Abstract:
A retainer is placed on a conduit to control movement of objects within the conduit in access-restricted areas. Retainers can prevent or allow movement in the conduit in a discriminatory fashion. A fork with variable-spacing between prongs can be a retainer and be extended or collapsed with respect to the conduit to change the size of the conduit. Different objects of different sizes may thus react to the fork differently, some passing and some being blocked. Retainers can be installed in inaccessible areas and allow selective movement in remote portions of conduit where users cannot directly interface, including below nuclear reactors. Position detectors can monitor the movement of objects through the conduit remotely as well, permitting engagement of a desired level of restriction and object movement. Retainers are useable in a variety of nuclear power plants and with irradiation target delivery, harvesting, driving, and other remote handling or robotic systems.

Description:
PRIORITY STATEMENT 
     This application is a continuation-in-part of, and claims priority under 35 U.S.C. §120 to, co-pending application Ser. No. 13/477,244 filed May 22, 2012, the contents of said application being incorporated by reference herein in their entirety. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with Government support under contract number DE-FC52-09NA29626, awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Elements, and specific isotopes thereof, may be formed by bombarding parent materials with appropriate radiation to cause a conversion to desired daughter isotopes. For example, precious metals and/or radioisotopes may be formed through such bombardment. Conventionally, particle accelerators or specially-designed, non-commercial test reactors are used to achieve such bombardment and produce desired isotopes in relatively small amounts. 
     Radioisotopes have a variety of medical and industrial applications stemming from their ability to emit discreet amounts and types of ionizing radiation and form useful daughter products. For example, radioisotopes are useful in cancer-related therapy, medical imaging and labeling technology, cancer and other disease diagnosis, and medical sterilization. 
     Radioisotopes having half-lives on the order of days or hours are conventionally produced by bombarding stable parent isotopes in accelerators or low-power, non-electricity-generating reactors. These accelerators or reactors are on-site at medical or industrial facilities or at nearby production facilities. Especially short-lived radioisotopes must be quickly transported due to the relatively quick decay time and the exact amounts of radioisotopes needed in particular applications. Further, on-site production of radioisotopes generally requires cumbersome and expensive irradiation and extraction equipment, which may be cost-, space-, and/or safety-prohibitive at end-use facilities. 
     SUMMARY 
     Example embodiments include systems for moving and managing objects through a nuclear reactor, where access may be limited. For example, irradiation targets, instrumentation, and/or other objects can be moved into, and maintained in, one or even several instrumentation tubes in a reactor during operation. Example systems include a traversable path connecting the instrumentation tube(s) and a retainer along such paths to control movement of irradiation targets or other objects traversing the path. Example embodiment retainers include any type of movement restrictor, including a retention mechanism that can limit or prevent movement along the path in a discriminatory fashion, to allow passage of only desired objects. For example, an example embodiment retention assembly includes a restriction fork that can move into and squeeze an area available for passage, preventing objects larger than the fork tines&#39; separation from passing. The fork tines can have variable separation, such that when the fork is retracted/extended the amount of area available for passage, and thus objects moveable through the path, changes as desired. For example, irradiation targets can be blocked or held in an instrumentation tube for irradiation while a driving plunger that pushed the targets is retractable through the fork. Example retainers can be compact and/or self-contained with their own motors and actuation/communication circuitry so as to be placeable in space-limited areas within nuclear power plants, including at instrumentation tube flanges. Example embodiments may further include position detectors to appropriately move and hold objects within the paths at desired, such as at irradiation positions for the creation of desired isotopes or blocking or reducing movement at other sensitive positions. Example embodiment retainers are useable in a variety of access-restricted and space-limited areas within any facility, and with a variety of different systems, including irradiation target delivery/harvesting/driving systems, instrumentation systems, and/or multiple-use systems, that can remotely and/or automatically move objects through the areas without direct user interaction. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Example embodiments will become more apparent by describing, in detail, the attached drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus do not limit the terms which they depict. 
         FIG. 1  is an illustration of a conventional commercial nuclear reactor. 
         FIG. 2  is an illustration of an example embodiment irradiation target system. 
         FIG. 3  is an illustration of an example embodiment retention assembly. 
         FIG. 4  is an illustration of an example embodiment restricting fork. 
         FIG. 5  is an illustration of an example embodiment retention assembly. 
     
    
    
     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. Several different embodiments not specifically disclosed herein fall within the scope of the appended claims; 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 in a spatial or physical relationship, 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, for example, 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 terms like “have,” “having,” “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. 
       FIG. 1  is an illustration of a conventional nuclear reactor pressure vessel  10  usable with example embodiments and example methods. Reactor pressure vessel  10  may be, for example, a 100+ MWe commercial light water nuclear reactor conventionally used for electricity generation throughout the world. Reactor pressure vessel  10  is conventionally contained within an access barrier  411  that serves to contain radioactivity in the case of an accident and prevent access to reactor  10  during operation of the reactor  10 . As defined herein, an access barrier is any structure that prevents human access to an area during operation of the nuclear reactor due to safety or operational hazards such as radiation. As such, access barrier  411  may be a containment building sealed and inaccessible during reactor operation, a drywell wall surrounding an area around the reactor, a reactor shield wall, a human movement barrier preventing access to instrumentation tube  50 , etc. 
     A cavity below the reactor vessel  10 , known as a drywell  20 , serves to house equipment servicing the vessel such as pumps, drains, instrumentation tubes, and/or control rod drives. As shown in  FIG. 1  and as defined herein, at least one instrumentation tube  50  extends into the vessel  10  and near, into, or through core  15  containing nuclear fuel and relatively high levels of neutron flux and other radiation during operation of the core  15 . As existing in conventional nuclear power reactors and as defined herein, instrumentation tubes  50  are enclosed within vessel  10  and open outside of vessel  10 , permitting spatial access to positions proximate to core  15  from outside vessel  10  while still being physically separated from innards of the reactor and core by instrumentation tube  50 . Instrumentation tubes  50  may be generally cylindrical and may widen with height of the vessel  10 ; however, other instrumentation tube geometries may be encountered in the industry. An instrumentation tube  50  may have an inner diameter of about 1-0.5 inch, for example. 
     Instrumentation tubes  50  may terminate below the reactor vessel  10  in the drywell  20 . Conventionally, instrumentation tubes  50  may permit neutron detectors, and other types of detectors, to be inserted therein through an opening at a lower end in the drywell  20 . These detectors may extend up through instrumentation tubes  50  to monitor conditions in the core  15 . Examples of conventional monitor types include wide range detectors (WRNM), source range monitors (SRM), intermediate range monitors (IRM), and traversing Incore probes (TIP). Access to the instrumentation tubes  50  and any monitoring devices inserted therein is conventionally restricted to operational outages due to containment and radiation hazards. 
     Although vessel  10  is illustrated with components commonly found in a commercial Boiling Water Reactor, example embodiments and methods are useable with several different types of reactors having instrumentation tubes  50  or other access tubes that extend into the reactor. For example, Pressurized Water Reactors, Heavy-Water Reactors, Graphite-Moderated Reactors, etc. having a power rating from below 100 Megawatts-electric to several Gigawatts-electric and having instrumentation tubes at several different positions from those shown in  FIG. 1  may be useable with example embodiments and methods. As such, instrumentation tubes useable in example methods may be at any geometry about the core that allows enclosed access to the flux of the nuclear core of various types of reactors. 
     Applicants have recognized that instrumentation tubes  50  may be useable to relatively quickly and constantly generate short-term radioisotopes on a large-scale basis without interfering with an operating or refueling core  15 . Applicants have further recognized a need to generate short-term radioisotopes and remove them from within access barrier  411  quickly, without having to shut down an operating nuclear reactor to access an area within access barrier  411 . Example methods include inserting irradiation targets into instrumentation tubes  50  and exposing the irradiation targets to the core  15  while operating or producing radiation, thereby exposing the irradiation targets to the neutron flux and other radiation commonly encountered in the operating core  15 . The core flux over time converts a substantial portion of the irradiation targets to a useful mass of radioisotope, including short-term radioisotopes useable in medical applications. Irradiation targets may then be withdrawn from the instrumentation tubes  50 , even during ongoing operation of the core  15 , and removed for medical and/or industrial use. 
     Applicants have further recognized a need for a maximized amount of radioisotope production within instrumentation tubes  50 , but also identified that such need is limited by relatively few and sensitive pathways through access barrier  411  during operation. Such pathways through access barrier  411  may require compatibility with existing instrumentation, including TIP probes that are inserted into instrumentation tubes  50  during TIP runs. Example embodiments and methods address this problem by permitting irradiation targets  250  to be inserted into and removed from instrumentation tubes  50  from a first access point, while reliably permitting TIP tubes to be inserted and removed at other instances from the instrumentation tubes  50  from a second access point. In this way, multiple operations and use of instrumentation tubes  50  can be safely achieved in an access-sensitive environment such as a nuclear power plant. 
       FIG. 2  is a schematic drawing of an example embodiment irradiation target delivery and retrieval system  1000  having a penetration pathway, a loading/offloading system, and a drive system.  FIG. 2  illustrates various components of example system  1000  in a loading configuration, parts of which are also described in US Patent Publication 2013/0170927, titled “Systems and Methods for Processing Irradiation Targets Through a Nuclear Reactor,” filed Dec. 28, 2011, said application incorporated by reference herein in its entirety. As shown in  FIG. 2 , example embodiment irradiation target delivery and retrieval system  1000  may include or use one or more elements to facilitate irradiation target loading, irradiation, and harvesting in a timely, automatic, and/or consumption-enhancing manner. System  1000  includes a penetration pathway that provides a path from outside access barrier  411  to instrumentation tube  50  for one or more irradiation targets, a loading/offloading system that permits new irradiation targets to be inserted and irradiated targets to be harvested outside access barrier  411 , and a drive system that moves irradiation targets between instrumentation tube  50  and loading/offloading in example embodiment system  1000 . 
     A penetration pathway in example embodiment system  1000  provides a reliable path of travel for irradiation targets  250  between an accessible location, such as an offloading or loading area outside access barrier  411  into one or more instrumentation tubes  50 , so irradiation targets  250  can move within the pathway to a position in or near an operating nuclear core  15  for irradiation. Example pathways can include many delivery mechanisms used alone or in combination, including tubing, frames, wires, chains, conveyors, etc. in example embodiment system  1000  to provide a transit path for an irradiation target between an accessible location and an operating nuclear core. As a specific example shown in  FIG. 2 , a penetration pathway may include penetration tubing  1100 , including  1100   a  and  1100   b,  running between, either in portions or continuously, a loading junction  1200  and instrumentation tube  50  in a nuclear reactor. 
     Penetration tubing  1100  may be flexible or rigid and sized to appropriately permit irradiation targets  250  to enter into and/or through penetration tubing  1100  and navigate various structures and penetrations in and within access barrier  411 . Penetration tubing  1100  may be continuously sealed or include openings, such as at connecting junctions. Penetration tubing  1100  may junction with other tubes and/or structures and/or include interruptions. One possible advantage of penetration tubing  1100  being sealed and securely mating at junctures and/or with any terminal/originating points is that penetration tubing  1100  better maintains pneumatic pressure that can be used for target withdrawal, and also may provide additional containment for irradiation targets  250  and any products (gas, fluid, solid, particulate, etc.) formed as irradiation products in example embodiment system  1000 . 
     Penetration tubing  1100  may be fabricated of a material that maintains its physical characteristics in an operating nuclear reactor environment and does not significantly react with or entrain materials from irradiation targets  250  coming into contact therewith, including, for example, aluminum, stainless steel, carbon steel, nickel alloys, PVC, PFA, rubber, etc. Penetration tubing  1100  may be cylindrical or any other shape that permits irradiation targets  250  to enter into and/or pass through penetration tubing  1100 . For example, penetration tubing  1100  may have a generally circular cross section with a  0 . 5 -inch diameter and smooth interior surface that permits spherical irradiation target  250  to roll within penetration tubing  1100 . One potential advantage of using such an example penetration tubing  1100  may be roughly matching diameters and geometries with instrumentation tubes  50  for consistent irradiation target movement therein; however, alternate geometries, shapes, and sizes for penetration tubing  1100 , or any other penetration pathway used in example embodiments, including those that limit movement, may be desirable, advantageous, and used. 
     Penetration tubing  1100  used in example embodiment system  1000  provides a route from an origin at loading junction  1200 , where irradiation targets may enter/exit penetration tubing  1100  outside of access barrier  411 . As shown in  FIG. 2 , for example, penetration tubing  1100  leads irradiation targets  250  from loading junction  1200  to access barrier  411 , which may be, for example, a steel-lined reinforced concrete containment wall or drywell wall or any other access restriction in conventional nuclear power stations. 
     Penetration pathways usable in example embodiment system  1000  provide a route through access barrier  411  and to reactor vessel  10  where irradiation targets  250  may enter an instrumentation tube  50 . For example, as shown in  FIG. 2 , penetration tubing  1100  penetrates access barrier  411  and extends to instrumentation tubes  50 . Penetration tubing  1100  may pass through an existing penetration in access barrier  411 , such as an existing TIP tube penetration, or may use a new penetration created for penetration tubing  1100 . Penetration tubing  1100  negotiates or passes through any other objects inside of access barrier  411  before reaching instrumentation tube  50 . 
     An annular reactor pedestal  412  may be present in a drywell  20  beneath reactor  10 , and penetration tubing  1100  is shown in  FIG. 2  passing through a penetration in pedestal  412 . It is understood that penetration pathways may follow any number of different courses and negotiate different obstacles in different reactor designs aside from the specific example path shown with penetration tubing  1100  in  FIG. 2 . Similarly, penetration pathways need not be consistent or uniform; for example, penetration tubing  1100  may terminate on either side of, and be connected to, a penetration in pedestal  412  to permit irradiation targets  250  to pass through the penetration between penetration tubing  1100 . 
     Penetration pathways useable in example embodiment system  1000  may terminate at or within an instrumentation tube  50 . As shown in  FIG. 2 , penetration tubing  1100  terminates at a flange  1110  at a base of instrumentation tube  50 , permitting irradiation targets  250  to pass from penetration tubing  1100  into instrumentation tube  50 . Similarly, penetration tubing  1100  may join with an indexer that provides access to several instrumentation tubes  50  from a single penetration through reactor wall  411  and/or pedestal  412 . Such a system is described in US Patent Publication 2013/0315361, titled “Systems and Methods for Processing Irradiation Targets Through Multiple Instrumentation Tubes in a Nuclear Reactor,” filed May 22, 2012, said application incorporated herein by reference in its entirety. 
     Penetration pathways useable in example embodiments may be pre-existing in part or in whole and/or installed during access to containment areas and/or restricted access areas in a nuclear power plant, such as during a pre-planned outage. For example, penetration tubing  1100  may be installed in access barrier  411  during an outage, with penetration tubing  1100  being passed through penetrations in access barrier  411  and pedestal  412 , moved and secured in an area within access barrier  411  and a drywell space  20  under reactor  10 , and secured to flange  1110 . Portions of penetration tubing  1100  extending outside access barrier  411  may be installed at loading junction  1200  at any time. Penetration tubing  1100  may be secured at various points inside access barrier  411  and/or divert around existing equipment to minimize congestion or clutter in a drywell  20  or other space bounded by access barrier  411  while preserving a traversable path for irradiation targets  250  to and from instrumentation tube  50 . Again, other penetration pathways, including wire guides, meshes, compartments, bored tunnels, etc. are useable in example embodiments to provide a path from outside an access-restricted area such as containment to an instrumentation tube of an operating nuclear reactor. 
     System  1000  may be dual purpose throughout and equally used with a TIP drive or other instrumentation and reactor components. Or system  100  may be exclusively dedicated to isotope production and harvesting with its own driving mechanism, pathways, reservoirs, etc. and excluding use with other instrumentation or a TIP drive. Or system  1000  may be exclusive in some part and shared in others. For example, outside of pedestal  412  and/or drywell  20 , example embodiment system  1000  may be dedicated to irradiation target production and harvesting. Within pedestal  412  and drywell  20 , space may be at a premium and installation of new dedicated components and/or movement of other components may be undesirable, such that example system  1000  may use and share pathways with conventional TIP drives and instrumentation. A shared functionality system is described in US Patent Publication 2013/0177125, titled “Systems and Methods for Managing Shared-Path Instrumentation and Irradiation Targets in a Nuclear Reactor” by Heinold et al., filed Dec. 10, 2012, that application being incorporated by reference in its entirety herein. 
     Because example embodiment systems may be dual-use and/or because it may be desirable to retain irradiation targets  250  within instrumentation tubes without requiring constant fill or blockage of one or more penetration pathways, example embodiment systems can include a holding mechanism at flanges  1110  to preserve irradiation targets  250  within instrumentation tubes  50  for desired amounts of time and/or at desired axial levels. 
       FIG. 3  is an illustration of an example embodiment retention assembly  650  useable in example embodiment systems. As shown in  FIG. 3 , example embodiment retention assembly  650  may be installed about a flange  1110  of an instrumentation tube  50  ( FIG. 2 ). For example, retention assembly may include a frame  659  that provides an air-tight enclosure attached directly to flange  1110  and an associated instrumentation tube. A penetration pathway,  1100  may extend completely or partially through example embodiment retention assembly  650 , and irradiation targets  250  may travel therein. Example embodiment retention assembly  650  may include a restricting fork  651  or other retaining structure that blocks movement of irradiation targets  250  through penetrations pathway  1100  and/or into/out from instrumentation tubes. As shown in  FIG. 3 , restricting fork  651  is moveable between at least two positions  651   a  and  651   b , selectively restricting irradiation target  250  movement. 
       FIG. 4  is an illustration of an example embodiment restricting fork  651 . As shown in  FIG. 4 , restricting fork  651  includes two prongs  6511  that are shaped and sized to pass around opposite sides of a penetration pathway in which irradiation targets pass. Prongs  6511  include variable separation so as to provide variable blockage to irradiation targets passing or blocked therebetween. For example, each prong  6511  may include a matching interior surface divot  6512  to provide a greater separation d 2  and more passage room between prongs  6511  at interior surface divots  6512 . At other points, prongs  6511  may be spaced with lesser separation d 1  that prevents irradiation targets of a particular size from passing between prongs  6511 . Lesser separation d 1  may still permit movement of other features, such as plunger  1350  ( FIG. 3 ) to pass therebetwen, thereby permitting discriminatory movement within penetration pathway  1100  ( FIG. 3 ). It is understood that while example embodiment retention assembly  650  may use a restricting fork  651  to provide discriminatory hold and pass functionality to penetration pathways, other devices may similarly achieve this goal in example embodiments, including a choke valve, magnets, retractable blocking post, tube seal, etc. 
     Divots  6512  may be placed at appropriate transverse positions to intersect penetration pathways  1100  ( FIG. 3 ) only at a retracted position  651   a  ( FIG. 3 ). As such, in retracted position  651   a  ( FIG. 3 ), restricting fork  651  may permit irradiation targets  250  ( FIG. 3 ) to pass through prongs  6511 , while in an extended position  651   b  ( FIG. 3 ), restricting fork  651  may restrict or crimp penetration pathway  1100  ( FIG. 3 ) to prevent irradiation targets  250  ( FIG. 3 ) from passing therebetween. 
     Example embodiment restricting fork  651  may further include a driving adapter  6514  that attaches to a piston or other reliable driving mechanism to move fork  651  into desired positions. For example, driving adapter  6514  can be a simple ring that engages with a piston to move fork  651  between positions  651   a  and  651   b  ( FIG. 3 ). Example embodiment restricting form  651  may further include an O-ring seal  6513  that seals an interior of frame  659  of example embodiment retention assembly  650 . In this way, retention assembly  650  may preserve an air-tight seal about flange  1110  and ensure any pneumatic driving fluid, such as that provided through pneumatic lines  510  and  509 , is directed into penetration pathways to move irradiation targets. 
     As shown in  FIG. 3 , example embodiment retention assembly  650  further includes a driving force to move restricting fork  651  to desired positions. For example, one or more motors  656  can be connected to a crankshaft and piston that drives restricting fork  651  back and forth perpendicular to penetration pathway  1100 . Such motors  651  can provide high reliability of positioning and movement of restricting fork  651  as well as statuses and commands to remote users. Motors  656  may provide functionality and be configured similarly to motors, driveshafts, and pistons of the incorporated Heinold co-pending application. Of course, any other co-located or remote driving devices may be used to move restricting fork  651  into and between desired positions, including pneumatic air sources, fail-safe solenoids, etc. 
     Restricting fork  651  can move relative to penetration pathway  1100  holding irradiation targets  250  and plunger  1350 , from positions  651   a  to  651   b  and vice versa, depending on a desired restriction and passability of penetration pathway  1100 . Plunger  1350  in example embodiment driving systems may be sized smaller than irradiation targets  250  in order to freely pass between restricting fork  651  in any position, whereas irradiation targets  250  may not. For example, restricting fork  651  may have prongs  6511  spaced d 1  apart, where d 1  corresponds to a length greater than a diameter of plunger  1350 . Irradiation targets  250  in example systems may be sized larger than d 1  but smaller than d 2  created by surface divots  6512 , such that irradiation targets  250  will be moveable in penetration pathway  1100  only when restring fork  651  is in pass position  651   a.    
     In the example of  FIG. 3 , irradiation targets  250  may be driven into and through example embodiment retention assembly  650  and into an instrumentation tube. Retention fork  651  may be in pass position  651   a  during this driving, such that irradiation targets  250  and plunger  1350  can pass through retention fork  651  in penetration pathway  1100  and into an instrumentation tube, without fork  651  restricting such insertion. Once irradiation targets  250  are above restricting fork  651 , motors  656  can drive retention fork  651  forward into hold position  651   b , where penetration pathway  1100  is restricted. For example, penetration pathway may be a flexible plastic that restricts when contacted by restricting fork  651  and/or penetration pathway  1100  may include a gap that allows prongs of restricting fork  651  to selectively block irradiation targets  250  without permitting their escape from penetration pathway  1100 . 
     As shown in  FIG. 5 , plunger  1350  may be withdrawn from irradiation targets  250  while restricting fork  651  maintains irradiation targets  250  in static desired axial positions for irradiation. Plunger  1350  may be entirely withdrawn from retention assembly  650  and penetration pathway  1100 , allowing their use with other systems while irradiation targets  250  are maintained in an instrumentation tube for irradiation. Fork  651  and motors  656  may be configured with sufficient strength and power to hold several pounds of irradiation targets  250  aligned axially within an instrumentation tube and bearing on prongs of fork  651 , while also permitting movement of restricting fork  651  between desired positions. One or more positioning irradiation targets  251  may be used to prop up irradiation targets  250  to desired axial positions within an instrumentation tube, and/or to demarcate where a chain of harvestable irradiation targets  250  ends/begins. When irradiation is complete, or at any other point when evacuation of an instrumentation tube is desired, motors  656  can drive fork  651  back into pass position  651 a, and irradiation targets  250  may descend into penetration pathway  1100  via gravity, with or without the aid of plunger  1350 . 
     Example embodiment retention assembly  650  may further include a position detector that detects whether irradiation targets  250  and/or plunger  1350  are in a desired position for movement of restricting fork  651 . As shown in  FIGS. 3 and 5 , example embodiment retention assembly  650  may include one or more reed switches  655  positioned about penetration pathway  1100  that detect a presence of a magnetic member therein and report a signal to an operator and/or motor  656  to indicate such detection. Reed switches  655  may be configured to detect presence of a magnet  1351  positioned within plunger  1350  and/or TIP cable  1305 , or a presence of a ferromagnetic positioning irradiation target  251 , for example. 
     Based on the position of these magnetic features, reed switches  655  may trip only when irradiation targets  250  are in desired positions for fork  651  to move between positions  651   a  and  651   b.  For example, a magnet  1351  in plunger  1350  may be configured and positioned such that it trips reed switches  655  only at a location where plunger  1350  has driven all irradiation targets  250  beyond restricting fork  651 , as shown in  FIG. 3 . Upon trip, reed switches  655  may actuate motors  656  to move restriction fork  651  from pass position  651   a  to hold position  651   b , allowing plunger  1350  to be withdrawn while retaining irradiation targets against prongs of fork  651 . Still further, reed switches  655  may detect several positions of magnet  1351  based on magnetic field strength, such as an approach of magnet  1351  toward reed switches  655 , and report such positioning and/or approach to a user, motors  656 , and or other automated components. In this way, TIP cable  1305 , plunger  1350 , and/or any TIP drive pushing the same may be slowed when such an approach is detected, permitting greater positioning accuracy, magnetic detection, and/or preventing overshoot or inertial lifting of irradiation targets  250  and plunger  1350  beyond desired positions. 
     Reed switches  655  and/or motors  656  may be self-powered and -contained as a relatively small unit in example embodiment retention assembly  650  about flange  1110 , so as to permit installation of several example embodiments at several instrumentation tubes, if desired, without requiring excess wiring or power. Still further, a shared wiring system and/or wireless communications units may communicatively connect example embodiment retention assemblies  650  to a user or other manual or automatic devices for coordination within an example embodiment system  1000  to irradiate and harvest desired isotopes. Example embodiment retention assembly  650  may reliably provide selective movability to irradiation targets  250  through penetration pathway  1100 , and, as such, is fabricated of materials that are resilient in operating nuclear power plant conditions. For example, retention assembly  650  may be useable in the systems described in incorporated application Ser. No. 13/477,244 at flange  1110  to replace and/or be used with various mechanisms described therein for controlling irradiation target movement. 
     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, the locations of retention devices that allow movement or holding of irradiation targets or other system components are not limited to the specific systems shown and described in the figures—other specific devices and systems for reliably preserving irradiation targets in an instrumentation tube are equally useable as example embodiments and fall within the scope of the claims. Furthermore, it is understood that example systems and methods are useable in any type of nuclear plant with access barriers that prevent unlimited access to the reactor, including known light water reactor designs, graphite-moderated reactors, and/or molten salt reactors, as well as any other nuclear plant design. Such variations are not to be regarded as departure from the scope of the following claims.