Method of manufacturing neutron-shielding tubes and neutron shielding tubes obtained by the same

Abstract: A method of forming a neutron-absorbing tube or a section of such tube, and the resulting tube. An elongate, generally rectangular metal ingot (16) having a hollow interior is formed with at least one elongate metal divider (26) in the interior forming chambers (28,30). The chambers (28,30) are filled with a uniformly dispersed mixture of finely divided boron and particles of a finely divided metal, the ingot is then soaked to an elevated temperature and hot rolled to form a thin, rigid neutron-absorbing sheet having opposite metal edge portions and an elongated metal spacer portion at each metal divider. The sheet is then longitudinally bent at each spacer portion. A tube is formed by welding one or more bent sheet(s) along the side edge portions.

Description: Background of the Invention This invention relates to neutron-absorbing or neutron-shielding material, and in particular to a novel process of forming such material into a tube, and the resulting tube. In U.S. Patent No. 4,027,337, which is assigned to the assignee of the present application and the disclosure of which is incorporated herein by reference, disclosed is the production of neutron-absorbing or shielding material comprised of a thin, rigid sheet having a neutron-absorbing material, preferably boron carbide, surrounded by aluminum plate. In forming the sheet, an ingot is first formed with a hollow interior, and a mixture of neutron-absorbing material, such as boron carbide powder, and a finely divided metal powder, such as atomized aluminum, is installed in the hollow interior of the ingot. Thereafter, the ingot is sealed, heated to a temperature below the melting point of the metal powder, and is then hot rolled to reduce its thickness a desired amount. The hot rolling causes the particles of metal powder and boron carbide to become metallurgically bonded together so that in subsequent use the material retains its neutron-absorbing properties. A problem with the material of Patent No. 4,027,377 is its inability to be bent and retain its neutron-absorbing capacity at the area of the bend. Consequently, the process of U.S. Patent No. 4,751,021 was developed to provide a sheet that can be bent. However, the bend cannot be abrupt, and therefore a relatively gentle bend is the result, with the neutron-absorbing boron carbide core being offset to one side of the sheet. That patent requires precision in manufacture, and although a quite satisfactory neutron-absorbing sheet is produced, the process is expensive. Further prior art is represented by US 3,103,478. This patent is directed to formation of a nuclear reactor control rod having either a series of lineal-spaced areas of neutron-absorbing material (Figure 2), or a series of staggered but overlapping areas of neutron-absorbing material (Figure 4). In the simple version of Figures 1 and 2, the rod is formed by forming a strip of material with a series of holes. There is no disclosure of forming the rod into a tube (since it is essentially an elongated article). Also, in connection with the staggered version of Figures 3 and 4, there is no spacing across the width at all, and while there is no bending of any kind disclosed since a rod is formed, if there were bending, it obviously could not be at an area where there is no shielding material. Summary of the Invention The present invention relates to a method of making sections of a neutron-absorbing tube, or a tube itself, through a series of steps. First, an elongate, generally rectangular metal ingot is formed having a hollow interior. At least one elongate metal divider is installed in the hollow interior to form at least two chambers in the interior of the ingot. The chambers are then filled with a substantially uniformly dispersed mixture of a finely divided neutron-absorbing boron compound and a finely divided metal powder. Thereafter, the ingot is soaked to bring it to an elevated temperature below the melting temperature of the metal powder. With the temperature of the ingot thus-elevated, the ingot is hot rolled to reduce its thickness to form a thin, rigid neutron-absorbing sheet having opposite metal edge portions and an elongated metal spacer portion at the location of each metal divider. Finally, the sheet is longitudinally bent at each spacer portion. Thus, in a first aspect the invention provides a method of making a section of neutron-absorbing tube, comprising the steps of: a. forming an elongate, generally rectangular metal ingot having a hollow interior with at least one elongate metal divider in said interior to form at least two chambers in said interior; b. filling said chambers with a substantially uniformly dispersed mixture of a finely divided neutron-absorbing boron compound and a finely divided metal powder; c. soaking the ingot to bring it to an elevated temperature below the melting temperature of the metal powder; d. hot rolling the ingot to reduce its thickness to form a thin, rigid neutron-absorbing sheet having opposite metal edge portions and an elongate metal spacer portion (40) at each metal divider; and e. longitudinally bending the sheet at each spacer portion (40). In accordance with the preferred form of the invention, only one metal divider is installed in the ingot, and the bending step includes longitudinally bending the sheet to an L-shaped cross section. A tube is formed by making a second, substantially identical section and joining the two sections at their metal edge portions to form the tube. Typically therefore, the method defined above will further comprise the step of joining, via said metal edge portions, the section to one or more further sections made by the same method so as to form a neutron-absorbing tube. Preferably, one of the metal edge portions of each of the sheets is also bent to an L-shaped cross section before the sheets are joined and welded along their side edges to form a tube. In accordance with the preferred form of the invention, when the sheet is formed and before bending of the sheet, one of the metal edge portions of the sheet is formed with a greater width than the other edge portion. That greater-width edge portion is the edge portion which is then bent to an L-shaped cross section. In accordance with a second form of the invention, a neutron-absorbing tube is formed by forming an elongate, generally rectangular metal ingot having a hollow interior, and a series of elongate metal dividers are installed in the interior of the ingot to form at least four chambers in the interior. Those chambers are then filled with the uniformly dispersed mixture of the finely divided neutron-absorbing boron compound and metal powder, the ingot is soaked to bring it to an elevated temperature, and the ingot is then hot rolled to reduce its thickness to form a thin, rigid neutron-absorbing sheet having opposite metal edge portions and an elongated metal spacer portion at the location of each of the metal dividers. The sheet is then longitudinally bent at each of the spacer portions, and the metal edge portions are joined to form a tube. In this form of the invention, also, it is preferred that the formation of the metal edge portions of the sheet is such that one of the metal edge portions has a greater width than the other edge portion so that the one metal edge portion can be bent to an L-shaped cross section before the two edge portions are joined. Brief Description of the Drawings The invention is described in greater detail in the following description of examples embodying the best mode of the invention, taken in conjunction with the drawing figures, in which: Figure 1 illustrates the cross section of a neutron-absorbing tube according to the invention and made in accordance with the method of the invention, Figure 2 illustrates, in an end view similar to a cross section, the formation of an elongate, generally rectangular metal ingot having a metal divider in the interior, and Figure 3 illustrates the cross section of the ingot of Figure 2 after the ingot has been soaked and hot rolled to reduce its thickness to form a thin, rigid neutron-absorbing sheet. Description of Examples Embodying the Best Mode of the Invention A neutron-absorbing tube according to the invention is shown generally at 10 in Figure 1. Preferably, as will be described in greater detail immediately below, the tube 10 is formed of a pair of thin, rigid neutron-absorbing sheets 12 and 14 which are identical to one another, one of which is shown in Figure 3, after rolling, trim and prior to bending. Each of the sheets 12 and 14 is formed from a metal ingot 16, as shown in Figure 2. The metal ingot 16 is formed in an elongate, generally rectangular fashion from aluminum, therefore having aluminum faces 18 and 20, and opposite aluminum side edges 22 and 24. The ingot can be formed in accordance with the disclosure of incorporated U.S. Patent No. 4,027,377, or in accordance with any other process for formation of a similar ingot. Once the ingot has been formed, at least one elongate, metal divider 26 is installed in the interior of the ingot to form the ingot into at least two chambers 28 and 30. The divider 26 shown in Figure 2 is actually formed of one or more metal elements, and preferably is made of the same material as the metal ingot 16. Alternatively, the divider 26, side edges 22 and 24, and aluminum faces 18 and 20 can be formed as a single unit. Also, each metal divider 26 may, alternatively, be formed of a single length of metal, and as explained above, more than one metal divider may be installed in the hollow interior of the ingot 16, in order to form additional chambers. For example, if two metal dividers are installed, the dividers, in combination with the metal ingot, would form three chambers, three dividers would form four chambers, and so forth. Once the ingot has been thus-formed, the chambers 28 and 30 are filled with a substantially uniformly dispersed mixture of a finely divided neutron-absorbing compound and a finely divided metal powder. The mixture preferably is in accordance with that disclosed in detail in incorporated U.S. Patent No. 4,027,377. Other similar materials, however, may be formed as well so long as they exhibit the neutron-absorbing capacity after a sheet has been formed. Once the ingot has been formed and filled, it is soaked to an elevated temperature which is below the melting temperature of the metal powder. In accordance with the preferred form of the invention, the ingot 16 is soaked for a period of time sufficient to raise the temperature of the ingot to between 427°C-455°C (800 and 850° Fahrenheit) throughout, for rolling. In the rolling operation, the ingot 16 is hot rolled to reduce its thickness. The thickness is preferably reduced in a series of passes through a rolling mill. During that process, the mixture of boron compound and metal powder become metallurgically bonded together or sintered to form the sheets 12 and 14. The resulting sheet has thin, rigid neutron-absorbing areas 32 and 34, opposite metal edge portions 36 and 38, and an elongated metal spacer portion 40 at the location of each metal divider 26 in the ingot 16. The edge portions of the sheet 12 are trimmed as necessary so that preferably the combined widths of the edge portions 36 and 38 approximates the width of each spacer portion 40, although the combined widths can be wider or narrower, as well. After the sheet 12 has been formed, it is longitudinally bent at each spacer portion 40, preferably to an L-shaped cross section, as illustrated for each of the sheets 12 and 14 forming the tube 10 of Figure 1. At the same time, the wider edge portion 36 is also bent to an L-shaped cross section. The two sheets 12 and 14 are then longitudinally joined at their edge portions 36 and 38 by welding, as shown at welds 42 in Figure 1. It is preferred that the welds 42 not be located at the bends of the edge portions 36, but rather be formed on an unbent portion of the edge portion 36 to assure the greatest possible strength for the formed tube 10. Alternatively, if the edge portion 36 is not bent, the edge portions 36 and 38 can be abutted and welded where they abut. The tube 10 is therefore formed with neutron-absorbing areas 32 and 34 of the sheets 12 and 14 being unbent, but with bends in the spacer portions 40 and edge portions 36 being wholly within solid metal areas. The bends therefore do not affect the integrity of the neutron-absorbing core portions or areas of the sheet. Although the tube 10 is preferably formed of two sheets 12 and 14 formed and welded in accordance with the process set forth above, the tube 10 can also be formed from a single sheet having three metal dividers 26 installed in a metal ingot 16 forming four chambers in the ingot. After the ingot is filled and soaked, it is hot rolled in the same manner as described above, resulting in a sheet having opposite edge portions 36 and 38 similar to the sheet 12, but having three of the metal spacer portions 40 dividing adjacent neutron-absorbing areas. The thus-formed sheet is bent at each of the spacer portions to an L-shaped configuration, and also the edge portion 36 is bent to an L-shaped configuration, forming a tube appearing the same in cross section as the tube illustrated in Figure 1. Since the tube is formed from a single sheet, only one longitudinal weld 42 is required to weld the edge portions 36 and 38 together to complete the tube. Although the tube 10 is shown in its preferred form as being generally square in cross section, the tube can be formed with a rectangular cross section. In addition, the tube can be formed with more than four sides, in a pentagonal, hexagonal or other configuration. However the tube is formed, it is important that the tube be formed with longitudinal bends being located wholly within a metal spacer portion dividing adjacent neutron-absorbing areas of the sheet or sheets forming the tube. Each of the metal dividers 26 may be formed of a single length of metal, or may be formed of one or more lengths of metal. However the dividers are configured, the ultimately-formed sheet is as illustrated in Figure 3, with one or more spacer portions 40 being formed of metal, so that the sheet 12 may be bent at the spacer portions. It is also preferred that the sum of the widths of the edge portions 36 and 38 equal that of the spacer portion 40, so that when each of the sheets 12 and 14 is bent, the resulting formed tube 10 is essentially symmetrical. Again, however, that relationship is not critical, and the sum of the widths can be greater or less than the width of the spacer portion 40. A square tube 10 is illustrated in the drawing figures, manufactured from two of the sheets 12 having a central spacer portion 40. If a rectangular tube is desired, it will be evident that the metal divider 26 can be offset in the ingot 16 toward one or the other of the side edges 22 or 24 so that one of the chambers 28 or 30 is larger in the other, resulting in a sheet 12 with the spacer portion 40 offset toward one or the other of the edge portions 36 or 38. It will be evident to one skilled in the art that other shapes can be effected as well by simple location of the ultimate spacer portion or portions 40 and bending of the sheet or sheets 12 thereat. The width of the spacer portion 40 is formed to the minimum extent necessary so that the sheets 12 and 14 can be bent without bending or affecting the integrity of the neutron-absorbing areas 32 and 34. The width of the various spacer portions 40 will be evident to one skilled in the art, given the disclosure of the present application. Various changes can be made to the invention within the scope of the following claims.

Claims: A method of making a neutron-absorbing tube (10) or a section (12, 14) thereof, comprising the steps of: a. forming an elongate, generally rectangular metal ingot (16) having a hollow interior with at least one elongate metal divider (26) in said interior to form at least two chambers (28, 30) in said interior; b. filling said chambers (28, 30) with a substantially uniformly dispersed mixture of a finely divided neutron-absorbing boron compound and a finely divided metal powder; c. soaking the ingot (16) to bring it to an elevated temperature below the melting temperature of the metal powder; d. hot rolling the ingot (16) to reduce its thickness to form a thin, rigid neutron-absorbing sheet (12, 14) having opposite metal edge portions (36, 38) and an elongate metal spacer portion (40) at each metal divider (26); and e. longitudinally bending the sheet at each spacer portion (40). A method according to claim 1, further comprising joining, via said metal edge portions (36, 38), the section (12, 14) to one or more further sections (12, 14) made by the method of claim 1 so as to form the neutron-absorbing tube (10). A method according to claim 1 or 2, wherein the or each ingot (16) has one metal divider (26), and in step "e" the or each sheet (12, 14) is longitudinally bent to an L-shaped cross-section. A method according to claim 1, wherein in step "a" the elongate, generally rectangular ingot (16) has at least three metal dividers (26) to form at least four chambers (28, 30) in the ingot (16), and further comprising the step (f) of: joining said metal edge portions (36, 38) of the sheet to form a neutron-absorbing tube (10). A method according to claim 4, wherein the ingot (16) has three metal dividers (26), and in step "e" the sheet (12, 14) is longitudinally bent to an L-shaped cross section at each metal spacer portion (40). A method according to any one of the preceding claims, wherein in step "d" the or each sheet (12, 14) is formed so as to have one metal edge portion (36) of greater width than the other edge portion (38). A method according to any one of the preceding claims, wherein step "e" further includes bending one of said metal edge portions (36, 38) of the or each sheet (12, 14) to an L-shaped cross section. A method according to claim 7, further comprising joining the bent metal edge portion (36) of one sheet (12, 14) to the unbent metal edge portion (38) of the same or a different sheet (12, 14). A method according to any one of claims 2 to 8, wherein metal edge portions (36, 38) are joined by welding. A neutron-absorbing tube (10) or section (12,14) made according to the method of any one of the preceding claims.

Inventors: ELLIOTT RANDELL, HERRICK WILLIAM, ELLIOTT, RANDELL, HERRICK, WILLIAM
Applicants: AAR CORP, AAR CORPORATION
Application year: 1993
Main classification: G21C21
IPC codes: G21C21, G21C7, B22F7, G21C19