Patent Description:
Radioactive isotopes, or radioisotopes, are used commercially in a variety of industries, such as medicine where gamma rays emitted by radioactive elements are used to detect tumors, in the food industry where food is sometimes irradiated by exposure to gamma rays to kill certain bacteria, in agriculture, pest control, and in archeology where radiocarbon dating uses carbon-<NUM> to measure the age of carbon-bearing items. Radioisotopes are highly unstable and readily decay, emitting radiation in the form of alpha, beta and gamma rays. A class of radioisotopes is produced as a by-product of typical nuclear power plant operation.

The production of commercially valuable radioisotopes, such as Co-<NUM>, Ac-<NUM>, and W-<NUM>, requires the capture of at least one neutron by a target material placed inside a reactor core. Cobalt-<NUM> (Co-<NUM>), for example, produces high energy gamma rays, which may be used for radiotherapy, equipment sterilization, and food irradiation. Co-<NUM> is a synthetic radioisotope of cobalt that is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidic cobalt isotope Co-<NUM>. Actinium-<NUM> (Ac-<NUM>) is produced by the decay of thorium-<NUM>. Ac-<NUM> can be used in nuclear medicine for treatment of malignancies. Tungsten-<NUM> (W-<NUM>) is produced in a nuclear reactor by irradiation of tungsten oxide with thermal and high energy neutrons. W-<NUM> is used to produce Rhenium-<NUM> (Re-<NUM>), a high energy β-emitting radioisotope which has shown utility for a variety of therapeutic applications in nuclear medicine, oncology, radiology, and cardiology.

The rate of production of the desired radioisotope depends on the numbers and energy spectrum of neutrons surrounding the target material (e.g., Co-<NUM>, thorium-<NUM> etc.) and the probability that the target material captures neutrons within the energy range. An example of this phenomena is revealed by the neutron "capture cross section" measurements for Co-<NUM>, shown in <FIG> as a function of neutron energy. The neutron capture cross section is shown to be about <NUM> barns (b) at <NUM> eV. The neutron capture cross section increases dramatically at the capture resonance present at a neutron energy of approximately <NUM> eV where it increases to approximately 7000b. In U-<NUM> fission, the most probable neutron energy is <NUM> MeV. Ideally, the number of neutrons in the <NUM> eV or below range needs to be maximized in order to maximize the rate at which Co-<NUM> is produced. This can be done by simply increasing the number of fissions that occur. This may be accomplished by increasing the amount of U-<NUM> present in the fuel. However, the economic cost of that approach significantly increases the cost of the radioisotope being produced.

Another method is to increase the amount of <NUM> eV neutrons surrounding the target material without needing to increase the fission rate. This can be accomplished by surrounding the target material with an optimal amount of neutron energy moderating material with a high neutron lethargy and slowing down more of the neutrons with higher energies that wouldn't normally be captured by the Co-<NUM> target before they diffuse away from the area of the target. This effectively increases the number of fission neutrons that are in the desired energy range to maximize the neutron captures in the target material. Optimizing the amount and distribution of water surrounding the Co-<NUM> target will allow the average neutron energy spectrum around the target to be controlled to maximize the Co-<NUM> production rate. The same approach can be used to increase the production rates of other desired radioisotopes.

One way to accomplish the desired shifting of the fission neutron energy spectrum around an irradiation target is to change the amount of neutron energy moderator, such as water, or other material with a low neutron capture cross section and a high neutron scattering cross section, between the target and the neutrons produced by fission. The hydrogen in a water moderator is very effective at slowing down high energy neutrons, but not capturing them so they can't interact at all with the irradiation target material. Adjusting the amount of water surrounding the irradiation target can be accomplished in a water cooled and moderated reactor core by controlling the geometry of the irradiation target. Materials other than water, such as low atomic number metallic substances with small neutron capture cross sections (e. Zirconium, Nickle, Graphite) may also be used to increase the neutron captures in the target material.

It has been shown by calculations performed by those skilled in the art that the use of pressurized water reactor (PWR) fuel assembly inserts similar to thimble flow plugs or burnable absorber rods containing rod-shaped slugs of Co-<NUM> for the production of Co-<NUM> can be used to produce commercially valuable amounts of Co-<NUM>. However, the predicted specific activity (SA) in Ci/cm<NUM> and the Ci per unit length of the current irradiation target shape of the Co-<NUM> in the irradiated material at the end of the desired irradiation period is less than needed to support economically favorable production useful for current application practices. Relevant prior art forming the basis for the pre-characterizing portion of claim <NUM> is found in document <CIT> and in document <CIT>.

The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, abstract and drawings as a whole.

The present invention resides in a radioisotope production capsule as defined in claim <NUM> to generate the maximum amount of desired radioisotope from irradiation targets designed to fit into the guide thimbles present in conventional reactors, such as PWR fuel assemblies, with the intent to benefit the radioisotope production economics.

A radioisotope production capsule used for this purpose includes an inner container for housing one of a target material and a neutron moderator, an outer container surrounding the inner container for housing the one of the target material and the neutron moderator not housed by the inner container, and cladding for isolating the target material from the neutron moderator. The neutron moderator, in various aspects, may be a coolant such as water.

In various aspects, the inner container is defined by an inner wall of a cladding material. In various aspects, the outer container is defined between an outer wall of the cladding material and the inner wall of the cladding material. The capsule also includes locking members for axially joining adjacent capsules within an insert component of a fuel assembly of a nuclear reactor. The locking members may in various aspects be mounted on the outer container. In certain aspects, the locking members may be quick-disconnect locking members.

The capsule may also include support members for holding the inner container in a desired position within the outer container. The support members may be posts that extend from an outer wall of the cladding material that forms the outer container to an inner wall of a cladding material that defines the inner container. The posts are preferably made of a cladding material that defines the inner container. The posts are preferably made of a material that expands at temperatures within a nuclear reactor to provide a pressure fit for the posts between the outer wall of the outer container and the inner wall of the inner container and contracts when the material is cooled to a temperature lower than the temperatures within a nuclear reactor.

The inner container is in various aspects a cylinder and the outer container is an annular cylinder concentric with the axis of the inner container. In certain aspects, the inner cylinder holds the neutron moderator and the annular cylinder holds the target material for irradiation. In certain aspects, the inner cylinder holds the target material for irradiation and the annular cylinder holds the neutron moderator.

A method for producing a desired radioisotope is described herein. The method includes providing at least one capsule, each capsule having an inner container and an outer container surrounding the inner container, and a cladding material isolating the inner container from the outer container, inserting a target material that will produce the desired radioisotope upon irradiation into one of the inner or the outer container, surrounding the target material with the cladding material to isolate the target material within the capsule, inserting a neutron moderator into the one of the remaining inner or outer container in which the target material was not inserted, inserting at least one capsule having target material and neutron moderator-filled inner and outer containers into an insert component of a nuclear fuel assembly, irradiating the target material to form the radioisotope from such target material, and removing the capsule from the insert component.

The modular radioisotope production capsules described herein allow the neutron energy spectrum to be optimized for the target capture cross section and the minimum reactor fuel assembly reactivity reduction with the capability to adjust the total activity contained in a module outer envelope.

The characteristics and advantages of the present disclosure may be better understood by reference to the accompanying figures.

As used herein, the singular form of "a", "an", and "the" include the plural references unless the context clearly dictates otherwise.

Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, lower, upper, front, back, and variations thereof, shall relate to the orientation of the elements shown in the accompanying drawing and are not limiting upon the claims unless otherwise expressly stated.

In the present application, including the claims, other than where otherwise indicated, all numbers expressing quantities, values or characteristics are to be understood as being modified in all instances by the term "about. " Thus, numbers may be read as if preceded by the word "about" even though the term "about" may not expressly appear with the number. Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description may vary depending on the desired properties one seeks to obtain in the compositions and methods according to the present disclosure. At the very least, each numerical parameter described in the present description should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Referring to <FIG> and <FIG>, a nuclear fuel assembly <NUM> commonly includes a plurality of fuel rods <NUM> (only a few rods are shown around the perimeter for clarity, but in use, the rods would occupy most of the fuel assembly space), and a set of guide thimble tubes <NUM>. Guide thimbles <NUM> are vacant tubes provided to hold control rods or in-core instrumentation used in a reactor (not shown). Each guide thimble <NUM>, as shown, includes a neck <NUM>, a mid-section <NUM> and a tapered end <NUM>. Several guide thimbles <NUM> are suspended from a plate <NUM>, which is itself suspended by a hold down spring <NUM> from the upper core plate <NUM> of the fuel assembly <NUM>. In the illustration shown in <FIG>, there are <NUM> guide thimbles <NUM> in each fuel assembly <NUM>. The number may vary depending on factors such as the size of the reactor.

<FIG> illustrates a section view of an exemplary thimble plug finger <NUM> dimensioned to fit within a guide thimble <NUM>. The plug finger <NUM> includes an outer sheath <NUM>, a cap <NUM> for closing the open end of finger <NUM>, and a tapered closed end <NUM>. The plug fingers <NUM> are meant to fit in the interior of the guide thimbles <NUM>, so the outer dimensions of the fingers <NUM> will be equal to or less than the inner diameter of the guide thimble mid-section <NUM>. Either the juncture between the mid-section <NUM> and the tip <NUM> or the mid-section <NUM> and the neck <NUM> may be opened for insertion of the plug fingers <NUM>.

An exemplary radioisotope production capsule <NUM> (see <FIG>) is shown positioned within the plug finger <NUM>. If there were no plug fingers <NUM> inserted into the fuel assembly guide thimbles <NUM> during reactor operation, the guide thimble tubes would be filled with the neutron moderator and coolant used in a PWR, such as water, with a flow rate that exceeds the neutron moderator and coolant flow through the fuel rods <NUM> included in the fuel assembly <NUM>. The plug fingers <NUM> block this flow and consequently increase the neutron moderator and coolant flow among the fuel rods <NUM>. Including the irradiation target material inside the plug fingers <NUM> allows the plug fingers <NUM> to both serve their intended function and to produce the desired radioisotopes.

While those skilled in the art will appreciate that a variety of different geometries for the plug fingers <NUM> and radioisotope production capsules <NUM>, <NUM> may be used, using the same geometry as the conventional fuel assembly guide thimbles <NUM> avoids the need to make any modifications to the fuel assembly mechanical design. This approach greatly reduces the costs associated with implementing the radioisotope production in commercial light water reactors (LWR) designs.

<FIG> illustrate an exemplary embodiment of modular radioisotope capsules <NUM> for insertion in a plug finger <NUM>. Each capsule <NUM> shown includes an inner container, such as inner cylinder <NUM> defined by inner cladding <NUM>. The inner cylinder in this embodiment houses the neutron moderator <NUM> and coolant, which in various aspects, is water. Surrounding the inner cylinder <NUM> is an outer container, such as outer annular cylinder <NUM> defined between the inner cladding <NUM> and an outer cladding <NUM>. In various aspects, the inner cylinder <NUM> is positioned such that its axis is concentric with the axis of the outer annular cylinder <NUM>. The annular cylinder <NUM> in this embodiment holds the target material <NUM> to be irradiated during the nuclear power generation cycle. The target material <NUM> will vary depending on the desired radioisotope to be produced, and the appropriate target material for production of the desired radioisotope can be selected. The target material <NUM> may be in any suitable form, including, but not limited to a solid block, a powder, pellets, spheres, or a liquid. The top and bottom ends of the inner container in this embodiment that holds the neutron moderator and coolant are open to the plug finger interior as indicated by in <FIG> by open ends <NUM>. The top and bottom ends of the outer container that holds the target material is closed by top and bottom extensions of outer cladding <NUM> to isolate the target material from the neutron moderator and coolant. In various aspects, two or more modular capsules <NUM> may be stacked axially within a plug finger <NUM> and may be connected to each other by locking rings or pegs <NUM> or a similar mechanism for joining adjacent capsules <NUM>.

<FIG> illustrate an alternative embodiment of a radioisotope production capsule <NUM>. Capsule <NUM> includes an inner container, such as inner cylinder <NUM> that holds target material <NUM> and an outer container, such as outer annular cylinder <NUM> that holds neutron moderator <NUM> and coolant in a PWR. Inner cladding <NUM> surrounds the top, bottom and sides of the target material and separates the inner cylinder from the annular cylinder. Outer cladding <NUM> surrounds the sides of the annular cylinder. If the irradiated target material is a solid, the cladding may simply be deposited on the target material. If the target material is a powder, pellet, sphere, or liquid, the cladding forms the walls of the container for the target material to isolate the target material from the neutron moderator and coolant.

Support members <NUM> support the inner cylinder <NUM> in position within the annular cylinder <NUM>. The support members may be any suitable device for either or both centering and rigidly mechanically supporting, the inner cylinder <NUM> within the annular cylinder <NUM>. For example, the support members <NUM> may be fingers or post-like structures that extend radially outwardly from the inner cladding <NUM> or which extend radially inwardly from the outer cladding <NUM> to connect with an engagement surface on the opposite cladding surface. Alternatively, the support members <NUM> may be made of a material that undergoes thermal expansion under the temperatures typical of a nuclear reactor to create a pressure fit, but which shrink enough as the surrounding temperature cools to allow the inner cylinder <NUM> to be removed from the annular cylinder <NUM>.

In various aspects, two or more modular capsules <NUM> may be stacked axially within a plug finger <NUM> and may be connected to each other by locking rings or pegs <NUM> or a similar mechanism for joining adjacent capsules <NUM>.

The capsule <NUM> or <NUM> design described herein for production of a radioisotope, such as Co-<NUM>, maximizes gamma radiation emission intensity by maximizing the conversion rate. The neutron moderator coolant which can be water, for example, in inner cylinder <NUM> or annular cylinder <NUM> enters and exits the plug finger <NUM> outer sheath <NUM> through small holes <NUM> that penetrate the finger <NUM> outer sheath <NUM> and tip <NUM> at the top and bottom, respectively, of the plug fingers <NUM>. (See <FIG>. ) As described above, the portions of the capsules <NUM>/<NUM> that contain the moderator and coolant are not capped so the top and bottom of the capsules in this portion are open to the liquid moderator and coolant environment inside the PWR thimble plug fingers <NUM>.

The claddings <NUM> and <NUM>, and claddings <NUM> and <NUM>, are made of a material that prevents the target material <NUM>/<NUM>, such as Co-<NUM>, from leaching out of the cylinder holding the target material into the cylinder that holds the neutron moderator.

The most appropriate material will depend on the target material and the desired radioisotope to be produced. Exemplary materials for Co-<NUM> include Ni and Zr. Exemplary materials for Ac-<NUM> and W-<NUM> include Zr and stainless steel. The material for the cladding will depend on factors such as the corrosion resistance of the target material relative to the irradiation of the target material and the need to avoid chemical reactions that would cause perforations in the cladding.

The ratio of the thickness of the annular cylinder <NUM>/<NUM> to the diameter of the inner cylinder <NUM>/<NUM> can be adjusted using commercially available nuclear design tools, such as software packages utilizing advanced nodal code (ANC™) for reactor core analysis or similar packages sold under the mark PALADIN®, which are understood by those skilled in the art to calculate the dimensions needed to maximize the rate of production of the desired radioisotope.

The maximum value of the outer diameter of the capsule <NUM> or <NUM>, indicated in <FIG> and <FIG> as Dm, is equal to the inner diameter (ID) of the plug finger <NUM>, shown in <FIG>. An example of the ID of the plug finger <NUM> is <NUM>. The length of the irradiation target material <NUM>/<NUM> contained in the capsule <NUM>/<NUM> is shown on <FIG> and <FIG> as the dimension Lt. This length is determined by the needs of the end-user of the radioisotope. For example, the irradiation activity of a selected radioisotope needed for a particular application may be calculated per unit length of the target material in the capsule <NUM>/<NUM>. For reasons explained in more detail below, the ideal diameter (Ds) of the inner cylinder <NUM>, or the target material <NUM> held within the inner cylinder <NUM> as shown in <FIG>, is less than or equal to the diameter (Dc) of the inner cylinder <NUM>, as shown in <FIG>.

One or more capsules <NUM> and <NUM> may be loaded in tandem into the interior of a plug finger <NUM>. <FIG> and <FIG> show exemplary modular capsules <NUM> and <NUM>, respectively, which may be spaced from each other, or which may be stacked one on top of the other, in the interior cavity of a plug finger <NUM>. In various aspects, locking members to mechanically join the adjacent capsule modules <NUM> or adjacent capsule modules <NUM> together, such as locking rings <NUM> or <NUM>, are provided, for example, to minimize the potential for mechanical vibration of the modules from causing failures of the outer sheath of the plug fingers. An example of the type of connection method is use of a quarter turn quick disconnect design. Numerous examples of quick-disconnect designs are known in the art. The modules <NUM> or <NUM> can be joined until the total length and total activity meets the end user needs.

The connected or stacked modules can be spaced from each other and separated within the fingers <NUM> to allow the module capsules <NUM> or <NUM> that are harvested after irradiation to properly fit inside shipping containers used to transport the irradiated material from the production reactor to a final processing facility. Any suitable means can be used to separate the modular capsules from each other, such as the support members <NUM> that hold the capsules from the sides, mounts on the outer container rims or on the joining mechanisms <NUM>/ <NUM> (e.g., the quick disconnect member on the rim) to separate adjacent modules axially within a plug finger <NUM>, or a similar mechanical attachments known to those skilled in the art. A suitable design of the attachment or suspension supports will allow the capsules <NUM>/<NUM> and target material <NUM>/<NUM> to be easily withdrawn at low temperatures to facilitate removal of the target material from the capsules <NUM>/<NUM> in an irradiated material processing facility.

In practice, one or more capsule <NUM> modules would be placed in each of a plurality of plug fingers <NUM>. Each single plug finger <NUM> loaded with one or more capsules <NUM> would be inserted into the mid-section <NUM> of a guide thimble <NUM>. Each of the individual guide thimbles <NUM> in an array of guide thimbles, as shown in <FIG>, may receive a plug fingers <NUM> which itself has been loaded with one or more capsules <NUM>. In various aspects, only one or a few of the guide thimbles <NUM> in an array of guide thimble need be used to house the plug fingers <NUM> and capsules <NUM>. Thus, a large number of capsules <NUM> may be inserted, via multiple plug fingers <NUM>, into one or multiple guide thimbles <NUM> and exposed to the radiation within a nuclear reactor to produce the desired radioisotope.

Alternatively, one or more capsule <NUM> modules would be placed in each of a plurality of plug finger <NUM>. Each single plug finger <NUM> loaded with one or more capsules <NUM> would be inserted into the mid-section <NUM> of a guide thimble <NUM>. Each of the individual guide thimbles <NUM> in an array of guide thimbles, as shown in <FIG>, may receive a plug fingers <NUM> which itself has been loaded with one or more capsules <NUM>. In various aspects, only one or a few of the guide thimbles <NUM> in an array of guide thimble need be used to house the plug fingers <NUM> and capsules <NUM>. Thus, a large number of capsules <NUM> may be inserted, via multiple plug fingers <NUM>, into one or multiple guide thimbles <NUM> and exposed to the radiation within a nuclear reactor to produce the desired radioisotope.

In a third alternative approach, one or more capsule <NUM> modules would be placed in one or more single plug finger <NUM> and one or more capsule <NUM> modules would be placed in one or more different single plug finger <NUM>. The plug finger <NUM> loaded with one or more capsules <NUM> and the plug finger <NUM> loaded with one or more capsules <NUM> would be inserted into the mid-sections <NUM> of different guide thimbles <NUM>. Each of the individual guide thimbles <NUM> in an array of guide thimbles, as shown in <FIG>, may receive a plug fingers <NUM> which itself has been loaded either with one or more capsules <NUM> or with one or more capsules <NUM>. In various aspects, only one or a few of the guide thimbles <NUM> in an array of guide thimble need be used to house the plug fingers <NUM> with capsules <NUM> or <NUM>.

Following irradiation, the capsules <NUM>/<NUM> and plug fingers <NUM> must be removed from the guide thimbles <NUM>. The guide thimbles would be withdrawn from the fuel assembly <NUM> by known means. The mid-section <NUM> of a withdrawn guide thimble <NUM> may opened, for example, by removing either or both of the tip <NUM> and the neck <NUM>. The plug finger or fingers <NUM> would be removed from the guide thimbles <NUM> and the capsules <NUM> and <NUM> would be removed from the plug fingers <NUM> and transported to a radioisotope production facility. In one aspect, the capsules <NUM>/<NUM> may be harvested from the plug fingers <NUM> by cutting the fingers <NUM> into appropriate length to fit into a transport container. Upon arrival at a production facility, the irradiated target material will be removed from the capsule and the desired radioisotope will be separated by known techniques from the irradiated material.

<FIG> illustrates an exemplary method that can be used to increase the activity contained within the module shown on <FIG>. The method involves irradiating (e.g., simultaneously or sequentially) modules like those shown in <FIG> and <FIG> in the same reactor or in different reactor cores. Once the levels of the desired radioisotope have reached the desired activity levels and have been shipped to the processing facility, the linked capsule <NUM> or <NUM> modules may be uncoupled to produce individual modules. The contents of cylinders <NUM>, for example of capsule <NUM> contained in the modules shown in <FIG> may be pushed out of the center of the module and into cylinder <NUM> of the module of capsule <NUM> shown in <FIG>. The diameter of the inner cylinder <NUM> in this procedure will be smaller than the diameter of inner cylinder <NUM>. The material that is received in cylinder <NUM> will be prevented from passing completely through the bottom of the cylinder <NUM> by the rims and locking rings <NUM> around the bottom of cylinder <NUM>. This will increase the net activity and activity per unit length of the annular cylinder <NUM> irradiation capsule <NUM> module to essentially be the sum of the activity of both the capsule <NUM> and <NUM>. This approach may be used to construct tubular irradiation sources with user defined axial source strength distributions that may be needed to maximize the end user desired radiation dose distribution.

While the capsules <NUM> and <NUM> have been described as having inner cylinders and annular outer cylinders, other shapes may be used. Cylinders fit best with existing fuel assembly insert components but the concept described of a container housing a target material for irradiation adjacent to (for example, either surrounded by or positioned within) a container of a neutron moderator, both housed in a capsule that can be inserted into insert components for a nuclear fuel assembly so that radiation from the nuclear reactor can be absorbed by the target material to produce a desired radioisotope is not limited to cylinders and may vary depending on the geometry of the reactor components.

The modular capsule <NUM>/<NUM> designs and associated methods -allow the maximum amount of desired radioisotope production with the minimal disruption in fuel assembly power distribution and minimal detrimental fuel assembly enrichment impacts. The methods and capsule designs described herein allow desired radioisotopes to be produced within an existing fuel assembly using existing guide thimble insert designs.

The modular radioisotope production capsules <NUM>/<NUM> described herein allow the neutron energy spectrum to be optimized for the target capture cross section and the minimum reactor fuel assembly reactivity reduction with the capability to adjust the total activity contained in a module outer envelope.

While the modular capsules <NUM>/<NUM> have been described as being inserted into guide thimbles <NUM>, they may in addition, or alternatively, be installed into other existing fuel assembly inserts, such as wet annular burnable absorber assemblies. The modular capsule <NUM>/<NUM> designs provide the radioisotope product supplier with a significant increase in product flexibility in terms of source activity levels and the distribution of activity levels within a source assembly.

Claim 1:
A radioisotope production capsule (<NUM>, <NUM>) comprising:
an inner container (<NUM>, <NUM>) for housing one of a target material (<NUM>, <NUM>) and a neutron moderator (<NUM>, <NUM>);
an outer container (<NUM>, <NUM>) surrounding the inner container (<NUM>, <NUM>) for housing the one of the target material (<NUM>, <NUM>) and the neutron moderator (<NUM>, <NUM>) not housed by the inner container (<NUM>, <NUM>); and,
cladding (<NUM>, <NUM>) for isolating the target material (<NUM>, <NUM>) from the neutron moderator (<NUM>, <NUM>),
characterized by
further comprising locking members (<NUM>) for axially joining adjacent capsules (<NUM>).