Abstract:
Irradiation target holders are configured to fit in open locations inside of an operating commercial nuclear core. Holders can be placed with ends at vertical bottom and top of the core or any position therebetween to directly expose holders to nuclear fuel reactions. Holders have ends and overall shape that can join with existing reactor structures, while fitting closely with fuel and moderator and being easily removable from the same. Holders are fabricated of any reactor-compatible material that will retain irradiation targets and daughter products. Holders securely retain irradiation targets and daughter products of any shape or phase throughout reactor operation. Holders can be installed during reactor outages and irradiated during operation without risk of movement or interference with operation. After a desired period of operation and irradiation, holders can be harvested from the core independent of other core structures and fuel.

Description:
BACKGROUND 
       [0001]    As shown in  FIG. 1 , a nuclear power station conventionally includes a reactor pressure vessel  10  with various configurations of fuel and reactor internals for producing nuclear power. For example, vessel  10  may include a core shroud  30  surrounding a nuclear fuel core  35  that houses fuel structures, such as fuel assemblies  40 . Core  35  may be bounded vertically by top guide  45  and core plate  70 . Fuel assemblies  40  may extend between and seat into core plate  70  and top guide  45 , which may include several openings shaped to receive ends of assemblies  40 . Other core structures, such as control elements and instrumentation tubes, may likewise extend through and/or between core plate  70  and/or top guide  45 . One or more control rod drives  1  may be positioned below vessel  10  and connect to control rod blades or other control elements that extend among fuel assemblies  40  within core  35 . 
         [0002]    An annular downcomer region  25  may be formed between core shroud  30  and vessel  10 , through which fluid coolant and moderator flows into the core lower plenum  55 . For example, in US Light Water Reactor types, the fluid may be purified water, while in natural uranium type reactors, the fluid may be purified heavy water. In gas-cooled reactors, the fluid coolant may be a gas such as helium, with moderation provided by other structures. The fluid may flow upward from core lower plenum  55  through core  35 . In a boiling water-based reactor, a mixture of water and steam exits nuclear fuel core  35  and enters core upper plenum  60  under shroud head  65 . 
         [0003]    Nuclear reactors are refueled periodically with new fuel to support power operations throughout an operating cycle. During shutdown for refueling, the vessel  10  is cooled, depressurized, and opened by removing upper head  95  at flange  90 . With access to the reactor internals, equipment may be shifted or removed and some or all of fuel bundle assemblies  40  may be replaced and/or moved within core  35 . Maintenance on other internal and external structures may be performed during such an outage. 
         [0004]    As shown in  FIGS. 2A and 2B , one or more fuel support castings  48  may sit on and/or extend through core plate  70 . Casting  48  may include several orifices  49  to receive fuel assemblies and/or control elements, aligning them with respect to one another and with core plate  70  and directing coolant up through such components. Casting  48  may accommodate several fuel assemblies in various orifices  49  while maintaining other space on core plate  70 . For example, an instrumentation tube  50  may penetrate core plate  70  and be positioned next to casting  48 , allowing tube  50  to extend vertically adjacent to several fuel assemblies positioned in casting  48 . 
         [0005]    Similarly, one or more source holder penetrations  75  may extend into core plate  70  adjacent to casting  48 . Source holder penetration  75  may hold a startup source, such as a sealed Californium or Plutonium-Beryllium isotope that emits substantial and detectable neutron spectra, which reliably begins the nuclear chain reaction in a new core with completely fresh fuel, or after excessively long shut-down periods when spontaneous fission is unreliable in burnt fuel. Co-owned “General Electric Systems Technology Manual,” Dec. 14, 2014, Chapter 5.1, describes helpful technological context and is incorporated by reference herein in its entirety. As seen in the top-down view of  FIG. 2B , source holder penetration  75  may position the source in a desired static relation with instrumentation tube  50 , permitting detection of neutrons from a source in penetration  75  to compare to neutrons generated through fission during startup, and fuel assemblies in casting  48 . In this way, core plate  70  and casting  48  may radially/horizontally align several different core components at a base of a core and ensure they maintain desired positioning throughout an axial/vertical extent of the core. 
       SUMMARY 
       [0006]    Example embodiments include holders for materials that are to be subject to irradiation in free core positions while sealed in a nuclear reactor core. Example embodiments can include lower and/or upper ends that mate with or otherwise join to reactor components to position holders within the core, in close proximity to neutron-generating fuel and moderator. Holders may robustly seal in irradiation targets and daughter products produced through irradiation with neutron flux, such as in internal cavities of any shape or size that houses desired targets. As an example, a holder may be shaped to minimally join with an existing core plate and/or fuel castings at a bottom of the core and span up to a top guide opening at a top of the core, resulting such an example holder being secured in, but easily removable from, the core at either end, while positioning the holder in an otherwise open space in the core. Such a space may be vacated by an unused startup source holder, for example. Irradiation targets may absorb neutron flux encountered at a position within the holder. 
         [0007]    Example methods include installing and irradiating target holders in operating nuclear reactors. Holders can be placed directly within a fuel core in example methods, without any structure between the holders and fuel and/or moderator, for higher irradiation by radiation encountered in the core during operation. For example, holders can be placed in positions vacated by conventional core components, and holders may be specifically shaped and dimensioned to be compatible with such positions. During operation, the installed holder may remain stationary within the core and generate larger amounts of desired daughter products through absorption and potentially radioactive decay without significantly contributing to reactivity where installed. Following an operational cycle of several months or other period of operation, the holders can be retrieved from the nuclear core without involvement with fuel or other core structures, yet holders may remain shielded in a moderator during such operations, allowing safer and easier handling and harvesting. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0008]    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. 
           [0009]      FIG. 1  is an illustration of a related art nuclear power vessel and internals. 
           [0010]      FIGS. 2A and 2B  are illustrations of a related art core plate and fuel support casting. 
           [0011]      FIG. 3  is an illustration of an example embodiment incore irradiation target holder installed in a nuclear reactor core. 
           [0012]      FIG. 4  is a detail illustration of an example embodiment incore irradiation target holder installed between fuel castings. 
           [0013]      FIGS. 5A and 5B  are illustrations of cross sections of example embodiment incore irradiation target holders. 
           [0014]      FIG. 6  is a graph demonstrating higher activation in example embodiment incore irradiation target holders. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Because this is a patent document, 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. 
         [0016]    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. 
         [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    Applicants have recognized that most methods for generating materials through neutron capture in a nuclear reactor insert irradiation targets into fuel or instrumentation tubes, or form irradiation targets as existing core structures like control blades. Applicants have recognized that these methods tend to tie generation to reactor operations, requiring the targets to be moved and harvested with fuel, or require complex configurations to interact with instrumentation tubes or existing core structures. Applicants have further newly identified that startup holder positions in most nuclear reactors have a distinct functionality that is no longer needed following operation of the reactor. To overcome these newly-recognized problems as well as others, the inventors have developed methods and systems that independently place irradiation targets directly into a nuclear fuel core without impacting fuel or other core structures or operation. These methods and systems may provide new functionality to startup holder positions and other core locations available during operation. 
         [0021]    The present invention is irradiation target holders for use in a nuclear reactor 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. 
         [0022]      FIG. 3  is an illustration of an example embodiment incore irradiation target holder  100 . As shown in  FIG. 3 , holder  100  may span core  35  in a vertical or axial dimension between core plate  70  and top guide  45 , irrespective of other core internals. For example, holder  100  may be shaped and sized to fit among several fuel assemblies, instrumentation tubes, control elements, etc. typically found in a nuclear core. Although example embodiment holder  100  is shown spanning an entire vertical distance from core plate  75  to top guide  45 , it is understood that partial extension is possible with proper connections. 
         [0023]    As shown in  FIG. 3 , example embodiment incore irradiation target holder  100  is configured to insert into—to securely mate with—a source holder penetration  75  in core plate  70 . For example, source holder penetration  75  may be an existing hole or other aperture in core plate  75  into which a startup source holder is originally placed and later removed by the plant operator or other servicer; or for example, source holder penetration  75  may be a penetration never used for any purpose or made ad hoc during an outage or other period of access to core  35 . Source holder penetration  75  may be one or more inches deep with an approximate one-inch diameter and may extend entirely or partially through core plate  70 . Source holder penetration  75  may be placed in other structures besides a core plate  70 ; however, placement of source holder penetration  75  provides vertical clearance above penetration  75  that is not blocked by other core internals such as fuel assemblies, fuel castings, instrumentation, flow devices, etc. common to nuclear cores. 
         [0024]    Example embodiment holder  100  may seat into penetration  75  through gravity, operator insertion, and/or under the force of a spring or other retention lock or mechanism during both installation and operation. For example, holder  100  may screw into penetration  75 , lock into the same through a tang-and-mating, or simply sit through gravity in penetration  75 . As such, an axially lower end of example embodiment holder  100  may be specifically shaped, sized, or otherwise configured to match a desired penetration  75  for insertion. 
         [0025]    As shown in  FIG. 3 , if example embodiment holder  100  runs a vertical length of core  35  holder  100  may seat into a top hole  145  in top guide  45 . For example, in a boiling water reactor, core  35  may be approximately 13 feet or longer, and holder  100  may extend all or any of this distance. Top hole  145  may be similar to source holder penetration  75  in that it may be preexisting or newly formed. Top hole  145  may be aligned and pre-purposed for retaining a startup source holder in conjunction with penetration  75  in core plate  70 . Example embodiment holder  100  may seat into top hole  145  through operator insertion. As such, an axially higher end of example embodiment holder  100  may be specifically shaped, sized, or otherwise configured to match a desired top hole  145  for insertion and retention. 
         [0026]    Holder  100  may be under the force of a spring or bias or other locking mechanism provided during installation and/or operation. For example, holder  100  may seat into top hole  145  due to a spring in penetration  75  biasing example embodiment holder  100  upward vertically into hole  145 . A spring in top hole  145  may similarly bias example embodiment holder  100  downward axially into opposite penetration  75 , permitting a desired degree of axial securing. 
         [0027]    Example embodiment incore irradiation target holder  100  may further include one or more casting fins  110  that extend radially—horizontally—or otherwise with respect to core  35  to mate with fuel castings nearby. As shown in  FIG. 4 , a simplified detail of a base of example embodiment holder  100 , fins  110  may be captured by a side of fuel casting  48 . For example, fuel casting  48  may include a slot configured to receive a part of a startup source holder or other core component, and fin  110  may be shaped and sized to fit within such a slot. Holder  100  may include, for example, four perpendicular fins  110  that insert into up to four adjacent castings  48 . Example embodiment holder  100  may thus seat between and into several adjacent casting  48  that anchors one or more fuel assemblies  40 , such that holder  100  is positioned adjacent to assemblies  40  extending upward in a vertical or axial direction. 
         [0028]    While penetration  75  and hole  145  may provide axial securing to holder  100  shaped to seat therein, fins  110  shaped to seat into an adjacent casting  45  may provide rotational securing and/or prevent radial translation of holder  100 . Fins  110  may lock into or removably seat in casting(s)  48  at other angles and positions in order to orient holder  100  at other positions and/or mate with other structures entirely to take advantage of other existing spaces and securing penetrations within a nuclear core. Similarly, example embodiment holder  100  may include any or neither of fins  110  and an end seating into penetration  75  ( FIG. 3 ) to achieve a desired positioning and level of securing within a nuclear core. 
         [0029]    Through the above-described example features, an example embodiment holder  100  may include any number of retaining features that are very similar to existing structures in startup holders that mate with other core features like a core plate and top guide, in order to replace the same without modification and/or disruption of existing core features. An operator or other servicer may install example embodiment holder  100  during an outage or other access period in combination with such existing core features. For example, a reactor may be operated for a period of months or years to sustain a nuclear chain reaction that generates heat that is in turn converted to electricity. The reactor may then be shut down by terminating the nuclear chain reaction, and operators can access the reactor internals for maintenance and refueling. During such an outage, reactor internals, one or more fuel assemblies  40 , and potentially any unnecessary startup source may be removed and/or shuffled within the core, and fresh fuel may be added. In the same timeframe, example embodiment holder  100  may be installed where the startup source was or would have been within the nuclear core. The reactor may then be brought back to operation to sustain the nuclear chain reaction and irradiation inherent therein, and example embodiment holder  100  may remain in the installed position during such operation and irradiation and retrieved at a later time, such as during a subsequent outage. 
         [0030]    As shown in  FIG. 3 , example embodiment holder  100  may include an internal cavity  150  that houses one or more irradiation targets  151  that convert to a desired daughter product when exposed to radiation in an operating nuclear reactor. For example, internal cavity  150  may be an integrally-formed housing within holder  100  into which an irradiation target  151  may be inserted at fabrication and removed through destruction of holder  100 . Similarly, internal cavity  150  may be selectively opened and/or segmented to allow segregation of multiple desired targets at differing positions and nondestructive removal. Compatible designs of fuel rod bodies and irradiation target holders are shown in co-owned patent publications 2009/0122946 published May 14, 2009 to Fawcett et al.; 2009/0135983 published May 28, 2009 to Russell, II et al.; and 2009/0274260 published Nov. 5, 2009 to Russell, II et al., which are useable as central portions of example embodiment holder  100 , these publications being incorporated herein in their entireties. 
         [0031]    Example embodiment incore irradiation target holder  100  may otherwise be fabricated of materials that substantially maintain their physical properties in an operating nuclear reactor environment so as to preserve positioning and containment of irradiation targets  151  retained in internal cavity  150 . For example, holder  100  may be fabricated of stainless steel, a zirconium alloy, and aluminum alloy, etc. If fuel casting  48 , core plate  70  and/or tope guide  45  are fabricated of one material, such as stainless steel, example embodiment holder, at least in structures that directly contact these core structures, may be another material, such as zirconium alloys, in order to enhance material compatibility and eliminate voltaic potential and fouling. Such materials may further have minimal impact on radiation, having minimal scattering and absorption cross-sections for neutron flux encountered in a reactor. 
         [0032]    Example embodiment holder  100  may match geometries of startup source holders at vertical ends, so as to mate with existing core structures that retain such startup holders; however, the remainder of holder  100  may be any shape that maximizes desired daughter material production in core  35 . For example, as shown in the cross-section of  FIG. 5A , internal cavity  150  may be round, or as shown in  FIG. 5B , cruciform. Internal cavity  150  may similarly be helical, square, planar, etc. and extending in any degree in a horizontal position in order to accommodate irradiation targets  151  of a matching shape and/or maximize radiation exposure at desired positions within a nuclear fuel core. Internal cavity  150  may further include a moderator and/or coolant such as a liquid water reservoir  152  shown in  FIG. 5A  or other structure that enhances geometry, irradiation, and/or cooling of any irradiation targets  151  contained in example embodiment holder  100 . 
         [0033]    Example embodiment holder  100  may be relatively small, such as cylindrical as shown in radial cross-section in  FIG. 5A  and approximately half to a full inch in diameter. If holder  100  is up to 13 feet in axial length and spans an entire vertical length of core  35 , internal cavity  150  may be approximately 8 feet in axial length to match lengths of fueled sections of the core. Even this smaller example sizing may accommodate, for example, 250 cubic centimeters of irradiation targets. Or for example, as shown in radial cross-section in  FIG. 5B , with a larger cruciform cavity  150 , 1-2 inches in total arm length, 570 cubic centimeters of irradiation targets may be accommodated. Depending on the parent irradiation target, these sizes may enable several thousands of curies of activity for a produced radioisotope or several moles of atoms of a produced isotope from a parent material and sufficient irradiation. 
         [0034]    As shown in  FIG. 3 , source holder penetration  75  and/or top hole  145  may be intentionally positioned within core  35  to receive a startup source holder, and thus either or both penetration  75  and top hole  145  may be free post-startup or in the event such startup sources are not used at startup. Penetration  75  and top hole  145  may further provide an open passage between the two for accommodating a startup source holder, providing close proximity to fuel elements or fuel assemblies  40  ( FIG. 4 ) generating large amounts of neutron flux during operation. This open passage may typically be readily accessible with fuel out during refueling outages every 2-3 years, and this passage may typically be underwater or otherwise shielded with fuel. 
         [0035]    Based on the above characteristics of existing source holder penetrations  75  and/or top holes  145 , example embodiment incore irradiation target holder  100  may take advantage of vacated passages between holder penetration  75  and top hole  145  following startup to generate desired daughter products from irradiation targets, including industrially-valuable elements and radioisotopes. Particularly, in the case of parent material Cobalt-59, significant irradiation with thermal neutrons for an operating cycle in holder  100  placed between bundles in a typical startup holder position within a nuclear core will generate large amounts of Cobalt-60, which is medically useful for its high-energy gamma rays. Of course, other irradiation targets, like iridium-193 or any other non-fissionable isotope with an atomic number under 90 and an appreciable thermal neutron absorption cross-section, such as a cross-section exceeding one barn, are useable as irradiation targets in example embodiments. 
         [0036]    Accessing such a holder  100  may be relatively simple during fuel movements in an operational outage, when source holder locations can be readily exposed through fuel moves. Advantageously, holder  100  may be entirely separate from any fuel in the core and shielded from operators during such accessing by a moderator such as coolant light water or other shield. This permits easy and safe handling of example embodiment holder  100  in a fuel core without involvement with nuclear fuel. 
         [0037]      FIG. 6  is a graph showing improved yields from use of example embodiment holder  100  at existing source holder penetrations  75  and/or top holes  145  between fuel assemblies compared to an expected best-yield at a corner fuel rod in a fuel assembly.  FIG. 6  reports activation levels in Curies per gram of irradiation target versus axial level for two different positions within a same simulated core with all other variables constant. As shown in  FIG. 6 , over the same amount of time in the same core, the example embodiment holder containing a same mass of Cobalt-59 irradiation targets will achieve higher activation—a higher percentage of nuclides converted to Cobalt-60—when positioned between fuel assemblies at a source holder location as compared to a corner rod position in a fresh fuel bundle. This improvement is seen at every axial position, due to improved moderator and neutron flux access at the source holder positions with which example embodiments are compatible. 
         [0038]    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 available source holder locations, in several different types of reactor designs, are compatible with example embodiments and methods simply through proper dimensioning 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.