Patent Number: 046831169
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the nuclear art and it has particular relationship to nuclear reactors for supplying electrical power. In the interest of safety and reliability, the cost of constructing and installing a nuclear power plant has increased to a high level. In view of the high investment demanded, it is desirable that the economics of the power reactor be such as to moderate the rate structure to the extent practicable. It is accordingly an object of this invention to provide a nuclear reactor for electrical power purpose in whose use and operation material reduction in rates as compared to current reactors shall be obtainable. SUMMARY OF THE INVENTION In reactors in accordance with the teachings of the prior art, the nuclear core of a reactor includes a plurality of fuel assemblies. Each fuel assembly includes a plurality of elongated fuel rods and in addition a plurality of so-called "thimbles" which are essentially tubes. Control rods are moveable into and out of the thimbles of some, but not all, of the fuel assemblies. The thimbles also serve the purpose of securing each fuel assembly into an integrated unit. For this purpose the thimbles are secured to the top and bottom nozzle of each fuel assembly. In prior art reactors, all fuel assemblies, those which receive or guide control rods and those which do not, are alike. They all have thimbles which serve to secure the assembly into an integrated unit. The assemblies which receive control rods are herein referred to as controlled assemblies. The assemblies which do not have control rods are referred to herein as non-controlled assemblies. The controlled assemblies are described as having control-rod or thimble locations or positions and the non-controlled assemblies are described as having non-control-rod locations or positions. This invention arises from the realization that substantial advantages can be achieved by structuring the noncontrolled assemblies differently from the controlled assemblies. The expression control rods as used in this application and in the art in its general sense refers to three kinds of rods: control rods, grey rods and water displacement rods. The rods of the three types are mounted in clusters on a spider. Each control-rod and grey-rod cluster is associated with a separate fuel assembly into whose thimbles the control rods are moveable. The water displacement rods are moveable into thimbles in a plurality of neighboring fuel assemblies. The different rods and their mounting are described in application Ser. No. 715,125 filed Mar. 26, 1985, to Franklin D. Obermeyer and Luciano Veronesi for Nuclear Reactor assigned to Westinghouse Electric Corporation and incorporated herein by reference. This invention concerns itself predominantly with fuel assemblies which receive control rods or grey rods; i.e., with rods each of whose clusters are associated, and penetrate into, a particular fuel assembly. However, fuel assemblies in accordance with this invention may be structured so that they also receive water displacement rods. The fuel-assembly guide-thimble geometry in controlled assemblies is fixed and constricted by the control rod array of the associated clusters. The thimbles in each such assembly must be precisely positioned so that the control rods do not blind as they are moved inwardly and outwardly of the thimbles. Non-controlled assemblies, particularly those which do not receive any rods, are not so constricted thus affording flexibility as to their construction and design. In arriving at this invention it has been realized that while there must be similarities between controlled and non-controlled assemblies, there may be marked differences. Advantage may be taken of these differences to provide an improved reactor. The non-controlled fuel assemblies must be compatible with the controlled assemblies in the following respects. Both types of assemblies: 1. Must have the same pressure gradient along the assemblies (vertically from bottom to top) to minimize cross-flow between assemblies. 2. Must have the same cross-sectional envelope dimensions. 3. Must interface with the upper and lower core plates in the same way. 4. Must interface with the refueling machine gripper in the same way. 5. Must have grids as disclosed, for example, in Andrews U.S. Pat. No. Re. 28,079 and the grids must be located substantially in the same positions along the assemblies. 6. Must meet the same seismic requirements and respond to a loss-of-coolant accident (LOCA) in the same way. The non-controlled assemblies need not be similar to, or compatible with, the controlled assemblies in at least the following respects: 1. The number and spacing of the fuel rods need not be the same. 2. The overall or aggregate length of the fuel rods need not be the same. 3. The guide thimbles are not required in the non-controlled assemblies. 4. The structural members or tie rods in non-control-rod locations, which bind an assembly into an integrated unit, need not include provisions for insertion of control rods so that, for example, their cross-section thickness may be greater than that of thimbles. 5. In non-controlled assemblies, the structural members can be located anywhere in the fuel assembly cross-section and not necessarily in the thimble locations of the controlled assemblies. 6. The number of structural members and the number of fuel rods in non-controlled assemblies need not be the same as the number of thimbles and fuel rods in controlled assemblies. 7. It is not necessary that the top nozzle of a non-controlled assembly accommodate the spider of a control-rod cluster during a scram when rods are fully inserted. Therefore, the vertical length of the top nozzle may be reduced so that longer fuel rods can be accommodated. 8. The top nozzle adapter plate of a non-controlled assembly need not include provisions for attaching hollow guide thimbles. This invention results from evaluation of the above-stated limitations and freedoms. This evaluation has led to the creation of a nuclear reactor whose controlled and non-controlled fuel assemblies are different. Advantage is taken of the above freedoms to provide non-controlled assemblies which endow the nuclear reactor according to this invention with marked advantages over prior-art reactors in which all assemblies are alike. In accordance with this invention, there is provided a nuclear reactor whose non-controlled assemblies have structural members which are hollow tubes or cladding at least certain of which contain burnable neutron absorbers or neutron poison. Typically, the neutron absorbers may be pellets of gadolinium compounds or of borosilicate glass, which may contain a high percentage of boron 10 or boron carbide in a matrix of aluminum oxide (see, for example, application Ser. No. 352,731 filed Feb. 26, 1982 to William G. Carlson et al. for Burnable Neutron Absorbers and assigned to Westinghouse Electric Corporation). The structural member is sealed throughout and protects the neutron absorbers from the coolant and the coolant cannot distribute the poison throughout the reactor. A burnable neutron absorber includes neutron absorber material; e.g. boron or gadolinium, in such low concentration that it burns out in a given time during the fuel cycle. For example, a burnable neutron absorber may burn out during the first year of a three-year cycle. After that, it no longer absorbs neutrons substantially. The burnable neutron absorber is thus effective only during the early part of the cycle when the reactivity of the fuel is high. The neutron-absorber material in control rods on the other hand is in such high concentration that the quantity of material which is converted during the life of the reactor does not materially reduce the effectiveness of the control rods. In the construction of a non-controlled fuel assembly, a skeleton including the bottom nozzle, and the structural members, held together by grids is provided. The structural members have end plugs at the bottom by which they are secured to the bottom nozzle. They are open at the top. The structural members are bulged above and below each grid. Then the neutron absorber pellets and a spring for holding the pellets is inserted in each structural member. After evacuation, heating and back-filling with an inert gas, an end plug is secured to each structural member. The fuel rods are then inserted in the skeleton and the top nozzle is mounted and the structural members secured to it. The thickness of the structural members in the non-controlled fuel assemblies is greater than the thickness of the thimbles which serve as structural members in the controlled assemblies. Typically, a thimble in a controlled assembly, which is a hollow circular cylinder, has an OD of 0.484 inch and an ID of 0.448 inch or a thickness of 0.018 inch, while a structural member of a non-controlled assembly has an OD of 0.484 inch and an ID of 0.423 inch or a thickness of 0.031 inch. There are substantially fewer structural members in the non-controlled assembly than in the controlled assembly. Typically, there are 24 thimbles, 1 instrumentation tube and 264 fuel rods in a controlled assembly having 17.times.17 or 289 total locations. In a non-controlled assembly according to this invention having the same number of locations, there are only 8 structural members and one instrumentation tube. Fuel rods are inserted in the other locations so that there are 304 fuel rods. As far as fuel content is concerned, the effect of increasing the number of fuel rods in the non-controlled assembly is to increase the effective fuel length for the overall assembly by 8.7 inches. The fuel cycle cost is reduced by 1.2%. The KW/ft is reduced by approximately 6% and the pellet-clad interaction (PCI) margin is increased, resulting in improved reliability. The reduction in the KW/ft also increases the margin between the operational flow of coolant and the design limit at which there is overheating. It has been prior-art practice in reactor fuel bundles, which do not include thimbles for control rods, to provide tie rods which are cylinders filled with nuclear fuel. Tie rods of this type have the disadvantage that they grow non-uniformly during operation of the reactor and become disengaged from the nozzles. The structural members, according to this invention, grow more uniformly so that the structural integrity of the non-controlled fuel assemblies remains sound.