Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Patent
1999-02-04
2000-11-21
Jordan, Charles T.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
376443, 376451, 376453, G21C 318, G21C 310, G21C 308
Patent
active
061513768
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a nuclear fuel assembly with a substantially square cross section for a light water reactor comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate.
BACKGROUND OF THE INVENTION
In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and to prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each extending along the main part of the length of the fuel assembly.
Together with a plurality of other fuel assemblies, the fuel assembly is arranged in a core. The core is immersed into water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby, in a boiling water light-water reactor, part of the water is transformed into steam. The percentage of steam increases towards the top of the fuel assembly. Consequently, the coolant in the lower part of the fuel assembly consists of water whereas the coolant in the upper part of the fuel assembly consists both of steam and of water. This difference between the upper and lower parts gives rise to special problems which must be taken into consideration when designing the fuel assembly.
This problem can be solved by providing a flexible fuel assembly which in a simple manner may be given a shape where the upper part of the fuel assembly differs from the lower part so that optimum conditions may be obtained. A fuel assembly for a boiling water reactor with these properties is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may in a simple manner be given different designs in its upper and lower parts.
Also in a light-water reactor of pressurized-water type, it may be desirable to design the fuel assemblies so that each fuel assembly comprises a plurality of fuel units stacked on top of each other. As described above, each of the fuel units then comprises a plurality of fuel rods extending between a top nozzle and a bottom nozzle. A fuel assembly for a pressurized-water reactor, however, comprises no fuel channel.
One factor which must be taken into consideration when designing fuel units which are of the order of magnitude of 300-1500 millimeters long is that fission gases are formed during nuclear fission. In addition, the column of fuel pellets expands because of the heat formed in the fuel pellets. To take care of the fission gases and the thermal expansion of the column of fuel pellets, normally a relatively large space, an axial gap, is formed above the uppermost fuel pellet in the cladding tube in known full-length fuel rods, that is, fuel rods of the order of size of 4 metres long. The axial gap is of the order of size of 200-300 millimeters long. To this axial gap, the fission gases may thus diffuse and the column of fuel pellets may expand inwardly here. Thus, the axial gap contains no fissionable material.
Another factor which must be taken into consideration when designing axial gaps is that the temperature of the cladding tube in this region is lower than in the rest of the cladding tube because no fuel pellet is arran
REFERENCES:
patent: 3669833 (1972-06-01), De Boeck et al.
patent: 3671393 (1972-06-01), Williams
patent: 4678924 (1987-07-01), Loriot et al.
patent: 4914679 (1990-04-01), Tomiyama et al.
Nylund Olov
Schrire David
ABB AB Atom AB
Jordan Charles T.
Mun K. Kevin
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