Abstract:
Filtering systems and methods remove debris from coolant in a nuclear reactor setting. One or more filters are installed outside coolant reservoirs specifically where coolant will flow toward the reservoir, such as during a transient or other coolant leak event. Useable filters permit coolant through-flow while catching, straining, diverting, or otherwise removing debris from the coolant without significant interference with the coolant flow. 
     Filters can be installed at any location in a flow path for coolant flowing toward the reservoir, including pipes draining into a suppression pool, floor or personnel platform gratings, areas around main steam legs or steam generators, in a reactor drywell, etc. One or more filters are installed by securing the filter in a coolant flow path into a coolant source. Installation and maintenance can be performed during any maintenance period.

Description:
BACKGROUND 
       FIG. 1  is a schematic of a conventional nuclear power station containment building  36  that houses a reactor pressure vessel  42  with various configurations of fuel  41  and reactor internals for producing nuclear power. Reactor  42  sits in a drywell  51 , including upper drywell  54  and a lower drywell  3  that provides space surrounding and under reactor  42  for external components and personnel. Several different pools and flowpaths constitute an emergency core coolant system inside containment  36  to provide fluid coolant to reactor  26  in the case of a transient involving loss of cooling capacity in the plant. 
     For example, containment  36  may include a pressure suppression chamber  58  surrounding reactor  42  in an annular or other fashion and holding suppression pool  59 . Suppression pool  59  may include an emergency steam vent used to divert steam into suppression pool  59  for condensation and heat sinking, to prevent over-heating and over-pressurization of containment  36 . Suppression pool  59  may also include flow paths that allow fluid flowing into drywell  54  to drain, or be pumped, into suppression pool  59 . Suppression pool  59  may further include other heat-exchangers or drains configured to remove heat or pressure from containment  36  following a loss of coolant accident. An emergency core cooling system line and pump  10  may inject coolant from suppression pool  59  into reactor  42  in order to make up lost feedwater and/or other emergency coolant supply. 
     Lines taking coolant from suppression pool  59 , either for injection into reactor  42  via ECCS line  10  or for heat exchanging outside containment  36  or other uses, conventionally use an intake strainer in suppression pool  59  to filter debris found in suppression pool  59 . For example, US Patent Publication 2011/0215059 to Carr et al. and 2006/0219645 to Bilanin et al., discuss strainers submerged and used in suppression pool  59  to prevent debris from entering ECCS pumps and reactor  42 . The US NRC&#39;s “Resolution of Generic Safety Issues: Issue 191: Assessment of Debris Accumulation on PWR Sump Performance (Rev. 2) (NUREG-0933, Main Report with Supplements 1-34)” of Mar. 29, 2012 further discusses the effects of debris from suppression pool  59  on reactor and emergency system chemistry and operation. The disclosures of each of these publications are incorporated herein in their entireties. 
     As shown in  FIG. 1 , other emergency cooling systems are useable with reactor  42 , including a gravity-driven cooling system pool  37  that can further provide coolant to reactor  42  via piping  57  and/or a passive containment cooling system pool  65 . Any or all discussed safety system may be used alone or in any combination in various reactor designs, each to the effect of preventing overheating and damage of core  41 , reactor  42  and all other structures within containment  36  by supplying necessary coolant, removing heat, and/or reducing pressure. 
     SUMMARY 
     Example embodiments include filtration systems and methods that remove debris from coolant in a nuclear power reactor. Example embodiments are useable in a wide variety of nuclear power reactor setups that include some sort of reactor vessel and an associated coolant source into which leaking or released coolant can flow, such as a suppression pool, for example. Example embodiments include one or more filters installed outside the source at specific locations where coolant will flow toward the source. Example embodiments may use filters that facilitate coolant flow and debris removal without absorbing or otherwise impeding coolant flow, such as fuel-assembly-type filters having fixed channels. 
     Some example locations for installation may include pipes or tubes draining into a suppression pool, floor or personnel platform gratings, areas around main steam legs or steam generators, and/or any other location in a flow path for escaping coolant. A filter in example embodiments can be installed in an “open-air” or voided location where the filter will contact coolant only in the instance of a leak, transient, or other operating condition that causes coolant to flow through areas that are otherwise not occupied by coolant, such as in a reactor drywell, for example. 
     Example methods include installing example filtering systems by affixing a filter at a location known to be a coolant flow path into a coolant source. Installation and maintenance of any installed filter can be performed during any maintenance period, such as during an operational outage for refueling, for example. Example methods may install one or more filters at locations that are not within coolant and thus may not require any special adaptation for a coolant environment; for example, if a coolant is water, no diving or submersible activity may be required for installation. 
    
    
     
       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 a schematic of a related art nuclear power containment and internals. 
         FIG. 2  is a schematic of a nuclear power containment using an example embodiment system 
         FIG. 3  is an illustration of an example embodiment filter. 
     
    
    
     DETAILED DESCRIPTION 
     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 or methods. 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. 
     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. 
     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. 
     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. 
     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. 
     Applicants have recognized problems in existing debris-mitigation systems for nuclear power stations. Particularly, Applicants have recognized that various strainers and filters placed in coolant sources may easily become clogged when their operation is most critical to plant safety and/or may not adequately prevent smaller debris from being injected into a reactor in the instance of an emergency. Such smaller debris may be readily created in transient situations due to failure of plant components and/or operation of rarely-used safety systems. When a strainer immersed in a suppression pool becomes clogged, emergency injection of coolant into a reactor may be compromised; similarly, when smaller debris is allowed into a reactor during emergency injection, additional system failure, and even increased fuel damage, may occur due to such debris. 
     The present invention is systems and methods for debris mitigation in a nuclear power reactor setting that use at least one filtering device installed at a position external to a coolant for the reactor. The present invention uses filters that are effective at removing debris from coolant but do not substantially impede coolant flow when coming into contact with coolant. Example embodiments discussed below illustrate just a few different options useable with this invention. 
       FIG. 2  is a schematic of a reactor containment showing installation of an example embodiment filtering system  100 . Although  FIGS. 1 and 2  show differing containment designs housing like-numbered components, it is understood that either design, as well as several other containment designs, are useable with example embodiment systems. 
     As shown in  FIG. 2 , one or more debris filters  150  are outfitted in drywell  51  surrounding reactor pressure vessel  42 . Filters  150  are strategically placed outside of any coolant sources, in areas where coolant may flow from drywell  51  into a coolant source such as suppression pool  59 . For example, filters  150  may be placed along flooring  80  inside of drywell  51  where fluid coolant would be likely to flow in drywell  51 , including platforms, personnel walkways, stairways, and/or grating. Or, for example, filters  150  may be placed at an entryway into downcomer tube  81 , which flows into suppression pool  59  housed in a torus or suppression chamber  58 , where fluid coolant would be likely to flow before entering suppression pool  59 . 
     Although example embodiment system  100  shows filters  150  in flooring  80  and at downcomer tubes  81 , it is understood that other recognized flow paths for coolant filtering are useable in example embodiments. In another containment design or different transient scenario, likely coolant flows may be at other positions, and filtering may be provided at these other positions by placing filters  150  in such coolant flows. For example, filters  150  may be placed about main steam legs, reactor pressure vessel bottoms, and/or jet pumps if these areas are likely to experience fluid coolant flow that may flow into a coolant source for reactor emergency cooling. In this way, a user can determine any likely coolant flow path into an emergency coolant source for a given transient and containment configuration and install filters  150  in these areas of containment before such transient occurs. 
     Because example embodiment system  100  may use filters  150  installed outside coolant, for example, not underwater in suppression pool  59 , filters  150  may not become clogged with sediment or other materials present in coolant sources. Filters  150  outside of coolant sources may further filter coolant much sooner and closer in proximity to where coolant flow originates within containment. As such, filters  150  may intercept and filter only coolant flow that exists in transient scenarios, keeping filters  150  relatively less clogged, and filters  150  may prevent debris, especially smaller debris, from being entrained in coolant flows in containment to flow into coolant sources. For example, filter  150  placed at a head of downcomer tubes  81  may filter coolant flowing from a coolant leak into drywell  51  before such coolant can flow into suppression pool  59 . This may prevent or reduce any debris from flowing into suppression pool  59  and ultimately into any injection point, such as reactor  42 , in a transient scenario. 
     Because example embodiment system  100  may use filters  150  installed within accessible areas of drywell  51 , installation and maintenance of system  100  may be achieved during plant construction and regular maintenance or refueling outages. For example, personnel may be able to ready access filters  150  installed in open-air spaces that may become fluid flow paths during transients, such as in flooring  80  or downcomer tubes  81 . Personnel may thus regularly inspect, clean, and or replace filters  150  without need to drain suppression pool  59  or perform underwater maintenance. 
     Several different filter materials and configurations are useable as filters  150 . Filters  150  may be chosen based on anticipated flow volume and debris type at their area of installation. For example, filters with rigid, open channels like those used in connection with fuel bundles may be installed as filters  150  in  FIG. 2 . An example of such a filter  150  is shown in  FIG. 3 . Filter  150  of  FIG. 3  is described in co-owned U.S. Pat. No. 8,317,035 to Elkins et al., the contents of which is incorporated herein in its entirety. Filters from Elkins may be effective at trapping smaller debris typically found in recirculating reactor coolant that may escape outside of reactor  42  during a transient or that may be created in particularly energetic transients. Filters  150  otherwise permit fluid flow into coolant sources like suppression pool  59 , such that coolant can be injected into reactor  42  via ECCS line  10  or otherwise used to mitigate temperature and pressure with less clogging or other damaging potential from entrained debris. Similarly, other strainers, screens, etc. can be used for filters  150  depending on anticipated coolant flow, including perforated metal plates, wire meshes, fiberglass filters, etc. 
     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, a variety of different reactor and containment designs are compatible with example embodiments and methods simply through proper dimensioning and placement of example embodiments—and fall within the scope of the claims. Such variations are not to be regarded as departure from the scope of these claims.