RELAP4 analysis of data from loca experiments in the fix loop. Case F6-267, a pump trip experiment with dryout

The work presented is part of a series of calculations eith RELAP 4, Mod3, Best Estimate Version, for the analysis of four experimental cases in the FIX loop. The present report refers to case F6-267. The transient was initiated by pump trip together with power decrease. The cooling system, attached to the main loop containing the rod bundle, was kept fully open up to 2 seconds transient time, then fully closed to 3.6 sec, then fully open to about 8 sec, to be gradually closed after that time. The RELAP results are in remarkably good agreement with experimental data up to 8 seconds transient time. Above that time the comparison is irrelevant because of incomplete knowledge of the coolant flow. However, the main events, including dryout, occurred before 8 seconds so that the latter part of the transient is not very important anyway. In this case there was no reason to study various choking options or contraction coefficients as the valves to the blowdown system remained closed. Only one complete RELAP transient was therefore carried out. The calculated and experimental times for the onset of dryout agree well. One should also notice that the dryout test in this case is uninfluenced by gap and bottom flashing, which both occur after dryout. The agreement between calculated and experimental times for the onset of dryout suggests that the GE dryout correlation is useful and that its main variable, the steam quality in the bundle, is well calculated. Some indirect support as to the calculated steam quality is given in this case also by the good agreement for the mass flow into the bundle and for the pressure drop along the bundle. - The maximum temperature increase for the rod surface was about 100 deg C in the calculation and 60 deg C in the experiment. This result gives an indication (but a weak one) of conservative behaviour of RELAP. (author)