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

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