Development of a method for solving downcomer boiling issues in RELAP5/MOD3.2

Abstract

Experimental study and RELAP5 analysis have been performed to investigate boiling phenomena in the downcomer with Direct Vessel Injection (DVI), which is a new Safety Injection System (SIS) of Advanced Power Reactor 1400 (APR1400). From the experiment, we visually observe the vapor jetting near the heated wall with small bubble migration to the bulk region and liquid circulation. Also, RELAP5 calculations using different nodal schemes have been carried out to identify the problem of downcomer boiling in RELAP5/MOD3.2. The calculation results are compared with experimental data in aspects of water level, void fraction, wall temperatures and phase velocities. The double-nodal scheme with top-bottom connections with 4cm-gap near heated wall produces best results among several nodal scheme including the radial single-nodal scheme and the double-nodal scheme with all radial connections. TRAC analysis has been performed to identify 1-D limitation of RELAP5/MOD3.2 and the calculation results are compared with experimental ones and RELAP5/MOD3.2 results. TRAC predicts very high vapor velocity than the experimental data. The proposed nodal scheme, the double-nodal scheme with top-bottom connections with 4cm-gap near heated wall, is applied to the LOCA analysis of APR1400 to solve the downcomer boiling problem produced in RELAP5/MOD3.2. The analysis results of the proposed nodal scheme are compared with those of single-nodal scheme and other nodal schemes in terms of downcomer collapsed level, mass flow of broken coldleg, core collapsed level, and void fraction. As a result, the single-nodal scheme analysis has the same problem of the experiment in collapsed level, mass flowrate. The proposed nodal scheme shows no reheating in core during reflood phase while the 1-D nodal scheme does. When we perform simulations with the reduction of SI flow and containment pressure, each reduction by 5% does not result in the reheating.