Patent Number: 053655559
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Illustrated schematically in FIG. 1 is an exemplary boiling water reactor (BWR) plant conventionally including a reactor pressure vessel 10 disposed in a drywell 12 which in turn is disposed in a containment vessel 14, with a wetwell or suppression pool 16 contained therein. The vessel 10 conventionally includes a nuclear reactor core 18 and a steam separator assembly 20 disposed thereabove, with a steam dryer 22 disposed above the separator assembly 20. The vessel 10 is filled with water 24 to a normal or nominal level L.sub.n at an elevation suitably above the core 18 which is typically within the intermediate region of the steam separator assembly 20. The vessel 10 conventionally includes additional components such as, jet pumps 26 located in the annular downcomer region circumferentially surrounding the core 18 for circulating the water 24 upwardly through the core 18. The core heats the water for generating steam 24a which flows upwardly into the steam separator assembly 20. Moisture is removed from the steam 24a in the assembly 20, and additional moisture is removed therefrom in the dryer 22, as is conventionally known, prior to discharging the steam 24a from the vessel 10 through a conventional main steam nozzle 28. The vessel 10 also includes conventional feedwater nozzles 30 which channel relatively cold feedwater into the vessel 10, the feed water being discharged above the core 18, when required, from conventional spargers (not shown). During operation of the reactor illustrated in FIG. 1, the level L of the water 24 in the vessel 10 is maintained at the nominal level L.sub.n well above the core 18 and at the required location through the steam separator assembly 20. Since the water level L will vary during operation, it must be monitored so that the nominal level may be maintained as well as for providing a water level signal to various conventional safety systems which may be activated as required in the event of deviations of the water level from the nominal level L.sub.n. In accordance with the present invention, a system 32 is provided for measuring the water level L in the vessel 10 which, therefore, is a part thereof. The system 32 further includes a reference leg or pipe 34 containing a predetermined and substantially constant reference column of water therein having a first reference height H.sub.1, with a top or reference level L.sub.r disposed vertically above the nominal level L.sub.n of the reactor water 24 in the vessel 10. A first or narrow range (NR) variable leg or pipe 36 includes a first or upper pressure tap 36a disposed in flow communication with the vessel 10 at a predetermined first death or level D.sub.1 below the reference level L.sub.r and below the nominal level L.sub.n, and further includes a first upper port 36b preferably disposed below the first tap 36a. A first differential pressure monitor 38, also referred to as a level transmitter, is disposed in flow communication with the reference leg 34 and the first port 36b of the first leg 36 for determining differential pressure therebetween to indicate the level L of the water in the vessel 10 above the first tap 36a. The reference leg 34, the first variable leg 36, and the first monitor 38 are conventionally configured and function conventionally for monitoring and indicating the level L of the water 24 in the vessel 10 using the fundamental fluid hydrostatic relationship of the pressure gradient in a fluid at rest being directly proportional to its density, with the pressure difference in the fluid at corresponding elevations therein being interrelated. Accordingly, by measuring the differential pressure between a known constant column of water, i.e. in the reference leg 34, and the pressure in a varying column of water represented by the level L of the water 24 in the vessel 10 as measured at the first port 36b, an accurate indication of the level L in the vessel 10 may be determined in a conventional fashion. FIG. 2 illustrates in more particularity the system 32 which further includes a conventional, thermally insulated steam leg or pipe 40 having an inlet port or tap 40a disposed in flow communication with the vessel 10 above the first tap 36a and above the nominal level L.sub.n of the water 24, shown in phantom in FIG. 2. The steam leg 40 further includes an outlet port 40b which is preferably disposed above the inlet port 40a so that the steam leg 40 is inclined upwardly away from the vessel 10. A conventional cold condensing chamber 42 has an inlet disposed in flow communication with the steam leg outlet port 40b for receiving steam 24a therethrough from the vessel 10 to form condensate in the relatively cold chamber 42 for maintaining the reference level L.sub.r therein. The chamber 42 also includes an outlet at a bottom thereof disposed in flow communication with the reference leg 34 for discharging thereto the condensate formed in the condensing chamber 42. As the steam 24a condenses in the chamber 42 it will partially fill the chamber 42 until the water level reaches the steam leg outlet port 40b, with the excess condensate spilling from the chamber 42 downwardly by gravity through the steam leg 40 for return to the vessel 10. In this way, the column of water in the reference leg 34 is maintained substantially constant and provides the reference level L.sub.r. The reference leg 34, the cooperating steam leg 40, and the condensing chamber 42 are conventional in structure and in operation. Accurate measurement of the water level L in the vessel 10 requires that the column of water in the reference leg 34 be accurately maintained by the condensing chamber 42. However, it has been observed in some operating nuclear reactor plants, that upon depressurization of the vessel 10 under certain conditions, for example, below about 32 kg/cm.sup.2 g, that water level measurement may temporarily falsely read high as indicated above in the Background section. It has been discovered that this aberration appears to be caused by bubbles being buoyed upwardly from the reference leg 34 due to degassing of non-condensable gas dissolved in solution therein. The vessel 10 conventionally includes a non-condensable gas such as oxygen and hydrogen which over the course of time, for example, several months, flows through the steam leg 40 into the condensing chamber 42 wherein the concentration of non-condensable gas therein becomes relatively high and enters into solution into the water therein. Upon depressurization of the vessel 10, it is believed that the non-condensable gas begins to degas from the water in the reference leg 34, decreasing its effective density and thereby artificially reducing the effective height H.sub.1 thereof, and the reference level L.sub.r, which results in an artificially high water level reading from the first monitor 38 for example. In accordance with the present invention, a second variable leg or pipe 44 is provided below the first variable leg 36 and has a second, or lower, pressure tap 44a disposed in flow communication with the vessel 10 through the bottom skirt of one of the jet pumps 26, for example, at a predetermined second level or depth D.sub.2 below the reference level L.sub.r, below the nominal level L.sub.n, and below the first tap 36a. The second variable leg 44 further includes a second or lower port 44b disposed below the second tap 44a so that the second variable leg 44 is inclined downwardly away from the vessel 10. A second differential pressure monitor 46 is disposed in flow communication with the first leg 36 and the second port 44b of the second variable leg 44 in accordance with the present invention for determining differential pressure therebetween to indicate the level L of the water 24 in the vessel 10 between the first and second taps 36a, 44a when the water level falls below the first tap 36a to a lower level L.sub.1 shown in solid line in FIG. 2. The first leg 36 is inclined downwardly away from the vessel 10 for containing the reactor water 24 therein up to the first tap 36a to provide a predetermined second reference column of water having a second height H.sub.2 for the second monitor 46. The second monitor 46 is joined to the first variable leg 36 at any suitable location between the first tap 36a and first port 36b by an extension leg 36e thereof. But for the extension leg 36e joining the second monitor 46 to the first variable leg 36, both the second monitor 46 and the second variable leg 44 are conventional in configuration and function, with the second monitor 46 also being conventionally known as a fuel zone range (FZR) monitor 46 since the second tap 44a is disposed below the active fuel of the core 18, with the first tap 36a being disposed substantially above the top of the active fuel of the core 18. In this way, the water level L within the core 18 may be accurately measured. In a conventional water level measurement system, the second monitor 46 is not connected to the first variable leg 36, but is instead connected directly to the reference leg 34. The level measurement ranges of the first monitor 38 and the second monitor 46 are then suitably selected and calibrated for measuring the water level L in the vessel 10. Since both monitors 38 and 46 are joined to the common reference leg 34 in a conventional system, the spurious level measurement notch discussed above will occur in both monitors. However, in accordance with the present invention, by using the first variable leg 36 instead of the constant reference leg 34 with the second monitor 46, the notch problem will be reduced or eliminated. More specifically, since the notch problem appears to be caused by the degassing of non-condensable gas in the reference leg 34, using the first variable leg 36 instead will improve level measurement in the monitor 46. During normal operation when the normal level L.sub.n of the water 24 is maintained in the vessel 10 as illustrated in phantom line in FIG. 2, the first tap 36a is normally underwater, which prevents the non-condensable gas within the vessel 10 from accumulating in any portion of the first variable leg 36. This is in contrast to the reference leg 34 which communicates with the vessel 10 through the steam leg 40 and condensing chamber 42 which allow the non-condensable gas to accumulate in the chamber 42 over time and increase the concentration of the gas in the water contained in the reference leg 34. During abnormally low levels of the water 24 within the vessel 10 when the accuracy of its measurement is most important, the second monitor 46 in accordance with the present invention provides improved accuracy by using the first variable leg 36 instead of the reference leg 34. Once the level of the water 24 drops below the first tap 36a, the second monitor 46 may then be used to accurately indicate the water level L in the vessel 10 using the first variable leg 36 which is now filled with reactor water up to the first tap 36a with the predetermined water column height H.sub.2. Of course, by joining the second monitor 46 to the first variable leg 36 instead of the reference leg 34, the second monitor 46 is ineffective for measuring water level when the level is above the first tap 36a, in this case the water level being measured instead by the first monitor 38. The system 32 preferably also includes a third variable leg or pipe 48 having a third or middle pressure tap 48a disposed in flow communication with the vessel 10 at a predetermined third level or depth D.sub.3 below the reference level L.sub.r and below the first tap 36a, and above the second tap 44a, with the third leg 48 also including a third or middle port 48b which is disposed below the third tap 48a. A third differential pressure monitor 50 is disposed in flow communication with the reference leg 34 and the third port 48b of the third leg 48 for determining differential pressure therebetween to indicate level L of the water 24 in the vessel 10 above the third tap 48a. The third monitor 50 is joined to the reference leg 34 by an extension 34e thereof to provide a predetermined reference column of water having a height H.sub.3 for use as a reference for measuring water level in the vessel 10 down to the third tap 48a. The third variable leg 48 and the third monitor 50 are conventional in configuration and function, with the third monitor 50 also being conventionally referred to as a wide range (WR) monitor 50. In this way, the three monitors 38, 46, and 50 may be calibrated for use over different as well as overlapping vertical ranges. In the exemplary embodiment illustrated in FIG. 2, the first tap 36a is disposed adjacent to the steam separator assembly 20, at the bottom thereof for example, the third tap 48a is disposed adjacent the top of the core 18 and above the second tap 44a, which is disposed below the core 18. In this way, the first or NR monitor 38 indicates the water level L from the nominal level L.sub.n down to about the first tap 36a, the third or WR monitor 50 indicates the water level L from the nominal level L.sub.n down to about the third tap 48a, and the second or FZR monitor 46 indicates the water level L from about the first tap 36a down to about the second tap 44a when the level drops below the first tap 36a. This arrangement of the taps 36a, 44a, and 48a and the operating ranges of the three monitors 38, 46, and 50 is conventional except for joining the second monitor 46 to the first variable leg 36 through the extension 36e thereof instead of to the reference leg 34, such as the third monitor 50 joined thereto through the extension 34e. In this way, the three monitors 38, 46, and 50 may be conventionally used to monitor water level in the vessel 10 and control conventional safety systems as desired. Since the third monitor 50 uses the reference leg 34, the notch problem can still effect the accuracy of water level measurement therefrom. Accordingly, for those plants which require safety related low water level trips generated by the wide range third monitor 50, the system 32 may further include a fourth differential pressure monitor 52 disposed in flow communication between the first variable leg 36 and the third variable leg 48 for determining differential pressure therebetween to indicate level of the water in the vessel 10 between the first and third taps 36a, 48a when the water level falls below the first tap 36a. In this embodiment, the fourth monitor 52 is suitably joined through an extension 48e of the third variable leg 48 to provide a predetermined reference column of water having a height H.sub.4 up to the first tap 36a, and suitably joined to the first variable leg extension 36e. Although the fourth monitor 52 is unable to measure water level when it is above the first tap 36a, when the water level drops below the first tap 36a the reference column of water captured in the first variable leg 36 is used for the fourth monitor 52, as it is for the second monitor 46, for more accurately determining water level in the vessel 10. As shown in FIG. 2, the reference leg 34 and the variable legs 36, 44, and 48 are all preferably inclined downwardly away from the vessel 10 to allow any gas bubbles therein to escape by being buoyed upwardly for return to the vessel 10, with the first variable leg 36 also being inclined downwardly for capturing water therein to provide the reference column heights H.sub.2, H.sub.4 when the water level in the vessel 10 drops below the first tap 36a. If desired, the signals from the monitors 38 and 50 may be conventionally automatically analyzed by electronic means to detect the well known notching signature and provide an alarm for the plant operator. While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims: