Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-12-23
2003-02-04
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S434000, C376S439000, C376S443000, C376S444000
Reexamination Certificate
active
06516043
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a fuel assembly used for a core of a D-lattice type boiling water reactor (hereinafter, referred to as a “BWR”), a reactor core using the fuel assembly, and a fuel spacer and a channel box used for the fuel assembly.
A gap structure between fuel assemblies, called a D-lattice structure, has been used for a core of a BWR. In the D-lattice core, a gap between the adjacent two of the fuel assemblies on the side where a control rod for power adjustment is inserted (on the control rod side) is wider than a gap between the adjacent two of the fuel assemblies on the side where the control rod is not inserted (on the anti-control rod side).
For a BWR, water in a core acts as a coolant and a neutron moderator. In general, the moderating action for neutrons becomes large in a region in which water entirely, continuously exists, and accordingly, water between fuel assemblies plays a large role for moderating neutrons.
One important factor associated with a reactor core is a linear heat generation rate. The maximum linear heat generation rate, which is the maximum value among the linear heat generation rates in a reactor, becomes important for design of the reactor. If the maximum linear heat generation rate becomes excessively large, the center temperature of the corresponding fuel rod becomes excessively high. In this case, there is a possibility that the thermal integrity of fuel pellets and a cladding tube constituting the fuel rod cannot be ensured. From the viewpoint of the safety of a reactor, a specific limited value of the maximum linear heat generation rate is determined. To keep a thermal margin state of a reactor, the maximum linear heat generation rate may be desirable to be made as small as possible.
One important factor associated with a fuel assembly is a fuel assembly critical power. In a core of a BWR, water flows in the lower portion of a fuel assembly, being boiled while flowing in the vicinity of fuel rods, and flows out of the upper portion of the fuel assembly. It may be considered that, at the upper portion of the fuel assembly, the rate of steam becomes large and the surfaces of the fuel rods are covered with liquid films. As the power of the fuel assembly is increased, the liquid film on the surface of any one of the fuel rods is initially lost by evaporation. The power at the time when the liquid film is initially lost is called the critical power. The critical power varies depending on the flow rate of coolant flowing through the fuel assembly. A reactor is operated while it is usually checked that the power of each of the fuel assemblies is less than the critical power.
Taking into account the above-described circumstances, a fuel assembly has been designed such that a suitable fuel enrichment distribution is set by preparing a plurality of kinds of fuel pellets or a suitable concentration distribution of burnable poison added to fuel rods is set, to make small a “relative axial peaking factor of a fuel assembly” or a “local peaking factor” which is the relative power peaking of each fuel rod in a cross section of the fuel assembly, thereby improving the critical power characteristic, and enhancing the safety margin and fuel economy of the reactor.
In a D-lattice core, as described above, there is a difference between a gap between the adjacent two of the fuel assemblies on the control rod side and a gap between the adjacent two of the fuel assemblies on the anti-control rod side. During usual operation, since most of the control rods are pulled out, the effect of moderating neutrons on the side where a gap between the adjacent two of the fuel assemblies is wide (on the control rod side) is larger than that on the side where a gap between the adjacent two of the fuel assemblies is narrow (on the anti-control rod side). When fuel assemblies are loaded in the D-lattice core, the power obtained from a fuel rod near a wide gap between the fuel assemblies (on the control rode side) is different from that obtained from a fuel rod near a narrow gap between the fuel assemblies (on the anti-control rode side). Accordingly, the value of the local peaking factor becomes relatively larger and thereby the maximum linear heat generation rate tends to be made larger. As a result, the above-described fuel enrichment distribution or the concentration distribution of burnable poison must be finely set, so that the degree of freedom in design of fuel assemblies is reduced.
To solve the above problem, the structure called a C-lattice has been proposed. In the C-lattice core, since the gap between the fuel assemblies on the control rod side is equal to that on the anti-control rod side, the degree of freedom in design of the C-lattice core becomes larger than that of the D-lattice core. To be more specific, in the C-lattice core, it is possible to relatively easily obtain the optimum structure in terms of energy efficiency. For example, the discharge exposure of fuel (energy obtainable from fuel in a unit weight) in the C-lattice core can be larger several percentage than that in the D-lattice core. In this way, the C-lattice core is superior to the D-lattice core in terms of fuel economy.
However, since there have been a number of functioning D-lattice cores, attempts have been made to improve these D-lattice cores for enhancing the fuel economies thereof. One of such prior arts has been disclosed in Japanese Patent No. 2791132. The prior art provides a fuel assembly for a D-lattice core including fuel rods placed in a square lattice array of 9×9 (9-rows/9-columns), in which the fuel rod pitch is reduced, and a distance between the outermost fuel rod and a channel box on the control rode side is made smaller than a distance between the outermost fuel rod and a channel box on the anti-control rode side, whereby a difference in gap between fuel assemblies on the control rod side and the anti-control rod side is made small. With this configuration, it is possible to make the core characteristic of such a D-lattice core close to that of a C-lattice core while adopting the same fuel rods and control rod drive mechanism as those having been used in the conventional D-lattice core.
The above-described prior art D-lattice core, however, has the following problem. Since the fuel rod pitch becomes small, cooling water less flows between the fuel rods, to reduce the heat removal performance by cooling water. As a result, it is difficult to ensure the thermal margin at the same linear heat generation rate. To be more specific, the thermal margin of the fuel assemblies in the D-lattice core according to the above-described prior art is made smaller than that of the fuel assemblies in the conventional D-lattice core.
Another problem of the above-described prior art D-lattice core is as follows: namely, in general, fuel rods and water rods placed in a square lattice array are bundled with fuel spacers at a plurality of axial positions and the fuel spacers each have holding members (for example, cylindrical members) for holding the fuel rods and water rods such that they are spaced from each other at specific gaps, and therefore, if the fuel rod pitch is changed, the pitch of the holding members must be correspondingly changed. In other words, according to the prior art D-lattice core, the existing fuel spacers cannot be used and new fuel spacers must be used.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a fuel assembly for a D-lattice core, which is capable of achieving the fuel economy comparable to that of a C-lattice core without reducing the thermal margin, and of using the existing fuel spacers.
(1) The present invention provides a fuel assembly including a plurality of fuel rods placed in a square lattice array of 9-rows/9-columns and at least one water rod, wherein the fuel rod pitch is in a range of 14.15 mm to 14.65 mm; and means for offsetting and holding a fuel bundle composed of the fuel rods and the water rod is provided in such a manner that the center in a cross section (cross section
Aizawa Yasuhiro
Aoyama Motoo
Chaki Masao
Haikawa Katsumasa
Koyama Junichi
Carone Michael J.
Hitachi , Ltd.
Keith Jack
Mattingly Stanger & Malur, P.C.
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