Patent Number: 044407173
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a nuclear reactor vessel 10 and reactor vessel closure head 12 vertically disposed in a concrete reactor cavity 14. Reactor vessel 10 is connected to the other components of the nuclear steam supply system (not shown) by a reactor coolant inlet nozzle 16 and outlet nozzle 18. Prior to start-up of the reactor, the entire volume within reactor vessel 10 is filled with a fluid coolant such as water. During normal reactor operation, the liquid coolant enters reactor vessel 10 through inlet nozzle 16, follows flowpath 20 down the outside of core support barrel 22, up through core support assembly 24 through fuel assemblies 26, collectively core 29, to remove heat generated therein, through fuel alignment plate 28 and exits from reactor vessel 10 through outlet nozzle 18. A small portion of the flow through core 29 passes up through shroud 110 into the plenum beneath reactor vessel closure head 12. Reactor vessel 10 coolant inventory may be affected by changes in the state of the coolant or by changes in the quantity of inventory in the reactor coolant system. Reactor vessel 10 coolant inventory may be decreased by a loss of reactor coolant system fluid due to a break in the system. Reactor vessel 10 coolant inventory may also be decreased by contraction due to system cooling as caused by a break in a steam line. Reactor vessel 10 coolant inventory may also be decreased due to displacement of water by noncondensables or steam as the result of flashing. The coolant inventory above fuel alignment plate 28 is available for cooling core 29 should one of the above incidents cause a decrease in reactor vessel 10 coolant inventory. Heated junction thermocouple level measurement system 32 monitors the liquid level in reactor 10 above fuel alignment plate 28. This liquid level represents the portion of total volume above fuel alignment plate 28 that is occupied by liquid coolant. As shown in FIG. 2, heated junction thermocouple level measurement system 32 is mounted above fuel alignment plate 28. During normal operation of the pressurized water reactor, heated thermocouple junction level measurement system 32 is covered with and senses liquid coolant and the level is not particularly difficult to establish. During accidents which affect the level of coolant, the coolant in the reactor vessel 10 is not necessarily a subcooled liquid but rather may consist of a two-phase fluid. The two-phases are liquid and gas. When the coolant is water the liquid phase is water and the gaseous phase is steam. Steam bubbles rise to the top of the vessel due to buoyant forces thereby forming a steam region above a water level. Liquid level measurement in reactor vessel 10 is further complicated by the varying void fraction of the two-phase coolant. FIG. 2 shows heated thermocouple junction level measurement system 32 mounted between the upper guide structure support plate 30 and fuel alignment plate 28. Electrical leads from heated junction thermocouple level measurement system 32, collectively conductors 48, 50, 52, 54 and 56, exit through vessel head 12 by way of seal plug 34. In this position, heated junction thermocouple level measurement system 32 monitors the level of coolant above the core. An embodiment of the axial spacing of sensors 36 has sensors equally spaced over the range of level sensing of heated junction thermocouple level measurement system 32. The sensors need not be equally spaced over the level sensing range. FIG. 3 is a vertical section of a sensor 36. Pins 38, 40, 42, 44 and 46 connect to conductors 48, 50, 52, 54 and 56, respectively, in receptacle 58. Conductors 48, 50, 52, 54 and 56 further pass through seal plug 60 entering housing 62. Conductors 48 and 56 are copper leads to heater coil 64. A controllable power supply (not shown) is electrically connected to pins 38 and 46. Chromel conductor 54 and Alumel conductor 52 are joined within housing 62 to form unheated thermocouple junction 66. Alumel conductor 52 extends beyond unheated thermocouple junction 66 to within heater coil 64 and joins with Chromel conductor 50 forming heated thermocouple junction 68. Heated thermocouple junction 68 is located central to the 1 inch wound inconel heater 64. A thermally conductive and electrically insulating means such as ceramic insulating material 70 fills the housing through at least the length containing the thermocouple junctions. Unheated thermocouple junction 66 is located approximately 4.5 inches above heated thermocouple junction 68 so that there is substantially no effect by heat input from heater 64 on unheated thermocouple junction 66. Unheated thermocouple junction 66 and heated thermocouple junction 68 are wired in series. The net voltage generated by thermocouple junctions 66 and 68 is a function of the temperature difference between the two thermocouple junctions. Heated thermocouple junction 68 will generate a voltage representative of its temperature. Unheated thermocouple junction 66 will also generate a voltage representative of its temperature. When the fluid surrounding housing 62 is liquid, heated thermocouple junction 68 remains at approximately the same temperature as unheated thermocouple junction 66 as the heat transfer from heater 64 through the thermally conductive and electrically insulating means 70 and housing 62 to the surrounding fluid is very good. The net voltage across pins 40 and 44 is approximately zero. When the fluid surrounding housing 62 is gaseous, the heat transfer from heater 64 through thermally conductive and electrically insulating means 70 and housing 62 to the gaseous fluid is not as good and the temperature of heated thermocouple junction 68 will rise above the temperature of unheated junction 66 producing a net voltage across pins 40 and 44 representative of the temperature difference between junctions 66 and 68. Thus, there will be a substantial difference in heat transfer between housing 62 and the surrounding fluid depending upon whether the coolant is liquid or gaseous. The differential temperature measurement between unheated thermocouple junction 66 and heated thermocouple junction 68 is used to indicate coolant level. In addition to having available the net voltage generated by unheated thermocouple junction 66 and heated thermocouple junction 68, a voltage representative of the absolute temperature of both unheated junction 66 and heated junction 68 is available. The voltage representative of the temperature of unheated thermocouple junction 66 is available across pins 42 and 44. The voltage representative of the temperature of heated thermocouple junction 68 is available across pins 40 and 42. The temperature of unheated thermocouple junction 66 indicates the temperature of the coolant in the reactor head region regardless of whether the coolant is water or steam. When supplied to an appropriate calculator, this coolant temperature can be utilized in the calculation of subcooled margin. Subcooled margin provides an indication of the margin to saturation conditions in the reactor coolant system. The temperature of heated thermocouple junction 68 is used to control the power delivered to heater 64. The heater power controller 165 shown in FIG. 4 is designed to maintain a constant heater power sufficient to insure that a voltage signal is generated by heated thermocouple junction 68. To protect heater coil 64, heater power is reduced when the temperature of heated thermocouple junction 68 rises above a preselected value. There are two heater power controllers per channel. Each heater power controller controls the power delivered to four heater coils. During testing it was found that liquid impinging on housing 62 in the region of heater 64 caused spurious cooling. It was further found that the effects of temperature decrease due to cooling were more rapid than the effects of temperature increase caused by heater 64. Since housing 62 could be cooled by spurious water droplets more rapidly than it could be heated by heater 64, it was imperative that spurious water droplets be kept off housing 62 in the region of heater 64 to obtain an accurate water level. To overcome the effects of spurious water droplets, a 4 inch long stainless steel splash guard 72 was disposed to surround housing 62 in the region of heater 64. Splash guard 72 is joined to housing 62 by continuous weld or braze 74 at one end and continuous weld or braze 76 at the other end. Splash guard 72 is generally cylindrical with circumscribing recessed regions 80 and 84 near the top and bottom thereof. The portion of housing 62 disposed within splash guard 72 is in fluid communication with the coolant outside splash guard 72 through upper lateral inlet-outlet ports 78 in recessed region 80 and lower lateral inlet-outlet ports 82 in recessed region 84 as stainless steel plug 85 seals the upper end of splash guard 72 and stainless steel plug 83 seals the lower end of splash guard 72. Splash guard 72 acts as a stilling chamber to provide a nonturbulent interface between liquid and gaseous coolant surrounding housing 62 within the range of level sensed by sensor 36. Inlet-outlet ports 78 and 82 permit the coolant level within splash guard 72 to fluctuate with the coolant level outside splash guard 72. The fluid splashing outside splash guard 72 impinges on splash guard 72 thereby eliminating the spurious cooling effect experienced without splash guard 72. Should coolant splash or condense on housing 62 above splash guard 72 then run down housing 62 onto weld or braze 74, stainless steel plug 85 and splash guard 72, heated thermocouple junction 68 will not experience spurious temperature decreases. The lower surface of lower lateral inlet-outlet ports 82 are at the top of stainless steel plug 83 and allow virtually complete drainage of the volume within splash guard 72 when the liquid coolant level drops below inlet-outlet ports 82. Upper lateral inlet-outlet ports 78 and lower lateral inlet-outlet ports 82 are multiple small ports sized to permit the coolant level inside splash guard 72 to respond to rapid changes in the level of coolant outside splash guard 72 within an acceptably short period of time. The number and size of inlet-outlet ports 78 and 82 will vary depending upon the volume enclosed by splash guard 72, the rate at which the coolant level outside splash guard 72 is expected to change and the time period acceptable for the level to equalize inside and outside splash guard 72. Generally the number and size of upper lateral inlet-outlet ports 78 are the same as the number and size of lower lateral inlet-outlet ports 82. In one application having a four inch long, three-eighths inch outside diameter splash guard, lateral inlet-outlet ports 78 and 82 were two in number, diametrically opposed across the splash guard and one-eighth inch in diameter. It was found that for a rapid coolant level change outside the splash guard the level inside the splash guard equalized in a fraction of the time required for the thermocouples to respond. Since inlet-outlet ports 78 and 82 are sized for a rapid change in coolant level and since a rapid change in coolant level is a worse case than a slower change in coolant level, inlet-outlet ports 78 and 82 function adequately for slower changes in coolant level. It is not necessary to enclose unheated thermocouple junction 66 within splash guard 72 as unheated thermocouple junction 66 does not discriminate between liquid or gaseous fluid surrounding housing 62. When the fluid surrounding housing 62 is a two-phase fluid, the saturation temperature detected by unheated thermocouple junction 66 will be the same for both the liquid and gaseous phases. The unheated thermocouple junction thereby compensates for temperature changes in the coolant and no further temperature compensation is necessary in the heated junction thermocouple level measurement system 32. FIG. 4 shows a vertical section through separator tube 86. Separator tube 86 encloses eight fixedly mounted sensors 36 and functions as a stilling chamber to hydraulically separate the two-phase fluid into a liquid phase and a gaseous phase. It is this liquid level that is measured by the heated junction thermocouple level measurement system 32. The level of liquid measured is indicative of the liquid inventory above the core. Separator tube 86 is a tubular element with end 88 completely enclosed. End 88 prevents rising bubbles of steam from passing through the separator tube and disturbing the liquid-vapor interface thereby giving a flase indication of level. Lateral inlet-outlet ports 90 and 92 permit fluid communication between the volume enclosed by separator tube 86 and the volume within support tube 100. Upper lateral inlet-outlet ports 90 and lower lateral inlet-outlet ports 92 are multiple inlet-outlet ports sized to permit the coolant level inside separator tube 86 to respond to rapid changes in the level of coolant outside separator tube 86 within an acceptably short period of time. The number and size of inlet-outlet ports 90 and 92 will vary depending upon the volume enclosed by separator tube 86, the rate at which the coolant level outside separator tube 86 is expected to change and the time period acceptable for the level to equalize inside and outside separator tube 86. Upper lateral inlet-outlet ports 90 permit fluid communication between the volume enclosed by separator tube 86 and the volume within support tube 100 at or above the level of inlet-outlet ports 78 of uppermost sensor 36a. Lower lateral inlet-outlet ports 92 permit fluid communication between the volume enclosed by separator tube 86 and the volume within support tube 100 at or below the level of inlet-outlet ports 82 of lowermost sensor 36h. Due to a decrease in pressure in reactor vessel 12 or a change in water level, water may pass through inlet-outlet ports 90 and 92 or water that has flashed to steam may pass through inlet-outlet ports 90 and 92 to equalize the water level inside and outside separator tube 86. Heated junction thermocouple level measurement system 32 is mounted in the plenum above core 29 where coolant tends to move vertically and laterally toward outlet nozzle 18. In this location heated junction thermocouple level measurement system 32 is subjected to cross flow as the reactor coolant exits through nozzle 18. To prevent steam bubbles entrained in a two-phase fluid cross flow from entering lateral inlet-outlet ports 90 and 92 of separator tube 86, separator tube 86 is enclosed by and fixedly mounted in support tube 100. Support tube extension 101 supports but does not hydraulically seal against separator tube 86. Support tube 100 is a tubular element with end 108 completely enclosed. It is not necessary that end 108 be completely enclosed. Lateral inlet-outlet ports 102 are spaced throughout the length of support tube 100. Lateral inlet-outlet ports 102 are sufficient in size and number to permit the coolant level inside support tube 100 to respond to rapid changes in the level of coolant within reactor vessel 10 above the core within an acceptably short period of time. Lateral inlet-outlet ports 102 in the lower portion of support tube 100 are axially offset from lateral inlet-outlet ports 92 of separator tube 86 because at least the lower portion of heated junction thermocouple level measurement system 32 is subjected to crossflow. Lateral inlet-outlet ports 102 in the upper portion of support tube 100 may be axially offset from lateral inlet-outlet ports 90 of separator tube 86 depending upon the crossflow in this region in a particular application. In some applications, support tube 100 may be located in a generally cylindrical, otherwise unused, control element assembly shroud 110. Shroud 110 is a tubular element open on the bottom that has lateral inlet-outlet ports 112 at least near the top to permit liquid level fluctuations to equalize inside and outside shroud 110. Shroud 110 further prevents steam bubbles in the cross flow from disrupting the coolant level measured by heated junction thermocouple level measurement system 32 but is not necessary for heated junction thermocouple level measurement system 32 to function properly. One embodiment of the cross sectional spacing of sensors 36 within separator tube 86 and support tube 100 is shown in FIG. 5 taken through line 5--5 of FIG. 4. A uniform cross sectional spacing is used because it efficiently utilizes the space within separator tube 86; the cross sectional spacing of sensors 36 need not be uniform. FIG. 6 shows an alternate embodiment of heated junction thermocouple level measurement system 32 in which hydraulic plug 94 divides the length of separator tube 86 into two level sensing sections 96 and 98 at the level of internal physical structure 114. This embodiment may be used when the internal physical structure 114 of the reactor could restrict coolant flow resulting in two liquid levels. Level sensing section 96 is above hydraulic plug 94 and internal physical structure 114 while level sensing section 98 is below hydraulic plug 94 and internal physical structure 114. Each level sensing section 96 and 96 has lateral inlet-outlet ports 90 or 90' at the top of the respective level sensing section and lower inlet-outlet ports 92 or 92' at the bottom of the respective level sensing section. Each level sensing section 96 and 98 contains at least one sensor 36 so that two separate liquid levels may be monitored. This concept can be extended to sense more than two liquid levels when the physical structure surrounding support tube 100 would impede the coolant flow. When separator tube 86 is divided by hydraulic plug 94 to measure two liquid levels, it is necessary to similarly divide support tube 100 into two corresponding level measuring PG,17 sections. Support tube extension 101 in this embodiment hydraulically seals against separator tube 100. The two level sensing sections remain 96 and 98. Upper level sensing section 96 has lateral inlet-outlet ports 102 in support tube 100 at least near the top and bottom thereof. Lower level sensing section 98 also has lateral inlet-outlet ports 102 at least near the top and bottom thereof. The level may equalize inside and outside level sensing sections 96 and 98 of support tube 100 through inlet-outlet ports 102. Inlet-outlet ports 102 are axially offset from lateral inlet ports 90, 90', 92 or 92' at least where support tube 100 is subjected to cross flow. In those applications where support tube 100 is located in shroud 110, shroud extension 111 hydraulically divides shroud 110 into the same level sensing sections 96 and 98. Also during accident conditions, heated junction thermocouple level measurement system 32 can be used to detect the presence of noncondensable gas bubbles in the reactor vessel head, thereby providing information to the operator for use in controlling the reactor vent system. In addition to being used during normal operation and accident conditions, the heated junction thermocouple level measurement system 32 can be used by the operator to determine if a recovery operation has been successful. Heated junction thermocouple level measurement system 32 can also be used to determine when sufficient reactor coolant has been removed from the primary system to permit refueling operations. Heated junction thermocouple level measurement system 32 has in some applications been designed to be handled in the same manner as other top mounted incore instrumentation devices during refueling operations and can be retrofitted into operating reactors and reactors under construction.