Patent Number: 044141777
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION The drawing illustrates a single length of tubing in the system. It is to be understood that the system will normally consist of a multiplicity of such tubes. Each would monitor a different sensing elevation within a reactor vessel. The length of small diameter metal tubing 10 within the pressurized vessel has an open end 11 at the sensing location where the condition of coolant is to be monitored. The tubing is brought out of the vessel to its exterior. This will be typically accomplished by a welded vessel penetration through a wall section 12. The portion of the length of tubing 10 exterior to the vessel walls 12 is shown as being wrapped in insulation 13, which prevents cooling of the liquid. The exterior or outer end of the length of tubing 10 is closed, and while liquid and/or gaseous coolant might enter it, no coolant flows through the length of tubing. As generally illustrated in the drawings, the tubing would lead to an insulated housing 14 which mounts a pressure transducer 15 capable of producing signals as a function of the pressure sensed within housing 14. The transducer 15 is operably connected to the interior of the length of tubing 10, and pressure sensed by it will be identical to the pressure at the open end 11, less the static head difference, which can be electronically compensated. The thermocouple leads 16 are preferably brought from the interior of the vessel through the interior of the length of tubing 10. The leads 16 and leads 17 from the pressure transducer 15 are preferably directed to temperature and pressure readout devices shown respectively at 18 and 20. The sensed signals are fed to an analyzer 21, which might be a microprocessor programmed to compare the magnitude of each signal to known values representing saturated temperatures of the coolant at the measured pressures. Any desirable output can be obtained from the analyzer 21. As an example, it might be provided with annunciators 22, 23, and 24 which respectively signal normal coolant conditions, abnormal void fraction conditions, and low liquid level. During normal use of the reactor with the coolant pumps operable, the appratus would provide constant measuring of temperature and pressure within the vessel interior. When either the temperature or pressure measured by the apparatus exceeds the programmed limits, the system will annunciate to indicate a possible condition of two-phase flow. If the analyzer determines that superheated steam exists at the open end 11 of the length of tubing 10, the annunciator 24 would indicate that liquid level has dropped below the elevation of the open end 11. This system is also capable of sensing liquid level by placing the open ends 11 of a number of lengths of tubing 10 at incremental elevations within the vessel to be monitored. If the primary reactor coolant pumps are turned off, the void fraction flow that commonly occurs within the reactor vessel will separate. As the liquid coolant level falls below the open end 11 of each length of tubing 10 the sensed temperature and pressure values for that individual length of tubing 11 will become superheated steam. This will be recognized by the analyzer 21 and indicated by annunciator 24. By relating this condition to the known elevation of the open end 11, liquid level can be accurately determined. Practical utilization of this system in a reactor core requires space availability, as well as physical access into the vessel and associated piping. Current design of boiling water and pressurized water reactors have sufficient vertical space in their cores for ten to eighteen sensing lines comprising lengths of tubing as generally discussed. It appears practical to transition such lines out of the vessel through the existing borate injection standpipe or other possible locations, such as flange fittings. When designing the apparatus, it is necessary to assure an adequately prompt response to void fractions caused by rapidly dropping pressure because of voiding in the length of tubing 10. Since the system is designed for detection of small and medium break loss of coolant accidents, there should not be a rapid drop in coolant loop pressure. However, if these were to occur, the pressure transducer 15 would reduce erratic and dropping pressure readings. The analyzer 21 can be programmed to activate the appropriate annunciator when such conditions occur. The problem of response time could be eliminated completely by sizing the interior of the length of tubing 10 sufficiently large to preclude slug-flow effects. In other words, the inside diameter of the tubing would be sufficiently larger that the surface tension of the liquid fluid within the length of tubing would be insignificant and not impede fluid flow, whether the coolant is at a gaseous or liquid state. Another approach to this condition would be to backfill the sensing tube with inert gas and use a movable plug to separate the gas backfill from the reactor fluid.