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

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