Patent Application
20090135990
A wide variety of radioactive isotopes are used for medical, industrial, research, and commercial applications. In general, radioisotopes may be produced by irradiating target isotope materials with nuclear particles. The target atoms either transmute directly into the desired isotope, or a radioisotope is produced through a chain of absorption and decay that subsequently generates the desired radioactive product.
Two different technologies are used to provide the source of radiation for radioisotope production: nuclear reactors, which produce a flux of neutrons; and particle accelerators or cyclotrons, which produce a flux of charged particles, usually protons, but sometimes electrons or other particles. For example, medical and industrial radioisotopes have been produced since 1957 by Canada's National Research Universal (NRU) reactor at the Atomic Energy of Canada's (AECL's) Chalk River Laboratories in Ontario, Canada. The NRU produces a high percentage of the world's medical and industrial radioisotopes, including molybdenum-99, a critical isotope used for medical diagnoses. Other exemplary radioisotopes used for medical, industrial, research and commercial applications include thallium-204, which is used for medical cardiac imaging; calcium-45, which is used in bone growth studies; iridium-192, which is used for nondestructive testing of construction and other materials; cobalt-60, which is used to destroy cancer cells, to disinfect surgical equipment and medications, and the sterilization of food supplies; thulium-170, which is used for portable blood irradiations for leukemia, lymphoma treatment, and power source; gadolinium-153, which is used for osteoporosis detection and SPECT imaging; nickel-63, which can be used for the creation of long-life batteries; and americium-241, which is used in smoke detectors.
In general, specimen rods containing isotope targets are inserted through penetrations in the NRU in a continuous process and subject to irradiation therein, so as to produce isotopes at a desired specific activity for use in nuclear medicine and/or industrial applications. The isotope targets are then irradiated during operation of the nuclear reactor. After irradiation, the radioisotope is recovered and used for preparing various radiopharmaceuticals for nuclear medical procedures.
Exemplary embodiments of the present invention are directed isotope production rods. Isotope production rods may replace one or more of the fuel rods in a fuel bundle. The isotope production rods include an irradiation target such a cobalt 59, iridium 191, etc. that when irradiated within a nuclear reactor produce a radioactive isotope such as cobalt 60, iridium 192, etc. As discussed above, these radioactive isotopes have various beneficial applications.
Other exemplary embodiments of the present invention are directed to the placement of one or more isotope production rods in a fuel bundle. The placement may be based on any one of, or a combination of, numerous factors such as relative location of core-monitoring equipment, the type of radioactive isotope being produced, the half-life or length of decay of the radioactive isotope, the neutron absorption rate of the target isotope to produce the radioactive isotope, the desired specific activity of the radioactive isotope being produced, the amount of neutron flux in different areas of the fuel bundle, the duration that the target isotope/radioactive isotope is expected to remain in the reactor until removed (i.e., harvested), etc.
In one embodiment, an arrangement of fuel bundles in a nuclear reactor includes a gamma detector and at least one fuel bundle having an isotope production rod. In particular, the fuel bundle includes an outer channel, and a plurality of fuel rods disposed in fuel rod positions within the outer channel. The fuel rods include nuclear fuel for sustaining a nuclear reaction in the nuclear reactor. Also, as mentioned above, at least one isotope production rod is included in the outer channel. The isotope production rod includes an irradiation target, which emits gamma particles if irradiated, and the isotope production rod is disposed in one of the fuel rod positions within the outer channel based on the location of the gamma detector with respect to the fuel bundle.
In another embodiment, the arrangement of fuel bundles in a nuclear reactor includes at least one fuel bundle having at least one isotope production rod. In particular, the fuel bundle includes an outer channel, and a plurality of fuel rods disposed in fuel rod positions within the outer channel. The fuel rods include nuclear fuel for sustaining a nuclear reaction in the nuclear reactor. Also, as mentioned above, at least one isotope production rod is included in the outer channel. The isotope production rod includes an irradiation target, which emits gamma particles if irradiated, and the isotope production rod is disposed in one of the fuel rod positions within the outer channel based on the absorption and the half-life of a radioactive isotope to be produced by irradiating the irradiation target.
The present invention will become more apparent by describing, in detail, exemplary embodiments thereof with reference to 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 exemplary embodiments of the present invention.
Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments may be embodied in many different forms and should not be construed as being limited to the example embodiments set forth herein. Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail to avoid the unclear interpretation of the example embodiments. Throughout the specification, like reference numerals in the drawings denote like elements.
It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it may be directly on, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. 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, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.
Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, 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.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The fuel rods 18 and 19 with at least a pair of water rods 22 and 24 may be maintained in spaced relation to each other in the fuel bundle 10 by a plurality of spacers 20 provided at different axial locations in the fuel bundle 10 so as to define passages for reactor coolant flow between fuel rods 18, 19 in the fuel bundle 10. There may typically be between five to eight spacers 20 spaced along the entire axial length of the fuel bundle 10 for maintaining the fuel rods 18, 19 in the desired array thereof. Spacer 20 may be embodied as any type of spacer, for example, ferrule-type spacers or spacers of the type described and illustrated in U.S. Pat. No. 5,209,899.
In
Rod segments 110 may be attached between the upper and lower end pieces 120, 130 and to each other so as to form the entire axial length of the rod assembly 100. In an example, a rod segment 110a, a rod segment 110b and one each of the upper and lower end pieces 120, 130 may be connected by adaptor subassemblies 300 at connections points along the axial length of the rod assembly 100 where the rod assembly contacts spacers 20. Although only three spacers 20 and adaptor subassemblies 300 are shown in
In this exemplary embodiment, the rod segments and adaptor subassemblies are constructed of a material which is corrosion resistant and compatible with the other reactor components. An exemplary material may be a zirconium alloy, for example.
Desirably, a portion of each spacer 20 contacts the rod assembly 100 at each of the adaptor subassemblies 300 so as to substantially cover adaptor subassemblies 300 and/or connection points 115 between rod segments 110, or substantially covers an adaptor subassembly 300 or connection point 115 connecting a given rod segment 110 and one of the upper and lower end pieces 120 and 130. Accordingly, the consequences of fretting may be eliminated. While fretting may still occur, the fretting wear on the rod assembly 100 occurs on the adaptor subassembly 300, instead of on a segment 110a, b. Accordingly, this may eliminate potential release of contents from within a given rod segment 110 to the reactor coolant.
Next, the structures associated with the multi-segment fuel rod 100 to produce radioactive isotopes will be described with respect to
Views I and II show a plurality of containment structures 600 within rod 100 that are housing multiple different targets, shown as a liquid, solid and gas isotope target 620 within a single rod segment 110A. Further, enlargements I and II illustrate indicia 650 that can be placed on the containment structures 600 within a given rod segment 110A, 110C, 110E, etc. As shown, the indicia 650 can indicate whether or not the isotope target is in solid, liquid or gas form, and can also provide the name of the target isotope and/or the name of the isotope to be produced due to irradiation, for example (not shown in
Rod segments 110B and 110D are shown to contain nuclear fuel 660, as shown in enlargements III and IV, for example. Of course in an alternative, the multi-segment rod 100 can be composed of a plurality of rod segments 110 in which no segment 110 includes nuclear fuel, or no segment 110 includes a target container or assembly container 600 as previously described. Enlargement V of rod segment 110E illustrates a container assembly 600 which includes a target that is in gaseous form. Enlargement VI of rod segment 110F illustrates a container assembly 600 within the rod segment 11 OF that includes a target 620 in liquid form. Enlargement VII of rod segment 110G illustrates a container assembly 600 which includes a solid target 620, shown as a single column of Co-59 BBs, which can be irradiated to produce the desire isotope, in this case, Co-60. Each of the container assemblies 600 can thus be prepackaged with the target 620 isotope material in solid, liquid or gas form, for insertion into a corresponding rod segment 110 of the multi-segmented rod 100, for example. As will be appreciated any of the radioactive isotopes discussed in the Background of the Invention section may be produced, and that the present invention is not limited to these isotopes.
Further, since each of the container assemblies 600 are sealed by end plugs 630 at one end 612 and by exterior threads 601 and an O-ring 602 at the first end 611 (as will be shown in
In
The male adaptor plug 330 may be made of a material that is corrosion resistant and compatible with the other reactor components, such as a zirconium alloy, as is known in the art.
The female adaptor 350 includes an interior cavity 358. A surface of the cavity 358 may include a plurality of mating threads 356 for receiving corresponding threads (see
As shown in
In another aspect, as the threads of the elongate section 338 engage the corresponding mating threads 356 within the cavity 358 of the female adaptor plug 350, the recessed break line 360 aligns with the intermediate member 339 of the male adaptor plug 330. Since the diameter of the intermediate member 339 is less than a diameter of the cylindrical section 333, this represents a ‘weakened area’ that facilitates cutting, snapping or breaking of the adaptor subassembly 300 of
The end piece assembly 500 may be fabricated of solid Zircaloy and does not necessarily have any nuclear fuel (enriched uranium) or poisons (gadolinium) loaded therein, since axial flux near the top and bottom of a fuel bundle such as fuel bundle 10 of
In an exemplary embodiment of the present invention, various ones of the rod segments 110 may include a container assembly 600 therein, as shown previously in
Referring to
Referring now to
Container 610 may house one or more irradiation targets 620. The irradiation target 620 shown in
In another aspect, the container 600 houses irradiation target 620 therein, having a first end 611 that has a pilot hole 603 for removing the irradiation target 620 after irradiation. The first end 611 may include exterior threads 601 and an O-ring 602 that is used for sealing container 600 when inserted into a piece of equipment. Pilot hole 603 has interior threads to aid in the removal of container 600 from the rod segment 110.
The irradiation target 620 may be a target selected from the group of isotopes comprising one or more of cadmium, cobalt, iridium, nickel, thallium, thulium isotope, for example, or any other isotope having an atomic number greater than 3. For example, the target may be cobalt-59, which when irradiated in the nuclear reactor for a period of time becomes cobalt-60. Desirably, a given segment 110 and/or container assembly 600 may include indicia or indicators thereon to indicate what irradiation target 620 is loaded in that rod segment 110/container 600, for example, and/or what isotope is to be produced from that target. The specific methodology in which one or more of the rod assemblies 100 containing irradiation targets is irradiated within a fuel bundle (such as fuel bundle 10 of
Unlike
As also shown in
Accordingly, as shown in
As an example, the rod assembly 100′ may contain a plurality of irradiation targets at various locations within different sized rod segments 110, and still maintain the same length of a standard full length fuel rod 18 or part length rod 19 within the fuel bundle 10 of
Additionally as shown in
The smaller, two-piece mini-subassembly 300a of
Similarly, in
Accordingly, the upper end piece assembly 1000 and lower end piece assembly 1100 provide reusable and removable lower and upper end pieces which can facilitate quick repairs or removal of designated rod segments 110 within the rod assembly 100′.
Accordingly, the adaptor subassembly 300b in
As previously described, each of the rod segments 110 may have identification marks or indicia thereon that identify the contents that are within that particular rod segment 110. Alternatively, the identification marks can be labeled on the container assemblies 600/600′ within a given rod segment 110, for example.
In another aspect, the threaded screw length of the elongate sections 338/338A/338B of
In a further aspect, male adaptor plugs 330 and 330′, and/or male connector 330″ may be oriented in the same direction for ease of extraction of a given rod segment 110. For example, segments 110 having male adaptor plugs 330, 330′ and/or 330″ may all be loaded and/or arranged in a given rod assembly 100/100′ so that the male adaptor plugs/connector 330, 330′, 330″ of the segment 110 extend vertically upward toward the top of bundle 10, to facilitate grasping by a suitable tool for removal, installation, for example. In the event the rod segment 110 is dropped, it would land with side having the female adaptor plug 350, 350′ and/or 350″ down, so as to reduce the chance that the male end snaps or breaks.
For the purposes of the following discussion, fuel rods containing isotope targets for isotope production will be referred to as isotope production rods even though these rods may contain nuclear fuel, and the fuel rods that do NOT contain isotope targets will simply be referred to as fuel rods. Furthermore, the isotope production rods are not limited to those embodiments discussed above; instead any isotope production rod that may replace a fuel rod in a fuel bundle may be used. For example, the isotope production rods disclosed in U.S. application Ser. No. Unknown, filed Unknown (and entitled “CROSS-SECTION REDUCING ISOTOPE CANNISTER” by the inventor of the subject application, hereby incorporated by reference in its entirety, may be used.
Next, placement of isotope production rods within a fuel bundle according to embodiments of the present invention will be discussed.
Studies have shown that the fuels rods in the vicinity of the core-monitoring instruments 2 are the largest contributors to the response of these instruments. Accordingly, in a first embodiment of the present invention, the isotope production rods are positioned as far as practical from the core-monitoring instruments 2. By positioning the isotope production rods away from the core-monitoring instruments 2 according to this first embodiment, the gamma particles produced by the isotope must attenuate through the material between the isotope and the core-monitoring instruments 2. As a result, the isotope production rods will have the least affect on the core-monitoring instruments 2 and help prevent inaccurate readings.
In keeping with the first embodiment, a first rod position filled with an isotope production rod is the (1,1) rod position. After that positions in rows 1 and 2 and columns 1 and 2 may be filled, keeping a maximum distance possible from the core-monitoring instruments 2 near the (10,10) position.
However, distance from the core-monitoring instruments 2 is not the only consideration. In other embodiments, isotope production considerations may be accounted for in addition to or alternatively to this core-monitoring instrument consideration. The isotope production considerations include the type of radioactive isotope being produced, the half-life or length of decay of the radioactive isotope, the neutron absorption rate of the target isotope to produce the radioactive isotope, the desired specific activity of the radioactive isotope being produced, the amount of neutron flux in different areas of the fuel bundle, the duration that the target isotope/radioactive isotope is expected to remain in the reactor until removed (i.e., harvested), etc. Namely, the above described factors determine the amount of the radioactive isotope having desired characteristics produced until the radioactive isotope is removed from the reactor (i.e., harvested).
The affect of the isotope production considerations will be more readily understood by discussing some examples. In particular, the production of cobalt-60 from cobalt-59 will be discussed followed by a discussion of producing iridium-192 from iridium-191. However, it will be understood that the present invention is not limited to these two examples.
In the case of cobalt-60, the half-life or length of decay is quite long, 5.27 years. Also, the cross-section of cobalt-59 is relatively small at 19 barns, and therefore, cobalt-59 is a slow absorber of neutrons. Still further, most medical uses of cobalt-60 prefer higher specific activity, which is achieved by greater neutron absorption. In view of these absorptions factors, a larger amount of cobalt-60 with high specific activity will be produced by placing cobalt-59 in the highest areas of neutron flux for the longest time practical. For example, the time between reactor shutdowns, and therefore the time between possible harvesting of the isotope production rods, is generally 1-2 years. Given these isotope production considerations, it is expected that the cobalt-60 producing isotope production rods will remain in the nuclear reactor for 4-6 years if placed in areas of highest neutron flux.
As is known, neutron flux is highest at the corners of a fuel bundle and decreases radially towards the center of the fuel bundle. Therefore, given the core-monitor instrument consideration and the isotope production considerations for the production of cobalt-60, the isotope production rods are positioned as follows according to one embodiment:
for 1 isotope production rod, position at the (1,1) rod position;
for 2 isotope production rods, position at the (1,1) and either of the (10,1) and (1,10) rod positions; or positions at the (10,1) and (1,10) rod positions if bundle diagonal symmetry is preferred;
for 3 isotope production rods, position at the (1,1), (10,1) and (1,10) rod positions;
for 4 isotope production rods, position at the (1,1), (10,1), (1,10), and either of the (2,1) and (1,2) rod positions;
for 5 isotope production rods, position at the (1,1), (10,1), (1,10), (2,1), (1,2) rod positions; etc.
As will be appreciated, the (10,10) rod position is in an area of high neutron flux, but has been avoided as a candidate for an isotope production rod because of the proximity to the core-monitoring instruments 2. However, it will be understood from
Next, the case of producing nickel-63 from nickel-62 will be described. In the case of nickel-63, the half-life is very long, 92 years. However, the cross-section of nickel-62 is similar to that of cobalt-59 at 15 barns. Consequently, radial rod locations to place nickel-62 are the same as Cobalt-59 placement. However, the optimal time of irradiation to obtain maximum specific activity is approximately 35 years. Because it is not practical to irradiate fuel bundles for 35 years, the maximum fuel assembly life is desirable as the duration until harvesting. Consequently, fuel assemblies should target an 8-10 year irradiation period.
Next, the case of producing thulium-170 from thulium-169 will be described. In the case of thulium-170, the half-life is approximately 134 days. However, the cross-section of thulium-169 is much higher than the previous examples at 125 barns. This combination of absorption rate and decay rate identify the preferred configuration to be locations of high neutron flux (see cobalt-60) but irradiation lengths optimally designed for 1 year, ideal in an annual cycle refueling campaign.
Next, the case of producing iridium-192 from iridium-191 will be described. In the case of iridium-192, the half-life or length of decay is quite short, 74.2 days. Also, the cross-section of iridium-191 is relatively large at 750 barns, and therefore, iridium-191 is a fast absorber of neutrons. Still further, most medical uses of iridium-191 prefer higher specific activity, which is achieved by greater neutron absorption. As discussed above, the time between reactor shutdowns, and therefore the time between possible harvesting of the isotope production rods, is generally 1-2 years. Given the short half-life of iridium-192 and the fast neutron absorption characteristics of iridium-191, it would be preferable to harvest the iridium-192 producing isotope production rods in 4 months even if placed in a high neutron flux area of the fuel bundle. However, given the practical considerations of harvesting, it is expected that iridium-192 producing isotope production rods may remain in the nuclear reactor for 1-2 years. Accordingly, it is desirable to place the iridium-192 producing isotope production rods in the areas of the fuel bundle within lower neutron flux locations. If the iridium-192 producing isotope production rods were placed in areas of high neutron flux, then, given a 4-6 year harvesting time, very little iridium-192 would exist as most of the iridium-191 would have been converted to iridium-192 and have already decayed into other materials.
As discussed above, neutron flux is highest at the corners of a fuel bundle and decreases radially towards the center of the fuel bundle. Slightly higher than average neutron flux locations exist directly near the water rod locations because of the water rods. Therefore, given the core-monitor instrument consideration and the isotope production considerations for the production of iridium 192, the isotope production rods are positioned as follows according to one embodiment:
for 1 isotope production rod, position at either of the (3,3) and (8,8) rod position;
for 2 isotope production rods, position at the (3,3) and (8,8) rod positions;
for 3 to 6 isotope production rods, positions at the (3,3), (3,4), (4,3), (8,8), (8,7), and/or (7,8) rod positions; etc.
As will be understood from the above examples, various factors influence the placement of isotope production rods in a nuclear reactor. For instance, the isotope production rods will be positioned in areas of greater neutron flux the longer the half-life of the radioactive isotope being produced. Similarly, the isotope production rod will be positioned in areas of lesser neutron flux the shorter the half-life of the radioactive isotope being produced. However, even short half-life radioactive isotopes may have their isotope production rods positioned in areas of higher neutron flux if the expected duration the isotope production rods will remain in the nuclear reactor is commensurately as short as the half-life. Still further, even short half-life radioactive isotopes may have their isotope production rods positioned in areas of higher neutron flux if their neutron absorption rate is commensurately lower. Yet still further, it may be desirable to produce a lesser amount of the radioactive isotope in order to obtain higher specific activity in the amount of radioactive isotope produced. Accordingly, this may mitigate against placing isotope production rods in lower neutron flux areas.
The exemplary embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. For example, while several of the relative relationships between the isotope production considerations have been explicitly mentioned above, many more will be readily apparent to those skilled in the art based on this disclosure. Such variations are not to be regarded as departure from the spirit and scope of the exemplary embodiments of the present invention, 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.
