Design and performance testing of a PC-based control system for nuclear waste vitrification

Abstract

The high computing and informationprocessing power of a General Purpose Digital Computer (GPDC) in the form of a Personal Computer (PC) has become available throughout the world, leading to the emergence of the PC as a very powerful and economical tool in computing applications. Simultaneously, the availability of industrialgrade versions of the PC and PC addon cards for several important I/O applications has made it popular in processcontrol applications. In the work reported here, introduction of digital controls using a Personal Computer for Nuclear Waste Vitrification by the PotGlass Process in a multizone induction furnace has been studied. For this purpose, a simplified process model has been developed; the control problem has been formulated; and the control algorithms have been consolidated into a working flowsheet. A laboratoryscale PCbased control system working in supervisory mode has been designed and assembled. Software for implementing control algorithms and other control functions has been developed. The control system has been tested under simulated laboratory conditions. The simplified process model has been developed based on material balance on the process pot and offgas pot condenser considered as one system. Based on this model, operational experience, and empirical relations, the control algorithms have been consolidated in the form of a working flowsheet and pseudocodes. The PCbased control system consists of a Personal Computer, an active dataacquisition card for multiplexing, thermal sensing, digitaltoanalog conversion, and relays. The open/short status of contacts and analog data from feedsafety checking lines are fed to the dataacquisition board after amplification. Relays are used to switch furnacepower lines and feedflow lines, while the digitaltoanalogconverted (DAC) output is used to change setpoints in the furnacepower lines and feedflow lines. Software has been implemented using GWBASIC as a task under DOS. It has been implemented as a foreground job that initiates a TIMERinterrupted background job. The foreground job accepts initial setup information and oversees, monitors, and guides the process advance in real time until the end of the process campaign or until some safety violation forces termination of the process advance. The background job performs data acquisition, safety checks, raises a software flag TRIP = 1 in case of a safety violation, and takes corrective action in such cases. The frequency of the background job is determined by the TIMER argument. The control system has been tested under simulated laboratory conditions by checking:

correctness of acquired data and analog output data, correctness of switching operations, raising of the TRIP flag to 1, and taking corrective actions in case of safety violation.

The study concludes that digital controls using Personal Computer–based control systems are highly suited to controlling complicated processes like vitrification. The study has also demonstrated that complex control schemes can be implemented using inexpensive and locally available electronic hardware.