Radiation source for nondestructive inspection, and method and apparatus for manufacturing same

Abstract

An irradiation target is formed into a sphere. The spherical irradiation target can be iridium metal containing natural or enriched iridium. The radiation source can be manufactured by manufacturing a spherical irradiation target, accommodating the spherical irradiation target in a rotating capsule, and rotating an axial flow impeller by a downward flow of a reactor primary coolant, whereby the rotating capsule is rotated. This radiation source provides an improved nondestructive inspection image having a high geometric resolution, and has no radiation source anisotropy and also has high target recyclability.

Claims

1. A method for manufacturing a radiation source for a nondestructive inspection, the method comprising: manufacturing a spherical irradiation target by dropping molten iridium into a liquid; accommodating the spherical irradiation target in a rotating capsule; and rotating an axial flow impeller by a downward flow of a reactor primary coolant, whereby the rotating capsule is rotated.

2. The method for manufacturing a radiation source for a nondestructive inspection according to claim 1, wherein a plurality of the spherical irradiation targets are loaded into the rotating capsule in a plurality of layers.

3. An apparatus for manufacturing a radiation source for a nondestructive inspection, comprising: a rotating capsule that accommodates a spherical irradiation target; and an axial flow impeller that is rotated by a downward flow of a reactor primary coolant, whereby the rotating capsule being rotated by the axial flow impeller.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram schematically showing a first half of an iridium 192 (.sup.192Ir) manufacturing procedure according to an embodiment of the present invention;

(2) FIG. 2 is a diagram schematically showing a second half of the same;

(3) FIG. 3 is a chart showing a nuclear evaluation of iridium targets obtained by enriching iridium 191 (.sup.191Ir) for describing the principle of the present invention;

(4) FIG. 4A is a diagram showing the effect (self shielding effect) of an .sup.191Ir reaction cross section for the same purpose;

(5) FIG. 4B is a chart showing the rate of generation of .sup.192Ir in outer spheres for the same purpose;

(6) FIG. 4C is a chart showing the rate of generation of .sup.192Ir in inner spheres for the same purpose;

(7) FIG. 5 is a chart showing a result of an evaluation of the rotatability of a rotating capsule by a reactor primary coolant for the same purpose; and

(8) FIG. 6 is a schematic diagram showing a cross section of an .sup.192Ir manufacturing rotating capsule usable in the foregoing embodiment.

DESCRIPTION OF EMBODIMENTS

(9) An embodiment of the present invention will be described in detail below with reference to the drawings. It should be noted that the present invention is not limited by the description of the following embodiment or practical examples. Components of the following embodiment and practical examples include ones readily conceivable by those skilled in the art, substantially identical ones, and ones so-called within the range of equivalency. The components disclosed in the following embodiment and practical examples may be combined as appropriate, and may be selectively used as appropriate.

(10) FIG. 1 schematically shows a first half of an .sup.192Ir manufacturing procedure according to the embodiment of the present invention.

(11) As shown in steps A and B of FIG. 1, iridium (Ir) microspheres 12 with a diameter of 1 mm, a diameter tolerance of ±0.02 mm, and a weight of 12 mg or so, for example, are manufactured from iridium (Ir) metal 10 made of enriched iridium (Ir) that is 80%-enriched .sup.191Ir, for example.

(12) The manufacturing uses an iridium metal melting technique. The Ir microspheres 12 can be manufactured by dropping molten iridium into a liquid (such as water).

(13) Alternatively, the Ir microspheres 12 can be manufactured by machining, using a fine turning machining technique.

(14) Next, as shown in step C of FIG. 1, 28 Ir microspheres 12 are loaded per layer as appropriately spaced at distances of, e.g., 2 mm along a 20-mm-diameter inner circumference of a cylindrical aluminum heat medium 14 having a diameter of, e.g., 25 mm. As shown in step D of FIG. 1, for example, eight such layers are loaded in a stacked manner vertically at appropriate distances to provide a total of approximately 220 irradiation targets, for example.

(15) Next, as shown in step E of FIG. 1, the eight layers of heat media 14 loaded with the Ir microspheres 12 that are irradiation targets are sealed in a rotating capsule (also referred to as an irradiation ampule) 20. As shown in step F of FIG. 1 (=step A of FIG. 2), the rotating capsule 20 is inserted into a reactor 40, and irradiated with neutrons while being rotated by a downward flow of the reactor primary coolant. The reason why the rotating capsule 20 is not fixed but rotated is to make the neutron irradiation uniform. The present embodiment utilizes the downward flow of the reactor primary coolant, and therefore does not need an electric motor for rotation, an external power supply, a connection cable thereof, or the like, and can be configured with low cost. Moreover, the installation operations are simple and easy since the cableless rotating capsule 20 is simply put in the passage of the reactor primary coolant.

(16) Next, as shown in step B of FIG. 2, the irradiated rotating capsule 20A taken out of the reactor 40 is disassembled. As shown in step C of FIG. 2, the irradiated heat media 14A including the irradiated Ir microspheres 12A are taken out. Then, as shown in step D of FIG. 2, the approximately 220 irradiated Ir microspheres 12A are taken out. Next, as shown in step E of FIG. 2, each of the irradiated Ir microspheres 12A is loaded into a container 50. As shown in step F of FIG. 2, the container 50 is measured and checked for a predetermined γ-ray dose (for example, 13 Ci). As shown in step G of FIG. 2, the container 50 is sealed. As shown in step H of FIG. 2, the sealed containers 50A are transported in a transportation container 52.

(17) FIG. 3 shows a nuclear evaluation of Ir targets containing enriched .sup.191Ir in examining the present invention.

(18) To manufacture .sup.192Ir, a thermal neutron flux density of approximately 1 to 2×10.sup.14 (n/cm.sup.2) is needed regardless of whether to enrich .sup.191 Ir. For example, in the case of transportation in every two months, the irradiation time can be 40 days.

(19) Next, FIG. 4 shows a result of an examination of the effect (self-shielding effect) of an .sup.191 Ir reaction cross section similarly in examining the present invention.

(20) The reaction cross section of Ir with neutrons is greater than that of uranium, and Ir microspheres in front block neutrons (self-shielding). Suppose that, as shown to the left in FIG. 4A, neutrons come in one direction (in the diagram, from the left), and Ir microspheres are arranged on an inner circle and an outer circle as shown in a cross-sectional view shown to the right in FIG. 4A. In such a case, the amount of generation of .sup.192Ir is nonuniform as shown in FIG. 4B (the rate of generation of .sup.192Ir in the outer spheres) and FIG. 4C (the rate of generation of .sup.192Ir in the inner spheres). Rotation makes the amount of generation uniform since Ir microspheres are irradiated with neutrons in all directions. Note that Ir microspheres are desirably loaded only on the outer circle since the amount of generation differs between the inner circle and the outer circle.

(21) Moreover, FIG. 5 shows a result of an evaluation of the rotatability of the rotating capsule by the reactor primary coolant similarly in examining the present invention.

(22) Since the flow rate in a gap portion is 1000 times the rotation speed or more, the flow at the surface of the cylindrical container passes through the gap portion without rotation. A mechanism for converting the axial flow into a rotational flow is therefore needed. In this system, an excessive axial force as shown in FIG. 5 is found to occur due to loss of the flow colliding with the cylindrical container and loss occurring in the gap portion. To rotate the cylindrical container, it is then found that a thrust bearing for supporting the axial force (axial load of approximately 50 N or more for a gap of 7 mm) needs to be applied.

(23) FIG. 6 shows a state where an .sup.192Ir manufacturing rotating capsule is inserted into a primary coolant channel running vertically inside the reactor 40.

(24) An inner capsule 30 made of A5052, for example, and accommodating the rotating capsule 20 is accommodated in an outer tube 22 made of A6063, for example. The inner capsule 30 is then inserted into a reactor primary coolant channel. The shaft of the inner capsule 30 is rotatably supported in the outer tube 22 at the top and bottom by bearings 26 made of SUS304, for example, and bearing holders 24 made of A5052, for example. The shaft is further equipped with an axial flow impeller 32 similarly made of A5052. In the diagram, the reference numeral 34 denotes a mesh made of SUS304, for example, and the reference numeral 36 denotes a mesh holder made of A5052, for example. The axial flow impeller 32 is thus rotated by the downward flow of the reactor primary coolant, whereby the inner capsule 30 and the rotating capsule 20 therein are also rotated.

(25) The foregoing radiation source has a dose of, e.g., 13 Ci in view of the fact that the intended amount of radioactivity upon shipping from the reactor is 10 Ci. However, a radiation source of 39 Ci, i.e., 1.3 times of 30 Ci can be manufactured in view of future demand.

(26) In the foregoing embodiment, the radioactive isotope is described to be iridium Ir 192. However, the type of radioactive isotope is not limited thereto, and other radioactive isotopes such as cobalt Co 60, cesium Cs 127, ytterbium Yb 169, selenium Se 75, and thulium Tm 170 may be used. The size and the number of layers of the heat medium 14, the number of microspheres, and the like are not limited to the foregoing embodiment, either.

REFERENCE SIGNS LIST

(27) 10 . . . iridium (Ir) metal 12, 12A . . . iridium (Ir) microsphere 14, 14A . . . heat medium 20 . . . rotating capsule 30 . . . inner capsule 32 . . . axial flow impeller 40 . . . nuclear reactor 50, 50A . . . container 52 . . . transportation container