Abstract:
A supplementary injection device is installed in a nuclear power plant to draw coolant and inject coolant using an entraining fluid. The injection device can be a venturi or other passive device operable at relatively low fluid pressure that draws coolant through suction at the venturi narrowing point and mixes the coolant with the fluid for injection. The injection device is operable with a known BWR design, where the device is attached to a steam connection to the main steam line of the reactor, a coolant connection drawing from suction lines to a suppression cool or condensate tank, and an outlet connection injecting into the main feedwater lines. In a BWR, the injection device is operable without electricity and at a wide range of pressures, even less than 50 pounds per square inch, to maintain coolant levels in the reactor.

Description:
BACKGROUND 
       [0001]      FIG. 1  is a schematic diagram of a conventional commercial nuclear power reactor and various safety and cooling systems for the same. As shown in  FIG. 1 , a reactor  10  is positioned inside of a containment structure  1 . During operation of reactor  10 , liquid water coolant and moderator enters the reactor  10  through main feedwater lines  60  that are typically connected to a heat sink and source of fluid coolant, like a condenser cooled by a lake or river. Recirculation pump  20  and main recirculation loops  25  force flow of the liquid down through a bottom of the reactor  10 , where the liquid then travels up through core  15  including nuclear fuel. As heat is transferred from fuel in core  15  to the liquid water coolant, the coolant may boil, producing steam that is driven to the top of reactor  10  and exits though a main steam line  50 . Main steam line  50  connects to a turbine and paired generator to produce electricity from the energy in the steam. Once energy has been extracted from the steam, the steam is typically condensed and returned to the reactor  10  via feedwater line  60 . 
         [0002]    In the instance that recirculation pump  20  fails and/or liquid coolant from main feedwater lines  60  are lost, such as in a station blackout event where access to the electrical grid is cut off, reactor  10  is typically tripped so as to stop producing heat through fission. However, significant amounts of decay heat are still generated in core  15  following such a trip, and additional fluid coolant may be required to maintain safe core temperatures and avoid reactor  10  overheat or damage. In these scenarios, active emergency cooling systems, such as a Reactor Core Isolation Cooling (RCIC) turbine  40  or higher-output High Pressure Injection Cooling (HPIC) turbine, for example, operate using steam produced in core  15  by decay heat to drive turbines. Flow from main steam lines  50  is diverted to RCIC lines  55  in this instance. RCIC turbine  40  may then drive an RCIC pump  41 , which injects liquid coolant from a suppression pool  30  or condensate storage tank  31  into main feedwater line  60  via RCIC suction line  35  and injection line  42 . The injected liquid coolant maintains a coolant level in reactor  10  above core  15  and transfers decay heat away from core  15 , preventing fuel damage. Saturated steam coming off RCIC turbine  40  can be condensed in suppression pool  30  by venting into suppression pool  30  via RCIC exhaust line  43 . 
         [0003]    RCIC turbine  40  typically requires a minimum steam pressure of 150 pounds/square inch in order to drive RCIC pump  41  to inject liquid coolant into main feedwater line  60  via injection line  42  and suction line  35 . Pressure in main steam lines  50  from an outlet of reactor  10  will typically drop below 150 pounds/square inch after  8 - 20  hours of shutdown, at which time RCIC turbine  40  and other higher-pressure injection systems will not function. At this time, lower-pressure shutdown coolant injection systems (not shown) are activated and run off electricity from the electrical grid, or, in the station blackout event, emergency diesel generators. As long as an electricity source is available, lower-pressure injection systems can maintain safe temperatures and fluid level in core  15  until cold shutdown can be achieved or transient circumstances have ended and core  15  can resume generating power through fission. Regulatory bodies worldwide typically require these active systems, including RCIC systems and electricity-powered lower-pressure delivery systems, as the sole mechanisms to avoid core overheat and damage in transient scenarios involving loss of coolant and/or loss of offsite power. 
       SUMMARY 
       [0004]    Example embodiments include methods and systems for cooling a nuclear reactor post-shutdown with a passive injection device connected to the reactor that injects a coolant into the reactor or a steam generator for the same using a local energetic fluid to drive the injection. Example embodiment injection devices work using fluids having pressure ranges with lower limits below those used in the operating nuclear reactor and those used to drive conventional coolant injection systems post-shutdown. The local energetic fluid may be supplied by the reactor itself; for example, in a Boiling Water Reactor (BWR) the passive injection device may use steam created by heating a coolant in the reactor. Similarly, in a Pressurized Water Reactor the passive injection device may use steam from a steam generator and inject coolant into the same. Example embodiment injection devices can passively inject coolant, without moving parts or electricity, using the local energetic fluid to suction and/or entrain the coolant and delivering the mixed fluid and coolant to the reactor. For example, an injection device may be a venturi that accelerates the fluid to create a pressure drop and draw the coolant into the fluid flow, which is then injected into the reactor. An example venturi may include a fluid inlet receiving the energetic fluid source, which then flows through a narrowing section to cause the acceleration and pressure drop, a coolant inlet at the narrowing section through which the coolant is drawn and entrained, and an outlet where the mix is injected into the nuclear reactor. For example, in a light water reactor, the coolant can be liquid water drawn from a suppression pool or other condensed source. 
         [0005]    Example methods include installing a passive, low-pressure-compatible injection device between a coolant source and the reactor and supplying the same with an energetic fluid. For example, a venturi can be installed off an RCIC line connected to a main steam line of a BWR, with the venturi on an RCIC suction line where the venturi can draw water from a suppression pool or condensate tank and inject the water into the reactor using steam from the main steam line. Example methods may further include operating one or more valves to selectively operate the injection device by providing it with fluid connection to the various coolant and fluid sources. Such operation may be executed any time coolant injection into the reactor is desired, such as post-shutdown following a complete station blackout transient after reactor pressure has dropped to levels at which RCIC and other active injection systems cannot operate, in order to maintain coolant to the reactor for several days or weeks following such a transient. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0006]    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. 
           [0007]      FIG. 1  is a schematic diagram of a conventional commercial nuclear reactor coolant injection system. 
           [0008]      FIG. 2  is a schematic diagram of an example embodiment passive low pressure coolant injection system. 
           [0009]      FIG. 3  is an illustration of an example embodiment venturi useable in example embodiment systems. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    This is a patent document, and general broad rules of construction should be applied when reading and understanding it. Everything described and shown in this document is an example of subject matter falling within the scope of the appended claims. Any specific structural and functional details disclosed herein are merely for purposes of describing how to make and use example embodiments. Several different embodiments not specifically disclosed herein 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 example embodiments set forth herein. 
         [0011]    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. 
         [0012]    It will be understood that when an element is referred to as being “connected,” “coupled,” “mated,” “attached,” or “fixed” to another element, it can 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.). Similarly, a term such as “communicatively connected” includes all variations of information exchange routes between two devices, including intermediary devices, networks, etc., connected wirelessly or not. 
         [0013]    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 with words like “only,” “single,” and/or “one.” It will be further understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, steps, operations, elements, ideas, and/or components, but do not themselves preclude the presence or addition of one or more other features, steps, operations, elements, components, ideas, and/or groups thereof. 
         [0014]    It should also be noted that 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, so as to provide looping or other series of operations aside from the single operations described below. It should be presumed that any embodiment having features and functionality described below, in any workable combination, falls within the scope of example embodiments. 
         [0015]    Applicants have recognized that plant emergency power systems, including local batteries and emergency diesel generators, may become unavailable in confounding combination with loss of access to the electrical grid during certain plant transients. That is, a transient event that cuts offsite power may also render unusable emergency diesel generators. In such a situation, active high-pressure injection systems, such as RCIC turbine  40  and pump  41 , can provide fluid coolant flow to a reactor  10  to remove decay heat from the same for several hours; however, once reactor pressure falls below the high-pressure injection systems&#39; operating pressure (typically within a day of the transient event), low-pressure injection systems must be initiated to provide liquid coolant makeup to reactor  10 , which is still generating large amounts of decay heat. If emergency diesel generator and local power grid access are unavailable, conventional low-pressure injection systems cannot be operated, and battery-based systems are insufficient to prevent eventual loss of liquid coolant level in core  15  due to decay heat, greatly increasing the risk of fuel damage. 
         [0016]    As such, Applicants have recognized an unexpected need for reliable reactor liquid coolant injection that is available without batteries or the electrical power grid starting almost a day after, and continuing several weeks after, a transient event that cuts both offsite power and local emergency power generation. Applicants have identified that using a steam source, such as low pressure steam from reactor  10  at below 150 pounds/square inch, may power some devices capable of injecting liquid coolant into reactor  10 , at lower but sufficient flow rates to prevent core  15  from becoming uncovered or overheated for weeks, with proper device and system engineering. Example embodiment systems and methods discussed below address and overcome these problems identified by Applicants in unique and advantageous ways. 
         [0017]      FIG. 2  is a schematic drawing of an example embodiment passive low-pressure injection system  100  useable in conventional and future water-cooled nuclear power plants. It is understood that although example embodiment  100  is shown using light water as a liquid coolant in a conventional BWR, other plant and coolant types are useable as example embodiments. Reference characters shared between  FIGS. 1 and 2  label plant components that may be in existing systems, and whose redundant description is omitted. 
         [0018]    As shown in  FIG. 2 , example embodiment system includes a low-pressure injection device  110  that is operable to inject coolant from a source, such as suppression pool  30  and/or condensate storage tank  31 , into reactor  10 . Low-pressure injection device  110  is operable at pressures below those required to operate conventional high-pressure systems, such as RCIC turbine  40 , in order to provide parallel cooling to reactor  10  at lower pressures. Low-pressure injection device  110  may be operable at pressures where conventional high-pressure systems operate, additionally allowing low-pressure injection device  110  to supplement such higher-pressure systems. 
         [0019]    For example, low-pressure injection device  110  may be a venturi device that receives steam from reactor  10 , passes the steam through a venturi that accelerates the steam and causes a suction/pressure drop, thereby drawing and entraining liquid coolant from suppression pool  30  and/or or condensate storage tank  31 , and then injects the resultant steam-liquid mixture into reactor  10  to makeup liquid coolant volume of reactor  10 . Such an example venturi tube for low-pressure injection device  110  is shown in  FIG. 3 . For example, as shown in  FIG. 3 , relatively lower-pressure steam from a reactor  10  can be routed into venturi  110  from main steam diversion line  155 . In a narrowing section  111  of venturi  110 , the steam may increase velocity with resultant pressure drop, or suction, under Bernoulli&#39;s principle. In this example, the suction draws liquid coolant from suction diversion line  135  into venturi  110 , where the coolant is entrained in the steam flow through venturi  110 . Venturi  110  may include a diffuser section  112  that decreases flow velocity and increases pressure of the resulting liquid coolant/steam flow to that necessary for injection into reactor  10  via injection diversion line  142 , or to some other desired pressure and velocity for compatibility with example embodiment systems. The liquid coolant may also condense a significant portion of steam flow through venturi  110  when mixing, yielding even more liquid coolant for injection into reactor  10 . Venturi  110  may be sized in a diameter and length and otherwise configured, such as in angle of narrowing section  111  and/or presence of diffuser section  112 , to provide desired flow characteristics to reactor  10  given the arrangement, parameters, and anticipated transient conditions of example embodiment system  100  in which venturi  110  operates. 
         [0020]    Venturi  110  generally includes few or no moving parts and may provide suction and liquid coolant entrainment/injection passively as long as a minimally pressurized steam flow from reactor  10  is connected to venturi  110 . For example, venturi  110  may be operable to draw and entrain fluid from suppression pool  30 /condensate tank  31  at about 150 to 50 pounds per square inch or less, well below an operating pressure of RCIC turbine  40 . Similarly, venturi  110  may be operable at pressures well above 150 pounds per square inch to supplement or replace any RCIC turbine  40  and pump  41  or other high-pressure injection systems. Further, venturi  110  may have very few energy losses, permitting efficient energy transfer from pressurized steam flow to liquid coolant injection. For example, with typical decay heat generated by commercial nuclear reactors, venturi  110  may be able to reliably inject sufficient liquid coolant to maintain coolant level above core  15  for several days or weeks before pressure in reactor  10  would be inadequate to operate venturi  110  and maintain required liquid coolant injection. Additionally, venturi  110  may be relatively simple and reliable, requiring no outside power or moving parts, so as to present very little opportunity for failure, even in transients involving emergency conditions and total station blackout, with easy installation and fabrication. 
         [0021]    Although the example embodiment of  FIG. 3  shows a particular venturi for low-pressure injection device  110 , it is understood that other reliable low-pressure injection devices may be used instead of a venturi in example embodiment system  100 . For example, low-pressure injection device  110  could be a choke plate, a nozzle, aspirator, and/or any other device that can reliably and passively drive liquid coolant into reactor  10  using only lower-pressure steam. 
         [0022]    In an example embodiment coolant system  100 , low-pressure injection device  110  is connected to a steam source, a liquid coolant source, and a reactor inlet to deliver entrained liquid coolant. These sources and connections may be achieved in several flexible ways, depending on the arrangement of a reactor and associated coolant systems. As shown in  FIG. 2 , for example, low-pressure injection device  110  can be connected to a main steam line  50  of reactor  10 , via RCIC line  55  and an isolated main steam diversion line  155 . Suction diversion line  135  may connect low-pressure injection device  110  to liquid coolant sources such as suppression pool  30  and/or condensate makeup tank  31  via conventional suction line  35 . Low-pressure injection device  110  may inject its entrained liquid coolant back into injection line  42  via injection diversion line  142  for delivery to reactor  10  through main feedwater line  60 . Any or all of main steam diversion line  155 , suction diversion line  135 , and injection diversion line  142  may include valves that permit isolation or activation of low-pressure injection device  110  through automatic or manual valve activation. For example, simple swing check valves may be used in main steam diversion line  155 , suction diversion line  135 , and/or injection diversion line  142  to reliably operate low-pressure injection device  110  when desired. 
         [0023]    Of course, a venturi or other low-pressure injection device  110  may be placed in any configuration with access to a steam source, a liquid coolant source, and injection to reactor  10  in order to provide reliable low-pressure coolant injection in example embodiment system  100 , in approximate parallel with conventional active emergency cooling systems. For example, low-pressure injection device  110  could be positioned directly between a heat sink and liquid coolant source, such as a river or lake, and an inlet of reactor  10  with access to any steam source in order to drive liquid coolant into reactor  10 . Similarly, low-pressure injection device  110  could be positioned in direct parallel with RCIC turbine  40  and pump  41  and operate simultaneously with these or other systems, and/or be switched to exclusive use upon failure of these or other systems. 
         [0024]    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, although example embodiments are described in connection with BWRs using light water as a liquid coolant in nuclear power plants, it is understood that example embodiments and methods can be used in connection with any reactor cooling system where energetic fluid input can be used to entrain and inject a coolant into the reactor or a heat sink/steam generator of the reactor, including heavy-water, gas-cooled, and/or molten salt reactors. For example, superheated helium coolant could be diverted from a pebble bed reactor output and into an example embodiment injection device such as an orifice plate or venturi and be used to passively draw and entrain colder helium or another fluid coolant for injection into the reactor with relatively low pressures to maintain core temperatures and/or coolant flow. Such variations are not to be regarded as departure from the scope of the following claims.