RELAP4 analysis of data from loca experiments in the fix loop. Case C13-263, a top break experiment with 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 dryout. The present report refers to case C13-263. The transient was initiated by a top break of diameter 29 mm together with power and pump speed decrease. Two complete transients (0-20 sec) were calculated by RELAP. From 0 to 12 sec the first one, using the contraction coefficient 1, gave a break flow that was on the average 60 % above the measured flow. The other one, using the coefficient 0.7, gave a much better agreement with the measurements in that time region. From 12 to 20 sec there was a considerable overprediction of the break flow - also when the low coefficient value was used. His time region is, however, of less importance as dryout and other striking events occur early in the transient. The break flow results just igven were estimated from the level in the blowdown tank. As in other cases there was flashing in the gap region surrounding the bundle as well as in the bottom region of the loop before the bundle. Times at the onset of these events - and of dryout - are given. All in all there is a fairly good agreement between calculated and measured times. In particular the time at the onset of dryout is well calculated for the 0.7-case, which also gave the best calculation of the break flow and also of the system pressure during the first few seconds of the transient. The maximum increase of the rod surface temperature after dryout is about 25 deg C in the experiment. The corresponding result from RELAP for the 0.7-case is more than 100 deg C. A comparison seems relevant for these cases in view of practically equal times for onset of dryout. So there is probably a conservative trend in RELAP concerning the dryout situation. (author)