Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Patent
1997-09-25
1999-01-12
Jordan, Charles
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
376412, 376371, G21C 317, G21C 332
Patent
active
058598863
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a fuel assembly for a boiling water reactor comprising a bottom tie plate, a top tie plate, a first and a second group of vertical fuel rods, each comprising a column of fuel pellets surrounded by a cladding tube, wherein the second group of fuel rods has a shorter active length than the first group of fuel rods. The active length of a fuel rod means the height of the column of fuel pellets.
BACKGROUND OF THE INVENTION
A core in a nuclear reactor comprises a plurality of vertically arranged fuel assemblies. A fuel assembly comprises one or more fuel bundles. A fuel bundle comprises a plurality of vertical fuel rods arranged between a bottom tie plate and a top tie plate. The fuel rods contain a column of circularly cylindrical pellets of a nuclear fuel arranged in a cladding tube. At the bottom of the fuel rods, a bottom plug is arranged for insertion into holes provided therefor in the bottom tie plate, and at the top of the fuel rods, a top plug is arranged for insertion into holes provided therefor in the top tie plate. A small number of so-called supporting fuel rods retain the fuel bundle and are fixed to the bottom tie plate and the top tie plate. The fuel bundles or the fuel bundle are/is surrounded by a fuel channel which is normally designed with a square cross section.
During the burnup of the nuclear fuel, fission gases, which are contained within the fuel rod, are released. To prevent the pressure on the cladding from becoming too large, an expansion space is needed for the fission gases, a so-called fission gas plenum. The fission gas plenum should correspond to 5-10% of the volume of the fuel. For a fuel rod whose diameter is substantially constant and whose active length is 4 m, this means that the fission gas plenum should be 0.2-0.4 m. In full-length fuel rods, the fission gas plenum is arranged in the upper part thereof, above the reactor core, and hence exerts a minimum influence on the efficiency of the reactor. The active length of a fuel rod is the length of that part of the fuel rod which contains fuel pellets and does not comprise a fission gas plenum.
The core is immersed into water which serves both as coolant and as neutron moderator. During operation, part of the water changes into steam. At the lower part of the fuel assembly, there is only water, but the higher up in the fuel assembly, the higher the percentage of steam. When the percentage of steam rises, the neutron moderation deteriorates since steam is inferior to water as moderator. The reactivity of the reactor depends on the ratio between fuel and moderator. To improve the reactivity, the water/steam ratio in the upper part of the fuel assembly must be increased.
One way of obtaining a higher water/fuel ratio in the upper part of the fuel assembly is to replace the majority of the fuel rods with fuel rods which are tapering in an axial direction. The General Electric patent application EP-0 514 121 A1 discloses a fuel rod which comprises a lower column of fuel pellets with a relatively larger diameter, and an upper column of fuel pellets with a relatively smaller diameter and a cladding tube, which comprises a lower coarser part which surrounds the lower column of fuel pellets, an upper narrower part which surrounds the upper column of fuel pellets, and a transition portion between the lower and upper parts. The fuel rod described in the above EP publication has two fission gas plenums, one in the upper part of the fuel rod and one in the lower part of the fuel rod.
To obtain an optimum fuel-to-moderator ratio with tapering fuel rods, all or at least the majority of the fuel rods should consist of tapering fuel rods. The disadvantage of tapering fuel rods is that it is expensive to manufacture fuel pellets with two different diameters and tapering cladding tubes compared with ordinary fuel rods of uniform thickness where all the fuel pellets have the same diameter and the cladding tube is straight. Since the majority of the fuel rods consist of tapering fuel rods, the
ABB Atom AB
Jordan Charles
Lattig M. J.
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