Assessment of similarity of HFBR (High Flux Beam Reactor) with separate effects test

RELAP5/MOD2. 5 analysis of the HFBR (High Flux Beam Reactor) for a loss of power and coolant accident

“…It should be noted that the void fraction at the flow reversal point actually goes up to about 80%; however, the transient was characteristically fast and was difficult to capture on the plot file. This behavior is consistent with the results from the scaling study [Rohatgi and Slovik, 1990] which determined the flow reversal event would occur on the order of 0.04 s once a void was generated in a channel. The results from the fuel temperature predictions show that the temperatures would return to the operating conditions during a flow reversal and would not be a challenge to the fuel integrity.…”

“…These values straddle the highest level of safe flow reversal and the lowest CHF limit. This series of experiments has been shown to be conservative [Rohatgi and Slovik, 1990]…”

“…These values straddle the highest level of safe flow reversal and the lowest CHF limit. This series of experiments has been shown to be conservative [Rohatgi and Slovik, 1990]. A further margin of safety is used since the HFBR staff has selected the safe flow reversal criteria of 1.472E5 W/m 2 (46,700 3TU/hr-ft 2 ) as the safety limit although experimentally this heat flux did not go into CHF.…”

“…The CHF correlation was for natural convection in a vertical rectangular channel submerged in saturated liquid. This is the phenomenon of concern since it has been determined [Rohatgi and Slovik, 1990] that the flow reversal process occurs under 40 ms. Thus, the flow coast down phase does not play a dominant role in predicting CHF.…”

“…A separate effects test was performed in [Tichler, 1963] to demonstrate the thermal hydraulic behavior of the HFBR during a flow reversal event and to determine the heat flux limits (i.e., CHF conditions) where flow reversal can safely be experienced. The test facility has been recently reassessed [Rohatgi and Slovik, 1990] for similarity to the HFBR and it was concluded that it was more difficult to achieve flow reversal in the test than in the HFBR core. The test data demonstrated that a safe flow reversal (i.e., no CHF) would occur at 1.47xl0 B W/m 2 (46,700 Btu/hr-ft2) and burnout would occur at 1.51 x 10 s W/m2 (48,000 Btu/hr-ft2).…”