Irradiation target retention systems, fuel assemblies having the same, and methods of using the same

Example embodiments and methods are directed to irradiation target retention devices that may be inserted into conventional nuclear fuel rods and assemblies. Example embodiment devices may hold several irradiation targets for irradiation during operation of a nuclear core containing the assemblies and fuel rods having example embodiment irradiation target retention devices. Irradiation targets may substantially convert to useful radioisotopes upon exposure to neutron flux in the operating nuclear core and be removed and harvested from fuel rods after operation.

BACKGROUND

Example embodiments generally relate to fuel structures and radioisotopes produced therein in nuclear power plants.

2. Description of Related Art

Generally, nuclear power plants include a reactor core having fuel arranged therein to produce power by nuclear fission. A common design in U.S. nuclear power plants is to arrange fuel in a plurality of fuel rods bound together as a fuel assembly, or fuel assembly, placed within the reactor core. These fuel rods typically include several elements joining the fuel rods to assembly components at various axial locations throughout the assembly.

As shown inFIG. 1, a conventional fuel assembly10of a nuclear reactor, such as a BWR, may include an outer channel12surrounding an upper tie plate14and a lower tie plate16. A plurality of full-length fuel rods18and/or part length fuel rods19may be arranged in a matrix within the fuel assembly10and pass through a plurality of spacers20. Fuel rods18and19generally originate and terminate at upper and lower tie plates14and16, continuously running the length of the fuel assembly10, with the exception of part length rods19, which all terminate at a lower vertical position from the full length rods18.

As shown inFIG. 2, fuel elements25may be shaped in pellet-form and placed within the fuel rods18or19. These fuel elements25may be “stacked” within the fuel rod continuously to provide fuel through the length of the fuel rod18or19. The stacking of fuel elements25may permit expansion or other deformation of the fuel elements25during the operation cycle of the reactor core. Further, a gap21between the elements25and an inner wall23of the fuel rod18or19may accommodate gaseous fission products produced from the fuel elements25during operation of the reactor. Spring24at ends, typically at least an upper end, of the fuel element stack in the fuel rod may be present to further allow fission product accumulation and fuel element25deformation.

SUMMARY

Example embodiments and methods are directed to irradiation target retention devices and systems that may be inserted into conventional nuclear fuel rods and assemblies. Example embodiment devices may hold several irradiation targets for irradiation during operation of a nuclear core containing the assemblies and fuel rods having example embodiment irradiation target retention devices. Irradiation targets may substantially convert to useful radioisotopes upon exposure to neutron flux in the operating nuclear core and be removed and harvested from fuel rods18/19after operation.

An example embodiment irradiation target retention device may include one or more irradiation targets that may be inserted and held in retaining bores in the device during operation. Bores may be sealed by a cap or by other retention devices so as to provide multiple levels of containment to the irradiation targets and radioisotopes produced therein. In other example embodiments, irradiation targets may be removed from example embodiment retention devices by aligning exit spaces within the devices and removing irradiation targets therefrom.

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.

FIG. 3Aillustrates an example embodiment irradiation target retention device125that may makeup an irradiation target retention system. Irradiation target retention device125has dimensions that enable it to be inserted into conventional fuel rods (cladding tubes) used in conventional fuel assemblies. For example, irradiation target retention device125may have a maximum width of an inch or less and a maximum length of several feet. Although irradiation target retention device125is shown as cylindrical, a variety of properly-dimensioned shapes, including hexahedrons, cones, and/or prismatic shapes may be used for irradiation target retention device125.

Example embodiment irradiation target retention device125includes one or more axial bores130that extend partially downward into device125in an axial direction from a top end/top face128. Axial bores130may be arranged in any pattern and number, so long as the structural integrity of example embodiment irradiation target retention devices is preserved. Axial bores130may have a variety of dimensions and shapes. For example, axial bores130may taper with distance from top face128and/or may have rounded bottoms and edges.

Irradiation targets140may be inserted into one or more axial bores130in any desired number and/or pattern. Irradiation targets140may be in a variety of shapes and physical forms. For example, irradiation targets140may be small filings, rounded pellets, wires, liquids, and/or gasses. Irradiation targets140are dimensioned to fit within axial bores130, and/or axial bores130are shaped and dimensioned to contain irradiation targets140.

Irradiation targets140may be fabricated of a variety of materials that substantially convert into radioisotopes when exposed to a neutron flux encountered in example embodiment irradiation target retention devices125. For example, irradiation targets140may include Iridium-191, which may convert to Iridium-192 when exposed to neutron flux encountered in an operating nuclear reactor, and/or Cobalt-59, which may convert to Cobalt-60 when exposed to neutron flux encountered in an operating nuclear reactor, etc. Irradiation targets140may further be sealed containers of a material designed to substantially maintain physical and neutronic properties when exposed to neutron flux within an operating reactor. The containers may contain a solid, liquid, and/or gaseous irradiation target and/or produced radioisotope so as to provide a third layer of containment (other containments discussed below) within irradiation targets140.

A cap138may attach to top end/face128and seal irradiation targets140into axial bores130. Cap138may attach to top end128in several known ways. For example, cap138may be directly welded to top face128. Or, for example, as shown inFIG. 3B, cap138may screw onto top end128via threads129on example retention device125and cap138. Or, for example, cap138may attach to an top end128via a lock-and-key mechanism on cap138and device125. In any of these attachments, cap138may retain irradiation targets140within an axial bore130and allow easy removal of cap138for harvesting of irradiated irradiation targets140. Cap138may further have a flat face that seats against each axial bore130on top face128so as to prevent irradiation targets140or solid, liquid, or gaseous radioisotopes produced by irradiation targets140from intermingling with other irradiation targets140and/or escaping from axial bores130.

Example embodiment irradiation target retention device125is fabricated from a material designed to substantially retain its neutronic and physical properties when exposed to a neutron flux encountered in an operating nuclear reactor. Thus example embodiment irradiation target retention device125may not substantially interfere with neutron flux reaching irradiation targets140and may not chemically react with irradiation targets140or radioisotope produced therefrom. Example embodiment irradiation target retention device may be fabricated from, for example, a zirconium alloy, stainless steel, aluminum, a nickel alloy, Inconel, etc.

As shown inFIG. 4, example embodiment irradiation retention device125may be inserted into conventional nuclear fuel rods18and/or19(FIGS. 1 & 2) in the same manner as conventional fuel pellets may be inserted into fuel rods18/19and sealed therein. Example embodiment irradiation retention device125may substantially fill the nuclear fuel rod18/19, or alternatively, may not substantially fill nuclear fuel rod18/19and allow for empty space and/or nuclear fuel pellets to fill the remaining space of nuclear fuel rod18/19. A spring24may be positioned axially with example embodiment irradiation retention device125so as to maintain a constant position of device125while permitting minor expansion and/or shifting due to variable conditions encountered in an operating nuclear reactor.

A nuclear reactor including a fuel assembly with a fuel rod having an example embodiment irradiation target retention device125may be operated at normal power operation such that example embodiment irradiation target retention device125and irradiation targets140therein are irradiated by neutron flux present in the operating reactor. Because flux levels in the reactor are known, and depth of bores130(shown inFIG. 3) and placement and composition of irradiation targets140therein may be known, it is possible for one skilled in the art to calculate the specific activity of radioisotopes produced from irradiation targets140. Conversely, a person skilled in the art may calculate a bore130depth in order to affect optimal radioisotope production knowing operating flux levels and irradiation target140makeup.

Once irradiated and substantially converted into useful radioisotopes, irradiation targets140and example embodiment irradiation target retention device125may be removed from the nuclear reactor, for example, during reactor shut down. Example embodiment irradiation retention device125may be removed from irradiated fuel assemblies and fuel rods18/19and disassembled by removing cap138in order to harvest the irradiated irradiation targets140therein.

Rod18/19and example embodiment device125being capped and sealed provide at least a double containment for irradiation targets140. This provides insurance against irradiation target escape in the event of fretting of cladding of fuel rod18/19containing example embodiment irradiation target retention device125. Depending on placement of axial bores140, additional containment may be provided by the radial thickness of example embodiment irradiation target retention devices125.

As shown inFIG. 5, an alternative example embodiment irradiation target retention device225may be in a fuel element shape/cylindrical pellet-type configuration, although other shapes are useable for example embodiments. Example embodiment device225may be dimensioned so as to fit within a conventional nuclear fuel rod18/19, had have a maximum length such that several example embodiment irradiation target retention devices225may fit within a fuel rod18/19. For example, irradiation target retention device may have a length of a few centimeters or less.

Example embodiment irradiation target retention device225may otherwise share several characteristics with previously-discussed example embodiments, redundant portions of which are omitted. Example embodiment device225defines one or more bores230that extend into but not through example embodiment device225. Bores230may be filled with a desired irradiation target240that substantially converts to a radioisotope when exposed to neutron flux passing through example embodiment device225. Ingot-type example embodiment devices may further include a cap as described above with regard to previous example embodiments to contain irradiation targets240in bores230therein.

Alternatively, as shown inFIG. 6, instead of having a cap to retain irradiation targets240within bores230, example embodiment irradiation target retention device225may be sealed and/or contained by an empty device225and/or a slug226. Example target retention devices225may be tightly stacked with other example target retention devices225within a conventional nuclear fuel rod18/19. A gap21may further be present between example devices225/slug226and wall23of the fuel rod18/19. A spring24or other holding device may supply resistive pressure against a stack of example embodiment devices225in order to hold them substantially flush against one another in the fuel rod18/19. Because bores230may not pass entirely through example devices225, the bottom surface of each device may be largely flat so as to facilitate a containing seal against another example device225stacked immediately below.

A slug226may be placed between the spring24or other preloading device and the stack of example embodiment irradiation retention devices225in order to provide the same sealing structure for the topmost device225in the stack. Slug226may be substantially similar to example embodiment devices225, except it does not contain any irradiation targets so as to not leak targets onto spring24or any other tensioning device within rod18/19.

Example embodiment irradiation target retention devices225may permit several different types and phases of irradiation targets240to be placed in each device225and each bore230thereof. Because several example devices225may be placed at precise axial levels within the fuel rod18/19, it may be possible to provide a more exact amount/type of irradiation target240at a particular axial level within fuel rod18/19. Because the axial flux profile may be known in the operating reactor, this may provide for more precise generation and measurement of useful radioisotopes in irradiation targets240placed within example embodiment irradiation target retention devices225.

As shown inFIG. 7, yet a further example embodiment irradiation target retention device325may be substantially similar to ingot-type example embodiment retention devices225. However, example embodiment devices325may have one or more bores330that share a radial position about a central axis380of example embodiment devices325. Example embodiment devices325further include a hole385in the shared radial position that passes completely through example embodiment irradiation target retention device325, unlike bores330. Irradiation targets may not be placed in hole385.

Example embodiment irradiation target retention devices may further include a keyed slit395or other aperture positioned at central axis380. Keyed slit395may be shaped to permit a correspondingly shaped shaft to pass through example embodiment device395and rotate example embodiment device395about central axis380. The keyed slit395may be oriented in the same position with respect to the hole385in each example embodiment irradiation target retention devices325.

As shown inFIG. 8, because bores330and hole385may share the same radial position about a central axis380in example embodiment irradiation target retention devices325, if example devices325are stacked along axis380in fuel rod18/19, all holes385may be aligned at a single angular position so as to form an exit shaft390through the stack of example embodiment devices325. Further, because keyed slits395may also align and share a common orientation with holes385if example devices325are stacked, a tool having a keyed end corresponding to slit shape395may be passed into and through the stack of irradiation target retention devices325.

As shown inFIG. 9, in order to harvest radioisotopes produced by example embodiment irradiation target retention devices325after irradiation thereof in an operating nuclear core, the stack of example embodiment devices325may be oriented with bores330facing downward such that irradiation targets340may fall out of bores330by gravitational action alone. Selected example embodiment devices325stacked within fuel rod18/19may then be rotated about central axis380until all holes385, and thus exit shaft390, align with a desired bore330of an unrotated device325within the stack. Irradiation targets340and radioisotopes present therein may fall from bore330through exit shaft390for harvesting.

Stacked example embodiment devices325may be rotated by a keyed tool396moved into keyed slit395at a desired axial distance. Thus the particular irradiation target retention device emptied through exit shaft390may be selected by the axial distance the keyed tool396is moved into keyed slits395. Because all keyed slits395may be oriented similarly with respect to holes385, exit shaft390may be rotated consistently to a bore330to be emptied. Further, a bottom-most (after turning the stack downward) example irradiation target retention device325may lack any irradiation targets340such that irradiation targets340will not fall from the bottom-most device325while emptying a stack of example embodiment devices325.

Example embodiment irradiation target retention devices may be rotated by other mechanisms and lack a central keyed slit395. For example, external sleeves may rotate individual retention devices325in a stack to desired angular positions to drain irradiated irradiation targets from exit shaft385. Similarly, holes385need not contemporaneously align in a stack of example embodiment retention devices325; an irradiation target may fall into an unaligned hole385that is later aligned with a lower hole385, such that irradiation target340may fall in increments through a stack of example embodiment devices until harvested.

Although example embodiment retention devices may be inserted into BWR-type fuel rods and fuel assemblies in example embodiments, it is understood that other types of fuel and power plants may be useable with example embodiment retention devices. For example, PWR, CANDU, RBMK, ESBWR, etc. type reactors may include fuel rods that can accommodate example embodiment retention devices in order to irradiate irradiation targets therein.

Example embodiments 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, the word “assembly” is used throughout to denote a collection of fuel rods in example embodiments, but terms like “bundle” may also be used interchangeably, and example embodiments may be useable with fuel bundles lacking all components typically found in a finished fuel assembly. Or, for example, other fuel types, shapes, and configurations may be used in conjunction with example embodiment irradiation target systems. 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.