JPH0236918B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Purpose of the Invention [Field of Industrial Application] This invention relates to a neutron shield for a nuclear power plant, particularly a liquid metal cooled fast breeder reactor.

A fast breeder reactor has a large number of fuel assemblies arranged in the center of the reactor core, and a neutron shield arranged around the radial outer periphery of the fuel assemblies. This neutron shield reflects neutrons from the fuel assembly, prevents their leakage, and
It has the function of shielding fast neutrons and gamma rays to protect the core external structure. As a core component, the neutron shield is required to remain sound throughout its lifetime.

[Prior Art] As shown in FIG. 12, a conventional neutron shield 101 includes a cylindrical rod-shaped inner shield 105 inside an outer cylinder 104 made of a hexagonal tube provided between a handling head 102 and an entrance nozzle 103. are stacked and packed, and coolant passages 106 and 107 are provided inside and outside the internal shielding body 105.

The neutron shield 101 having such a configuration is placed outside the central fuel assembly group in the core of a nuclear reactor plant. The neutron density decreases as you move toward the core, and therefore the temperature distribution is highest at the center and decreases toward the periphery.When the neutron shield 101 is loaded at such a core position, Among the circumferential surfaces of the neutron shield 101, a temperature difference occurs between the circumferential surface facing toward the center of the core and the circumferential surface on the opposite side, and the circumferential surface on the core center side expands greatly due to thermal expansion. deformation (warping) occurs in the outward direction of the

[Problems to be Solved by the Invention] However, in such a neutron shield 101, the inner shield 105 is long, and the inner shield 105 is separated from the adjacent inner shield 105 by the unevenness of the cylindrical surface at the end. It has a structure that is strongly connected and has a small allowable relative displacement, and this structure has high rigidity.
In addition, swelling and gamma heat generation under high irradiation were large, and there was a concern that this would adversely affect the core curvature and the entrance nozzle 103 and the intermediate pad portion 108 of the outer cylinder 104 during core group vibration.

Further, when replacing the neutron shield 101, there is a possibility that the neutron shield 101 may not come out of the reactor core due to its deformation, which may also have an adverse effect on handling equipment.

This invention has been made in view of the above circumstances, and is capable of reducing the stress generated in the entrance nozzle during core curvature, and also capable of reducing γ heat generation of the internal shield, and
It is possible to alleviate the stress in the intermediate pad part due to core group vibration, further reduce the occurrence of swelling, improve the structural integrity even when there is a temperature difference, and improve the handling performance of the neutron shield. It is an object of the present invention to provide a neutron shield that can be improved.

(b) Structure of the Invention [Means for Solving the Problems] Corresponding to this object, the neutron shield for a fast breeder reactor of the present invention has a structure in which the neutron shielding body for a fast breeder reactor of the present invention has an outer shell and an outer cylinder of the outer shell. A neutron shield for a fast breeder reactor, comprising a plurality of internal shields stacked and packed in series, each of the internal shields having a short column shape, and having a coolant flowing in the axial direction. It is provided with a through hole for passing the coolant through, and a groove for passing the coolant through the upper end surface or the lower end surface, and the contact surface with another adjacent internal shield is composed of one or more curved surfaces or a combination of curved surfaces and flat surfaces. The inner shielding body located corresponding to the pad portion of the outer cylinder is characterized in that the outer diameter thereof is larger than that of the other internal shielding bodies.

Hereinafter, details of the present invention will be explained with reference to the drawings showing one embodiment.

In FIGS. 1 and 2, 1 is a neutron shield, and the neutron shield 1 has an outer shell 2. In FIG. The outer shell 2 consists of a hexagonal outer cylinder 3, an entrance nozzle 4 fixed to the lower end of the outer cylinder 3, and a handling head 5 fixed to the upper end of the outer cylinder 3.

An intermediate pad portion 6 is provided at the center of the outer cylinder 3.

Inside the outer cylinder 3, internal shielding bodies 7 are stacked and packed. The internal shield 7 includes five types of internal shields 7a,
7b, 7c, 7d and 7e, all of the internal shields 7a, 7b, 7c, 7d and 7e have a short columnar shape, are provided with holes through which the coolant passes in the axial direction, and have holes on the outer surface. It also has grooves for passing coolant,
In addition, the contact surface with another adjacent internal shield is composed of a curved surface or a plurality of planes formed along the curved surface.

That is, the inner shield 7a located at the lowest end is the third
As shown in the drawings and FIG. 4, it has a short cylindrical shape and is provided with a cylindrical protrusion 8 on the lower surface. The protrusion 8 fits into the upper end of the entrance nozzle 4. Upper end surface 9
is dish-shaped and consists of a flat surface 11 at the center and a tapered surface 12 at the periphery. When this upper end surface comes into contact with the adjacent internal shielding body 7b, the internal shielding body 7a
This is a surface for allowing relative rotational displacement between the inner shielding body 7b and the inner shielding body 7b around the trigonal axes, and is preferably a curved surface, but in this embodiment, a flat surface formed along such a curved surface It is shaped like a plate.

A through hole 1 is provided in the center of the internal shield 7a in the axial direction.
0 is formed, and grooves 20 are formed radially on the upper end surface 9. Through hole 10 and groove 20
are for passing coolant.

As shown in FIGS. 5 and 6, the internal shielding body 7b has a short cylindrical shape, and has an upper end surface 13 and a lower end surface 14.
is composed of a central flat surface 15 and a peripheral tapered surface 16, both of which are convex. A through hole 10 is formed in the central part of the internal shielding body 7b in the axial direction, and grooves 20 are formed radially in the upper end surface 13.

As shown in FIGS. 7 and 8, the internal shielding body 7c has a short cylindrical shape, and has an upper end surface 17 and a lower end surface 18.
Both are plate-shaped and consist of a central flat surface 21 and a peripheral tapered surface 22. A through hole 10 is formed in the central part of the internal shield 7c in the axial direction, and grooves 20 are formed radially in the upper end surface 17 and the lower end surface 18.

As shown in FIGS. 9 and 10, the internal shielding body 7d has a short cylindrical shape, the upper end surface 23 is a flat surface, and the lower end surface 25 is convex with a flat surface 26 at the center and a tapered surface 27 at the periphery. It consists of. A through hole 10 is formed in the central part of the internal shielding body 7d in the axial direction, and a groove 20 is also formed in the axial direction in the side surface.

As shown in FIG. 11, the internal shielding body 7e is filled in a position corresponding to the intermediate pad 6 of the outer cylinder 3, and its structure is the same as that of the internal shielding body 7c, except that the internal shielding body 7e is the internal shield 7
The diameter is larger than c.

[Function] The neutron shield 1 configured as described above is arranged around the fuel assembly group of the nuclear reactor core. The neutron shield 1 receives neutron irradiation from the core core and generates heat, and among the side surfaces of the neutron shield 1, the core-side side and the outer side are in different temperature states, and as a result, each internal shield 7a 7e are curved, but the internal shields 7a to 7e are short, so 1
The amount of deformation per inner shield remains extremely small. Further, each internal shielding body contacts the adjacent internal shielding body at a curved surface or a group of planes similar to the curved surface, and can be rotated relative to each other, so that the rigidity of the neutron shielding body is kept small. The coolant that has flowed into the neutron shield 1 from the entrance nozzle 4 flows out from the handling head 5, but in the meantime flows through the through holes 10 and grooves 20 to cool the respective internal shields 7.

(c) Effect of the invention As described above, in the neutron shield of the present invention, the internal shield is not a long one, but is divided into short pieces, and the fitting portion of each adjacent internal shield is divided into short pieces. The rigidity of the internal shield can be reduced by combining and laminating a concavo-convex structure having a surface consisting of a spherical surface or other curved surface, or a combination of other curved surfaces, flat surfaces, or tapered surfaces that form a pseudo surface of such a curved surface. In addition, due to the contact between the spherical surface and the tapered surface, each internal shield can move freely without interfering with its function, and as a result, the neutron shield does not bend easily, reducing the bending stress that acts on the neutron shield when the core curves. By bending and absorbing the neutron shield itself, it is possible to reduce the stress on the entrance nozzle due to core curvature, and avoid damage due to bending stress of the neutron shield due to bending due to swelling, temperature differences, etc. It is possible to effectively improve the structural integrity. Moreover, by providing the internal shielding body loaded at the level of the outer cylinder intermediate pad of the split type internal shielding body, an internal shielding body with an outer diameter (slightly larger diameter) that is different from that of other internal shielding bodies.
The gap between the outer cylinder middle pad and the inner shield becomes smaller, and the lateral external force acting on the neutron shield is effectively propagated to the adjacent neutron shield through the inner shield at the outer cylinder middle pad. As a result, it is possible to alleviate stress in the intermediate pad portion against core group vibration.

Furthermore, by providing a plurality of grooves forming a coolant flow path in one part or all of the laminated fitting parts of the divided internal shield, the temperature rise due to γ heat generation of the internal shield can be reduced. At the same time,
The radial temperature gradient (ΔT) of a single internal shield can be reduced. Furthermore, since the internal shield of the present invention has great flexibility, even if the neutron shield is deflected, the deflection will be absorbed by the middle part of the neutron shield, and the top of the neutron shield will be absorbed by the middle part of the neutron shield. The upper surface of the handling head is no longer tilted, or the upper surface of the handling head is not flush with the upper surface of another handling head adjacent to it, and there is no difference in level between the upper surface of the adjacent handling head.
There is no difficulty in grasping the handling head, and due to the large flexibility of the neutron shield, there is no catch when pulling out the neutron shield, which makes it easier to handle the fuel during in-reactor handling of the neutron shield. The handling performance of in-furnace relay equipment, fuel inlet/output equipment, cleaning equipment, ex-furnace fuel storage equipment, etc. when handling replacement equipment or outside the reactor (on-site) will be greatly improved.

[Brief explanation of drawings]

FIG. 1 is an explanatory longitudinal cross-sectional view showing a neutron shield according to an embodiment of the present invention, FIG. 2 is a sectional view taken along the line A-A in FIG. 1, and FIG. Figure 4 is a sectional view of the − part in Figure 3;
5 is a plan view of the internal shield 7b, and FIG. 6 is a plan view of the internal shield 7b.
7 is a plan view of the inner shield 7c, FIG. 8 is a sectional view of the inner shield 7d, and FIG. 9 is a plan view of the inner shield 7d.
Figure 0 is a cross-sectional view of the - part in Figure 9, Figure 11 is an enlarged partial cross-sectional view of the - part in Figure 2, and 1.
FIG. 2 is a longitudinal sectional view showing a conventional neutron shield. 1... Neutron shield, 2... Outer shell, 3... Outer tube, 6...
Intermediate pad, 7...Inner shield, 9, 13, 17,
23...Top end surface, 10...Through hole, 11, 15, 2
1, 26...Plane, 14, 18, 25...Lower end surface, 2
0...Groove.

Claims (1)

[Claims] 1. A neutron shield for a fast breeder reactor, comprising an outer shell and a plurality of internal shields stacked and packed in series in the axial direction within the outer cylinder of the outer shell,
Each of the internal shields has a short columnar shape, has a through hole through which the coolant passes in the axial direction, and has a groove through which the coolant passes through the upper end surface or the lower end surface, and is connected to other adjacent internal shields. The contact surface with the outer cylinder is composed of one or more curved surfaces or a combination of curved surfaces and flat surfaces, and the inner shield located corresponding to the pad part of the outer cylinder has an outer diameter smaller than that of the other inner shields. A neutron shield for fast breeder reactors that is characterized by its large size.