Patent Number: 047553510
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and has particular relationship to fuel assemblies. A fuel assembly includes a skeleton in which fuel rods are supported and into which control rods and the like penetrate. The skeleton includes an upper or top nozzle, a lower or bottom nozzle and a plurality of egg-crate-like grids spaced between the nozzles. The grids are composed of interlaced straps. The nozzles and straps are held together by thimble tubes (some of which receive the control rods) which extend between the nozzles and to which the grids are secured. Each fuel rod is held in a column of aligned pockets in the grids between oppositely-disposed springs and dimples. The springs are each mounted on a part of a strap bounding a pocket and urge the rods into firm engagement with the dimples in the part of the opposite strap bounding the jacket. The fuel assemblies are mounted between the upper and lower core-support plates of the reactor. This invention is described in this application as integrated into a nuclear reactor of the pressurized water type (PWR) to which it is uniquely applicable. It is to be understood that embodiment of this invention into reactors of other types is within the scope of equivalents of this invention. During normal operation of a PWR, the flow of coolant, which may be as high as 50 feet per second, through each fuel assembly produces a net upward force of substantial magnitude on the assembly that would cause vertical movement, i.e., would cause the assembly to rise, if the assembly were not restrained. In accordance with the teachings of the prior art, that restraint is provided by a plurality of heavy leaf springs mounted on the top nozzle. These springs are compressed by the upper core plate and produce a restraining force that is larger than the assembly lift forces by a specified magnitude. The force which the springs counteract may be as high as 1500 to 2000 pounds. The compressive load supplied by the springs varies over the assembly lifetime because of thermal and irradiation-induced differential growth of the assembly relative to the upper core plate and spring irradiation-induced relaxation. The leaf springs are costly and complicate the structure and use of the top nozzle and of the fuel assembly as a whole. They constitute an appreciable increase, formidable in its demands, of the number of parts which must be assembled to construct a fuel assembly and maintained during its life. Because the springs are mounted on the corners of the upper nozzle but engage the core plate a distance from the corners, the upper nozzle must have substantial depth so that it can withstand the high bending loads exerted by the springs. The length of the fuel rods which can be accommodated by the prior-art assembly is correspondingly reduced. The leaf springs exert a reactive upward load on the upper core plate and on the upper internals of the reactor even when the coolant is not flowing as, for example, after refueling when the vessel head has been reinstalled. This upward load exacerbates the difficulty of tensioning the bolts which secure the head to the body of the reactor pressure vessel following refueling or initially. The resilience and dimensions of the leaf springs are materially affected by the thermal conditions in the reactor and by neutron irradiation. The downward load on the lower core-support plate, which is impressed solely by the leaf springs, thus varies in normal operation as the temperature within the reactor varies and also changes progressively, during the life of the reactor as a result of neutron irradiation. It is an object of this invention to overcome the above-described disadvantages and drawbacks of the prior art and to dispense with the heavy holddown springs in the fuel assemblies of a nuclear reactor. It is an object of this invention to provide a nuclear fuel assembly for a nuclear reactor which shall not include the heavy holddown springs of prior-art assemblies. SUMMARY OF THE INVENTION In accordance with this invention a fuel assembly is provided which, instead of being held down by heavy leaf springs, while the coolant is flowing through the reactor, is permitted to move; i.e., float, in a controlled manner so that the upper nozzle is in engagement with the upper core-support plate of the reactor. Typically, the maximum clearance required to accommodate axial assembly growth, both thermal and resulting from irradiation is about 11/2 inches (Anthony--U.S. Pat. No. 4,078,967, in column 4, lines 20-24, puts the growth between the cold beginning of life of a fuel assembly and its hot end of life, including variations between assemblies resulting from tolerances, at 13/4 inches in a 170 inch-long assembly). The length of the fuel assembly must then be 11/2 inches (or 13/4 inches in Anthony's example) shorter than the distance between the inward surfaces of the core plates since the maximum movement which can occur under the force of the coolant is then 11/2 inches (or 13/4 inches in Anthony's example). Lateral motion of the fuel assembly is restrained, at all times, by alignment pins. The pins typically extend from the upper and lower core plates and engage in holes generally coaxially in the top and bottom nozzles. This structure may be reversed so that the pins extend from the top and/or bottom nozzles and engage in holes in the upper and/or lower core plates. The pins and the cooperative holes should be of such length as to accommodate the full movement of the assembly. In accordance with an aspect of this invention the pin-hole units are provided with means for assuring that the pins, which are slidable in the holes, fit snugly, so that vibration under the action of the flowing coolant is precluded. Unless suppressed, movement of the fuel assembly would occur because of pin-hole tolerance and cross-flow induced motion at the inlet of the lower nozzle. The springs on the top nozzle are thus dispensed with. Load pads are provided to interface with the upper core plate and to provide clearance above the nozzle to accommodate inserts in the thimbles such as control assemblies, burnable neutron absorbers, etc. In accordance with a further aspect of this invention, the minimum downward loading on the lower core plate demanded by the specifications is provided. For this purpose at least one snubber is interposed between the bottom nozzle and the lower core plate. The snubber essentially includes a spring-loaded hydraulic piston with seal rings which is connected to the lower nozzle. The piston is slidable in a cylinder under force exerted on the nozzle. Between the piston and the cylinder there is a compression spring. The spring exerts just enough force to urge the cylinder into engagement with the lower core plate with the force demanded by the design specifications when the fuel assembly is driven upwardly by the hydraulic force of the upwardly-flowing coolant so that the top nozzle is in engagement with the upper core plate under pressure. The spring is dimensioned to apply the required minimum down load to the lower core plate. Typically the spring may exert a force of about 50 to 100 pounds. Because the coolant is under high pressure (2000 pounds per square inch), coolant penetrates into the cylinder. The movement of the piston of the snubber is damped by the coolant in the cylinder. If the lift forces on the assembly are removed (loss of coolant), leakage of fluid from the snubbers controls the rate at which the assemblies can drop. Leakage can be highly restricted to assure long drop times or small holes can be drilled in the seal ring to accurately control the discharge rate for faster drops. The limited maximum vertical motion assures that structural grids on adjacent assemblies do not have elevation mismatches which could cause interference at grid corners. This is most important near the assembly vertical center. The alignment pins prevent such interference near the top and bottom of the assembly. Because, in the practice of this invention, the fuel assembly moves while in the core, the effect of inadvertent movement must be considered. The anticipated initial upward movement of the assemblies occurs at zero power with all control rods inserted. Therefore, concerns for reactivity/power effects are not pertinent. Even if some assemblies did not move upwardly when anticipated but did so later while at power, the effect would be to reduce reactivity because the fuel would be nearer to the parked control rods since the assemblies would move upwardly and encompass greater lengths of control rods. The reverse movement, i.e., lowering of the assemblies when flow is reduced, also does not produce any operational problems. The maximum reactivity insertion due to all assemblies dropping is small (less than 20 pcm or 2% power). The snubbers assure that even for this worst case, insertion is very slow and well within the response capability of the protection system. Normally downward movement occurs when the pumps are shut down after the reactor is subcritical. In the practice of this invention the upward force on the upper core plates may actually be reduced. In accordance with prior-art practice, the holddown springs are dimensioned to provide downward force which is greater than the maximum practicable upward force. In accordance with the invention, the loading on the upper core plate cannot exceed the hydraulic lift forces on the assembly plus the reaction force of the snubber springs. Requirements for holddown of the lower internals can be met by selecting the spring constant of the snubber springs to provide adequate downward loading per assembly. This invention has the following advantages: 1. The structure of the top nozzle is simplified by elimination of the leaf springs and their securing mechanisms. The depth of the top nozzle is reduced because it need not be constructed to withstand the considerable pressure of the leaf springs developed by coolant flowing typically at a velocity of 50 feet per second. 2. Because the depth of the top nozzle is reduced, the length of the fuel rods may be correspondingly increased without increasing the length of the fuel assembly. This increase in length could potentially be as much as 2 inches. The linear heating rates and power peaking factors are correspondingly reduced. 3. Because the leaf springs are dispensed with, the upward load on the upper internals is removed when the coolant flow is interrupted, as during refueling. The bolts which secure the head to the body of the pressure vessel are more readily tensioned. 4. A constant preset downward load, as required by design specifications, is applied to the lower core plate. 5. Since the snubber springs are dimensioned to provide substantially lower forces than the leaf springs, (50 to 100 pounds compared to 500 pounds) their sensitivity to radiation is far less critical.