RELAP4 - analysis of data from loca experiments in the fix loop. Case D6-259, a bottom break experiment without dryout

The work presented is part of a series of calculations with RELAP4, Mod3, 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 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 D6-259. The transient was initiated by a bottom break of diameter 7 mm together with power and pump speed decrease. In addition a safety valve (diam. 15 mm) in the top was open for about five seconds during the first part of the transient. Total transient time was 20 seconds. Two main RELAP calculations were performed. In addition a lot of auxiliary REALAP tests were done for this particular case. Table choking (Moody) gave a break flow (subcooled) of 1.06 kg/s whereas the momentum flux equation gave 3.7 kg/s. Neither value agrees with the measured result, 2.7 kg/s. The flow of saturated steam through the safety valve was 1.9 kg/s with sonic choking and 2.4 kg/s with Moody choking. In the latter case there were also water droplets in the medium. The calculated results agree well with the measured result 2.1 kg/s. In spite of the disagreement for the break flow - and partly due to its smallness, several parameters inside the loop were well calculated by RELAP, especially with the momentum equation (and droplets in the steam dome). The mass flow, for example, was correct to within 10 % for most of the transient. It should be noted also that dryout was absent in the calculations as well as in the measurements. (author)