Induced nuclear reactions: processes – systems – and elements – Control component for a fission reactor – Wherein concentration of the reactivity affecting material...
Reexamination Certificate
2001-03-12
2003-10-21
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Control component for a fission reactor
Wherein concentration of the reactivity affecting material...
C376S327000
Reexamination Certificate
active
06636580
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a nuclear reactor and, more particularly, to a control rod used in a pressurized water reactor.
2. Related Art
For better understanding of the present invention, background techniques thereof will first be described in some detail. A representative example of a fuel assembly employed in a pressurized water reactor is shown in
FIG. 3
of the accompanying drawings. As is shown in the figure and also well known in the art, a fuel assembly
31
includes a plurality of control rod guide tubes
34
fixedly held at both ends by an upper nozzle
32
and a lower nozzle
33
, respectively, a plurality of fuel rods
35
, and a plurality of supporting lattices
36
through which the control rod guide tubes
34
and the fuel rods
35
are inserted, wherein individual control rods
42
constituting, for example, a control rod cluster
41
, as shown typically in
FIG. 4
, are inserted into the control rod guide tubes
34
from the above or withdrawn therefrom for the purpose of adjusting or regulating the reactor power. As is well known in the art, the number and the disposition of the control rod guide tubes
34
and the fuel rods
35
differ in dependence on the type or species the fuel assembly
31
.
Referring to
FIG. 5
, the control rod cluster
41
is composed of a spider
43
operatively coupled to a driving shaft of a control rod driving unit (not shown) and a plurality of vanes
44
mounted radially on the outer peripheral surface of the spider
43
, wherein the control rods
42
are held vertically in the upright state by means of these fingers
45
, respectively. Disposition of the fingers
45
and hence that of the control rods
42
corresponds to the disposition of the control rod guide tubes
34
in the fuel assembly
31
. As can be seen in
FIG. 6
, each of the control rods
42
includes a cladding tube
51
formed of stainless steel and hermetically closed at both ends thereof by a top end plug
52
connected to the finger
45
as mentioned above and a bottom end plug
53
. Accommodated within the cladding tube
51
is a rod-like neutron absorber
54
which is formed of a neutron absorbing material such as an Ag—In—Cd (silver-indium-cadmium) alloy or boron carbide or the like and which is pressed downwardly against the bottom end plug
53
by a hold-down spring
55
disposed within the cladding tube
51
at a top end portion thereof.
At this juncture, it is to be mentioned that when the control rod
42
of the structure described above is inserted into the control rod guide tube
34
of the fuel assembly
31
loaded in the reactor, the neutron absorber
54
disposed within the control rod cladding tube expands in the axial direction as well as in the radial direction under irradiation of neutrons. Furthermore, there is great likelihood that the soundness or integrity of the cladding tube
51
is impaired due to the irradiation. In the cladding tube
51
itself, the neutron irradiation does increase gradually from the bottom end thereof toward the top end. On the other hand, with regard to the neutron absorber
54
, it is noted that a lower end portion
54
a
thereof among others undergoes noticeable expansion in both the axial and radial directions, as mentioned above. In that case, expansion of the neutron absorber
54
in the axial direction can be absorbed by contraction of the hold-down spring
55
. Accordingly, the integrity of the cladding tube
51
can be protected against impairment due to the expansion of the neutron absorber in the axial direction. By contrast, expansion of the neutron absorber
54
in the radial direction can not be absorbed by the hold-down spring
55
. For this reason, such arrangement has heretofore been adopted to allow the expansion of the neutron absorber in the radial direction so that the diameter d, of the lower portion
54
a
of the neutron absorber
54
is reduced over a length L in the axial direction as compared with a diameter d, of the other ordinary portion of the neutron absorber.
Now referring to
FIGS. 7 and 8
, description will be made in detail of a method of determining magnitude of the diameter reduction (d
0
−d
1
) in the lower portion
54
a
of the neutron absorber in the radial direction and the axial length L thereof (i.e., length of the reduced-diameter portion in the axial direction). First referring to
FIG. 7
which is an exaggerated section of a control rod, clearance between the neutron absorber and the cladding tube is enlarged at the lower portion
54
a
of the neutron absorber due to the reduction of diameter when compared with the clearance at the upper portion of the neutron absorber. Consequently, the cladding tube
51
is more likely to undergo deformation due to irradiation-induced creep at the location corresponding to the reduced-diameter portion of the neutron absorber, and initially the section form of the cladding tube becomes flattened. Magnitude of such deformation or strain of the cladding tube has to be suppressed enough to fall within a range of elastic deformation in order to ensure insertability of the control rod into the control rod guide tube. In other words, the strain of the control rod guide tube must be held so as not to exceed a strain equivalent to the yield strength or yield capability of the material forming the cladding tube. Further, any decrease in volume of the neutron absorber as a whole due to the reduction of diameter must essentially exert no influence on the neutron absorbing capability. Under the circumstances, the diameter reduction is determined by taking into account the requirements mentioned above.
On the other hand, as the neutron irradiation of the control rod progresses, the neutron absorber expands gradually not only in the axial direction but also in the radial direction to be ultimately brought into contact with the cladding tube, whereby an internal pressure is applied to the cladding tube consequently, the diameter of the cladding tube increases, bringing about strain in the circumferential direction. Thus, the length of the reduced-diameter portion or the lower portion
54
a
of the neutron absorber is so determined that in the state where the cladding tube has undergone enough neutron irradiation to expand, the strain induced in the circumferential direction in the cladding tube portion which corresponds to the lower end position of the reduced-diameter portion is substantially equivalent to the strain induced in the cladding tube portion corresponding to the upper end position of the reduced-diameter portion (which may be considered as corresponding to the lower end position of the ordinary diameter portion of the neutron absorber). More specifically, since the neutron irradiation dose has a distribution profile such that the dose attenuates along the longitudinal axis of the control rod in the upward direction, the length of the reduced-diameter portion is determined so that difference in the expansion due to difference in the neutron irradiation dose between the lower end portion of the neutron absorber having the ordinary diameter and the diameter-reduced lower end portion of the neutron absorber is equivalent to the diameter reduction. In this conjunction,
FIG. 8
illustrates graphically a relation between locations or positions of a cladding tube along the longitudinal axis thereof as viewed from the bottom end of the neutron absorber and strains induced in the cladding tube in the circumferential direction.
As is well known in the art, various types of control rods are available. In a typical conventional control rod, the length L, the diameters d
0
and d
1
mentioned previously have heretofore been selected, for example, such that d
0
≈8.7 mm, d
1
≈d
0
−0.13 mm and that L≈300 mm.
However, because the control rod cluster
41
constituted by an assembly of the control rods
42
is driven stepwise by the control rod driving unit, shock produced upon stepwise driving of the control rod cluster
41
acts repetitionally on the reduced-diam
Murakami Kazuo
Suzuki Shigemitsu
Yoshigai Kyohichi
Carone Michael J.
Gottlieb, Rackman and Reisman, P.C.
Keith Jack
Mitsubishi Heavy Industries Ltd.
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