Simulation of a strongly asymmetric ''loss of load'' event, using the coupled 3D code R/P/C

Framatome-ANP has developed S-RELAP5, a RELAP5/Mod2 based thermal hydraulic code, and PANBOX, a 3D core kinetics code. By coupling both codes, a powerful neutronic and thermal hydraulic plant model was developed, which is capable of calculating extremely complex transients, particularly events bearing strongly asymmetric phenomena. The capability of the code system has been tested by recalculation of several transients that occurred in Siemens built PWRs. The most complex transient was a loss of load combined with a temporary coastdown of one main coolant pump, which is presented here. Since the measured values from the data recording system of the plant were available, the calculation could be compared to measured parameters. The key phenomenon of the transient is a highly asymmetrical neutronic condition, which was caused by: the drop of 5 control rod pairs in an asymmetric pattern upon detection of ''loss of load''. the coastdown of one man coolant pump, due to failure to connect to the auxiliary bus, which allowed coolant in one loop to stagnante and cool. Subsequent reactivation of that pump forced a plug of cold water into one side of the core. The physical progress of the transient is strongly dependent on this double asymmetry; therefore, a 3-D calculation is indispensable for an accurate simulation. The calculated results are in good agreement with measurements and represent an important contribution to code validation. (author)