Conversion of the WSU reactor to a mixed core of TRIGA- FLIP and ...

3.2. THE FLIP FUEL EXPERIENCE AT WASHINGTON STATE UNIVERSITYThomas A. Lovas, Washington State UniversityConversion of the WSU reactor to a mixed core of TRIGA-FLIP and TRIGA-Standard fuel was undertaken in February, 1976.The reactor is of General Electric design and originally operatedwith MTR-type fuel at a maximum power of 100 Kilowatts.In 1967, the power output was increased to 1 Megawatt andpulsing capability was added by a conversion to TRIGA 20%enriched fuel in four-rod fuel bundles and installation of anadapted transient rod system. The General Electric reactivitycontrol system consisting of three boral control blades andone stainless steel servo blade was retained, as were the gridplate and core support structure. The conversion in 1976 to70% enriched, 1.5% Erbium loaded FLIP fuel achieved the objecrivesof excess reactivity restoration and improved fluxlevels in experimental facilities,.Associated with the FLIP loading, three major modificationswere made to the control and instrumentation systems.The first was the addition of a fuel temperature scram systemas required by the Technical Specifications for operation withFLIP fuel. The scram system is based on a Calex Voltsensorfed from Acromag Thermocouple transmitters, providing bothfast and slow scram signals. The second modification was ashortening of the shafts for the boral control elements. Theoriginal MTR fuel had, an active fuel, length of 24 inches andthe control elements were installed to provide a 0.5 inch fuelmeat overlap. Conversion to TRIGA fuel with a 15 inch activelength and nearly the same core centerline position resultedin an excessive element overlap of 5.0 inches. In order toremove the negative reactivity addition when elements areinitially withdrawn, the shafts were shortened by 4 inches,providing a 1 inch overlap. Subsequent element calibrationsrevealed that the reactivity characteristics over the firstfew inches of blade travel were significantly improved.The third modification was the replacement of the transientrod hold-down device. The hold-down device supplied byGeneral Atomic in 1967 had an inside diameter above the rod3-21

travel region of 1.25 inches, the same value as the outsidediameter of the transient control element. A new hold-downdevice was machined by General Atomic with an inside diameterthrough the top portion of 1.3125 inches, allowing the controlelement to be fully withdrawn and removed for inspection withoutcomplete disassembly of the guide tube and hold-down system.Fuel loading commenced February 9, with installation ofthe three-rod cluster with guide tube for the transient rodand installation of two clusters containing instrumented fuelelements. After operational tests of the transient rod systemand calibration of the fuel temperature systems, loading tocritical was continued until criticality was obtained with aloading of 12 Standard and 9 FLIP elements. Data collectedduring the approach to critical experiment is shown in Figure1. Additional elements were then added to achieve a fullyoperational core loading of 9 FLIP and 19 Standard elementsfor a total of 35 FLIP rods and 75 Standard rods. Figure 2describes the fully loaded core including experimental facilities.Upon completion of control element calibrations, it wasverified that core excess reactivity was $7.98 with a shutdownmargin of $2.53. The transient rod worth was determinedas $3.45 over the fifteen inches of travel. No adjustmentwas made of the transient element to limit the travel andreactivity worth.Full power tests included measurement of core temperatures,excess reactivity changes, temperature and void coefficients.The relationship between fuel temperature and powerlevel for the mixed core is shown in Figure 3, and the peakindicated fuel temperature at 1 MW during initial full poweroperation was 349°C. The power coefficient of the TRIGA-FLIPmixed core without Samarium in the FLIP region and no Xenonwas determined as $2.45 to 1 MW. Figure 4 was developed inthe course of a determination of the temperature coefficient3-22

of reactivity, with the coefficient ranging from a value of-$0.005/°C at low power levels to a value of -$0.0136/°C atnear peak steady state power levels.The observed value of-$0.0136/°C is quite near the expected temperature coefficientof -$0.014/°C derived in the Safety Analysis Report for themixed core conversion.The void coefficient of reactivity was measured in thewet tube location of Core Position C-7.Results indicated anapproximate core void coefficient of reactivity of -$1.42 x10" 2 /7 o void.A summary of the significant core parameters before andafter the conversion to a mixed core is given in Table 1.characteristics of the mixed core all compared quite favorablywith the expected values given in the Safety Analysis Report.During initial full-power operation it was observed thatfluctuations of approximately 2% occurred in power level indicationsat power levels between 900 KW and 1 MW.Mean powerlevel and core temperatures maintained quite constant withoutmanual or automatic adjustment of reactivity control systems.The power fluctuations have been extensively investigated atother facilities fueled with mixed FLIP and Standard and all-FLIP cores with the conclusion that the disturbance is due to2voiding effects and poses no operational hazards.The results of pulsing tests completed between March 4and March 31 for a variety of pulsed reactivity insertions aregiven in Table 2.Figures 5 and 6 describe the pulse outputand fuel temperatures from a series of pulses performed March15.The representative pulse data shown in Figures 5 and 6indicate a change in the slope of each function in the neighborhoodof a pulsed reactivity insertion of $2.00.It ishypothesized that the inflection in the curves reflects thecomplexity of temperature, prompt neutron lifetime and promptnegative temperature coefficient relationships in a mixed coreduring pulses of insertion levels greater than $2.00.TheBubble.3-23

formation in the fuel region, particularly in the contiguousblock of FLIP fuel, is much more pronounced at insertionsabove $2.00. The formation of a greater quantity of bubblesis indicative of boiling at the surface of the fuel claddingand represents a possible shift in the heat transfer modeaway from natural convection. The heat transfer characteristicsunder pulsing conditions vary significantly from thesteady state case and are particularly complex in the analysisof an inhomogenously fueled core.A second aspect of the pulsing performance with mixedFLIP and Standard fuel is the occurrence of power oscillationsin the 1 to 4 MW range on the tail end of reactor pulses. The3occurrence of such oscillations has been reported previouslyin mixed cores and the WSU experience parallels the reportedfindings.The phenomenon may be identified as occurring only afterpulsed reactivity insertions of between $2,00 and $2.50, thelicense limit. Figure 7 represents the post-pulse poweroscillations from data collected by a photomultiplier tube andoscilloscope camera recording the tail of a $2.50 pulse. Thefirst oscillation in the pulse tail occurs approximately 0,4seconds after the pulse peak is attained and is generallyrepeated one or more times by rapid oscillations followed bya final rise approximately 0.7 seconds after the initial.Since the oscillations only occur in the pulse tail and atsuch a low power level relative to the pulse peak, no indicationof the oscillations can be obtained from the recordingoscillograph routinely utilized in the pulsing mode for peakpower and energy release measurement.The characteristics of the post-pulse oscillations indicatethat the oscillations are the result of voiding effectswithin the FLIP region associated with film boiling duringhigh reactivity, temperature, and power level pulses. Theoscillations do not appear to be correlated with the peakvalue characteristics described above.3-24

The thermal flux levels in the most commonly used experimentalfacilities increased between 257o and 307 o as a resultof the conversion. Measurements made with a self-powered neutrondetector indicated a peak flux of ~8.5 x 1012nv in thevertical wet tube located in Grid Position D-8. Measurementsin the same position prior to the core change revealed a level12of ^6.0 x 10 nv at the peak. The improvement of the experimentalfacility flux level met the anticipations from preconversioncalculations and analysis.Operation of the reactor since initial startup with themixed core has totalled only forty megawatt-days. The rate ofexcess reactivity loss over this operating period has beencalculated at $0,011 per megawatt day, reflecting the accumulationof primary poisons in the FLIP fuel. More data fromcontinuing operations will be necessary to determine an equilibriumrate of excess loss.In summary, the observed performance of the mixed corehas been quite satisfactory and analysis has indicated nosignificant deviations from the design predictions and specifications.No major operational problems have developed sincethe conversion was completed.3-25

Table 1:WSU Reactor CharacteristicsAll-Stam dardMixed FLIP-StdExcess reactivityShutdown marginPower coefficientPrompt neutron lifetimeTemperature coefficient(per °C)Void coefficient(per % void)Pulse peak power($2.50 addition)Maximum indicatedfuel temperature($2.50 addition)Available thermal flux(axial average nv, D-8)$6.03 $7.98$5.54 $2.53$3.20 $2.4539 usee 28 usee-$1.71 x io- 2 -$1.36 x 10" 2-$1.31 x 10" 2 -$1.42 x 10" 2970 MW 1850 MW347°C 449°C5.6 x 10 12 6.17 x 10 12Table 2: Pulse Data, 3/4-31/76p, $ No. of PulsesAvg. PeakFuel Temp., CAvg. PeakPower, MW1.10 2 235.5 7.111.15 1 248.0 16.591.25 3 271.7 60.001.50 6 309.3 155.301.75 4 333.8 304.802.00 3 367.3 622.302.25 3 403.0 1197.302.50 11 442.4 1850.503-26

1.0Figure 1:Approach to Critical Experiment Core 30AInverse multiplication vs. fuel rod addition+->03i.0.8-0.6-C3Oo .0.4 _Blades in0.2BladesoutAB1CIC#2/T20"T40 60Fuel Rods InstalledFigure 2:WSU TRIGA Core Layout2 3 4 5 6 7 R/ / / / / /S s s S SC S S sir F F - IDEFG/s\ )9-ission>hmbrS S F IIF JJ F so 0s S F F F s/7S S S S s/GammaCICChmbr © /m / / / / #1*Control Element"T80TSm©mInstrumentedJT|wlRotatorTube100FliD orStandardFuel BundleReflectorElementRabbitNeutronSourceFuel RodTransient RodWet Tube3-27

Figure 3:Fuel Temperature vs.Power Levelf T —I-200 400 600Power, KW8001000Figure 4:Average Temperature Coefficient vs,Average Core Temperature"TOO 200Average Fuel Temoerature, °Cm3-28

2000500-Figure 5:Peak Indicated FuelTemp vs. Reactivity Insertion301Figure 6:Peak Power and EnergyRelease vs. ReactivityInsertion* 1500-aOJIN3«3oo1)5400'S-

2.4-1Figure 7:Post-Pulse Power Oscillationsu>Ios-cu3:oQ_S_o-t->ures

References1. Amendment I_ tc> Safety Analysis Report of October, 1966.Washington State University, Nuclear Radiation Center,Pullman, Washington. May, 1974.2. Randall, J. D. , e_t al. , "A Study of Power Fluctuationsin a FLIP Fuel Reactor using the Technique of NoiseAnalysis," Texas A&M University, in Papers andAbstracts, TRIGA Owners' Conference III, 1974.3. Schumacher, R. F., et al., "Post-Pulse Power Oscillationsin a 35 FLIP Element, Mixed TRIGA Core," TRIGAOwners' Conference IV, 1976.3-31