Patent Number: 055330746
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is a system for determining the level of the coolant in a nuclear reactor when that reactor is depressurized. The "level" of the coolant means the relative location of the top surface of the coolant with respect to a fixed, preselected point. Depressurized means, in this context, that the higher, operating pressure has been relieved so that the reactor pressure vessel can be opened to the atmosphere inside the reactor containment. The coolant may still be under a pressure other than atmospheric pressure (14.7 psi), but the pressure in the depressurized vessel is the same as that in the reactor containment. "Determining" is essentially the process of calculating a level based on pressure readings. Therefore, the actual level--or depth--of the coolant is not measured directly but is inferred by making a calculation using an algorithm based on pressure data and other information that correlates the pressure of the coolant at specific locations to its level. The fact that pressure and depth or level are related is well established. For example, barometric pressure is given in units of inches, corresponding to the height ("level") of a column of mercury; the weight of the column, which is directly proportional to its level, is also proportional to the pressure it exerts. Levels of coolant in a reactor are somewhat more complicated. The weight of the coolant is a function of its temperature (as is the internal volume of the coolant piping) and its chemical make up, since the coolant will not be pure water but is likely to be a weak solution of boric acid, usually given in pans per million (ppm). Both temperature and boron concentration are factors that affect the coolant density. Temperature will also affect the geometry of the coolant piping, which can cause the level of coolant in the piping to change. The actual coolant level will be referred to simply as the "coolant level;" the level that is determined from pressure transmitter data will be referred to as the "calculated level" or sometimes the "determined level." Both levels are given with respect to a stationary reference point that is arbitrarily chosen. For example, the level can be stated in terms of inches or feet above the top of the core or from the lowest point in the system. The term "loop" refers to a closed system of components and piping that are all in fluid communication. In a typical nuclear reactor system (see FIG. 1), there is one pressure vessel 10 containing the nuclear core, which is a collection of fuel elements, and one or more coolant loops 12 that conduct coolant to and from the pressure vessel. Each loop has its own pump 14 and the coolant system has a pressurizer 16. Each loop of a PWR will have a heat exchanger 18 included in the loop. Each loop of a BWR will be connected to a steam turbine (not shown) rather than a heat exchanger. One set of sensors 22 is placed in a lower location, below the lowest expected level of coolant, preferably a location in the coolant loop 12 but preferably not in a part of the coolant loop piping where air can be trapped. The other set of sensors 20 is placed at a location higher than the highest expected coolant level, preferably at the top of pressurizer 16 or in the piping leading directly into it and as close as possible to pressurizer 16. Because the inside of the pressurizer 16 (and piping leading into it) communicates with the reactor coolant piping 12, the difference in pressure between these two points must be due to the coolant. This first sensor system 20 is referred to as the "dry" side of the level detection system. The second set of sensors 22, also preferably electronic pressure transmitters, are installed in coolant loop 12. The part of the coolant loop 12 leading to the reactor vessel 10 is called the "cold" leg and the part leading from the reactor vessel 10 is called the "hot" leg. Transmitters can be installed at various locations in either or both legs. There is also an intermediate leg 24 between the heat exchanger and the reactor coolant pump. Although pressure transmitters can also be connected to intermediate leg 24, inaccuracies in the pressure sensed in the intermediate leg can be caused by restricted flow from the steam generator tubes and the pump adaptor casing, so positioning sensors in this leg is not preferred. Preferably, each set of sensors comprises a plurality of electronic pressure transducers 40 (see FIG. 2) selected for durability, reliability and sensitivity range under reactor conditions. Plural transducers provide assurance that, if any one or more tranducers 40 are or become defective, the remaining transducer or transducers will continue to provide level indicating signals. Each transducer 40 is preferably mounted using a threaded or tubing type seal as is well known in the art of process instrumentation. The dry side transducers 40 are preferably selected for sensitivity in the range of pressures from about 30 inches of mercury vacuum to about 15 PSIG. The wet side tranducers 40 are preferably selected for sensitivity in the range of pressures from about 30 inches of mercury vacuum to about 50 PSIG. Each pressure transducer 40 issues an electrical signal carried by cable to a control area, such as the reactor control room, where the signal is fed into a controller that includes a microprocessor 30 and a display 32 (FIG. 1). The electrical signals from each transmitter can be monitored at intervals by a multiplexer or monitored continuously. If monitored at intervals, four to ten times per second is preferred in order to measure coolant levels in essentially real time. Signals are checked for quality. A signal meeting a quality check is an accurate signal. If a signal is not within an expected range, or if the difference between two transducers of a transducer pair or between the transducers of one pair and a second pair located in a different part of coolant loop 12 is too great, the signal may not be a quality signal. Suitable alarms can be given to alert the operator that there is a signal from one transducer that is more than a preselected amount (2%, for example) different from a corresponding transducer of a pair of transducers, is inconsistent with expectations, or is obviously erroneous. Any signal that does not meet these threshold quality checks is disregarded, and the signals of other transducers 40 are used instead. Preferably, of each pair of transducers 40 at each location where the signal differs by more than a small amount, such as one percent, the more conservative signal is selected, as shown in FIG. 2, as long as the signals do not differ by more than a small amount (to pass a quality check), such as two percent or less. For example, from the first set of sensors 20, located at pressurizer 16, the more conservative signal would be that corresponding to the higher pressure, and from the second set of sensors 22, located in coolant loop 12, the more conservative signal would be that corresponding to the lower pressure. The combination of these two selections would produce a calculated coolant level that would, if at all, err on the low side. If the actual level differs from the calculated level, the former would tend to be somewhat higher. If a pair of transducers 40 is placed in intermediate leg 24 rather than or in addition to other locations in coolant leg 12, the lower of their two signals is selected because they would also be wet side sensors. Often, intermediate leg 24 is physically lower than the remainder of the coolant loop, as illustrated in FIG. 1; therefore, depending on the range of coolant level, signals from sensor 22 or signals from a sensor 26 in intermediate leg 24 would be selected by microprocessor 30. For example, if the coolant level were low enough, the intermediate leg sensors would be used because there may be no coolant in the balance of coolant loop 12. Otherwise, coolant loop sensors 22 would be used and are preferred. The selected high and low signals are then used to calculate the differential pressure which is then converted to a calculated coolant level. The calculation includes factors for compensating for coolant temperature and chemistry (boron concentration), which affect coolant density and piping geometry. As an example of an algorithm for calculating coolant level from pressure readings from pressure tranducers plus temperature and boron concentration data, the following formula can be used: EQU Level (ft)={[K.sub.1 +(T.sub.w -80)(K.sub.2)+(K.sub.3 C.sub.B)](P.sub.2 -P.sub.1)}+E.sub.p2 where K.sub.1 equals 2.314512, the ft/PSI conversion for pure water at 80.degree. F.; PA1 K.sub.2 equals 0.0062784, the ft/PSI-.degree.F. conversion factor for any temperature between 80.degree. F. and 180.degree. F.; PA1 K.sub.3 equals 6.944X10.sup.-9, the ft/PSI-ppm correction factor for any boron concentration; PA1 T.sub.w is the temperature of the water in .degree. F. input by the operator (in the range 80.degree. F. to 180.degree. F.); PA1 C.sub.B is the boron concentration in ppm input by the operator; PA1 E.sub.p2 is the actual elevation in feet where the instrument P.sub.2 is located; PA1 P.sub.1 is the pressure (PSI) at the high point (dry side); and PA1 P.sub.2 is the pressure (PSI) at the low point (wet side). The operator will be able to input various measurement points (the identification of wet side transducers), water temperatures, boron concentrations, alarm setpoints, and units of measurements of the microprocessor through an interface device 50. Microprocessor 30 will be designed to verify signal quality and to display 32 the calculated coolant level digitally or in graphical form to the operator. When the operator is not actively operating interface device 50, it will provide warnings and displays using light emitting diodes (LEDs) or liquid crystal displays (LCDs). The alarms included in the system comprise high and low level alarms, temperature and boron concentration update alarms that are connected to a timer, instrument quality alarms, and alarms that warn the operator that the level is below the elevation of an instrument tap. It will be apparent to those skilled in the art that many modifications and substitutions may be made to the foregoing preferred embodiments without departing from the spirit and scope of the present invention. For example, fiber optic signals could be used instead of or in addition to electrical signals from the pressure tranducers, and a variety of different displays of the coolant level could be employed. However, the scope of the invention is defined by the appended claims.