3.2. THE FLIP FUEL EXPERIENCE AT WASHINGTON STATE UNIVERSITY Thomas A. Lovas, Washington State University

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1 3.2. THE FLIP FUEL EXPERIENCE AT WASHINGTON STATE UNIVERSITY Thmas A. Lvas, Washingtn State University Cnversin f the WSU reactr t a mixed cre f TRIGA- FLIP and TRIGA-Standard fuel was undertaken in February, The reactr is f General Electric design and riginally perated with MTR-type fuel at a maximum pwer f 100 Kilwatts. In 1967, the pwer utput was increased t 1 Megawatt and pulsing capability was added by a cnversin t TRIGA 20% enriched fuel in fur-rd fuel bundles and installatin f an adapted transient rd system. The General Electric reactivity cntrl system cnsisting f three bral cntrl blades and ne stainless steel serv blade was retained, as were the grid plate and cre supprt structure. The cnversin in 1976 t 70% enriched, 1.5% Erbium laded FLIP fuel achieved the bjecrives f excess reactivity restratin and imprved flux levels in experimental facilities,. Assciated with the FLIP lading, three majr mdificatins were made t the cntrl and instrumentatin systems. The first was the additin f a fuel temperature scram system as required by the Technical Specificatins fr peratin with FLIP fuel. The scram system is based n a Calex Vltsensr fed frm Acrmag Thermcuple transmitters, prviding bth fast and slw scram signals. The secnd mdificatin was a shrtening f the shafts fr the bral cntrl elements. The riginal MTR fuel had, an active fuel, length f 24 inches and the cntrl elements were installed t prvide a 0.5 inch fuel meat verlap. Cnversin t TRIGA fuel with a 15 inch active length and nearly the same cre centerline psitin resulted in an excessive element verlap f 5.0 inches. In rder t remve the negative reactivity additin when elements are initially withdrawn, the shafts were shrtened by 4 inches, prviding a 1 inch verlap. Subsequent element calibratins revealed that the reactivity characteristics ver the first few inches f blade travel were significantly imprved. The third mdificatin was the replacement f the transient rd hld-dwn device. The hld-dwn device supplied by General Atmic in 1967 had an inside diameter abve the rd 3-21

2 travel regin f 1.25 inches, the same value as the utside diameter f the transient cntrl element. A new hld-dwn device was machined by General Atmic with an inside diameter thrugh the tp prtin f inches, allwing the cntrl element t be fully withdrawn and remved fr inspectin withut cmplete disassembly f the guide tube and hld-dwn system. Fuel lading cmmenced February 9, with installatin f the three-rd cluster with guide tube fr the transient rd and installatin f tw clusters cntaining instrumented fuel elements. After peratinal tests f the transient rd system and calibratin f the fuel temperature systems, lading t critical was cntinued until criticality was btained with a lading f 12 Standard and 9 FLIP elements. Data cllected during the apprach t critical experiment is shwn in Figure 1. Additinal elements were then added t achieve a fully peratinal cre lading f 9 FLIP and 19 Standard elements fr a ttal f 35 FLIP rds and 75 Standard rds. Figure 2 describes the fully laded cre including experimental facilities. Upn cmpletin f cntrl element calibratins, it was verified that cre excess reactivity was $7.98 with a shutdwn margin f $2.53. The transient rd wrth was determined as $3.45 ver the fifteen inches f travel. N adjustment was made f the transient element t limit the travel and reactivity wrth. Full pwer tests included measurement f cre temperatures, excess reactivity changes, temperature and vid cefficients. The relatinship between fuel temperature and pwer level fr the mixed cre is shwn in Figure 3, and the peak indicated fuel temperature at 1 MW during initial full pwer peratin was 349 C. The pwer cefficient f the TRIGA-FLIP mixed cre withut Samarium in the FLIP regin and n Xenn was determined as $2.45 t 1 MW. Figure 4 was develped in the curse f a determinatin f the temperature cefficient 3-22

3 f reactivity, with the cefficient ranging frm a value f -$0.005/ C at lw pwer levels t a value f -$0.0136/ C at near peak steady state pwer levels. The bserved value f -$0.0136/ C is quite near the expected temperature cefficient f -$0.014/ C derived in the Safety Analysis Reprt fr the mixed cre cnversin. The vid cefficient f reactivity was measured in the wet tube lcatin f Cre Psitin C-7. Results indicated an apprximate cre vid cefficient f reactivity f -$1.42 x 10" 2 /7 vid. A summary f the significant cre parameters befre and after the cnversin t a mixed cre is given in Table 1. characteristics f the mixed cre all cmpared quite favrably with the expected values given in the Safety Analysis Reprt. During initial full-pwer peratin it was bserved that fluctuatins f apprximately 2% ccurred in pwer level indicatins at pwer levels between 900 KW and 1 MW. Mean pwer level and cre temperatures maintained quite cnstant withut manual r autmatic adjustment f reactivity cntrl systems. The pwer fluctuatins have been extensively investigated at ther facilities fueled with mixed FLIP and Standard and all- FLIP cres with the cnclusin that the disturbance is due t 2 viding effects and pses n peratinal hazards. The results f pulsing tests cmpleted between March 4 and March 31 fr a variety f pulsed reactivity insertins are given in Table 2. Figures 5 and 6 describe the pulse utput and fuel temperatures frm a series f pulses perfrmed March 15. The representative pulse data shwn in Figures 5 and 6 indicate a change in the slpe f each functin in the neighbrhd f a pulsed reactivity insertin f $2.00. It is hypthesized that the inflectin in the curves reflects the cmplexity f temperature, prmpt neutrn lifetime and prmpt negative temperature cefficient relatinships in a mixed cre during pulses f insertin levels greater than $2.00. The Bubble. 3-23

4 frmatin in the fuel regin, particularly in the cntiguus blck f FLIP fuel, is much mre prnunced at insertins abve $2.00. The frmatin f a greater quantity f bubbles is indicative f biling at the surface f the fuel cladding and represents a pssible shift in the heat transfer mde away frm natural cnvectin. The heat transfer characteristics under pulsing cnditins vary significantly frm the steady state case and are particularly cmplex in the analysis f an inhmgenusly fueled cre. A secnd aspect f the pulsing perfrmance with mixed FLIP and Standard fuel is the ccurrence f pwer scillatins in the 1 t 4 MW range n the tail end f reactr pulses. The 3 ccurrence f such scillatins has been reprted previusly in mixed cres and the WSU experience parallels the reprted findings. The phenmenn may be identified as ccurring nly after pulsed reactivity insertins f between $2,00 and $2.50, the license limit. Figure 7 represents the pst-pulse pwer scillatins frm data cllected by a phtmultiplier tube and scillscpe camera recrding the tail f a $2.50 pulse. The first scillatin in the pulse tail ccurs apprximately 0,4 secnds after the pulse peak is attained and is generally repeated ne r mre times by rapid scillatins fllwed by a final rise apprximately 0.7 secnds after the initial. Since the scillatins nly ccur in the pulse tail and at such a lw pwer level relative t the pulse peak, n indicatin f the scillatins can be btained frm the recrding scillgraph rutinely utilized in the pulsing mde fr peak pwer and energy release measurement. The characteristics f the pst-pulse scillatins indicate that the scillatins are the result f viding effects within the FLIP regin assciated with film biling during high reactivity, temperature, and pwer level pulses. The scillatins d nt appear t be crrelated with the peak value characteristics described abve. 3-24

5 The thermal flux levels in the mst cmmnly used experimental facilities increased between 257 and 307 as a result f the cnversin. Measurements made with a self-pwered neutrn detectr indicated a peak flux f ~8.5 x nv in the vertical wet tube lcated in Grid Psitin D-8. Measurements in the same psitin prir t the cre change revealed a level 12 f ^6.0 x 10 nv at the peak. The imprvement f the experimental facility flux level met the anticipatins frm precnversin calculatins and analysis. Operatin f the reactr since initial startup with the mixed cre has ttalled nly frty megawatt-days. The rate f excess reactivity lss ver this perating perid has been calculated at $0,011 per megawatt day, reflecting the accumulatin f primary pisns in the FLIP fuel. Mre data frm cntinuing peratins will be necessary t determine an equilibrium rate f excess lss. In summary, the bserved perfrmance f the mixed cre has been quite satisfactry and analysis has indicated n significant deviatins frm the design predictins and specificatins. N majr peratinal prblems have develped since the cnversin was cmpleted. 3-25

6 Table 1: WSU Reactr Characteristics All-Stam dard Mixed FLIP-Std Excess reactivity Shutdwn margin Pwer cefficient Prmpt neutrn lifetime Temperature cefficient (per C) Vid cefficient (per % vid) Pulse peak pwer ($2.50 additin) Maximum indicated fuel temperature ($2.50 additin) Available thermal flux (axial average nv, D-8) $6.03 $7.98 $5.54 $2.53 $3.20 $ usee 28 usee -$1.71 x i- 2 -$1.36 x 10" 2 -$1.31 x 10" 2 -$1.42 x 10" MW 1850 MW 347 C 449 C 5.6 x x Table 2: Pulse Data, 3/4-31/76 p, $ N. f Pulses Avg. Peak Fuel Temp., C Avg. Peak Pwer, MW

7 1.0 Figure 1: Apprach t Critical Experiment Cre 30A Inverse multiplicatin vs. fuel rd additin +-> 03 i C 3 O.0.4 _ Blades in 0.2 Blades ut A B 1 CIC #2 / T 20 "T Fuel Rds Installed Figure 2: WSU TRIGA Cre Layut R / / / / / / S s s S S C S S sir F F - I D E F G / s \ ) 9 -issin >hmbr S S F IIF JJ F s 0 s S F F F s / 7 S S S S s / Gamma CIC Chmbr / m / / / / #1 * Cntrl Element "T 80 T S m m Instrumented JT wl Rtatr Tube 100 FliD r Standard Fuel Bundle Reflectr Element Rabbit Neutrn Surce Fuel Rd Transient Rd Wet Tube 3-27

8 Figure 3: Fuel Temperature vs. Pwer Level f T I Pwer, KW Figure 4: Average Temperature Cefficient vs, Average Cre Temperature "TOO 200 Average Fuel Temerature, C m 3-28

9 Figure 5: Peak Indicated Fuel Temp vs. Reactivity Insertin 301 Figure 6: Peak Pwer and Energy Release vs. Reactivity Insertin * a OJ I N3 «3 1) 5400' S- <D a. E OJ I CD I/) < <D en «. a> 1000, %. a 10 " \ Reactivity Insertin, $ Reactivity Insertin, $

10 2.4-1 Figure 7: Pst-Pulse Pwer Oscillatins u> I s- cu 3: Q_ S_ -t-> u res <U ad Time, Sec

11 References 1. Amendment I_ tc> Safety Analysis Reprt f Octber, Washingtn State University, Nuclear Radiatin Center, Pullman, Washingtn. May, Randall, J. D., e_t al., "A Study f Pwer Fluctuatins in a FLIP Fuel Reactr using the Technique f Nise Analysis," Texas A&M University, in Papers and Abstracts, TRIGA Owners' Cnference III, Schumacher, R. F., et al., "Pst-Pulse Pwer Oscillatins in a 35 FLIP Element, Mixed TRIGA Cre," TRIGA Owners' Cnference IV,