Patent Publication Number: US-8542789-B2

Title: Irradiation target positioning devices and methods of using the same

Description:
BACKGROUND 
     1. Field 
     Example embodiments generally relate to fuel structures and radioisotopes produced therein in nuclear power plants and other nuclear reactors. 
     2. Description of Related Art 
     Radioisotopes have a variety of medical applications stemming from their ability to emit discreet amounts and types of ionizing radiation. This ability makes radioisotopes useful in cancer-related therapy, medical imaging and labeling technology, cancer and other disease diagnosis, medical sterilization, and a variety of other industrial applications. 
     Radioisotopes, having specific activities are of particular importance in cancer and other medical therapy for their ability to produce a unique and predictable radiation profile. Knowledge of the exact amount of radiation that will be produced by a given radioisotope permits more precise and effective use thereof, such as more timely and effective medial treatments and improved imaging based on the emitted radiation spectrum. 
     Radioisotopes are conventionally produced by bombarding stable parent isotopes in accelerators or low-power reactors with neutrons on-site at medical facilities or at nearby production facilities. The produced radioisotopes may be assayed with radiological equipment and separated by relative activity into groups having approximately equal activity in conventional methods. 
     SUMMARY 
     Example embodiments and methods are directed to irradiation target positioning devices and systems that are configurable to permit accurate irradiation of irradiation targets and accurate production of daughter products, including isotopes and radioisotopes, therefrom. Example embodiments include irradiation target plates having precise loading positions for irradiation targets, where the targets may be maintained in a radiation field, such as a neutron flux. Example embodiment target plates may further include holes and target spacing elements to further refine the positioning of irradiation targets of very small or large size within the field. Example embodiments may further include a target plate holder for retaining and positioning the target plates and irradiation targets therein in the radiation field. Example embodiment target plate holders may further include spacer plates to further refine the positioning of irradiation target plates within example embodiment target plate holders. Example embodiments may be fabricated of materials with known absorption cross-sections for the radiation field to further permit precise, desired levels of exposure in the irradiation targets. 
     Example methods configure irradiation target retention systems to provide for desired amounts of irradiation and daughter product production. Example methods may include determining a desired daughter product, determining characteristics of an available radiation field, configuring the irradiation targets within example embodiment target plates and target plate holders, and/or irradiating the configured system in the radiation field. 
    
    
     
       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 example embodiments herein. 
         FIG. 1  is an illustration of an example embodiment target plate. 
         FIG. 2  is an illustration of an example embodiment target plate and details of irradiation targets and spacers therein. 
         FIG. 2A  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 2B  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 2C  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 2D  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 2E  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 2F  is a detail of a loading position in the example embodiment target plate of  FIG. 2 . 
         FIG. 3  is a detail illustration of an example embodiment target plate having irradiation targets and spacers arranged therein in accordance with example methods. 
         FIG. 4  is an illustration of an example embodiment target plate holder. 
         FIG. 5  is a flow chart illustrating example methods of using target plates and target holders. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed illustrative embodiments of example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The example embodiments may, however, 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.). 
     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 language explicitly 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. 
     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. 
       FIG. 1  is an illustration of an example embodiment target plate  100 . As shown in  FIG. 1 , example embodiment target plate  100  may be a circular disk, or, alternatively, any shape, including square, elliptical, toroidial, etc., depending on the application. Target plate  100  includes one or more loading positions  101  where irradiation targets may be placed and retained. Loading positions  101  are positioned in target plate  100  at positions of known radiation levels when target plate  100  is subject to a neutron flux or other radiation field. As used herein “radiation level” or “radiation field” includes any type of ionizing radiation exposure capable of transmuting targets placed in the radiation field, including, for example, high-energy ions from a particle accelerator or a flux of neutrons of various energies in a commercial nuclear reactor. For example, if target plate  100  is placed in neutron flux at a particular position in an operating commercial nuclear reactor, exact levels and types of neutron flux at loading positions  101  are known, such that each position may correspond to a particular level of exposure given an exposure time. 
     In this way, loading positions  101  may be arranged in example embodiment target plate  100  so as to ensure irradiation targets at those positions are exposed to an exact and desired level of radiation exposure. As an example, it may be desirable to place loading positions  101  so that each position is exposed to an equal amount of neutron flux in a light-water reactor. Knowing the flux profile to which target plate  100  will be exposed and the relevant cross-sections, including absorption and scattering/reflection cross-sections, of target plate  100 , loading positions  101  can be arranged such that each loading position  101  receives equal irradiation, including, for example, having loading positions  101  be more frequent at an outer perimeter of target plate  100  where more flux is encountered, as shown in  FIG. 1 . 
       FIG. 2  is another view of example embodiment target plate  100  showing various example arrangements at loading positions  101  and irradiation targets  150  therein, wherein detailed views are provided in  FIGS. 2A-2F . One or more holes  102  that extend partially or completely through target plate  100  may be at a loading position  101  to hold one or more irradiation target  150 . Holes  102  may be any shape. 
     For example, as shown in the details of  FIG. 2A and 2C , holes  102  may be shaped to match a shape of irradiation targets  150  therein, including, for example, cylindrical holes  102  to hold cylindrical irradiation targets  150 . As a further example, as shown in the details of  FIG. 2D and 2F , holes  102  may be shaped as slits to hold disk or flat irradiation targets  150 . A number of irradiation targets  150  may be loaded into any hole  102  based on the estimated neutron flux profile at a loading position  101  of the hole. For example, loading positions  101  expected to be exposed to higher levels of radiation may include holes  102  having more irradiation targets  150  loaded therein. While example embodiments illustrate holes  102  at loading positions  101 , it is understood that other irradiation target retention mechanisms, such as an adhesive or containment compartment, for example, are useable to retain irradiation targets  150  at loading positions  101 . 
     A single hole  102  may be at a loading position  101 , as shown in the details of  FIG. 2A , for example, or multiple holes may be at a loading position  101 , as shown in the details of  FIG. 2C , for example. Example embodiment target plates  100  may include a variety of holes  102  of different shapes and numbers at different loading positions  101 . For example, in order to accommodate different shapes of irradiation targets  150  and based on the known flux profile to which target plate  100  is exposed, multiple square holes  102  may be placed at edge loading positions  101  while a single, cylindrical hole  102  may be at interior loading positions  101 . 
     Irradiation targets  150  may take on a number of shapes, sizes, and configurations and may be placed, sealed, and/or retained in holes  102  or other retaining mechanisms at loading positions  101  in a variety of ways. The size of the irradiation targets  150  may be adjusted as appropriate for their intended use (e.g., radiography targets, brachytherapy seeds, elution matrix, etc.). For instance, an irradiation target  150  may have a length of about 3 mm and a diameter of about 0.5 mm. Irradiation targets  150  may also be spherical-, disk-, wafer-, and/or BB-shaped, or any other size and shape, within different types of holes  102  in the same target plate  100 , as shown in  FIG. 2 . It should be understood that the size of the holes  102  and/or the thickness of the example embodiment target plates  100  may be adjusted as needed to accommodate the targets  150 . 
     Irradiation targets  150  are strategically loaded at the appropriate loading positions  101  based on various factors (including the characteristics of each target material, known flux conditions of a reactor core, the desired activity of the resulting targets, etc.) discussed in greater detail below, so as to attain daughter products from irradiation targets  150  having a desired concentration or activity level, such as a relatively uniform activity. 
     Irradiation targets  150  may be formed of the same material or different materials. Irradiation targets  150  may also be formed of natural isotopes or enriched isotopes. As used herein it is understood that irradiation targets  150  include those materials having a substantial absorption cross-section for the type of irradiation to which example embodiments may be exposed, such that irradiation targets  150  include materials that will absorb and transmute in the presence of a radiation field. For example, suitable targets  150  may be formed of cobalt (Co), chromium (Cr), copper (Cu), erbium (Er), germanium (Ge), gold (Au), holmium (Ho), iridium (Ir), lutetium (Lu), molybdenum (Mo), palladium (Pd), samarium (Sm), thulium (Tm), ytterbium (Yb), and/or yttrium (Y), although other suitable materials may also be used. Similarly, targets may be liquid, solid, or gaseous within appropriate containment at loading positions  101 , such as in holes  102 . 
     In order to preserve spacing among irradiation targets  150  and orientation of irradiation targets  150  within a known radiation field to which they are exposed, one or more spacing elements  105  may space and/or retain irradiation targets  150  within holes  102 . For example, as shown in the details of  FIG. 2B , a single target spacing element  105 A may be placed in a hole  102  to retain and space irradiation targets  150  at proper positions at loading positions  101 . Alternatively, as shown in the details of  FIG. 2E , one or more target spacing elements  105 B may be shaped like a dummy target and inserted into hole  102  to retain and space irradiation targets  150  at proper positions within a hole  102  at irradiation target loading position  101 . 
       FIG. 3  is an illustration of an example embodiment target plate  100  using target spacing elements  105 B, like those shown in the details of  FIG. 2E , at each loading position  101  having a hole  102 . As shown in  FIG. 3 , each hole  102  may be equally filled with a combination of target spacing elements  105 B and/or irradiation targets  150 . In accordance with example methods, discussed below, loading positions  101  at a periphery may contain an increased ratio of irradiation targets  150  to target spacing elements  105 B, whereas loading positions  101  may have a lower ratio, in order to produce daughter products of a desired activity. 
     Still alternatively, as shown in  FIG. 2D , target spacing elements  105 C may be shaped like wafers having a thickness sufficient to separate irradiation targets  150  in a slit-type hole  102 . The separation may space irradiation targets  150  at desired positions for irradiation. Other types of spacing and retaining elements, including caps, adhesives, elastic members, etc. may be useable as target spacing elements  105 . 
     Example embodiment target plate  100  and any spacing elements  105  therein may be fabricated from materials having a desired cross-section, in view of the type of radiation field to which example embodiments may be exposed. For example, example embodiment target plate  100  being exposed to a thermal neutron flux field may be fabricated of a material having a low thermal neutron absorption and scattering cross-section, such as zirconium or aluminum, in order to maximize neutron exposure to irradiation targets  150  therein. For example, if example embodiment target plate  100  is exposed to an aggregate neutron flux with a wide energy distribution, spacing elements  105  may be fabricated of a material, such as paraffin, having a high absorption cross-section for particular energy neutrons to ensure that irradiation targets  150  are not exposed to a neutron flux of the particular energy. 
     The above-described features of example embodiment target plate  100  and the known radiation profile to which target plate  100  is to be exposed may uniquely enable accurate irradiation of irradiation targets  150  used therein. For example, knowing an irradiation flux type and profile; a shape, size, and absorption cross-section of irradiation targets  150 ; and size, shape, position, and absorption cross-section of example embodiment target plate  100 , loading positions  101  on the same, and target spacing elements  105  therein, one may very accurately position and irradiate targets  150  to produce desired isotopes and/or radioisotopes. Similarly, one skilled in the art can vary any of these parameters, including irradiation target type, shape, size, position, absorption cross-section etc., in example embodiments in order to produce desired isotopes and/or radioisotopes. 
       FIG. 3  illustrates an example arrangement for target plate  100  where outer loading positions  101  will be directly exposed to higher levels of radiation when the target plate  100  is placed in a neutron flux, such as found in an operating nuclear reactor core. A greater number of irradiation targets  150  may be placed at each of the outer positions  101 , thereby resulting in more equal activity amongst the irradiation targets  150  in the outer loading positions  101 . Fewer irradiation targets  150  may be placed in each of the inner loading positions  101  to offset the fact that these irradiation targets  150  will be farther from the flux, thereby allowing irradiation targets  150  in the inner loading positions  101  to attain activity levels comparable to targets  150  in the outer loading positions  101 . It is understood, however, in light of the above discussion, that the example arrangement of  FIG. 3  may be altered in several ways so as to increase/decrease the resulting activity of each irradiation target  150  following irradiation. For instance, irradiation targets  150  formed of materials having lower capture cross-sections for a particular radiation field may be arranged at loading positions  101  that will be in closer proximity to the field, whereas irradiation targets  150  of materials with higher cross-sections may be positioned in example embodiment target plates  101  farther away from the field. 
       FIG. 4  is an illustration of an example embodiment target plate holder  200  that is useable with example embodiment target plates  100  described above. As shown in  FIG. 4 , example embodiment target plate holder  200  may include a body  201  that is insertable in a radiation field. Body  201  may be rigid or flexible. Body  201  may be shaped and/or sized to fit in areas where radiation fields may exist, including, for example, an instrumentation tube of a light-water reactor, a nuclear fuel rod, an access tube for a particle accelerator, etc. Similarly, multiple example embodiment target plates holders  200  may be inserted and/or placed together and body  201  may be sized and shaped to permit multiple insertions, for example, in a 4″ hole commonly found in nuclear reactors. Body  201  may further include one or more connectors  202  that may permit holder  200  to be attached to extensions or insertion devices, such as a snaking cable. 
     Body  201  holds at least one example embodiment target plate  100 . For example body  201  may include a shaft upon which target plates  100  may fit and be retained. Body  201  and parts thereof may be sized and shaped to match any of the various possible shapes of target plate  100 , including a square, circular, triangular, etc. cross-section. As shown in  FIG. 5 , one or more spacer plates  203  may be placed with target plates  100  in or adjacent to body  201 . Spacer plates  203  may separate and position target plates  100  at precise locations within example embodiment target plate holder  200  in order to achieve accurate exposure for irradiation targets  150  therein. Spacer plates  203  may have thicknesses that result in a desired degree of separation among target plates  100 . For example, if example embodiment target plates  100  are fabricated and configured to substantially absorb neutron flux passing therethrough, a thicker spacer plate  203  may separate target plates  100  in target plate holder  200  to ensure that plates have a minimal effect on each other&#39;s irradiation, so as to achieve more even irradiation of irradiation targets  150  therein. Alternatively, more spacer plates  203  may be placed at greater frequency to achieve the same spacing and/or exposure as thicker spacer plates  203 . Spacer plates  203  may be shaped and sized in any manner to achieve desired positions of target plates. Spacer plates  203  may be any shape, such as rectangular, triangular, annular, etc., based on positioning of target plates  100  in example embodiment target plate holder  200 . 
     Spacer plates  203  may further provide for securing irradiation targets  150  within example embodiment target plates  100  stacked consecutively with spacer plates  203  on body  201 . Spacer plates  203  may also be colored, textured, and/or bear other indicia that indicates their physical properties and/or the identities of irradiation targets  150  within target plates  100  placed adjacently. 
     Spacer plates  203  and body  201  may be fabricated of a material having a desirable radiation absorption profile. For example, spacer plates  203  and body  201  may have a low cross-section (e.g., approximately 5 barns or less) for thermal energy neutrons by being fabricated of a material such as aluminum, stainless steel, a titanium alloy, etc. Similarly, some spacer plates  203  and/or body  201  may be fabricated of materials having higher cross-sections for particular radiation fields, such as silver, gold, a boron-doped material, a barium alloy, etc. in thermal neutron fluxes. Spacer plates  203  may be strategically placed on body  201  based on its effect on the radiation field. For example, high cross-section (e.g., over 5 barns) spacer plates  203  placed on either side of target plates  100  may reduce or eliminate irradiation of irradiation targets  150  therein from the side, permitting a desired activity level of isotopes produced therefrom. Similarly, annular spacer plates  203  may provide for maximum irradiation of target plates  100  from a side. 
     The above-described features of example embodiment target plate holder  200  and spacer plates  203  and target plates  100  therein, and the known radiation profile to which target plate holder  200  is to be exposed may uniquely enable accurate irradiation of irradiation targets  150  used therein. For example, knowing an irradiation flux type and profile; a shape, size, and absorption cross-section of irradiation targets  150 ; precise positioning of irradiation targets  150  within radiation flux; size, shape, position, and absorption cross-section of example embodiment target plate  100  and spacing elements  105  therein; position of target plate  100  and spacer plate  203  within target plate holder  200 ; size, shape, and absorption cross-section of plate holder  200  and spacer plate  203 , one may very accurately irradiate targets  150  to produce desired isotopes and/or radioisotopes. Similarly, one skilled in the art can vary any of these parameters in example embodiments in order to produce desired isotopes and/or radioisotopes. 
       FIG. 5  is a flow chart of an example method of using example embodiment target plates  100  and/or target plate holders  200 . As shown in  FIG. 5 , the user determines a desired isotope/radioisotope to be produced, and amount to be produced, in example methods in S 110 . The desired isotope and amount thereof may be chosen based on any number of factors, including, for example, an available irradiation target, desired industrial application, and or an available radiation field. By virtue of correspondence between daughter product and parent nuclide, the user will also select the material and amount for irradiation targets  150  in S 110 . 
     In S 120 , the user will determine the characteristics of an available radiation field. The relevant characteristics may include type of radiation, energy of radiation, and/or variations of type and energy in a particular space. For example, the user may determine the level and variation of a neutron flux at a particular access point to a research reactor in S 120 . Alternatively, the user may determine the energy and type of ions encountered at a target stand in a particle accelerator in S 120 . 
     Based on the physical properties of the selected irradiation target  150  and the properties of the radiation field, both determined above, the user then configures target plate(s)  100 , irradiation target(s)  150 , target spacing element(s)  105 , target plate holder(s)  200 , and/or spacing plate(s)  203  in order to achieve an amount of irradiation necessary to produce a desired amount and/or activity of produced isotopes, in S 130 . Such configuration may include determining locations of loading positions  101  in target plate  100 , placing and positioning irradiation targets  150  in target plates  100  at loading positions  101  with target spacing elements  105 , and positioning target plates  100  in target plate holder  200  with spacing plates  203  to achieve a precise position of each irradiation target  150  within a radiation field. Additionally, such configuration may include selecting materials with known absorption cross-sections for a radiation spectrum relevant to the radiation field in order to achieve desired amounts of irradiation for irradiation targets  150  placed within that field. For example, a desired activity may be a substantially equal activity among several produced isotopes from several irradiation targets  150 . In S 130 , the user may also calculate an exposure time based on the configuration, radiation field properties, and irradiation target  150  properties to achieve a desired magnitude of irradiation for irradiation targets  150  placed in example embodiment devices in that field. 
     In S 140 , the user may then place the configured irradiation targets  150  in example embodiment devices configured in S 130  and place them into the determined radiation field so as to produce the desired isotopes and/or radioisotopes of a desired amount and/or activity. Alternatively, the user may deliver or otherwise provide the configured example embodiment devices for another to insert the irradiation targets  150  and irradiate them in the determined radiation field in S 140 . 
     Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied through routine experimentation and without further inventive activity. For example, although various example embodiment plates, holders, and spacers are used together with example methods of producing desired isotopes, each example embodiment may be used separately. Similarly, for example, although cylindrical example embodiments are shown, other device types, shapes, and configurations may be used in example embodiments and methods. Variations are not to be regarded as departure from the spirit and scope of the exemplary 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.