Patent Number: 055925227
Section: summary

TECHNICAL FIELD The present invention relates to a control rod for a nuclear reactor and particularly to a cruciform-shaped control rod having each wing divided into discrete compartments formed by structural stiffeners and which compartments contain neutron-absorbing material. BACKGROUND As well known, control rods in a nuclear reactor perform the functions of power distribution, shaping and reactivity control. This is accomplished generally by providing a plurality of control rods containing material for neutron absorption and manipulating the control rods within the reactor. Generally, a control rod used in a boiling water reactor (BWR) is provided in a cruciform shape and extends in complementary cruciform-shaped interstices between fuel channels. While there are a number of different control rod designs, two basic control rod designs have been typically employed previously in boiling water reactors. In one design, each control rod consists of a plurality, e.g., either 48 or 84, boron carbide absorber or hafnium rods, or a combination of boron carbide and hafnium rods, separated into four cruciform-shaped wings of either 12 or 21 tubes each. The tubes are enclosed in a perforated outer sheath which is welded to a full-length central tie rod which has a handle and connector/velocity limiter at the respective opposite ends of the rod. The perforated sheath affords a cooling medium to the tubes. If a tube fails in this design, the boron carbide powder is exposed to the reactor coolant. In another type of control rod, essentially square tubes are welded to one another to form the four wings of the cruciform-shaped control rods. Each wing contains 12 to 15 square tubes welded to one another and to a handle at one end and a connector/velocity limiter at the opposite end. The welds between the tubes and the tubes themselves provide the structural support for the control rod. The tubes serve as individual pressure vessels, as well as structural members subjected to all reactor induced loadings. In this design, the boron carbide powder is contained in sealed capsules inserted into the individual square tubes. One of the difficulties with the latter type of control rod construction is the magnitude of the welding required to weld each of the individual square tubes to an adjacent tube to provide the structural support necessary for the four wings of the control rod. While that construction has served well, it is quite expensive to manufacture. Another problem associated with the design of control rods is that the reaction of the neutron-absorbing material, e.g., boron carbide, with neutrons produces a helium gas. Thus, the tubes of each of the previously noted designs must have structural integrity to maintain the generated helium gas within the tube at increasing pressure throughout the lifetime of the control rod. It is known that the mechanical lifetime of a control rod is limited by the reactor burn-up and the corresponding helium pressure build-up in the tube containing the boron carbide exposed to the highest neutron flux. The magnitude and pressure of helium gas generated depends on the location of the tube within the control rod. That is, the boron carbide containing tube which has the highest exposure to neutrons in the control rod determines the mechanical life of the control rod. However, in each prior design, the neutron-absorbing material was contained in discrete tubes individually subjected to helium gas pressure build-up and without any relief. Consequently, the life of the control rod is dependent upon the structural integrity of only one of a large number of absorber tubes. DISCLOSURE OF THE INVENTION In accordance with the present invention, there is provided a control rod construction which requires considerably reduced manufacturing time and expense as compared with prior control rod designs, principally by reducing the amount of welding required to form the control rod. Additionally, the invention extends the mechanical life of the control rod by equalization of the pressures within the absorber tubes across each wing of the control rod, hence limiting the helium gas pressure within the control rod to an average pressure of the individual absorber tubes. Particularly, the present invention provides a control rod in cruciform shape wherein each wing of the cruciform-shaped control rod includes discrete compartments sealed from the surrounding environment. For example, each wing of the cruciform-shaped control rod includes laterally spaced, vertically extending structural or stiffening members interconnected at their opposite ends by generally horizontally extending structural or stiffening members defining a plurality of compartments. Neutron-absorbing material is disposed in each of the compartments. For example, hafnium rods or capsules containing boron carbide, or a combination of hafnium rods and boron carbide capsules, may be disposed in each of the compartments. The compartments are sealed on opposite sides by overlays of sheet metal secured to the vertical and horizontally extending stiffening members, thus sealing the compartments from the surrounding environment. In another aspect of the present invention, the compartments in each wing of the cruciform-shaped control rod communicate one with the other to afford an equalization of pressure within the entire wing of the control rod. More particularly, the capsules containing the boron carbide are designed to leak the helium generated by the boron-neutron reaction into the compartments. By providing openings or passages through the vertical members or stiffeners defining adjacent compartments, the helium leakage migrates between compartments, equalizing the pressure. As a consequence, variations in the generation of helium gas in the individual boron carbide capsules can be accommodated, thus extending the mechanical life of the control rod by providing an average pressure for all of the boron carbide capsules. In a preferred embodiment according to the present invention, there is provided a control rod for a nuclear reactor, comprising a cruciform control rod body having four elongated, substantially vertically extending wings arranged generally 90.degree. relative to one another, each wing including a plurality of elongated, generally vertically extending structural members spaced from one another and horizontally extending structural members adjacent opposite ends of the vertically extending members defining a plurality of vertically extending, side-by-side compartments, a sheath disposed along opposite sides of the vertically and horizontally extending members whereby the members and the sheath seal the compartments externally of the wing and a neutron-absorbing material disposed in each sealed compartment. In a further preferred embodiment according to the present invention, there is provided a control rod for a nuclear reactor comprising a control rod body having a plurality of elongated, laterally spaced, generally parallel structural members in part defining a plurality of side-by-side sealed compartments each containing neutron-absorbing material, and an opening in at least one of the members in part defining adjacent compartments enabling passage of gas under pressure generated by reaction of the neutron-absorbing material with neutrons from one of the adjacent compartments through the opening to another of the adjacent compartments to maintain the adjacent compartments under substantially equal pressure. Accordingly, it is a primary object of the present invention to provide a novel and improved control rod which can be readily, easily and inexpensively manufactured and which may extend the mechanical life of the control rod.