Patent Number: 046860770
Section: summary

This invention relates to a nuclear reactor installation. More particularly, this invention relates to a nuclear reactor installation having movable control rods. As is known, nuclear reactor installations have been constructed with a pressure vessel in which a reactor core is disposed, for example, for the heating of water to steam. In many cases, the reactor core has been constructed of straight vertical channel-like fuel elements which contain fissile material and vertically mobile control rods to which absorber rods are secured and positioned between the fuel elements. In addition, a conveying means has been provided to circulate a coolant from the pressure vessel under pressure through the reactor core. In one known installation, each control rod has been reciprocated vertically by means of a dedicated hydraulic reciprocating actuator. On the whole, such installations operate reliably but suffer from two considerable disadvantages. First, the reciprocating actuators are expensive to produce and maintain since the efficiency of the actuators depends largely upon satisfactory sealing tightness between the piston and cylinder of each actuator. In view of the large number of control rods in a reactor, for example about two hundred rods for a 1300 megawatts (MW) output boiling water reactor, the significance of these costs will be apparent. A second disadvantage is that the vertical adjustment of the control rods requires an elaborate form of control. Accordingly, it is an object of the invention to provide a nuclear reactor installation in which control rods may be driven and controlled in a relatively easy manner. It is another object of the invention to provide a reliable manner of controlling the movement of the control rods of a nuclear reactor installation. It is another object of the invention to provide a nuclear reactor installation with a system for controlling the movement of control rods which is simple to maintain. Briefly, the invention is directed to a nuclear reactor installation which is comprised of a pressure vessel and a reactor core within the vessel which includes at least one fuel element and a vertically movable control rod. In accordance with the invention, a guide rod is disposed within each control rod in order to define an annular gap. In addition, the guide rod has a tubular bottom which is disposed in immobile relation to the fuel element with a plurality of bores communicating an interior of the guide rod with the annular gap. A conveying means is also provided for conveying a coolant from the pressure vessel into the interior of the guide rod in order to move the control rod relative to the guide rod and fuel element. In addition to having the interior of the guide rod communicate with the annular gap via the communicating bores, an annular restrictor is provided at one end of the control rod to communicate the annular gap with the interior of the pressure vessel while a second annular restrictor is provided at the opposite end of the control rod to communicate the annular gap with the interior of the pressure vessel. During operation, the control rod is able to move in one axial direction in response to an increasing quantity of coolant in the guide rod interior while moving in an opposite axial direction in response to a decreasing quantity of coolant in the guide rod interior. The guide rod is constructed so as to be longer than the control rod. Hence, the guide rod extends upwardly through and beyond the control rod. The installation is such that each control rod moves along the associated stationary guide rod while the coolant, which is present in the pressure vessel in any instance, is used not only to drive the control rod but also to guide the control rod without contact. An accurate relation between the variable coolant throughflow and the vertical position of a control rod can thus be provided simply by the construction of the communicating bores and restrictors, and hence the bores and restrictors can be considered to comprise means whereby such position is varied as a function of coolant throughflow. In contrast to previously known installations, the drive for a control rod requires no special accuracy in manufacture and is virtually free from wear in operation. A further advantage is in the compactness of the arrangement which provides the drive and control system for each control rod. The installation is also constructed so that a level of liquid coolant is maintained within the pressure vessel. In this case, the conveying means has an intake disposed immediately below the coolant level to convey the coolant to the guide rods. In this embodiment, even a slight drop in coolant level in the pressure vessel renders the conveying means inoperative. Hence, the control rods can move automatically into a safety position. In order to enable the control rods to be retained at clearly defined positions, the communicating bores in each guide rod may be distributed vertically. Each guide rod may be constructed so that the top half is of smaller diameter than the bottom half with the communicating bores disposed in only the bottom half. In this embodiment, the control rod may have a removable guide cap on the top end which cooperates with the top half of the guide rod in order to form the upper restrictor. In this case, the restrictor provides a laminar flow of coolant. The use of a guide cap thus ensures satisfactory guidance of the control rod while also simplifying assembly and servicing. The top half of each guide rod may also be made of a decreasing diameter in an upward direction. This provides a very fine adjustment of the position of a control rod and leads to a very advantageous control system where the communicating bores are disposed along the guide rod. In one embodiment, each guide rod may have at least one adjustable continuous bore at a top end which communicates the interior of the guide rod with the interior of the pressure vessel. This permits a simple and accurate way of calibrating throughflow so that minor differences, for example, of weight, of the control rods can be compensated. Further, at least one restrictor may be provided for controlling a supply of coolant to the continuous bore in response to the coolant pressure. This restrictor may be disposed to open the continous bore in response to a shortfall of the coolant pressure. This feature enables the control rod to move rapidly into a bottom position which is usually a safety position. The installation may be constructed with a core support plate which is connected to and within the pressure vessel for supporting the fuel elements while a guide lattice is secured to and within the pressure vessel for guiding the top end of each guide rod. The arrangement of the guide lattice is such as to inhibit vibrations or hunting movements of the guide rods with a consequent increase in the general safety of the installation. Brief disturbances, such as brief variations in coolant pressure of throughflow or the formation of vapor in the coolant, could cause an alteration in the vertical position of the control rods and the rods would return only gradually to their reference or set-value position. Brief disturbances might, inter alia, impair the operation and control of the reactor. In order to ensure that the control rods return rapidly to the required position in such cases, a plurality of annular grooves are provided in each control rod in a transverse manner with each groove in facing relation to at least one bore of a respective guide rod. Consequently, the coolant issuing from the communicating bores is presented with further restrictions, on the one hand, between the outside surface of the guide rod and, on the other hand, the webs which remain between the annular grooves. In the event that the control rods become deflected because of brief disturbances, these restrictions ensure a rapid return of the control rods to their required position. In order to enhance the above effect, each control rod may have at least one stabilizing aperture connecting at least one of the annular grooves with the interior of the pressure vessel. In still another embodiment, the top half of each guide rod may be provided with a plurality of annular grooves while each control rod has at least one annular projection facing a respective top half of the guide rod in order to form the second annular restrictor. In this embodiment, a rapid return of the control rod to the required position can be achieved.