Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Encased with nonfuel component
Reexamination Certificate
2000-10-13
2002-09-03
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Encased with nonfuel component
C376S435000, C376S428000
Reexamination Certificate
active
06445759
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fuel assembly having fuel rods containing gadolinium as a burnable poison, and nuclear reactor. In particular, the present invention relates to a fuel assembly and nuclear reactor in which, without damaging thermal margin of the fuel rods, fuel economy is improved.
2. Description of Related Art
In a light-water type power reactor in which uranium or uranium dioxide is used as a nuclear fuel material, a fuel assembly is set at a necessary initial uranium enrichment, with the progress of burnup an excess reactivity of the reactor being decreased. From a viewpoint of safety, for the excess reactivity not to be too large, a material called a burnable poison of which negative reactivity decreases with the burnup is added to the fuel. Mainly in a boiling water reactor (BWR) and sometimes in a pressurized water reactor (PWR), gadolinium of atomic number 64 is used as the burnable poison, the gadolinium being added in the form of gadolinia that is an oxide of gadolinium to the nuclear fuel material.
A sectional structure of an existing fuel assembly is shown in FIG.
19
.
In the fuel assembly
21
, fuel rods
22
and
23
in which the nuclear fuel material such as uranium or the like is enclosed are arranged in a square grid pattern, for instance. A water rod
24
through the inside of which non-boiling cooling water flows during output operation is disposed in the neighborhood of the center. Furthermore, an entirety thereof is accommodated in a channel box
25
. In the figure, reference numeral
22
denotes a fuel rod that does not contain gadolinia, reference numeral
23
denoting a fuel rod (hereinafter referred to as gadolinia fuel rod) that contains gadolinia, respectively.
Usually, for four sets of fuel assemblies
21
each, one set of control rod
26
is disposed to constitute a reactor core. Inside the control rod
26
, a plurality of poison rods
27
in which a neutron absorber is enclosed are regularly arranged to be accommodated. A length, a number of pieces, a shape and an arrangement of the fuel rods
22
and
23
, and an existence, a number of pieces, a shape, an arrangement or the like of the water rod in the fuel assembly are different according to nuclear reactors. The structure of an existing fuel assembly is not restricted to the fuel assembly shown in FIG.
19
.
FIG. 20
is a sectional view in a vertical direction (axial direction) showing a structure of a fuel rod. In the fuel rods
22
and
23
, at the uppermost portion of a body in which fuel pellets
28
are regularly piled up, a plenum spring
29
is disposed to suppress a displacement of the pellets, the entirety being accommodated in a shield tube
30
. Lower and upper end plugs
31
and
32
seal lower and upper portions of the shield tube
30
respectively, inside of the shield tube
30
helium gas being enclosed under an appropriate pressure.
For the fuel rods, according to the position inside the fuel assembly, enrichment of fissile material and concentration (content) of gadolinia are determined.
FIG. 21A
is a transversal sectional view showing an arrangement of the fuel rods when the control rod
26
is disposed in the upper left position. Reference marks
1
,
2
,
3
,
4
and
5
denote the fuel rods
22
that do not contain gadolinia, G
1
, G
2
and G
3
denoting gadolinia fuel rods
23
, respectively. WR denotes the water rod
24
.
FIG. 21B
is a diagram showing, in a vertical direction (axial direction) of the fuel rods (
1
,
2
,
3
,
4
,
5
, G
1
, G
2
and G
3
), enrichment distributions of the fissile material (uranium for instance) and concentration distributions of gadolinia. In the figure, there are used the enrichments of uranium of A to G, and the gadolinia concentrations (additional ratio) of a to c. The gadolinia fuel rods
23
contain uniformly in the axial direction uranium of the enrichment of, for instance, B (the enrichment C is also satisfactory). Only the gadolinia concentrations are shown in FIG.
21
B.
The uranium enrichments decrease in the order of A>B>C>D>E>F>G, G being the enrichment of natural uranium. In the fuel rod
22
that does contain no gadolinia, there are disposed at the upper and lower ends the portions (natural uranium blankets) that contain only the natural uranium. There may be cases where the uranium enrichments have a difference in the axial direction.
The gadolinia fuel rods
23
, though usually disposed at the positions other than that of the outermost periphery of the fuel assembly, may be disposed at the positions of the outermost periphery. The gadolinia concentrations are in the decreasing order of a>b>c, a difference being given in some cases in the axial direction. Furthermore, whereas there are cases where the natural uranium blankets are disposed at the upper and lower ends, without the natural uranium blanket there may be disposed gas reservoirs that are called plenum space at the upper and lower ends.
Another existing arrangement of the fuel rods in the fuel assembly
21
is shown in FIG.
22
.
FIG. 22A
is a transversal sectional view showing an arrangement of the fuel rods when the control rod
26
is located in the upper left position. Reference marks
1
,
2
,
3
,
4
,
5
,
6
and V
1
and V
2
denote the fuel rods
22
that contain no gadolinia, G
1
, G
2
and G
3
denoting the gadolinia fuel rod
23
. The fuel rods designated by reference marks
1
,
2
,
3
,
4
,
5
,
6
, and G
1
, G
2
and G
3
are long-length fuel rods, the fuel rods designated by the reference marks V
1
and V
2
being short-length fuel rods shorter than the long-length fuel rods in fuel effective portion. WR denotes the water rod
24
.
FIG. 22B
is a diagram showing, in the vertical direction (axial direction) of the fuel rods (
1
,
2
,
3
,
4
,
5
,
6
, V
1
, V
2
, G
1
, G
2
and G
3
), enrichment distributions of the fissile material (uranium for instance) and concentration distributions of gadolinia.
In the figure, there are used the uranium enrichments of from A to G and the gadolinia concentrations of from a to c. The uranium enrichments are in the decreasing order of A>B>C>D>E>F>G, G being the enrichment of the natural uranium. In the long-length fuel rods, there may be disposed the portions that contain only the natural uranium at the upper and lower ends. Further, in the axial direction, the uranium enrichments may be differentiated. Though the gadolinia fuel rods
23
are usually disposed in the positions other than the outermost periphery of the fuel assembly, those may be disposed in the positions of the outermost periphery. The gadolinia concentrations are in the decreasing order of a>b>c, the concentrations being differentiated in the axial direction in some cases.
The fuel assembly shown in
FIGS. 22A and 22B
is designed to be suitable for the following fuel grid pattern (hereinafter referred as D lattice). That is, in the D lattice, a width of the non-boiling water region between outer walls of the channel boxes of the adjacent fuel assemblies is configured to be larger in a control rod insertion side than in the opposite side (non-insertion side). The fuel assembly is loaded with a different spacing from adjacent fuel assemblies in the reactor core comprising D lattice.
In the D lattice, the control rod insertion side, being larger in thermal neutron flux distribution in comparison with the non-insertion side, is likely to be high in local power. Accordingly, when a transversal cross-section of the fuel assembly is divided into two regions of a control rod side and an opposite-control rod side, in the fuel rods disposed in the region of the control rod side, in comparison with the fuel rods disposed in the region of the opposite-control rod side, the enrichment of the fissile material (uranium) is lowered. For instance, in the fuel rods
22
containing no gadolinia that are disposed at the outermost periphery on the opposite-control rod side, the uranium enrichment is set at the highest value of A. Whereas, t
Hayashi Yamato
Hida Kazuki
Hiraiwa Kouji
Hirano Yasushi
Saeki Jun
Behrend Harvey E.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Palabrica R.
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