Patent Publication Number: US-2023142980-A1

Title: Coolant cleanup and heat-sinking systems and methods of operating the same

Description:
RELATED APPLICATIONS 
     This application is a divisional of, and claims priority under 35 U.S.C. §§ 120 &amp; 121 to, U.S. patent application Ser. No. 16/582,638, filed Sep. 25, 2019, now U.S. Pat. No. 11,545,274, the original contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Nuclear reactors with high operating temperatures may use a fluid heat exchange media, such as a liquid metal or molten salt, for coolant. The heat exchange media may transfer heat from a reactor to a heat exchanger and/or turbine for energy extraction and electricity generation as well as act as a heat sink to remove decay heat or other unwanted heat during operation or a shutdown condition. Many reactor designs, including, for example, liquid sodium-cooled fast reactors, such as the PRISM reactor, use multiple loops of heat exchange media to efficiently transfer heat away from a reactor for electrical generation and cooling. One loop may be an intermediate loop that is heated in an intermediate heat exchanger and then passed through a steam generator connected to a turbine and generator. Any fluid heat exchange media, such as liquid lead or sodium, molten salts, etc. may be used for this heat exchange in the intermediate loop. 
     Intermediate loops using fluid media may benefit from cleanup of the heat exchange media to remove impurities or debris that may accumulate during operation in a nuclear reactor environment.  FIG.  1    is an illustration of a related art cleanup system  10  useable with an intermediate loop carrying a fluid heat exchange media. For example, system  10  may be a sodium cleanup loop useable with an intermediate coolant loop of a liquid sodium reactor or molten salt reactor. 
     As shown in  FIG.  1   , system  10  includes input  50  and output  67  that may connect to a same leg of an intermediate coolant loop, just far enough apart to prevent backflow or short-circuiting between the two, such as a few feet apart. Input  50  and output  67  may be intake from and returns to an intermediate coolant loop, removing and then re-supplying a relatively small amount of coolant from/to the intermediate loop. Pump  51  may push the fluid coolant through system  10 . Regenerative heat exchanger  60  may be used to initially cool an incoming coolant stream  61  with outgoing, cooler coolant that is to be resupplied to the intermediate loop by output  67 . The cooled coolant stream  62  may then flow to cooler  70 , which may be a series of smaller tubes with fins exposed to an open air fan  71  to convect away further heat. Cooler  70  may lower the temperature of the coolant sufficiently so that impurities, such as oxides, will solidify or precipitate from the fluid coolant. 
     Purifier  80  may include chemical reactants, catalysts, and/or mechanical filters like cold traps, mesh, or other filter media that removes impurities or debris, including precipitates that come out of solution, following cooler  70 . Bypass valves  81  and  82  may permit flow bypass of purifier  80 , allowing flow to be raised or lowered slowly, and otherwise controlled, through purifier  80  during startup or shutdown. Colder, filtered coolant then passes back through regenerative heat exchanger  60  through input  66  to reheat the coolant to near operating temperatures before being returned to an intermediate loop via output  67 , typically just downstream from inlet  50  in the intermediate loop. In this way, the coolant passed through system  10  for cleanup minimizes heat loss from the intermediate loop. 
     SUMMARY 
     Example embodiments include combined cleanup and heat removal systems and coolant loops joined to the such systems. The coolant loops may have a hot leg connecting between the reactor to a heat extractor like a steam generator or heat exchanger and a cold leg opposite the hot leg returning from the heat extractor to the reactor. Example embodiment cleanup and heat sink systems connect to the hot leg and/or cold leg and, depending on plant situation and/or operator input, function to remove impurities or debris from the fluid coolant flowing in the loop and/or remove a substantial amount of heat from the fluid coolant. The combined system may selectively create flow between the hot leg and the cold leg, which may bypass the heat extractor entirely to permit draining and shutdown operations on the same, even as the reactor is still generating large amounts of heat. Similarly, the combined system may work on a single leg and prevent significant heat loss while cleaning the coolant during normal reactor and heat extractor operation. Intermediate modes are also possible, depending on flow path creation, pumping, and/or cooler operations. Purification may be achieved with a cold trap, for example, cooler connected serially with an outlet, and potentially a regenerative heat exchanger, back into the coolant loop, while heat sinking may be achieved by the cooler, potentially operating in a larger-capacity mode, connected in parallel to a bypass outlet back into the coolant loop. 
     Because the combined system may selectively provide both cleanup and significant cooling to the coolant loop, the system may be structured to operate between both these modes in desired levels of combination. For example, a cooler in the system may switch between modes, or levels of, heat removal. One mode may remove only a small amount of heat from the coolant sufficient to solidify or otherwise precipitate impurities from the coolant, while another mode may sink significant amounts of heat from the coolant, potentially up to full decay heat or even reactor operational levels of heat. Such modality from impurity-removal to heat-sinking levels may be achieved by increasing forced convection, increasing flow path volume flow rate, changing heat sink media, etc. Similarly, inlet volume flow rate may be increased, pumping pressure may be increased, and/or flow paths connecting the hot leg and cold leg of the coolant loop while avoiding a purifier like a cold trap and any regenerative heat exchanger in the system may be created, such as by valves, between these modes. 
     Example embodiment coolant loops and cleanup/cooler systems are useable in a variety of plants and coolants, including fluid media like a liquid sodium coolant used in a PRISM reactor. Coolant loops may provide for entire bypass of a primary heat extractor like a steam generator by directly connecting hot and cold legs through the cleanup-cooler systems, allowing for isolation and draining of the heat extractor and related pumps for maintenance. The hot leg and cold legs may include portions filled with fluid columns extending vertically higher than cooler, which itself may be above the reactor, and the hot and cold leg in the loop may be positioned with slightly angled horizontal paths that decline back toward the reactor, to prevent backflow into the heat extractor. Example embodiments may thus be installed and operated with several types of coolant loops already existing with purifiers in nuclear reactors, simply by adding additional cooler capacity and/or additional outlets to opposing portions of the loop. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Example embodiments will become more apparent by describing, in detail, the attached drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus do not limit the terms which they depict. 
         FIG.  1    is an illustration of a related art coolant cleanup system. 
         FIG.  2    is an illustration of an example embodiment coolant cleanup and heat sinking system. 
         FIG.  3    is an illustration of an example embodiment intermediate loop useable with a nuclear reactor. 
     
    
    
     DETAILED DESCRIPTION 
     Because this is a patent document, general, broad rules of construction should be applied when reading it. Everything described and shown in this document is an example of subject matter falling within the scope of the claims, appended below. Any specific structural and functional details disclosed herein are merely for purposes of describing how to make and use examples. Several different embodiments and methods not specifically disclosed herein may fall within the claim scope; as such, the claims may be embodied in many alternate forms and should not be construed as limited to only examples set forth herein. 
     It will be understood that, although the ordinal terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited to any order by these terms. These terms are used only to distinguish one element from another; where there are “second” or higher ordinals, there merely must be that many number of elements, without necessarily any difference or other relationship. 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 or methods. As used herein, the terms “and,” “or,” and “and/or” include all combinations of one or more of the associated listed items unless it is clearly indicated that only a single item, subgroup of items, or all items are present. The use of “etc.” is defined as “et cetera” and indicates the inclusion of all other elements belonging to the same group of the preceding items, in any “and/or” combination(s). 
     It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” “fixed,” etc. to another element, it can be directly connected to the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” “directly coupled,” etc. 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.). Similarly, a term such as “communicatively connected” includes all variations of information exchange and routing between two electronic devices, including intermediary devices, networks, etc., connected wirelessly or not. 
     As used herein, the singular forms “a,” “an,” and the  are intended to include both the singular and plural forms, unless the language explicitly indicates otherwise. Indefinite articles like “a” and “an” introduce or refer to any modified term, both previously-introduced and not, while definite articles like “the” refer to a same previously-introduced term; as such, it is understood that “a” or “an” modify items that are permitted to be previously-introduced or new, while definite articles modify an item that is the same as immediately previously presented. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, characteristics, steps, operations, elements, and/or components, but do not themselves preclude the presence or addition of one or more other features, characteristics, steps, operations, elements, components, and/or groups thereof. As used herein, “axial” and “vertical” directions are the same up or down directions oriented with gravity. “Transverse” and “horizontal” directions are perpendicular to the “axial” and are side-to-side directions in a plane at a particular axial height. 
     The structures and operations discussed below may occur out of the order described and/or noted in the figures. For example, two operations and/or figures shown in succession may in fact be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Similarly, individual operations within example methods described below may be executed repetitively, individually or sequentially, to provide looping or other series of operations aside from single operations described below. It should be presumed that any embodiment or method having features and functionality described below, in any workable combination, falls within the scope of example embodiments. 
     The Inventors have newly recognized that cleanup systems may be used as a heat sink in a nuclear reactor, instead of merely removing impurities from coolant. The Inventors have further newly recognized that cleanup systems may be used as alternative or parallel coolant loops while intermediate coolant loops are drained and worked on, such as during plant maintenance. While these uses of cleanup systems are contrary to their established functions, the Inventors have recognized that they may solve long-standing problems of emergency cooling and operations maintenance that have traditionally been solved by using other systems and/or fully shutting down a plant. Example embodiments described below uniquely enable these solutions to these and other problems discovered by the Inventors. 
     The present invention is heat-sink purifier systems, nuclear reactors using the same, and methods of using the same. In contrast to the present invention, the few example embodiments and example methods discussed below illustrate just a subset of the variety of different configurations that can be used as and/or in connection with the present invention. 
       FIG.  2    is an illustration of an example embodiment decay heat removal system  100  useable in a commercial nuclear power plant. As seen in  FIG.  2   , several features of example embodiment system  100  may be similar to the related art system  10  of  FIG.  1   . In this way example embodiment system  100  is also useable in connection with an intermediate loop carrying a molten heat transfer medium, in a number of different nuclear plant designs. Example embodiment system  100  includes an additional, higher capacity inlet  150  from the intermediate loop, as well as an additional, higher capacity outlet  180  into the intermediate loop. Inlet  150  may, for example, be a valved connection to a hotter side or hot leg  4  ( FIG.  3   ) of an intermediate loop where coolant exits a reactor. Similarly, outlet  180  may be, for example, a valved connection to a colder side or cold leg  8  ( FIG.  3   ) of the intermediate loop where coolant enters the reactor. Inlet  150  and outlet  180  may be separated by great distances, potentially even at opposite sides of an intermediate loop. 
     Example embodiment decay heat removal system  100  has increased flow and heat transfer capacity to dissipate or sink a substantial portion of heat in the intermediate loop. As such, system  100  may act as a decay heat removal system by removing such heat form the intermediate loop and ultimately the reactor, instead of avoiding heat loss. To accommodate this large-scale heat sinking, additional or larger-scale cooler  170  and fan  171 , as well as additional parallel and/or higher-volume pump  151 , may be used to remove a substantial amount of heat from a larger amount of coolant directed through example embodiment system  100 . For example, system  100  may remove heat equivalent to about 7% of full rated thermal power of a plant. Of course, the amount of heat varies based on plant, one example may sink 5 megawatt-thermal heat from an 840 megawatt-thermal rated plant. Smaller values may also be achieved through selective activation of cooler and flow paths, such as for partial removal of decay heat in combination with other heat removal systems. 
     Selective activation may be achieved by, for example, cooler  170  including several parallel channels with fins to selectively accommodate larger flows, and/or fan  171  including several speeds or multiple fans or higher-pressure blowers that can be selectively activated to convect large amounts of heat. Or, for example, larger-scale cooler  170  may include other coolant media, submerged sections, counter-flow heat exchangers, printed-circuit heat exchangers, plate-and-frame heat exchangers, and other heat sinks in parallel that can be turned on to selectively dissipate large amounts of heat from the coolant. In this way, cooler  170  may seamlessly change from a purifying mode that removes little heat, such as 0.5 MW or less, from a coolant to a heat-sinking mode that removes much heat, such as around 5 MW or more, from the coolant. 
     Example embodiment decay heat removal system  100  may be scaled between increased decay heat removal and lower-level cooling useable for purification, such as cold trapping. For example, connections  150  and  180  may be shut off, such as by valves, during normal plant operations without excess heat loss, and system  100  may act as a purification system with purifier  80 , returning flow to outlet  67  and receiving flow from inlet  50  nearby in an intermediate loop. When additional cooling is necessary, such as during a transient involving reactor shutdown or loss of other cooling systems, connections  150  and  180  may be opened to enable larger coolant flows, and pump  151 , cooler  170 , and/or fan  171  may be increased in speed, number, and/or type, to increase heat dissipation from larger coolant flows. Similarly, valves  81  and/or  82  may be closed to avoid purifier  80  and/or reheater  60  when example embodiment system  100  is selectively scaled to decay heat sink levels. Closing off purifier  80  may create direct and/or exclusive coolant flow between connections  150  and  180 , improving heat sinking through example system  100  in the additional cooling state. In this way, example embodiment system is compatible with nearly any coolant loop using a cold trap or other purifier, while still providing optional functionality of a selectively-activatable increased heat sink. 
       FIG.  3    is an illustration of an example embodiment intermediate coolant loop  200  useable in nuclear reactors, including higher-temperature reactors such as a PRISM reactor or molten salt reactor. As shown in  FIG.  3   , example embodiment intermediate coolant loop  200  may interface with several related or conventional reactor components including reactor  1  housing core  2  with nuclear fuel. An intermediate heat exchanger  3  transfers heat from reactor  1  to intermediate coolant loop  200 , which in turn may transfer heat to an extractor like a steam generator  6  or heat exchanger for electricity generation. 
     As shown in  FIG.  3   , intermediate coolant loop  200  is interfaced with example embodiment decay heat removal system  100  ( FIG.  2   ) via inlet  150  and outlet  180 . For example, inlet  150  may take coolant from a bottom of hot leg  4 , where coolant first exits reactor  1  and has its highest energy, and outlet  180  may return coolant to a bottom of cold leg  8 , where coolant is returned to reactor  1  and has its lowest energy. For typical cold-trapping purification, inlets  50  and outlets  67  ( FIG.  2   ) might take from a same or nearby position on a same leg to prevent heat loss, unlike inlet  150  and outlet  180  that may be segregated at temperature extremes in example embodiment intermediate coolant loop  200 . During a transient state or when larger heat-sinking is desired, inlets  50  and/or outlets  67  may be closed, and inlet  150  and outlet  180  may be opened or enabled to remove heat from coolant that ultimately flows back through intermediate heat exchanger  3 , cooling reactor  1 . 
     Example embodiment intermediate coolant loop  200  can also be operable with intermediate pump  7  and steam generator  6 , or other heat extractor, drawing heat from the coolant to generate electricity. Intermediate pump  7  and/or steam generator  6  may optionally be deactivated and drained while coolant loop  200  still circulates coolant and sinks heat through inlet  150  and outlet  160 . For example, intermediate pump  7 , steam generator  6 , and/or portions of hot leg  4  and cold leg  8  may be drained into drain tank  5 , such as through opening drain valves to drive coolant by gravity into drain tank  5  and/or through active pumping. 
     Proper sloping of piping in hot leg  4  and cold leg  8  may permit draining of pump  7  and steam generator  6  and their associated piping. For example, horizontal piping of hot leg  4  and cold leg  8  may be at slight angles with respect to the vertical, such as slightly declined toward steam generator  6  and away from reactor  1  at 5-10 millimeters vertical drop per meter length. This decline may further prevent backflowing and ensure coolant looping only through a portion of example embodiment intermediate coolant loop  200  in combination with example positioning discussed below. 
     Hot leg  4  and cold leg  8  may be arranged such that a column of fluid in hot leg  4  may be at a vertical height  240  and fluid in cold leg  8  may be at a vertical height  280 . Columns of fluid in these legs may remain even though other portions of loop  200  are drained. Because of the presence of the columns of fluid at vertical heights  250  and  280  above inlet  150  on hot leg  4  and outlet  180  on cold leg  8 , coolant may still be circulated between intermediate heat exchanger  3  and a decay heat removal system  100  ( FIG.  2   ) via the lower portions of hot leg  4  and cold leg  8 . In this way, it is possible to repair or otherwise work on an emptied steam generator  6 , intermediate pump  7 , and/or any other drained portions of coolant loop  200  while still removing heat from reactor  1  via intermediate heat exchanger  3 . Of course, example embodiment coolant  200  with system  100  may also be used with a completely-filled loop. 
     Similarly, in  FIG.  3   , system  100 , or at least cooler  170  ( FIG.  2   ) of system  100 , may be placed at a vertical height  231  above intermediate heat exchanger  3  at vertical height  230 . The difference in vertical heights  230  and  231  may create natural circulation driving forces, where coolant heated at heat exchanger  3  rises due to lowered density, flows to cooler  170  and is cooled, increasing its density, which then flows by density difference back to heat exchanger  3 . This configuration and associated natural circulation may eliminate or reduce the need for active pumping, such as with pump  151  or  7 . If all other coolant-filled portions of system  100  are below elevations  240  and  280  of coolant columns, natural circulation will occur in loop  200  through heat exchanger  3  due to gravity and because voids cannot form in system  100  below. 
     As seen in  FIGS.  2  and  3   , example embodiment intermediate coolant loop  200  and example embodiment decay heat removal system  100  can be used with several types of nuclear reactors and existing components. Some functionality of loop  200  and system  100  may be achieved simply by increasing capacity of inlet  50  to that of inlet  150 , increasing heat sink capacity of a cooler, and adding an exclusive return outlet  180  to cold leg  8 . Loop  200  and system  100  may be used during typical reactor operation to remove impurities and/or debris from a relatively small stream of coolant, as well as being selectively scaled to remove all or a significant portion of decay heat or even operation heat from reactor  1  during a transient or non-electricity generating state, such as during an accident or plant maintenance. Similarly, multiple loops  200  and systems  100  are useable with a single reactor  1  to provide even larger amounts of heat transfer and sinking from reactor  1 . 
     Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied and substituted through routine experimentation while still falling within the scope of the following claims. For example, any number of different reactor types and thermodynamic cycles can be used with example embodiments, simply by allowing for different temperatures and coolants. Such variations are not to be regarded as departure from the scope of these claims.