Induced nuclear reactions: processes – systems – and elements – Reactor structures – Circulating fluid within reactor
Patent
1996-12-06
1998-07-07
Jordan, Charles T.
Induced nuclear reactions: processes, systems, and elements
Reactor structures
Circulating fluid within reactor
376439, 376462, G21C 1500, G21C 334
Patent
active
057780358
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for equalizing the cooling between less loaded and more loaded sub-regions of a fuel assembly or between fuel assemblies in a light-water nuclear reactor. The equalization of the cooling is achieved by mixing a coolant flow within a mixing cross section comprising four orthogonally arranged sub-regions which may have considerably different power levels because of different degrees of burnup or the effect from the surroundings. The mixing is made possible by creating a special flow pattern in a horizontal cross section in the mixing cross section. The flow pattern, in turn, is achieved with the aid of spacers, that is, those support means which fix the fuel rods in spaced relationship to each other and are arranged at a plurality of levels along the fuel rods.
BACKGROUND OF THE INVENTION
A fuel assembly in a boiling water nuclear reactor (BWR) consists of an elongated tubular container, often with a rectangular or square cross section, which is open at both ends forming a continuous flow passage, through which the coolant of the reactor is able to flow. The fuel assembly comprises a large number of equally elongated tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. At the top, the fuel rods are retained by a top tie plate and at the bottom by a bottom tie plate. To allow coolant in the desired manner to flow past the fuel rods, it is important to keep them at a distance from each other and prevent them from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction. Some fuel assemblies for boiling water reactors are divided into so-called sub-assemblies. Such a sub-assembly is disclosed in U.S. Pat. No. 4,795,608. Between the sub-assemblies there are openings allowing coolant to pass between the sub-assemblies.
A fuel assembly for a pressurized-water nuclear reactor (PWR) has, in principle, the same design as a fuel assembly for a boiling water nuclear reactor (BWR), apart from the fact that the fuel rods are not enclosed by any tubular container and the fact that their number is higher.
At the bottom each fuel assembly is provided with an essentially equally large amount of coolant, but higher up in the core it is desirable to transfer water from less loaded regions to more loaded regions in a fuel assembly or between fuel assemblies.
The lower the power output from a fuel assembly, or part of a fuel assembly, the less cooling it needs. Conversely, a higher power may be taken out from a fuel assembly, or part of a fuel assembly, the better its cooling is. The more the fuel is consumed in a fuel assembly, the lower is the power that can thereafter be extracted from the fuel assembly.
One way of achieving transfer of coolant from less loaded to more loaded fuel assemblies is disclosed in Swedish patent application No. 9003330-9. By irradiation-dependent growth of tabs arranged at spacers in the fuel assembly, the reduction is increased successively during the life cycle of the fuel assembly. In this way, coolant is transferred from older, less loaded assemblies to newer, more loaded assemblies. It is also known to create mixing of coolant locally between adjacently located fuel rods. This is done by means of mixing vanes in the form of tabs arranged at the downstream edge of the spacer, thus diverting the flow such that mixing occurs. Such mixing tabs are described, inter alia, in U.S. Pat. No. 3,395,077.
U.S. Pat. No. 3,749,640 shows four orthogonally arranged fuel assemblies for a pressurized-water reactor which are provided with spacers with mixing vanes. The mixing vanes are adapted to guide the flow concentrically around the intersectional points between adjacently located fuel elements. The flow control results in cold coolant being moved into and hot coolant being moved out of a concentric region with high power, and in cold coolant being moved into and hot coolant being moved out of th
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ABB Atom AB
Jordan Charles T.
Lattig Matthew J.
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