(A) study on the dynamic simulation of the forced circulation radwaste evaporating system

Abstract

Main operating problems of a forced circulation radwaste evaporating system in a PWR power plant are entrainment carryover into distillate and poor control against disturbances. The study focused to describe the mass and energy balance and the process control between and among major components of the given system, and to describe the cause and removal mechanism of entrainment carryover, as well as to understand, using case studies, the process and the extent of their effects and finally intended to derive some recommendation which may be useful to enhance the system performance. Entrainment carryover into distillate is essentially estimated with three correlations; Kister et. al.``s correlation for sieve tray operating in spray regime; Carpenter-Othmer``s empirical correlation for wire-mesh pad; and author``s correlation for disengaging height in the vapor body. Controllers such as P, PI, PID and fuzzy heuristic are also modeled. DYFLO, a dynamic simulation package of sequential modular approach is adopted to solve the relevant differential and algebraic equations. Decontamination factor or the quality of distillate depends largely on the vapor flow rate in the sieve tray column. Therefore, in order to sustain the design DF of 1,000, the step increase of steam supply, the main manipulated variable, shall be kept within 10% of the rated flow rate. Sieve tray acts merely as an entrainment remover, which was initially designed to adsorb some volatile material such as iodine. Therefore, it seems feasible to think of alternatives which are less sensitive to the vapor flow rate. One of such an alternative is to have two wire-mesh pads in series. All controllers bring the system to new steady states when the steam flow is either step-increased or sinusoidally changed. But, the PI control allows the vapor body level and the feed flow to oscillate 60 minutes or so. Thus PI control requires more frequent open and closed action of the feed flow control valve. Liqu...