Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Encased with nonfuel component
Patent
1997-06-23
1998-12-22
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Encased with nonfuel component
376435, G21C 3326
Patent
active
058526455
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to fuel assemblies for pressurized water nuclear reactors, and more particular by assemblies of the type comprising fuel rods distributed at the nodes of a square array, with at least most of the rods containing fuel material which, in most of the rods, is uranium oxide enriched in isotope 235 and/or in plutonium; the rods are generally carried by a skeleton having two endpieces interconnected by guide tubes and by rod-holding grids.
BACKGROUND OF THE INVENTION
Assemblies of this type are already known in which some (generally less than one in ten) of the fuel rods contain a neutron poison constituted by gadolinium at a mass concentration in the range of 5% to 12%. The gadolinium performs two functions. Because it absorbs neutrons, it serves to reduce the initial reactivity of the core after partial or total reloading with new fuel assemblies. The progressive disappearance of the gadolinium serves to compensate for the progressive using up of the fuel. By an appropriate distribution of fuel assemblies, containing gadolinium in the core, it is possible to achieve a more uniform radial distribution of power, and to do this throughout an operating cycle of the core prior to reloading.
The use of such assemblies in which rods that contain gadolinium all have the same concentration, does indeed have a favorable effect from the point of view of reducing reactivity at the beginning of each cycle, with a corresponding reduction in the concentration of boron that is required in the water, and with good radial neutron flux distribution. However, it is not possible to achieve satisfactory core management when it is designed to extend the duration of each core utilization cycle to periods of 18 months to 24 months, and/or when it is necessary to limit radial neutron leakage for better utilization of the fuel.
SUMMARY OF THE INVENTION
The present invention seeks in particular to provide a fuel assembly that makes more favorable core management possible, in which initial reactivity is high.
The invention makes use of the observation that the initial anti-reactivity (i.e., in a new rod) provided by the gadolinium present in the rod is not proportional to the concentration thereof, but increases much more slowly once that concentration exceeds about 1%.
Consequently, the invention proposes an assembly of the above-defined type in which the rods comprise, in addition to rods having a mass concentration of gadolinium lying in the range about 5% to about 10%, four to twelve rods having a gadolinium concentration in the range of 0.5% to 2%, and advantageously about 1%.
The anti-reactivity of the rods having low gadolinium concentration decreases much more quickly, starting from the beginning of the reactor cycle, than does the reactivity of rods having a considerably greater concentration. Thus, the rods having a low gadolinium content compensate for the initial high excess reactivity and provide extra margin on the moderator coefficient without thereby having an unfavorable effect on radial power distribution within a cycle.
It may be considered that the invention makes it possible to act independently on the two functions of the gadolinium: reducing initial reactivity, and controlling radial flux distribution. With a single concentration, the first function can be provided in a long cycle only with a high number of rods having 10% to 12% gadolinium, and that would deteriorate control of power distribution, in particular towards the end of a cycle as the gadolinium progressively disappears.
BRIEF DESCRIPTION OF THE DRAWINGS
The above characteristics, and others, appear more clearly on reading the following description of particular embodiments of the invention, given by way of example and with reference to the accompanying drawings, in which:
FIG. 1 is a schematic plan view showing the distribution of fuel rods in a first assembly of the invention;
FIG. 2 shows the variation of the anti-reactivity of a rod as a function of its Gd content, in two assemblies respectively c
REFERENCES:
patent: 4587090 (1986-05-01), Mochida et al.
patent: 4649020 (1987-03-01), Dehon et al.
Kerntechnik, vol. 52, No. 4, Aug. '88, Munchen DE, pp. 245-249, XP002000895 Francillon et al.; "Fuel management at Fragema: experience, assents, perspectives", See page 248.
Bonniaud Michel
Romary Jean-Michel
Compagnie General des Matieres Nucleaires
Framatome
Wasil Daniel D.
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