RELAP4 analysis of data from loca experiments in the fix loop. Case C12-245, a top break experiment without dryout

The work presented is part of a series of calculations with RELAP4, Mod 3, Best Estimate Version, for the analysis of four experimental cases in the FIX loop: The primary objective was to study the conditions around fryout. The main variable in this context was the rod surface temperature, which showed a steep rise when dryout occurred. Other variables such as pressure and mass flow were also examined. The present report refers to case C12-245. The transient was initiated by a top break of diameter 24 mm together with power and pump speed decrease. Three complete transients (0-20 sec) were calculated by RELAP. The first one, using a contraction coefficient 1, gave on the average a break flow about 35 % above the measured flow. This estimate is based on the level in the blowdown tank. The other two, using the coefficient 0.7 (the last one having also 3.6 deg C lower gap volum temperature), gave a much better calculation of the break flow. The most striking feature in the RELAP results was the presence of dryout - in contrast to the results from the measurements. However, it should be observed that the experiment was not far from dryout, because a change of the break diameter from 24 to 29 mm gave dryout. (author)