Patent Number: 055925227
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to FIG. 1, there is illustrated a representative example of a control rod, generally designated 10, according to the prior art. The control rod includes a generally cruciform-shaped, vertically extending central tie rod 12 and four wings which form continuations of the cruciform shape of the central tie rod 12. Each of the wings contains a plurality of tubes 14, for example, 84 stainless steel tubes, filled with boron carbide powder compacted to a percentage of its theoretical density. The tubes are seal-welded with end plugs on either end and act as pressure vessels to contain helium gas released by the boron-neutron capture reaction. The tubes are held in each wing by a stainless steel sheath 16 extending the full length of the tubes with the inner edges of the sheath secured by welding to the cruciform-shaped tie rod 12, the outer edge being wrapped around the outer tube. One or more hafnium tubes may also be employed in conjunction with the boron carbide-containing tubes 14. A handle 18 is provided at the upper end of the control rod, while a velocity limiter 20 and a coupling socket 22 are provided adjacent its lower end. As those of skill in this art will appreciate, the control rod is disposed between fuel bundles arranged to have interstices in a generally cruciform shape complementary to the shape of the control rod. By vertically manipulating the control rod, the power distribution, shaping and reactivity control of the reactor can be accomplished. In the illustrated prior control rod of FIG. 1, perforations 24 are provided in the external sheathing 16 around the control rod to permit a cooling medium to flow in and about the neutron absorber tubes. Additionally, the tubes may be formed of all boron carbide or all hafnium rods or combinations of boron carbide and hafnium tubes, depending upon the circumstances of the reactor. Referring now to FIG. 2, there is illustrated a portion of a control rod, generally designated 26, constructed in accordance with the present invention. It will be appreciated that the control rod 26 is generally in a cruciform shape similarly as the previously described prior art control rod, and has wings 28 disposed at right angles to one another about a centrally disposed tie rod 30. Only two of the wings 28 lying in a common plane are illustrated in FIG. 2, it being appreciated that additional wings 28 are similarly secured to the central tie rod 30 and project to opposite sides of the illustrated wings 28 at right angles thereto. Each wing in accordance with the present invention is formed of a plurality of vertically extending structural stiffeners or members 32 laterally spaced one from the other. Generally horizontally extending parallel structures or stiffeners 34 are disposed between the vertically oriented members 32 at opposite ends of the control rod. Alternatively, the upper and lower ends of the vertically extending members 32 may be welded to members 34 which extend the full length of each wing and thus overlie the ends of the vertical members 32 at each of the upper and lower ends. In either case, the vertically and horizontally extending members 32 and 34 form a plurality of laterally spaced, side-by-side s compartments in each wing 28 of the control rod 26. Thus, in the illustrated form, three compartments 36a, 36b and 36c are formed in each wing 28 of control rod 26. Overlying the compartments on opposite sides thereof there is sheathing 38 secured, e.g., by welding, to each of the vertical and horizontally extending members 32 and 34, respectively. The members 32 and 34 and sheathing 38, when secured to the members 32 and 34, form compartments 36 completely sealed from the surrounding environment. Neutron-absorbing material is disposed in each of the compartments 36. For example, a plurality of capsules 40, each containing boron carbide powder, may be disposed in columnar fashion in each of the compartments 36. Thus, the capsules may be superposed one over the other end-on-end and without securement to one another or to the members 32, 34 or sheath 36. Additionally, one or more elongated tubes containing neutron-absorbing material, e.g., a hafnium rod, extending the full height of the compartments may be provided. For example, the two outermost tubes in the outermost compartments 36c of the wings 28 may comprise hafnium tubes 42 containing hafnium rods. These tubes likewise need not be secured to the members 32, 34 or sheath 36. As illustrated in FIGS. 2 and 5, the central tie rod comprises a plurality of vertically superposed planar plates 44, each having a centrally located slot 46 in its lower end. By alternating the planar tie rod plates 44 at right angles to one another, it will be appreciated that a central upper edge portion of an underlying tie rod planar plate may be received in the slot 46 of the next-adjacent superposed planar tie rod plate 44. Each plate has its outermost edges 48 secured as by welding to the innermost vertically extending stiffener or member 32 of a wing lying in the same plane as the planar tie rod plate 44. Thus, for example, as illustrated in FIG. 2, the upper tie rod plate 44 has its opposite edges 48 secured to the innermost members 32 of the wings 28 lying 180.degree. one from the other. Thus, the wings 28 and the uppermost tie rod plate 44 lie in a common plane with one another and with the uppermost tie rod plate 44. The next-lower tie rod plate 44 is received in the slot 46 of the superposed tie rod plate and has its opposite edges 48 secured as by welding to the innermost members 32 of the right angularly related wings 28 lying in a common plane with one another and with the underlying plate 44. The planar tie rod plates 44 thus alternate at right angles to one another to form a generally cruciform-shaped central tie rod. Note that the tie rod plates are themselves not secured to one another and that the structural connection between the wings and the tie rod is through the vertically extending innermost members 32 of the wings 28. The foregoing construction has been found to be advantageous in the manufacture of the control rod. Whereas previously it was necessary to weld individual rods to one another to provide for the structural integrity of the control rod, the only welding required in the present invention is the welding of the members 32 and 34 to one another and the sheathing 38 to the members 32 and 34 in conjunction with its attachment to the central tie rod. Thus, the amount of welding is very considerably reduced as compared with the magnitude of the welding necessary for prior art control rods. In another aspect of the present invention and referring to FIGS. 3 and 4, the internal vertically extending members 32 are provided with leakage openings or apertures between adjacent compartments 36. Thus, as illustrated, vertically spaced leakage openings 50 are provided through the vertical members 32, affording communication between compartments 36a and 36b. Similarly, vertically spaced leakage openings 52 are provided through the other internal vertically extending member 32, affording communication between compartments 36b and 36c. These leakage openings are particularly useful for equalizing the pressure of the helium gas generated by the boron-neutron capture reaction as between the various compartments and capsules. In this form, the boron carbide capsules 40 are provided with leakage paths through their ends such that helium gas generated by the boron-neutron reaction may leak from the capsule into the sealed compartment 36. As a consequence, the pressure within each of the compartments is substantially equalized and the boron capsules need not provide structural integrity and strength necessary to maintain the generated helium gas within the capsule under pressure. As a further consequence, the mechanical life of the control rod is extended because the pressure within the compartments would constitute the average of the pressures generated by the boron-neutron reaction in the various capsules, eliminating any need to replace a control rod as a result of high pressures developing in one of the many boron carbide capsules. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.