Method and system for fine control of thermal power of a boiling water

Abstract

A method and a system for fine control of the thermal power of a nuclear boiling water reactor (BWR) with natural circulation. The reactor vessel, in which the core of such a reactor is enclosed, embodies an in-vessel part of a water level gauge containing a water level sensor, such that together with the out-of-vessel part of the system, which is a signal conditioning and processing unit, the effective water level in the vessel can accurately be measured and adjusted either automatically or by hand. The in-vessel part of the system comprises a water level gauge, such as a vertical standpipe, in which the 'steam-collapsed' or 'effective' water level is accurately created. The effective water level is typically defined for a reactor vessel of a normally operating boiling water reactor (BWR), while partly containing liquid water, partly a mixture of steam and water, and partly just steam. A ternary-coding, hybrid digital/analog thermocouple sensor with stand-alone capability is enclosed in the water level gauge, providing ternary-coded (-1, 0, +1) as well as analog (amplitude) signaling, such that by using an appropriate signal conditioning and processing unit, the water level in the gauge, relative to a reference level, is uniquely and very accurately determined. By the beneficial use of the analog information on water level contained in the signal transition curves, in addition to the digital information of the sensor signals, continuous water level readings can be obtained over the whole of a designed water level measuring range, this being independent of reactor water temperature or pressure. This accurate continuous level-measuring system is not limited to use in boiling water reactors, nor to water as a level-forming liquid.