RU2198440C1 - Method for producing gamma-ray source core - Google Patents

Abstract

FIELD: nuclear technology and power engineering; nuclear power reactors. SUBSTANCE: method is proposed to use capsules of irradiating assemblies of nuclear reactors to build up target radionuclides and to produce active cores of gamma-ray sources. Core that has cylindrical tube and cylindrical rod of diameter fitting inner diameter of tube is separately irradiated in reactor and upon irradiation they are placed together in common can. Capsule opened upon irradiation with cylindrical tube is proposed to be used as active core can to receive cylindrical rod extracted from other irradiated capsule whereupon capsule with combined components is encapsulated. Tube and rod are proposed to be made of same material such as metal cobalt; in order to enhance yield of gamma-quanta of produced active core cobalt surface in tube should amount to 0.8-0.9 of its surface in rod. In addition a number of separately activated tubes inserted one into other as well as separately irradiated fragments of mentioned components, such as bushings, rings, half- cylinders, and the like, are proposed to be used in core assembly process to produce source of high specific activity due to additional unlocking of material being irradiated. Capsules used for irradiating active core components are proposed to be made of material whose neutron capture sectional area is smaller than that of cobalt. EFFECT: enhanced total activity of source core, facilitated procedure of material irradiation, reduced material input. 6 cl, 3 dwg, 1 tbl

Description

 The invention relates to the field of nuclear technology, relates, in particular, to the design of capsules of irradiation assemblies for nuclear power reactors and can be used to produce target radionuclides and manufacture active cores for sealed sources of gamma radiation.

 It is known that the design of a typical source of gamma radiation for industrial plants [1, 2] is a sealed metal shell in which a capsule (one or more) with irradiated cobalt material (OKM) is placed, which is the active core of the source. The main technical and economic parameters of the source, which determine its demand in the world market, are the total activity and its cost, which is 50-75% determined by the cost of the work performed in the hot chamber for processing OKM and making the finished product from it - a gamma radiation source . The performance of the manufacturing technology in the hot chamber of the source and its main part, the active core, depends mainly on the combination of properties and characteristics of the applied OKM: overall dimensions, shape, physical condition, the number of single irradiated elements that make up the active core, and other parameters that determine it manufacturability.

 Analysis of the designs of active cores of typical sources of gamma radiation [1, 2, 4], currently used in industrial plants, shows that the cores are made of OKM in the form of metal disks (tablets), whose dimensions are small (diameter about 6 mm and thickness 1-2 mm), and methods for manufacturing active cores based on the processing of "disk" OKM have low technical and economic indicators. Reducing the number and complexity of technological operations for the manufacture of an active source core in a hot chamber is possible using a more technological than a set of disks, OKM with a minimum number of elements, which occurs, for example, in the method of manufacturing an active source core described in the literature [3] , which is made in the form of a sleeve of one material and inserted into it a cylinder of another material or composition of these materials. However, an active core made by this method has several disadvantages. Simultaneous irradiation of two different materials (europium and cobalt) with different physical characteristics in the same target (capsule) leads to a sharp decrease in the level of Co-60 production due to the strong screening of it with europium isotopes, because The neutron capture cross section for europium-154 (7700 barn) is significantly larger than for cobalt-60 (36 barn). This leads to the fact that the activity of such a combined core is mainly determined by the activity of europium isotopes, and cobalt-60 is used inefficiently in this design, although it determines one of the main characteristics of the gamma radiation source - the value of the γ-constant, which is cobalt-60 more than europium.

 Considering the specific physicochemical properties of the irradiated material, in the case of a violation of the sealing of the irradiated capsule of the active core with europium, there will be significant radioactive contamination of the chamber equipment, since europium oxide is a volatile powder (uncontrolled in the chamber) that can be removed from the contaminated chamber equipment acceptable levels even with the most thorough and repeated decontamination will be almost impossible. And the long half-life of europium isotopes (up to 16 years) will lead to an increased "lifetime" (in terms of the life of the camera equipment) level of radiation in the chamber and, therefore, will determine large dose loads on equipment and repair personnel, an extended repair period, etc.

 These circumstances impose additional increased requirements to ensure the tightness of the capsule of the active source core both for the period of its operation (three half-lives - 48 years) and during subsequent long-term storage after decommissioning during disposal (five periods - 80 years). In addition, the method is characterized by a complex and unsafe (europium oxide is a highly toxic substance) manufacturing technology of the starting target (core), the need for simultaneous preparation of two materials in the form of a charge, the presence of a dosing park, special welding (welding or soldering of cobalt metal) and press equipment for preparation desired composition and configuration of target elements.

 The closest analogue to the claimed method of manufacturing an active core of a gamma radiation source is the method described in the literature [4], which consists in placing full-bodied disks (tablets) assembled into a continuous column in an irradiated capsule (target). The capsule is closed with a lid, sealed by welding and then placed in the reactor core as part of the irradiation device. After irradiation of the capsule in the reactor, it is opened from one or both sides, the ends of the capsule are expanded and, using the hot chamber manipulators and special equipment, disks are removed from the capsule, for example, from Co-60. This operation is the most "dirty" and time-consuming, because a long period of irradiation of the disks in the reactor leads to their radiation swelling and jamming in the capsule, which prevents their free extraction. Then, the activity of the accumulated Co-60 radionuclide is measured in each disk, after which the operator, in accordance with the prepared scheme for manually loading one disk into one new capsule (sometimes interspersed with inactive pads to ensure the required uniformity of activity along the length of the capsule), receives the active source core. Thus, the active core made by this method is a continuous column of irradiated disks placed in another (new) capsule, because during the extraction of disks from the irradiated capsule in which they were irradiated in the reactor, for the above reasons, it comes into unsuitability and its further use is excluded.

The disadvantages of the closest analogue are:
- insufficient specific and full core activity, because the dense arrangement of the irradiated material (OM) in the volume of the irradiated capsule leads to the blocking of the inner nuclei of the target material by its outer layers, as a result of which the flux of thermal neutrons in the central region of the target material is much smaller compared to the flux incident on the target;
- irrational use of starting material, because as a result of the action of blocking effects in the central axial region of the disk column, the accumulation of the target radionuclide practically does not occur even after prolonged exposure. And, thus, this part of OM is not of commercial value, but due to its mass is essentially a “diluent” of the accumulated total activity in the capsule;
- long duration and low productivity of the technological process of manufacturing an active core according to this method due to the low manufacturability of OM, because the overall dimensions of the disks are small, and all operations are carried out in a hot chamber using simple equipment and exclusively manually (by manipulators).

 Due to the great responsibility, tension, and laboriousness of carrying out operations for manufacturing one core, their duration in real conditions strongly depends on the skill of the operator and, in the best case, is several hours. This causes large dose loads on the camera equipment (viewing systems, sealing elements, etc.), which shorten its life.

The tasks solved by the invention:
- increasing the total activity of the source core by obtaining OM with a higher specific activity and its compact placement in the capsule of the active core,
- improving the manufacturability of OM in order to reduce the number and duration of operations performed in a hot chamber in the manufacture of an active source core.

 - saving starting material.

где Δ - толщина стенки трубки, мм;
d - диаметр стержня, мм.The essence of the invention lies in the fact that in the inventive method of manufacturing an active core of a gamma radiation source, including separate capsular irradiation of core elements in the active zone of a nuclear reactor and their subsequent combination in an airtight case, a core is proposed consisting of a cylindrical tube and a cylindrical rod having a diameter mating with the inner diameter of the tube, irradiate separately in the reactor, and after irradiation combine with each other in one sealed case. It is proposed to use a capsule with a cylindrical tube, which is opened after irradiation, as a cover for the active core, into the cavity of which a cylindrical rod is removed from another irradiated capsule, and then the capsule with combined elements is sealed. In addition, it is proposed to irradiate the tube and the rod, the dimensions of which are related by the following ratio:

where Δ is the tube wall thickness, mm;
d is the diameter of the rod, mm

 It is also proposed that the tube and the rod be made of one material, for example, cobalt metal, and in order to increase the gamma-ray yield of the manufactured active core, the cobalt density in the tube should be 0.8-0.9 of its density in the rod. In addition, it is proposed in the process of core assembly to use a number of separately activated tubes inserted one into the other, and also to use separately irradiated fragments of these elements (for example, bushings, rings, half cylinders, etc.), which makes it possible to obtain a source with increased specific activity due to additional unlock OM. Capsules in which elements of the active core are irradiated are proposed to be made of a material having a neutron capture cross section less than that of cobalt.

 The advantages of the proposed method for manufacturing the active core of the gamma radiation source are that the irradiation in the reactor is not small discs (tablets), of which after irradiation, extraction from the capsule and sorting it is necessary to collect the active core, but only two elements: a cylindrical tube and a cylindrical a rod with a diameter equal to the inner diameter of the tube. The combination of these elements, quite long in length, simple in shape and easy to handle in a hot chamber, in comparison with a set of columns from a large number of small disks, is much simpler, faster and without much operator stress.

где Δ - толщина стенки трубки, мм;
d - диаметр стержня, мм.Thanks to the proposed configuration of the active core elements, the number and duration of previously labor-intensive assembly operations are significantly reduced, the technology of its manufacture is simplified, and the requirements for the level of skill of the operator are reduced. It becomes possible to use robotics and automation, a high processing speed of OM is ensured, and ultimately, the production efficiency of sources is increased by 5-10 times. The wall thickness of the tube and the diameter of the rod are selected from the conditions of operating in them the required specific activity of the accumulated radionuclide for a set period of irradiation and obtaining the full activity of the core. Therefore, the dimensions of the tube - wall thickness Δ and the diameter of the rod d should be interconnected by the following ratio:

where Δ is the tube wall thickness, mm;
d is the diameter of the rod, mm

 The calculations given in the table (in relation to the operating time of Co-60 in a RBMK-1000 type reactor) show that under the same conditions of target irradiation, the total activity of the core containing two separately irradiated active elements (tube and rod) exceeds the total activity of the analog core (option 1) by 28%, while the saving in starting material mass reaches 15%. The manufacture of active core elements from one material - cobalt, as well as the use of a material with a density of 0.8-0.9 of the density of the rod for the manufacture of the tube, reduces the screening effect of the tube wall on the output of gamma rays from the rod and thereby increase the efficiency of use Co-60 activity accumulated in it. And the use of a material with a neutron capture cross section less than that of cobalt-60, such as zirconium (0.18 barn) for capsule manufacturing, allows to reduce the level of neutron flux perturbation in the irradiation volume of the reactor and thereby provide the best physical conditions for the production of Co radionuclide in a reactor -60 with the highest possible specific activity.

 Examples of the proposed capsule designs with active core elements are illustrated in FIGS. 1, 2, 3, where FIG. 1 shows a longitudinal section through the structure of the capsule with a cylindrical tube, FIG. 2 shows a longitudinal section through the structure of the capsule with a cylindrical rod, FIG. 3 shows a longitudinal section of the design of the final capsule with an active core made by the proposed method. As you can see, one part of the radioactive material of the target (one core element) is made in the form of a cylindrical tube made of cobalt metal (item 2, figures 1, 3), and the second part of the radioactive material of the target (second element of the core) is made in the form of a cylindrical rod ( Pos. 9, FIGS. 2, 3) also made of cobalt metal, which has a diameter equal to the inner diameter of the cylindrical tube, but made with a minus tolerance to ensure subsequent alignment with the tube. A metal tube 2 having a central axial hole 3 is inserted into the capsule cover 1. The tube can be made of a segment of cobalt-59 metal pipe of a suitable size (assortment) or in the form of a sleeve having a split side surface due to folding of the cobalt sheet or tape on the corresponding rod - mandrel followed by its removal. The capsules are welded on both sides with a circular seam 6 by argon-arc welding, the upper 4 and lower 5 covers. Technological recesses 7 are made in each lid on the outside, having a diameter equal to or slightly larger than the inner diameter of the tube inserted into the capsule. In the case of the capsule with a cylindrical rod (see Fig. 2), the caps 8 do not have technological recesses, which makes it possible to easily distinguish the capsule with the rod from the capsule with the tube under the conditions of the protective chamber. The rod 9 of metal cobalt-59 is placed in the capsule cover freely without spacer liners. The rod can be made in one of the known ways: by grooving a workpiece on a lathe, rolling the wire to a predetermined size, followed by cutting lengths of the desired length, or from a length of wire with a diameter of a suitable standard assortment.

 Dismantling of capsules of both types after irradiating them in a reactor and manufacturing an active core is carried out remotely in a protective chamber, for example, described in the literature [5], which has a chain of separate boxes and special equipment. In capsules containing tubes, in one of the lids, a through hole with a diameter equal to the diameter of the tube inserted into it is drilled through the technological recess 7 in it and, thus, access is made to its central hole. Capsules containing rods are opened using a cutting device (such as a pipe cutter) by making an annular cut on one of the covers below the weld 6 (see Fig. 2, zone C). After removing the incised lid, the rod is removed from the capsule. The free location of the rod inside the capsule allows the operation to remove it to be carried out simply and quickly (for example, by tipping, without the use of complex equipment, as is the case with removing the disks. Then, the rod 9 is inserted into the drilled hole of the capsule in which the tube is located). The operation can This operation is preceded by a software calculation of the optimal combination of elements of the future active core of the source, based on its required activity 1. As you can see (see figure 3), the final capsule 1 is filled with a set of elements from OKM: a tube 2 and a rod 9 inserted into it. After combining the core elements in the capsule cover, a pre-prepared stopper 10 is inserted into the lid 4, which hermetically welded to the lid with a circular seam by argon-arc welding 11. In this form, the capsule is ready for use as the initial main structural element for the manufacture of a typical source of gamma radiation.

Thus, the application of the proposed method for manufacturing the active core of the gamma radiation source, as well as the designs of the starting targets that implement this method, allows you to:
- to improve the technical and economic indicators of the technology for the manufacture of active cores of gamma radiation sources in a hot chamber due to the production and processing of more technological OM,
- increase by 28% the full activity of the assembled core of the gamma radiation source due to the use of a radioactivated material of a more unlocked configuration and its separate irradiation in the reactor, thereby increasing the specific activity of the target radionuclide,
- Save up to 15% starting material.

Sources of information
1. Sources of alpha, beta, gamma, and neutron radiation. Catalog. V / O "Isotope", 1980

2. Sources closed with radionuclide cobalt-60 type GIC. Specifications TU 95.11052-82;
3. Patent RU 2035076, cl. 6 G 21 G 4/04, "Active core gamma radiation source and method for its manufacture.

 4. Patent RU 2107957, cl. 6 G 21 C 7/1, "Nuclear reactor neutron absorber" (close analogue).

 5. Patent RU 2112288, class 6 G 21 C 7/10, "Protective chamber".

Claims (6)

 1. A method of manufacturing an active core of a gamma radiation source, comprising separate capsular irradiation of core elements in the core of a nuclear reactor and their subsequent combination in a sealed case, characterized in that the cylindrical tube and the cylindrical rod with a diameter mating with the inner diameter of the tube, and as a cover use a capsule with a tube opened after irradiation, into the cavity of which a rod is removed from another capsule and the capsule is then sealed with the combined elements. 2. The method according to claim 1, characterized in that the tube is irradiated, the wall thickness of which is related to the diameter of the rod by the ratio:

where Δ is the tube wall thickness, mm;
d is the diameter of the rod, mm  3. The method according to claim 1 or 2, characterized in that the tube and the rod are irradiated, which are made of one material, for example, cobalt metal.  4. The method according to claim 1, or 2 or 3, characterized in that the density of the tube material is 0.8-0.9 of the density of the rod material.  5. The method according to claim 1, or 2, or 3, or 4, characterized in that the capsules in which the elements of the active core are irradiated are made of a material whose neutron capture cross section is smaller than that of the material of the core elements.  6. The method according to claim 1, or 2, or 3, or 4, characterized in that when assembling the active core, a series of tubes with conjugated diameters located on a common rod are used.

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