Abstract:
A method and system for measuring a temperature and liquid level of water within a Spent Fuel Pool (SFP) of a Light Water Reactor, without using external electrical power. The method and system may use a string of thermocouples attached to a cable and mounted in the spent fuel pool. The thermocouples supply their own power when heated by water that is exposed to the thermocouples within the spent fuel pool. The thermocouples may measure the temperature in the SFP. By measuring the temperature, the liquid water level of the SFP may be inferred by comparing a difference in temperature measurements of the thermocouples, as thermocouples exposed to ambient air will experience a noticeably different temperature from the thermocouples covered by water. The cable (with the thermocouples on one end) may terminate in a location that is remote from the SFP.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Example embodiments relate generally to nuclear reactors, and more particularly to a method and system for measuring a spent fuel pool (SFP) temperature and liquid level that may be accomplished without the use of external electrical power. The system may be particularly beneficial in the event a plant emergency that causes plant electrical power to be disrupted, or normal cooling of the spent fuel pools to otherwise become impaired. 
         [0003]    2. Related Art 
         [0004]      FIG. 1  is a cut-away view of a conventional boiling water nuclear reactor (BWR) reactor building  5 , which is one example of a light water reactor (LWR). It should be understood that this is merely an example, as example embodiments may be equally applied to other reactor design layouts, such as pressurized water reactors (PWR) or other LWRs. In the BWR, the spent fuel pool (SFP)  10  is a storage pool used to store spent fuel  7  that remains following the use of the fuel to power the BWR reactor  1 . The SFP  10  is generally positioned in a location adjacent to, and toward the top of, the reactor  1  (as shown in  FIG. 1 , the SFP  10  is located in secondary containment, outside of the steel containment vessel  3  and concrete shell  4  protecting reactor  1 ). It should be noted that in other reactor designs, the spent fuel pool may be located at a same plant elevation as the reactor  1 , or at an elevation that is below the reactor  1 . The spent fuel  7  is generally stored in the spent fuel pools  10  for a period of at least 5 years before being sent to reprocessing or cask storage. The SFP  10  is typically 40 feet or greater in depth, with a floor that is equipped to support the spent fuel  7 . About 8 feet of water (above the top of the spent fuel  7 , itself) is generally needed to keep radiation levels in the SFP  10  within acceptable limits. 
         [0005]    A flow of cooling water, provided by conventional fuel pool cooling and cleanup system (not shown), provides shielding from radiation and maintains the SFP  10  at cool temperatures that ensure the cooling water does not boil (thereby exposing the spent fuel to open air). Specifically, the conventional fuel pool cooling pumps transfer the water from the spent fuel pool to the fuel pool cooling and cleanup system. The conventional fuel pool cooling and cleanup system cools and cleans the water, using a heat exchanger and demineralizers (removing some radioisotopes, and other impurities). The spent fuel pool cooling pumps then send the cool, clean water back to the SFP  10 . 
         [0006]    During a serious plant accident, normal plant electrical power may be disrupted. In particular, the plant may be without normal electrical power to run the conventional spent fuel pool cooling pumps, operate the spent fuel pool cooling and cleanup system, and power fuel pool instrumentation. If electrical power is disrupted for a lengthy period of time, disruption in the use of the fuel pool cooling and cleanup system may cause water in the spent fuel pool to warm and eventually boil. When enough boiling occurs, water levels in the pool may drop to levels that no longer provide enough cooling water to effectively shield radiation that may be caused by the spent fuel. In very serious emergencies, water in the SFP  10  may boil and evaporate to the point that the spent fuel  7  may become exposed to open air. Furthermore, other events such as earthquakes, fires or explosions, may cause water to be lost from the spent fuel pool even in the absence of boiling. Such an emergency may pose grave dangers for plant personnel and the environment. 
         [0007]    Conventionally, the level of the SFP  10  is monitored indirectly via level measurement instruments (requiring external electrical power) at a skimmer surge tank (attached to the SFP  10 ). Also, the temperature of the SFP  10  is indirectly measured at the spent fuel pool cooling system via temperature instruments (also requiring external electrical power). Therefore, during a serious plant accident when plant power may be disrupted, the conventional temperature and level measurement instruments may not be used. This may cause plant operators to be unaware of the true temperature and/or liquid level of the water in the SFP  10 . By not knowing this information, preventative measures to cool and/or restore a volume of water to the SFP  10  may not be taken to prevent a serious accident from occurring. 
       SUMMARY OF INVENTION 
       [0008]    Example embodiments provide a method and/or system for measuring a spent fuel pool (SFP) temperature and liquid level without the use of electrical power. The method and system may include a cable, with string of thermocouples attached to the cable, that may be mounted along a side wall of the SFP. Each thermocouple may supply their own power (when heated from the water exposed to the thermocouple), such that external electrical power is not needed to operate the thermocouple. The thermocouples may measure the temperature in the SFP. By measuring the temperature, the string of thermocouples may also measure the liquid level within the SFP. Specifically, the liquid water level of the SFP may be inferred by comparing a difference in temperature measurements of the thermocouples, as thermocouples exposed to ambient air will experience a noticeably different temperature from the thermocouples covered by water. Therefore, the string of thermocouples may provide both temperature and liquid level information, without the need for external power. The cable (with the thermocouples on one end) may terminate in a remote location to provide temperature and level information to plant personnel who may be a distance away from the SFP during a potential plant accident. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other features and advantages of example embodiments will become more apparent by describing in detail, example embodiments with reference to the attached drawings. The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the intended scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
           [0010]      FIG. 1  is a cut-away view of one example design of a conventional light water nuclear reactor (LWR) reactor building; 
           [0011]      FIG. 2  is a schematic of a temperature and level measurement system, in accordance with an example embodiment; and 
           [0012]      FIG. 3  is a flowchart of a method of using the temperature and level measurement system, in accordance with an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
         [0014]    Accordingly, while example embodiments are capable of various modifications and alternative forms embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments. Like numbers refer to like elements throughout the description of the figures. 
         [0015]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0016]    It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
         [0017]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0018]    It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
         [0019]      FIG. 2  is a schematic of a temperature and level measurement system  30 , in accordance with an example embodiment. The system  30  may include a cable  20  with thermocouples (TC1-TCn) attached to a bottom end of the cable  20 . The end of the cable  20  with the thermocouples may be mounted vertically along a sidewall  10   a  of the SFP  10  via brackets  10   d . The cable  20  may extend down the side of the SFP  10  and to a depth that is near the floor  10   b  of the SFP  10 . The cable  20  and thermocouples (TC1-TCn) may be located a distance apart from spent fuel  7  within SFP  10 , so as not to interfere with the maintenance and/or movement of the spent fuel  7 . 
         [0020]    The thermocouples (TC1-TCn) may be used to take temperature measurements within the SFP  10 . In doing so, the liquid level  10   c  of the SFP  10  may be inferred by comparing the temperature readings of the thermocouples to each other. Specifically, thermocouples exposed to ambient air (such as TC10-TC12) will provide a noticeable different temperature reading than the thermocouples (TC1-TC9) below the water level  10   c . For instance, when the water in SFP  10  and the ambient air above SFP  10  is about room temperature, the thermocouples (TC1-TC9) below the water level  10   c  will normally provide a temperature reading that is noticeably below the temperature reading of the thermocouples (TC10-TC12) above the water level  10   c . Therefore, the water level  10   c  may be inferred as being located between thermocouple TC9 and TC10. By knowing the height (relative to the floor  10   b  of SFP  10 ) of thermocouples TC9 and TC10, the liquid water level  10   c  can therefore be determined. Furthermore, when a plant accident occurs and water in the SFP  10  heats up significantly (to temperatures that may reach boiling, at about 212° F.), the level of liquid in the SFP  10  may drop (due to boiling/evaporation of the heated water) to a lower level  10   e . Because the water would be hot during an accident, thermocouples below the water level  10   e  (specifically, thermocouples TC1-TC6) will provide a temperature reading that is higher than thermocouples exposed to air (specifically, thermocouples TC10-TC12, as well as thermocouples TC7-TC9 that were exposed to air due to water level boiling/evaporation). Therefore, the water level  10   e  may be inferred as being located between thermocouple TC6 and TC7. By knowing the height (relative to the floor  10   b  of SFP  10 ) of thermocouples TC6 and TC7, the liquid water level  10   e  can therefore be determined. 
         [0021]    The thermocouples TC may be spaced apart about equidistantly from each other along the cable  20 . Increasing the number of thermocouples (and, thereby, reducing the spacing between the thermocouples) may increase the accuracy of water level measurements of system  30 . Furthermore, prior to taking water level measurements, an accurate measure of the distance between the floor  10   b  of the SFP  10  and each thermocouple TC must be obtained for the system  30  to provide accurate liquid level measurements. 
         [0022]    Because ambient heat (from the water in the SFP  10 , and air above the SFP  10 ) may power the thermocouples (TC1-TCn), no external power source is required to operate system  30 . Wiring  22  may be attached to the top of the cable  20 . A terminal portion  21  of the wiring may extend to a remote location positioned a distance away from the SFP  10 . The remote location may include a simple multi-meter  24  or multiple voltage indicators  24  attached to the thermocouples TC via the wiring  21 . This may provide plant personnel with temperature and level measurement information at a location that is a safe distance from the potentially hazardous environment of the SFP  10  during a plant accident. 
         [0023]    A processor  25  may optionally be included in the system  30 . The processor  25  may be in communication with the simple multi-meter  24  or multiple voltage indicators  24 . The processor  25  may be used to compare temperatures output from the thermocouples TC in order to infer a liquid level in the SFP  10 . Alternative to using a processor  25 , the comparison of the temperatures output from the thermocouples TC may be done manually (by a person) in order to infer a liquid level in the SFP  10 . 
         [0024]      FIG. 3  is a flowchart of a method of using the temperature and level measurement system  30 , in accordance with an example embodiment. Step S 40  includes measuring a temperature of each thermocouple (TC1-TC12). Step S 42  includes comparing the temperatures of the thermocouples to each other. This comparison may include determining a noticeable temperature difference between two groups of the thermocouples (TC1-TC12). For example, when the liquid in SFP  10  is at  10   c  (as shown in  FIG. 2 ), thermocouples TC1-TC9 may share a common temperature reading and thermocouples TC10-TC12 may share a common temperature reading (though the temperatures of TC1-TC9 and TC10-TC12 may be noticeably different from each other). Step S 44  includes inferring the liquid level  10   c  of the SFP  10  based on the comparison of the thermocouple temperature readings. For instance, knowing that thermocouple groups TC1-TC9 and TC10-TC12 share a noticeable temperature difference, it can be inferred that consecutive thermocouples TC9 and TC10 bracket the water level  10   c . If TC9 and TC10 are a known distance from the floor  10   b  of the SFP  10 , the height of the water level  10   c  (somewhere between TC9/TC10) may then be determined. Step S 44  may be accomplished manually (by a person), or accomplished by processor  25 . 
         [0025]    Example embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the intended spirit and scope of example embodiments, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.