and anticipated processes of fuelbehavior during the transient are discussed.

INTRODUCTION

From 1994 to 1996, we performed seven pulse-irradiation experiments with high-burnup pressurized wa-ter reactor~PWR! fuels in the HBO test series1,2 andobserved fuel failures at a low enthalpy level in twoexperiments, HBO-1 and -5. In the test fuel rods, sig-nificant hydride deposition occurred below the oxide filmthat was generated in the cladding peripheral region. Brit-tle fracture can be seen in the cladding outer region,where a number of hydride clusters precipitated, and duc-

tile fracture appears in the inner region. Incipient crack-ing occurred in the outer, hydrided region and propagatedto the inner region. Fuel failure occurred in the experi-ments with test fuel rods from the highest elevationsin the core, and microcrack generation in oxide andhydride layers was observed in the experiments withrods from these relatively high elevations. Occurrenceof fuel failure in the HBO experiments correlates withthe sampling elevation and hence with the thickness ofoxide film and the severity of hydrogen deposition ofthe tested fuel rods. The fuel rods in the HBO serieswere sampled from a mother rod irradiated in a 48 MWd0kg U lead-use program, and these rods had conven-tional ~1.5% Sn! Zircaloy-4 cladding. However, fuelswith low-tin ~1.3% Sn! Zircaloy-4 cladding were adoptedfor 48 MWd0kg U regular use in Japanese PWRs. Ac-cordingly, we started a new series of experiments withthe fuels with the low-tin cladding, as TK test series.This paper presents results from the TK tests and dis-cusses anticipated processes of fuel behavior, includingpellet-cladding mechanical interaction~PCMI!, late-phase fuel deformation, and postfailure events. Finally,the paper describes pending questions in the field of studyand ongoing programs.

EXPERIMENTAL SETUP AND TEST CONDITIONS

The Nuclear Safety Research Reactor~NSRR! is amodified TRIGA annular core pulse reactor, whose sa-lient features are a pulsing power capability and a 22-cm-diam dry irradiation space located in the center ofthe reactor core to accommodate a sizable experiment.Details of the NSRR can be found in previous docu-ments.1,3The experimental capsule used for irradiated fuel*E-mail: [email protected]

50 NUCLEAR TECHNOLOGY VOL. 133 JAN. 2001

rod tests in the NSRR for pulse irradiations is a doublecontainer system. Figure 1 shows a schematic of the cap-sule. The outer capsule is a sealed container with 130-mminner diameter~i.d.! and 1250-mm height, and the innercapsule is a sealed pressure vessel with 72-mm i.d. and680-mm height. The capsule contains an instrumented testfuel rod with stagnant water at atmospheric pressure andambient temperature. Cladding surface temperatures weremeasured by 0.2-mm bare-wire R-type~Pt0Pt13%Rh!thermocouples spot-welded to the cladding. Coolant wa-ter temperature was measured by sheathed K-type~CA!thermocouples~1 mm in diameter! near the cladding sur-face at top of the test fuel rod and0or center of the fuelstack. A strain-gauge-type pressure sensor was installedat the bottom of the inner capsule to measure the in-crease in capsule internal pressure. In some experiments,

sensors for axial elongation of pellet stack and claddingtube or float-type water column velocity sensor were used.

Test fuel rods in the TK test series~TK fuel! hadbeen irradiated in the Takahama unit 3 reactor. The TKfuel has 173 17 geometry of types A and B.a They havedifferent cladding thicknesses and pellet outer diam-eters, as listed in Table I. The test fuel rod is illustrated inFig. 2. Since the TK test fuel in each experiment wassampled at a different elevation, oxide film thickness onthe cladding outer surface and concentration of absorbedhydrogen in the cladding interior varied. The test condi-tions, including rod sampling elevation and peak fuel en-thalpy, are listed in Table II. Unless otherwise noted, the

aTypes A and B fuel were manufactured by Mitsubishi HeavyIndustries, Ltd., and Nuclear Fuel Industries, Ltd., respectively.

Fig. 1. Schematic of test capsule.

TABLE I

Diametral Dimensions of Cladding and Fuel Pellets

Fuel Type

Cladding OuterDiameter

~mm!Thickness

~mm!P0C Gap

~mm!

Pellet OuterDiameter

~mm!Pellet Height

~mm!

A 9.5 0.57 0.085 8.19 10B 9.5 0.64 0.085 8.05 9

Fuketa et al. PWR HIGH BURNUP FUEL BEHAVIOR

NUCLEAR TECHNOLOGY VOL. 133 JAN. 2001 51

peak fuel enthalpy denotes the radial average peak fuelenthalpy per unit mass of fuel~Joules per gram or calo-ries per gram! in this paper. The table includes condi-tions of previously performed experiments in the HBOseries.

TEST RESULTS

PCMI Failure

Among the six experiments of the TK test series de-scribed in this paper, failure occurred only in test TK-2.A comparison between this experiment and subsequenttest TK-3 provides critical information regarding whetherfuel fails due to PCMI in the NSRR experiments. Thetest fuel rods of TK-2 and -3 were sampled from the sec-ond span~from the top! and fourth span of the same

mother rod. Because of the difference in the samplingelevation, the oxide layer thickness of the cladding rangesfrom 15 to 35mm for TK-2 ~23mm on average! and from4 to 12mm for TK-3 ~7 mm on average!. Fuel burnup is48 MWd0kg U for TK-2 and 50 MWd0kg U for TK-3.Both experiments were performed with the same pulsein the NSRR, and fuel enthalpy reached;414 J0g ~99cal0g! in TK-2 and 385 J0g ~92 cal0g! in TK-3.

After the earlier HBO-5 test resulted in fuel failure,the possibility of influence from cladding surface ther-mocouples on cladding failure was discussed. To ex-clude the possibility, cladding surface thermocouples werenot installed in TK-2. Instead of elongation sensors forcladding and fuel stack, a float-type water column veloc-ity sensor was used to measure mechanical energy gen-eration when the fuel rod failed during the experiment.Axial elongation sensors and cladding surface thermo-couples were installed for TK-3. The pulse-irradiation

Fig. 2. Schematic of a short-sized test fuel rod.

TABLE II

Test Conditions of TK and HBO Tests

Test Fuel Type SpanOxide Layer

~mm!Fuel Burnup~MWd0kg U!

Peak Enthalpy~J0g! Result

TK-1 A Fiftha 7 38 527 No failure, FGR5 20.0%TK-2 B Second 15 to 35 48 448 Failed at 251 J0g, 7% fuel dispersedTK-3 B Fourth 4 to 12 50 414 No failure, FGR5 10.9%TK-4 A Third 25 50 410 No failure, FGR5 8.3%TK-5 A Second 30 48 423 No failure, FGR5 11.1%TK-6 A Third 15 38 523 No failure, FGR5 16.2%

HBO-1 A Third 40 to 48 50.4 306 Failed at 251 J0g, 100% fuel dispersedHBO-2 A Fourth 30 to 40 50.4 155 No failure, FGR5 17.7%HBO-3 A Fifth 20 to 25 50.4 310 No failure, FGR5 22.7%HBO-4 A Sixth 15 to 20 50.4 209 No failure, FGR5 21.1%HBO-5 B Second 35 to 60 44 335 Failed at 322 J0g, 5% fuel dispersedHBO-6 B Fourth 20 to 30 49 356 No failure, FGR5 10.4%HBO-7 B Third 35 to 45 49 368 No failure, FGR5 8.5%

aThe first span is the highest.



conditions are identical in experiments TK-2 and TK-3,except the instrumentation and the state of the test fuelrods. Test TK-2 resulted in fuel failure and TK-3 did not.

Figure 3 shows transient histories of the NSRR’spower, signal from the water column velocity sensor, fuelrod internal pressure, and capsule internal pressure dur-ing the TK-2 test. During pulse irradiation, the water col-umn starts to move, and spikes appear in the rod and inthe capsule internal pressure histories simultaneously.These indicate that fuel failure occurred at that instant.When the fuel rod failed, fuel enthalpy reached 251 J0g~60 cal0g!. In the signal from the water column velocitysensor, a half-wavelength corresponds to 3-mm move-

ment of the float at the surface of the coolant water. Thethermal-to-mechanical energy conversion ratio, a ratio ofmechanical energy generated to the peak fuel enthalpyof the fuel, is estimated as;0.08%. A vertical crack overthe active fuel region was observed in the posttest exam-ination in the TK-2 fuel, as seen in Fig. 4. An enlargedphotograph of the cracking in TK-2 is shown in Fig. 5,and the appearance of the crack is similar to those inHBO-1 ~Ref. 1! and -5~Ref. 2!. An X ray of the posttestTK-2 rod shows that few fuel particles were dispersedinto the coolant water.

In the subsequent test TK-3, fuel failure did not oc-cur. During the TK-3 test, cladding surface temperaturereached;980 K at maximum, as shown in Fig. 6. Al-though relatively large fuel deformation occurred, thecladding survived in the experiment. The histories ofcladding surface temperature in the TK-3 test suggestthat the fuel failure in the previous TK-2 occurred whencladding surface temperature was still low early in thetransient.

In experiments TK-2 and -3, only the test fuel rodsampled from the higher elevation, with thicker oxidelayer and larger hydrogen pickup, failed at 251 J0g ~60cal0g!. The results from the two experiments indicate thatthe critical factor is whether cladding has enough ductil-ity to survive until the time that cladding temperaturereaches a certain level. In the experiments with test fuelrods sampled from the higher elevation, cracking oc-curred initially in the radially localized hydride layer ofthe cladding and then propagated during early stages ofthe transient when cladding temperature was still low.

According to the photo-image analyses, residual hoopstrains of failed claddings in HBO-1 and -5 are much lessthan 1%. A separate fast-transient tube burst experimentalso showed that cladding with a radially localized hy-dride layer fails with,1% of residual strain at room tem-perature.4 It can be expected that deformation of thecladding is limited to small amounts during the earlyphase, i.e., during the PCMI-loading process, and the de-formation increases with elevated cladding temperatureduring the late phase, i.e., post–departure from nucleateboiling ~DNB! phase.

Rod Deformation

The deformation of the cladding was most dramaticin test TK-1. The test fuel rod of TK-1 was sampled fromthe fifth span~from the top! of type-A fuel. The motherrod had been irradiated for two cycles, and burnup ofthe TK-1 fuel was 37.8 MWd0kg U. Because of the

Fig. 3. Transient records of test TK-2: reactor power and in-tegrated reactor power, signal from water column ve-locity sensor, fuel rod internal pressure, and capsuleinternal pressure as a function of time.

Fig. 4. Posttest appearance of the TK-2 test fuel.



relatively low burnup and low sampling elevation, oxidethickness of the cladding remained low at 7mm. Fuelenthalpy during pulse irradiation reached 523 J0g ~125cal0g! at maximum, and the rod did not fail. Figure 7shows transient histories of the cladding surface temper-ature during TK-1. Cladding surface temperature in-creased rapidly at the pulse, and DNB occurred. Thetemperature reached;860 K at maximum. Figure 8 showsthe posttest appearance of the TK-1 test fuel rod. Signif-icant swelling occurred over the active fuel region as isalso shown in Fig. 9. The increase in cladding diameteris 10% on average over the pellet stack region and 25%at maximum. The X ray in Fig. 8 shows the portion wherethe most significant deformation appeared. It shows thata gap did not form between the fuel pellets and the clad-ding inner surface, and the fuel pellets themselves ex-panded significantly. This fact suggests that the largeincrease in cladding diameter of the TK-1 rod was notcaused by a pressure increase in the fuel rod plenum~bal-looning! but was produced by pellet expansion~i.e., by

the PCMI!. Roughly polished radial and vertical crosssections of the posttest TK-1 fuel are shown in Fig. 10.Large cracks and openings are observed in the cross sec-tions of the posttest fuel pellets. The results of the earlierHBO tests indicated that rapid expansion of intergranu-lar fission gas caused grain-boundary separation, and itresulted in fuel pellet expansion and PCMI.

Figure 11 shows the pre- and posttest cladding outerdiameters of experiments TK-3 through TK-6. Figure 12shows residual hoop strains in the NSRR PWR fuel ex-periments as a function of peak fuel enthalpy, and thefigure includes data from MH, GK, and OI test series.1

The higher fuel enthalpies correlate with the larger strains,and those of TK-1 and -6 are extremely large due to thehigher enthalpy level of these two experiments. Thefission-gas-induced expansion has a significant role inthe large deformations. However, the information regard-ing pellet expansion and resulting rod deformation is

Fig. 5. Cracking observed in a radial cross section of posttest TK-2 fuel~chemically treated surface to show hydride clusters!.

Fig. 6. Transient histories of cladding surface temperature dur-ing test TK-3~No. 1: 33 mm below the axial center ofthe active fuel region, No. 2: 5 mm above the axial cen-ter of the active fuel region!.

Fig. 7. Transient histories of the cladding surface temperatureduring test TK-1~Nos. 1 and 2: 42 and 2 mm, respec-tively, below the axial center of the active fuel region;No. 3: 38 mm above the axial center of the active fuelregion!.



limited to that from the posttest measurement of residualcladding deformation, and we do not know how muchexpansion occurred during the early PCMI loading pro-cess. In the experiments that result in large deformation,i.e., 2% strain or higher, cladding surface temperatureincreased from 600 to 1000 K at maximum due to DNB.As described earlier, the large deformation occurs duringthe post-DNB phase. It can be expected that the fission-gas-induced expansion combined with thermal expan-sion provides PCMI loading to the cladding during theearly stage as well as in the later post-DNB phase, but atpresent it is not known how early and how fast pressure

Fig. 8. Posttest appearance of the TK-1 test fuel~X-ray shows the most expanded region!.

Fig. 9. Axial profile of pre- and posttest cladding outer diam-eters of the TK-1 test fuel.

Fig. 10. Radial and transverse cross sections of the posttest TK-1 test fuel~as roughly polished!.



elevations and reductions occur in fission gas accumu-lated on grain boundaries. Since rod deformation in theearly stage is critical information for evaluating the PCMIloading, a transient method to measure cladding outer di-

ameter during pulse irradiation is being developed witheddy current sensors. The method is being verified in out-of-pile testing and in NSRR experiments with unirradi-ated fuels and is to be applied to irradiated fuel tests inthe NSRR.

Fission Gas Release

After the pulse-irradiation experiments, rod-averagefission gas release~FGR! was measured for the test rodsby rod puncture and gas analysis. The FGRs during pulseirradiation are shown in Fig. 13 as a function of the peakfuel enthalpy. Except for HBO-2, -3, and -4, the higherFGR correlates with the higher peak fuel enthalpy. InHBO-2, -3, -4 and TK-1, FGR reached;20%, and thiscorresponds to all the fission gas accumulated in grainboundaries being released in these experiments. As canbe seen in Figs. 12 and 13, the experiments with highFGR resulted in large rod deformation, except in HBO-2,-3, and -4. This fact indicates the significant role of fis-sion gas in rod deformation. In HBO-2, -3, and -4, DNBdid not occur, and cladding temperatures remained in low.~A transient signal from the thermocouple in HBO-3showed;670 K at maximum, but the duration of stablefilm boiling was very short and must have been limitedto the local area.! Therefore, the significant role of fis-sion gas in rod deformation appears to occur only at hightemperatures, where cladding ductility is enhanced.

Fuel Fragmentation and Mechanical Energy Generation

The loading from fission gas expansion appears asgrain boundary separation in posttest micrographs of fuelpellets. Figure 14 shows the microstructure of the post-test TK-2 fuel pellet. The pictures show a horizontal, as-polished cross section of the pellet. Significant grainboundary separation can be seen in extensive areas ofthe cross section.

In the TK-2 test, which resulted in fuel failure, somefuel was dispersed from the fuel rod, and;7% of the

Fig. 11. Axial profile of pre- and posttest cladding outer di-ameters of the TK-3, -4, -5, and -6 test fuels.

Fig. 12. Residual hoop strain of cladding as a function of peakfuel enthalpy.

Fig. 13. Fission gas release as a function of peak fuel enthalpy.



total fuel was recovered as fragmented particles from cap-sule water after the experiment. A cross-sectional viewof the fuel particles is shown in Fig. 15, and scanningelectron microscopy~SEM! images are shown in Fig. 16.The collected fuel particles were not once molten, as canbe expected from the low maximum fuel temperature~;2100 K or lower! during pulse irradiation. Althoughthe fragmented particles remained in the solid phase, 22-Jmechanical energy was generated during test TK-2 andcorresponds to a thermal-to-mechanical energy conver-sion ratio of;0.08%. The estimation of mechanical workdue to rod internal gas release and expansion shows that

the gas does not have enough potential to produce thismechanical energy in TK-2. With an extreme assump-tion that all the rod internal gas reaches the maximumfuel temperature, gas internal energy is only 6.8 J, whichis well below the mechanical energy generated. This sug-gests that rapid steam generation due to thermal inter-action of dispersed fuel fragments with coolant water isthe primary source of the mechanical energy generatedduring the test. The postulated heat flux in this thermalinteraction was compared with those in separate-effectsexperiments with powder fuels, and the comparison cor-roborates that the heat flux in this process is realistic.5

Fig. 14. Fuel pellet microstructures observed in a radial cross section of the posttest TK-2 fuel.

Fig. 15. Cross-sectional view of fragmented fuel particles recovered from coolant water in test TK-2.



DISCUSSION, KEY QUESTIONS, ANDONGOING PROGRAMS

Figure 17 illustrates schematically the processes offuel behavior during these transients. Fuel pellet temper-ature increases promptly at the onset of the event, andfuel pellets expand rapidly due to thermal expansion andmost likely due to fission-gas-induced expansion. Thefission-gas-induced expansion is caused by thermal ex-pansion of fission gas accumulated in fuel grain bound-aries, as shown in Fig. 18. Then, expanded fuel pelletscontact the cladding inner wall and push on the surface.When hydrides precipitate in the cladding periphery, duc-tility of the cladding is reduced significantly due to thisradially localized hydride layer. Incipient cracking oc-curs in the cladding periphery, and the cracks propagateto the inside. This hydride-assisted PCMI failure occursin the early stage of the transient when cladding temper-ature is still low and the cladding ductility is low. In theNSRR experiments, the hydride-assisted PCMI failure oc-curred when cladding surface temperature was 400 K orlower. If the cladding survives this early phase, the be-havior proceeds to the late-phase, post-DNB process.Then, cladding temperature increases rapidly, and the duc-tility of the cladding decreases. Expansion of fuel pelletsresults in cladding with large deformation. If fuel rod in-ternal pressure is high enough, ballooning of the clad-ding and burst-type failure can be caused in the late phase.

When cladding fails during the transient, the contactof dispersed fuel pellets with coolant water produces me-chanical energy as a postfailure event. The grain bound-ary separation accelerates fuel fragmentation, resultingin a large surface area of finely fragmented fuel parti-cles. Although fuel particles are in solid form, thermalinteraction between fuel fragments and coolant water canproduce mechanical energy.

The results from the NSRR and CABRI REP-Na ex-periments6 have provided evidence that fuel failure ofhigh-burnup PWR fuel occurs at a low enthalpy level.The results indicate that decreased cladding ductility, fuelpellet expansion, and increased internal pressure drivenby fission gas expansion have significant roles in the fail-ure process. The NSRR experiments have also shown theoccurrence of postfailure events, including mechanicalenergy generation and fuel fragmentation. To understandhigh-burnup fuel behavior during reactivity-initiated-accident~RIA! conditions, further efforts are needed todetermine the following:

1. mechanical properties of cladding materials, inparticular, ductility reduction due to radially lo-calized hydride precipitation

2. role of fission gas in fuel pellet expansion, grainboundary separation, and fuel fragmentation

3. assessment of postfailure events

Fig. 16. SEM images of fragmented fuel particles recovered from coolant water in test TK-2.



4. post-DNB failure modes

5. influence of pulse-width and initial temperatureon the failure

6. code qualifications.

Hydrogen deposition, in particular, could be criti-cally important in the high-burnup PWR fuel behavior dur-ing an RIA. The effect of hydrides cannot be described byhydrogen concentration averaged in a horizontal cross sec-tion. The effect of radial and circumferential localizationof the hydride clusters is very important, as indicatedby photographic analysis,2 for the distribution of hydrideclusters.The influence of a radially localized hydride layeron cladding ductility is being examined in out-of-pile,separate-effects tests at JapanAtomic Energy Research In-

stitute~JAERI!. The separate-effects tests include fast-transient tube burst experiments4 at room and elevated tem-peratures and modified ring tensile tests on machinedspecimens.The results from in-reactor testsTK-2 andTK-3indicate that the critical factor is whether cladding hasenough ductility to survive until the time that cladding tem-perature reaches a certain level.Accordingly, data regard-ing brittle-ductile transition temperatures of hydridedcladding are important.

To examine hydride-assisted PCMI failure and toevaluate PCMI loading, the pellet expansion mechanismmust be clarified in the early phase of the transient aswell as cladding ductility reduction. As stated earlier, itis expected that the fission-gas-induced expansion com-bined with thermal expansion provides a PCMI load to

Fig. 17. Processes of fuel behavior during an RIA.



the cladding during the early stage. However, the role offission gas in pellet expansion remains speculative forthe moment.7 A key question about pellet behavior hasto do with fission-gas-induced expansion due to high-pressure loading of heated fission gas confined in grainboundaries. How early and how fast the pressure eleva-tion and reduction occur in fission gas in grain bound-aries should be determined. The pressure increase andsubsequent reduction in small fission gas bubbles arestrongly affected by a number of unknown parameters,such as bubble size, initial pressure, and threshold of pathopening.

To investigate the role of fission gas in grain bound-ary separation and fuel fragmentation as well as the fail-ure process, measurements on the radial distribution ofaccumulated fission gas and inter- and intragranular in-ventory are continuing with newly installed postirradia-tion examination devices, including an ion microanalyzer.An out-of-pile fuel pellet heating test called VEGA~ver-ification experiment of gas0aerosol release!8 under ac-cident conditions will also provide information on fissiongas states in high-burnup fuels.

High coolant temperatures and wide pulse widths inpower reactors under accident conditions can provide a

higher cladding temperature when the cladding takesPCMI loading. It is expected that a reduction of hydrideembrittlement occurs at elevated cladding temperatures.Wider pulse is not available in the NSRR, but a high-temperature, high-pressure test capsule9 is being de-signed to clarify the initial temperature effect. The narrowpulse of the NSRR gives a lower cladding temperature inthe early transient stage, but the pulse-width effect is cur-rently understood only in a qualitative manner. As forcladding temperature during transients, no data are avail-able from CABRI, and no code can simulate the temper-ature histories from NSRR tests at present. Lower thermaldiffusivity and gap conductance in high-burnup fuel pel-lets may reduce the difference of cladding peripheral tem-perature between narrow and wide pulses. The differencein thermal expansion between wide and narrow pulses isalso unknown due to a lack of quantitative informationregarding heat loss during transients.

CABRI REP Na-1 and Na-10 gave different ener-gies at the time of failure~failure energy! with a 9.5-ms~full-width at half-maximum! pulse in Na-1 and a 31-mspulse in Na-10~Ref. 7! Both experiments were per-formed with fuel rods with an 80-mm oxide layer andinitial oxide spallation. It can be expected that the pulse-width effect appears more dramatically in experimentswith test fuel rods with extremely embrittled cladding,such as cladding with preexisting oxide spallation. Sincethis kind of cladding fails with very small strain, smalldifferences in thermal expansion may give different fail-ure energies. A milder effect~smaller difference in fail-ure energy! may appear when unspalled 50 to 60 MWd0kg U PWR fuels are tested with narrow and wide pulses.The key question is strain at the time of failure, and thatis what we are working on with the eddy-current-typesensor for transient strain measurement.

Pending questions exist also in the late-phase, post-DNB process and in the postfailure events. The possibil-ity of post-DNB, burst-type failure depends on timing offission gas release and elevation of rod internal pressurein combination with cladding temperature increase. Re-garding fuel fragmentation and mechanical energy gen-eration after failure, the driving force of fuel expulsionfrom cladding, the contact mode of fuel and coolant, thesteam generation rate, etc., should be studied in experi-mental and in-reactor conditions.

CONCLUSIONS

High-burnup PWR fuel rod segments with low-tincladding~1.3% Sn! were subjected to pulse irradiation,and test specimens from higher elevations with thickeroxide layers, failed at values as low as 251 J0g ~60 cal0g!for fuel enthalpy. The results indicate that the critical fac-tor is whether cladding has enough ductility to surviveuntil the time that cladding temperature reaches a certain

Fig. 18. Sequence of fission-gas-induced expansion, PCMIloading, and grain boundary separation.



level. Greater fuel deformation occurred at higher fuelenthalpy levels and reached a 25% increase in claddingouter diameter at maximum. In the experiment resultingin fuel failure, fuel fragmentation and mechanical en-ergy generation were observed. In test TK-2, 7% of thefuel was dispersed into coolant, and the thermal-to-mechanical energy conversion ratio was 0.08%. Col-lected fuel particles were not previously molten. Theresults indicate vigorous thermal interaction between theparticles and coolant water.

To understand the failure process, the pellet expan-sion mechanism in the early phase of the transient mustbe clarified as well as cladding ductility reduction. Theexisting data suggest that the fission-gas-induced expan-sion combined with thermal expansion provides PCMIloading to the cladding in the early stage. A key questionremains about pellet behavior with regard to fission-gas-induced expansion due to high-pressure loading of heatedfission gas confined in grain boundaries.

ACKNOWLEDGMENTS

The authors would like to acknowledge and express theirappreciation to JAERI colleagues in the NSRR Operation Di-vision for pulse-irradiation experiments, the Department ofHot Laboratories for test fuel fabrication and postirradiationexaminations, and the Process Technology Division of the De-partment of Safety Research Technical Support for mass spec-trometry. Discussions with R. Meyer of the U.S. NuclearRegulatory Commission, F. Schmitz of France’s Institute forProtection and Nuclear Safety, and K. Ishijima of JAERI werealso invaluable for the study. The HBO and TK experimentswere performed as collaborative programs between JAERI,Mitsubishi Heavy Industries, Ltd., Nuclear Fuel Industries, Ltd.by using fuel rods from Kansai Electric Power Company, Inc.

REFERENCES

1. T. FUKETA, Y. MORI, H. SASAJIMA, K. ISHIJIMA, andT. FUJISHIRO, “Behavior of High Burnup PWR Fuel UnderSimulated RIA Conditions in the NSRR,”Proc. CSNI Special-ist Mtg. Transient Behavior of High Burnup Fuel, Cadarache,

France, September 12–14, 1995, NEA0CSNI0R~95!22, p. 59,Nuclear Energy Agency~1996!.

2. T. FUKETA, F. NAGASE, K. ISHIJIMA, and T. FUJISH-IRO, “NSRR0RIAExperiments with High-Burnup PWR Fuels,”Nucl. Safety, 37, 328~1996!.

3. T. FUKETA, H. SASAJIMA, Y. MORI, K. HOMMA, S.TANZAWA, K. ISHIJIMA, S. KOBAYASHI, H. KAMATA, andH. SAKAI, “Behavior of Pre-irradiated Fuel Under a Simu-lated RIA Condition,” JAERI-Research 95-078, Japan AtomicEnergy Research Institute~1995!.

4. T. FUKETA, F. NAGASE, T. NAKAMURA, H. UETSUKA,and K. ISHIJIMA, “NSRR Pulse Irradiation Experiments andTube Burst Tests,”Proc. 26th Water Reactor Safety Informa-tion Mtg., Bethesda, Maryland, October 26–28, 1998, NUREG0CP-0166, Vol. 3, p. 223, U.S. Nuclear Regulatory Commission~1999!.

5. T. SUGIYAMA, T. FUKETA, and K. ISHIJIMA, “Mechan-ical Energy Generation During High Burnup Fuel Failure Un-der Reactivity Initiated Accident Conditions,”Proc. 7th Int.Conf. on Nuclear Engineering (ICONE-7), ICONE-7070, To-kyo, Japan, April 19–23, 1999~1999!.

6. F. SCHMITZ and J. PAPIN, “High Burnup Effects on FuelBehavior Under Accident Conditions: The Tests CABRI REP-Na,” Nucl. Mater., 270, 55 ~1999!.

7. F. SCHMITZ and J. PAPIN, “REP-Na 10, Another RIA Testwith a Spalled High Burnup Rod and with a Pulse Width of30 ms,”Proc. 26th Water Reactor Safety Information Mtg., Be-thesda, Maryland, October 26–28, 1998, NUREG0CP-0166,Vol. 3, p. 243, U.S. Nuclear Regulatory Commission~1999!.

8. T. NAKAMURA, A. HIDAKA, K. HASHIMOTO, Y.HARADA, Y. NISHINO, H. KANAZAWA, H. UETSUKA,and J. SUGIMOTO, “Research Program~VEGA! on the Fis-sion Product Release from Irradiated Fuel,” JAERI-Research99-036, Japan Atomic Energy Research Institute~1999!.

9. T. FUKETA, T. NAKAMURA, and K. ISHIJIMA, “The Sta-tus of the RIA Test Program in the NSRR,”Proc. 25th WaterReactor Safety Information Mtg., Bethesda, Maryland, Octo-ber 20–22, 1997, NUREG0CP-0162, Vol. 2, p. 179, U.S. Nu-clear Regulatory Commission~1998!.

Toyoshi Fuketa ~BS, 1982; MS, 1984; and PhD, 1987, mechanical engi-neering science, Tokyo Institute of Technology, Japan! is a principal engineer ofthe Fuel Safety Research Laboratory at Japan Atomic Energy Research Institute~JAERI!. He has been involved in the Nuclear Safety Research Reactor~NSRR!project to study modes and consequences of failure of light water reactors andresearch reactor fuels. His research interests include fuel-coolant interactions,fuel failure mechanisms, and transient fission gas behavior.

Hideo Sasajima~BS, applied chemistry, Kanagawa University, Japan, 1982!is a senior engineer of the Fuel Safety Research Laboratory at JAERI. He hasexperience in fuel behavior study in two in-pile experiment programs in the



Japan Materials Testing Reactor and NSRR. His current research interests in-clude transient fission gas release from UO2 and mixed-oxide fuels.

Tomoyuki Sugiyama~BS, 1991; MS, 1993; and PhD, 1996, mechanical en-gineering, Tokyo Institute of Technology, Japan! is a research engineer of theFuel Safety Research Laboratory at JAERI. He has been engaged in the NSRRproject since 1996. In the project, he has performed modeling and separate-effects experiments regarding thermal interaction of solid fuel particles withcoolant.



PWR_TK-Fuketa-NT133-2001

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