NUREG/IA-0169 "Analysis of KS-1 Experimental Data on the Behavior of the Heated Rod Temperatures in the Partially Uncovered VVER Core Model Using RELAP5/MOD3.2

This report has been prepared as a part of the Agreement on Research Participation and Technical Exchange under the International Code Application and Maintenance Program. KS-i Test 35-1 data on the behaviour of the heated rod temperatures in the partially uncovered VVER Core model were simulated with RELAP5/MOD3.2 to assess the code, especially its non equilibrium (unequal phase temperatures) heat transfer models for modeling phenomena in partially uncovered core under Small Break LOCA conditions. The test has been carried out at experimental section KS-I of the test facility KS (RRC KI) in 1991. KS-1 experimental section (VVER Loop model) includes models of all main elements of VVER type reactor, loop hot leg model and cold leg simulator, and also horizontal SG tube bundle simulator with passive heat removal. Core model consists of 19 electrically heated rod simulators with diameter 9 mm and height 2.5 m. Test 35-1 models thermal and hydraulic processes during reflux condenser mode in primary circuit with low mixture level in partially uncovered VVER core under conditions of small residual heat power, middle pressure and counter current flow in the core. First a study of the effect of the hydraulic nodalization to the code calculations was performed using different number of hydraulic volumes for Core model. After the choice of proper nodalization and maximum user-specified time step, base case calculations were done for the test. The differences between code predictions for behavior of rod simulator temperatures along the height of Core model and test data are described and analyzed. Sensitivity studies were carried out to investigate the effects of modeling on the behavior of the rod simulator temperatures along the height of Core model.