Non-equilibrium two-phase flow simulation of an open feed water heater in the HPLWR power plant

Abstract

In 1996 the U.S Nuclear Regulatory Commission issued a notice that addressed the operation of pressurised water reactors (PWR’s) above the licensed power limit due to a decrease in the feed water temperature which may also affect the measuring accuracy of nuclear instrumentation. The designed feed water operating temperature range may be inaccurate due to various assumptions made, including that of an equilibrium steady state approach which represents the conditions of the open feed water heater (OFWH). The aim of the study is to simulate the open feed water heater for application in a high-performance light water reactor (HPLWR), using a non-equilibrium two-phase flow approach. The main reason is to evaluate the potential improvement in accuracy when predicting operating conditions compared to an equilibrium steady state approach. The simulation developed consists of three integrated sub-models used to evaluate the heat and mass transfer of the open feed water heater of the HPLWR power plant. The first sub-model is a feed water preparation model in which various streams entering the OFWH are premixed, the properties of the prepared feed water serve as the initial boundary conditions for the second sub-model. The second sub-model is a detailed two-phase flow model for the bubble formation and ascension. The model includes the formation of a bubble at an orifice due to superheated steam being injected into premixed saturated water. The ascension of the bubble is then simulated by tracking the heat and mass being transferred from the bubble to the water body as it condenses and depletes. In the third submodel, the heat and mass are transferred from a multiple bubble column to the premixed feed water. The nonequilibrium simulation model can furthermore be used to analyse transient effects by incorporating events such as mass flow and temperature variations. The results gained from the non-equilibrium integrated transient simulation model of the OFWH were then compared to a steady state model that was based on the OFWH as designed by Lemasson for the HPLWR power plant. The non-equilibrium integrated transient approach delivered similar results to that of the equilibrium steady state analysis. The energy transferred from the steam to the bulk liquid was predicted within 90% accuracy and the outlet temperature with an error 0.21%