Patent Number: 053234308
Section: description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION In a vessel 1 (FIG. 1) of the Dodewaard nuclear reactor in the Netherlands, provided with an inlet nozzle 2 for water and an outlet nozzle 3 for steam, is provided a reactor core 4, in which first control means or rods 5 are movable. In the vessel 1 there is formed an interface I between a liquid phase (L) and steam or gas phase (S), which may be rather turbulent. The interface I is far above the core 4 and above the chimney 8. Secured to the inner wall of the vessel 1 is a gauge 6 in which sensors are provided which are connected to the outside through a nozzle 7 in the top of the vessel. Inside the gauge 6, BICOTH sensor wires are provided according to the arrangement and design of FIG. 2A. For further details of the sensors, see Ara et al.'s U.S. Pat. No. 4,423,639. Further, low level sensor wires are provided for responding to very low levels of the water in the reactor. Inside the water level gauge 6, there exists an interface between liquid and steam phase, which corresponds accurately to the collapsed level in the vessel. When the interface is situated between two thermocouple junctions of a BICOTH sensor there will be a different temperature at the uppermost thermocouple junction of the pair than at the undermost thermocouple junction as a result of poor heat transfer to the surrounding steam area. This temperature difference produces a positive voltage output of the sensor in situations wherein the interface is between two junctions of a thermocouple pair, or a zero voltage output wherein it is under or above two such junctions. The sensor output signals are supplied to a signal processing and conditioning unit. FIG. 2A shows a BICOTH arrangement and design for the measuring of levels of the water-steam interface between +70 cm and -40 cm relative to a zero level which is situated exactly 55 cm above the top of the chimney. In this arrangement of FIG. 2A, eight BICOTH sensors are used. In another embodiment of the present invention according to FIG. 4, a sensor known as a TRICOTH sensor is used in which a central heater wire 21 is insulated by an insulating sheet 22 from a heater sheet 23, outwardly of which eight wires 24 are uniformly embedded in a sheet 25 of Al.sub.2 O.sub.3, which is enclosed by a cladding 26. In this embodiment the heating occurs uniformly to the eight wires such that they will all obtain substantially the same amount of heat. This arrangement also avoids electrical interference between the heater wire and the sensor lines. Apart from a common wire 24 of Alumel and a wire tc1 of Chromel, six of the wires 24 each comprise three thermocouple materials, such that with the arrangement of FIG. 5, as can be seen from FIG. 6, 23 different levels can be measured. The wire tc1 is used just to measure water temperature, forming a thermocouple at the bottom of FIG. 5 with the common return wire com. The wires 1a, 1b, 2a, com, tc1, 2b, 3a, 3b are the eight wires 24 in FIG. 5. At the bottom, the eight wires are interconnected. Each crossbar on the wires in FIG. 5 represents one of the above-mentioned thermocouple junctions. The table at the right in FIG. 6 indicates that 23 levels are uniquely distinguishable by the ternary code structure at the left in FIG. 6. The TRICOTH sensor is stand-alone; only one sensor needs to be used, having 6 ternary coding signal lines, as shown in FIGS. 4, 5 and 6. It uses three types of thermocouple materials, alumel, constantan, and chromel (FIG. 5), and produces positive signals, active zero signals, and negative signals in each sensor line, such that a given water level on the outside of the TRICOTH sensor generates uniquely a 6-digit ternary code (e.g., level 16 generates the code -+-0-0 as shown in FIG. 6). The TRICOTH sensor has a central coaxial heater which is the heat source over the length of the sensor and which is a central component for the generation of the sensor codes. Compared to the BICOTH system consisting of 8 different sensors, the stand-alone TRICOTH sensor has advantages of simplicity as well as digital and analog precision. This is explained as follows. Consider the TRICOTH system in an upright position, the water level at interval 16 (see FIG. 6), and the central heater switched on. Assume a stepwise axial temperature transition inside the sensor at the position of the water level, i.e., a lower temperature, almost the water temperature, below the water level, and a higher temperature (20 degrees C. higher for example) above the water level. Then, only the ternary code -+-0-0 is produced. However, due to axial conduction of heat, a smooth temperature transition curve exists inside the sensor instead of a stepwise transition. This temperature transition function moves with the water level. Therefore, analog signal transition functions (FIGS. 7 and 8) are created in the sensor lines. When the thermocouple junction of a sensor line is in range of the transition function, then the normalized amplitude of the sensor signal is an accurate analog measure of the position of the water level relative to that junction. Therefore, for all level intervals, the combination of ternary codes and analog signal information provides unique and continuous water level readings over the whole measuring range of the hybrid digital-analog TRICOTH sensor.