Patent Number: 044407173
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the sensing of liquid levels within a vessel and, in particular, to the sensing of the level of a fluid coolant above the core in a nuclear reactor vessel. When a nuclear reactor starts up from a cold start, the reactor vessel is completely filled with a fluid coolant such as subcooled water. During operation of the nuclear reactor, the fluid coolant is forced through the core to remove the heat generated therein. During normal operaton of a pressurized water reactor, the fluid coolant is subcooled and remains in a liquid state as it passes through the core. During abnormal operation of a pressurized water reactor due to a loss of pressure, the fluid coolant within the reactor vessel may change state to become a two-phase fluid mixture of water and steam. Due to the buoyancy of steam, the water in the volume above the core is particularly susceptible to being displaced by the two-phase fluid. The two-phase fluid in the reactor vessel head may be stagnant or turbulent two-phase flow. The two-phase fluid forms due to a decrease in pressure that results in a portion of the water flashing to steam. The difficulty of measuring the level of coolant above the core is more complex with a two-phase fluid than with a subcooled fluid. A level sensing device must be able to sense the level of liquid coolant regardless of whether the coolant is a subcooled liquid or a two-phase fluid. Prior art instrumentation such as heated thermocouples and differential pressure sensors have provided an ambiguous and sometimes indirect indication coolant inventory above the core. An accurate indication of coolant inventory above the core is an important indication of the core cooling conditions. An accurate indication of the coolant inventory above the core would provide a sound basis for operators to insure appropriate actions are taken to prevent the coolant level from dropping below the top of the core. Therefore, a need exists for an apparatus to provide a direct and accurate indication of the inventory of fluid coolant above the core in nuclear reactor vessels during abnormal operation as well as during normal operation. Such an apparatus would be useful in recognition of low reactor coolant levels and inadequate core cooling. SUMMARY OF THE INVENTION The present invention overcomes the disadvantages of the prior art by providing a direct and accurate indication of the coolant inventory above the core in a nuclear reactor vessel. The present invention also has several additional desirable features. Redundancy in the level measurement is provided in that two identical level measurement systems are utilized with the electrical signals generated by the redundant measurement systems processed by electrically separate channels of a microprocessor based control system. Furthermore, the present invention has no moving parts, no external piping, requires no in-containment electrons and can be retrofitted into existing reactors or reactors under construction. The heated junction thermocouple level measurement system utilizes the difference in heat transfer characteristics between heat transfer to a liquid and heat transfer to a gas or vapor to determine if liquid coolant is present above the core at the level of each sensor. The sensors are located in the upper guide structure of the reactor vessel. Signal processing equipment and display equipment are located outside containment. The heated junction thermocouple level measurement system has eight sensors. Each sensor has a heated thermocouple junction, an unheated thermocouple junction and a heater coil within the same housing; a portion of the housing is enclosed in a splash guard. The heated and unheated thermocouple of each sensor are physically displaced so that heat from the heater coil does not effect the voltage generated by the unheated thermocouple. The heated and unheated thermocouples are wired to provide the absolute temperature of both the thermocouple junctions as well as the differential termperature between the thermocouple junctions. With the unheated junction in the same housing as the heated junction, no additional temperature or pressure compensation is required. When liquid coolant surrounds the housing in the region of both the heated thermocouple and the unheated thermocouple, the temperature of both thermocouples will remain essentially identical as heat produced by the heater coil is transferred from the heater coil through the housing to the surrounding liquid coolant. Since the voltage produced by the heated thermocouple junction opposes the voltage produced by the unheated thermocouple junction, the net voltage output is small when liquid coolant surrounds both the heated thermocouple junction and the unheated thermocouple junction. In the absence of liquid coolant surrounding the housing in the region of the heated thermocouple junction, the heat produced by the heater coil does not transfer as well to the surrounding gaseous coolant causing the heated thermocouple junction temperature to rise above the unheated thermocouple junction temperature and a much larger net voltage output results. This condition indicates the liquid coolant level is below the heated thermocouple junction. A heater controller prevents the heated junction thermocouple from becoming excessively hot by ramping back the voltage to the heater coil when the uncovered heated thermocouple junction temperature reaches some pre-established limit. A portion of each housing is enclosed in a splash guard to prevent liquid coolant from splashing on the housing in the region of the heater coil or running down the housing because liquid splashing on the housing or running down the housing in the region of the heater coil causes temperature fluctuations. By eliminating spurious temperature fluctuations of the heated thermocouple junction a more accurate indication of liquid coolant level is achieved. The splash guard also acts as a stilling chamber. The splash guard has inlet-outlet ports near the top and near the bottom thereof to maintain fluid communication with the exterior of the splash guard. The top inlet-outlet ports in the splash guard are above the heated thermocouple junction and preferably beyond the heater coil. The bottom inlet-outlet ports in the splash guard are below the heated junction thermocouple and preferably beyond the heater coil. A plurality of sensors are vertically spaced in the reactor vessel above the core to give a coolant level indication over the entire height above the core. The vertical spacing is such that an incremental level indication is obtained with eight sensors spanning the approximately 15 feet from the reactor vessel head to the fuel alignment plate. The plurality of sensors are enclosed in a separator tube. The separator tube serves to collapse the level of two-phase fluid by separating the two-phase fluid into essentially a liquid phase and a vapor phase. It is this liquid level which is measured by the heated junction thermocouple level measurement system. The separator tube also acts as a stilling chamber. The bottom surface of the separator tube is enclosed to prevent rising bubbles of steam from passing through the separator tube and disturbing the liquid-vapor interface thereby giving a false indication of level. The separator tube has lateral inlet-outlet ports near the top and near the bottom thereof to maintain fluid communication between the interior of the separator tube and the coolant above the core and to permit liquid level changes to equalize inside and outside the separator tube. The lateral inlet-outlet ports are sized to permit rapid equalization of liquid level changes when water inside the separator tube flashes to steam due to a loss of pressure within the reactor vessel. The lateral inlet-outlet ports are sized based on a theoretical worst condition rate of decrease in the pressure and water level in the reactor vessel. The heated junction thermocouple level measurement system is mounted above the core near the outlet nozzle. The heated junction thermocouple level measurement system is subjected to cross flow as the coolant exits from the reactor vessel. To prevent steam bubbles entrained in a two-phase fluid cross flow from entering the lateral inlet-outlet ports of the separator tube, the separator tube is enclosed in a support tube. The support tube has lateral inlet-outlet ports throughout the length thereof to permit liquid level fluxuations to equalize inside and outside the support tube. The lateral inlet-outlet ports of the separator tube are axially offset from the lateral inlet-outlet ports of the support tube at least where crossflow exists. In some applications the support tube may be located in an otherwise unused control element assembly shroud. The control element assembly shroud has openings which permit liquid level fluxuations to equalize inside and outside the control element assembly shroud. The lateral inlet-outlet ports of the control element assembly shroud may be axially offset from the lateral inlet-outlet ports of the support tube. Redundant level monitoring is achieved by using more than one heated junction thermocouple level measurement system. Each heated junction thermocouple level measurement system is mounted in the reactor vessel above the core. Each heated junction thermocouple level measurement system has eight heated junction thermocouples. Associated with each heated thermocouple junction and within the same housing as the heated thermocouple junction are a heater coil and an unheated thermocouple junction. The electrical signals generated by the heated and unheated thermocouple junctions are processed by electrically independent microprocessor channels. The output voltages of each heated junction thermocouple level measurement system are processed by electrically independent channels of a microprocessor based control system. Each channel of the microprocessor based control system has two heater coil power controllers. The control system provides operator access to all thermocouple temperatures on a digital display, an output signal for trend recording the output of each of the thermocouple temperatures, test features for performing diagnostics, alarm outputs to the plant anunciator system, temperature outputs for calculating the subcooled margin and control of the heater coil power for each of the heater coils.