Patent Number: 047145846
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to head adapter extensions for use in the reactor vessel of a nuclear pressurized water reactor. Pressurized water reactors which are currently in use include a reactor vessel containing the reactor core and control rods which are movable relative to the core inorder to control its reactivity. Each control rod is suspended from a drive rod that extends through the reactor vessel head to a drive mechanism located above the head. In order to seal the vessel head at the location of such drive rod, there is provided a cylindrical housing which extends downwardly from the drive mechanism and through a bore in the head to a location within the upper portion of the reactor vessel. The portion of the housing which extends through the vessel head includes a head adaptaer secured in the associated bore, by welding, in order to form a seal between the housing and the vessel head. The housing further includes a head adapter, or housing, extension which opens, at its lower extremity, to the interior of the vessel and which is generally terminated, at its lower end, by a guide funnel that opens downwardly toward the interior of the vessel and that serves to guide a drive rod into the housing when the vessel head is installed on the vessel body. When such a reactor vessel is assembled, an annular passage which communicates with the interior of the reactor vessel will be created between the drive rod and the associated housing which passes through the vessel head. In a proposed new pressurized water reactor model, there will also be employed displacer rods which will be movable relative to the reactor core and which will be supported by a drive rod passing upwardly through the vessel head to a drive mechanism. Here again, each drive rod is to be surrounded by a housing having the form described above. The portion of each housing which passes through the vessel head is made of inconel in order to suitably fasten the vessel head, which is currently made of carbon steel clad with standard stainless steel, to the housing of the drive mechanism itself, which is made of standard stainless steel. BEcause of the expenses involved in the manufacture of inconel parts, this is achieved by forming the part of the housing which is connected to the vessel head as a short head penetration adapter made of Inconel and by then welding a stainless steel pipe to the upper end of the adapter, the upper end of the stainless steel pipe then extending, and being connected to, a further housing enclosing latch components of the drive mechanism. The head adapter and pipe are joined together by a full penetration weld and it is necessary to take account of the possibility of failure of that weld, which would result in the leakage of liquid out of the reactor vessel via the annular passage between the housing and its associated drive rod. As protection against the adverse consequences of such a weld failure, or of a leak due to other causes in the housing surrounding the drive rod, it is desirable to limit th cross-sectional area of the flow passage between the interior of the vessel and the source of such a leak to a value of no greater than approximately 5 cm.sup.2. To achieve this, it has previously been proposed to insert a cylindrical tube, known as a thermal sleeve, in the housing to surround the drive rod and to provide orifices between the thermal sleeve and the head adapter such that the cross-sectional area of the remaining flow passage from the interior of the reactor vessel to the region of the drive mechanism was restricted to the above-cited value. In this structure, an extension rod must be provided above the vessel head. The use of a such a thermal sleeve, however, is accompanied by a number of disadvantages, among which are tht it incrases the total number of components required and the total manufacturing cost. SUMMARY OF THE INVENTION It is an object of the present invention to avoid the above-mentioned difficulties. Another object of the invention is to simplify the structure of the housings associated with the drive rods of a nuclear reactor vessel. A further object of the invention is to eliminate the need for additional machining of the head adapter of such a housing. Another object of the invention it to provide a structurally simple solution to the problem of providing a restricted flow passage in the event of a leak in the drive rod housing. Yet another object of the invention is to reduce the total weight of such housing. Still another object of the inention is to provide a flow limiting struture which can be connected directly to the head adapter forming part of a drive rod housing. Still another object of the invention is to permit the use of larger diameter drive rods. A still further object of the invention is to make possible the provision of an ejection resisting component which will prevent a drive rod, and particularly the head adapter thereof, from being ejected from the pressure vessel in the event of a failure in the weld between the adapter and the vessel head. Yet another object of the invention is to minimize the number of components rquired to assure the desired flow limitation. The above and other objects are achieved, according to the present invention by a device for use in a nucler reactor which includes a pressure vessel having a vessel head, a core in the vessel, elements for controlling the reactivity of the core, drive rods which pass through the vessel head for displacing the elements, and a plurality of head adapters which pass through the vessel head, each head adapter forming part of a drive rod housing enclosing a respective drive rod, each housing enclosing a region which communicates with the interior of the vessel and which is closed at the top, the device being associated with a respective housing and constituting a housing extension including means for connecting the device to the head adapter forming part of the respective housing, and means for forming, around the associated drive rod and within the associated housing, a fluid passage having a cross-sectional area not exceeding a selected value at least upon the occurrence of a leak in the respective housing.