JPH02195295A - Atomic fuel assembly - Google Patents

Abstract

PURPOSE:To secure mechanical soundness by a construction wherein a plate and a nozzle frame are formed separately from each other and the plate is made movable only in the upward direction in relation to the nozzle frame, while a hold-down spring is incorporated between the plate and the nozzle frame. CONSTITUTION:A plate 11 has water flow holes 14 joining the upper part of a control rod guide tube 2, and although it is similar to a conventional nozzle plate in respect that it provides a flow channel for a coolant, it is formed as a body separate from a nozzle plate 12 and, accordingly, has a structure of making it movable only in the upward direction in relation to an upper nozzle frame 12. The upper nozzle frame 12 has engaging elements 12a, and springs 13 are engaged with spring fasteners on the lower side and with said engaging elements 12a on the upper side and thus incorporated in a state wherein they alternate on the two sides of an upper nozzle being opposite to each other. By making the nozzle plate movable in relation to the nozzle frame in this way, a change in elongation of a fuel assembly can be absorbed to a large extent.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a nuclear fuel assembly, particularly a fuel assembly for a pressurized wooden nuclear reactor. The present invention relates to the above-mentioned fuel assembly that ensures the mechanical integrity of the fuel assembly.

(Prior Art) A conventional fuel assembly for a pressurized water reactor has a large number of fuel rods (1) arranged in parallel as shown in Fig. 4, and a control rod guide pipe (2).
The fuel support part is supported by a plurality of support grids (3), and the upper and lower nozzles (41, (5)
), and a hold-down spring (6) is installed in the upper nozzle (4) to prevent the assembly from floating up due to fluid force during reactor operation against the upper core plate. attached on all four sides.

By the way, recently, research has been carried out to improve fuels at the expense of improving the economic efficiency of nuclear power generation, and high burnup reactor cores that extend the length of the operating cycle are attracting attention as an effective measure. A problem when using fuel to a high burnup as described above is that the amount of elongation of the fuel assembly increases.

That is, as a result of the fuel rod cladding tubes and control rod guide tubes in the fuel assembly being elongated in the axial direction by neutron irradiation during fuel combustion, the overall length of the fuel assembly increases.

The amount of increase in the total length, that is, the amount of elongation depends on the burnup of the fuel, and as the fuel burnup increases, the amount of elongation also increases. As a result, the distance between the upper part of the assembly and the upper core plate becomes shorter by the amount of elongation of the overall length of the assembly.

Therefore, as the amount of elongation of the assembly increases, the amount of displacement of the hold-down spring of the upper nozzle of the fuel assembly described above also increases. Furthermore, when the reactor is shut down, the reactor temperature decreases and the core dimensions become smaller, and the distance between the upper part of the fuel assembly and the upper core plate becomes even smaller.As a result, the permissible displacement required for the hold-down spring becomes wider ( .

(Problem to be Solved by the Invention) However, in the case of the conventional fuel assembly described above, the hold-down spring does not have sufficient countermeasures especially for the above-mentioned high burnup of the fuel, and therefore the fuel assembly If the elongation becomes too large, this spring will not only be strongly pressed and deformed and lose its spring force, but also have a problem in that its deflection as a spring will become small and its original hold-down function will be impaired.

Therefore, in order to deal with this and consider increasing the burnup of the fuel, as a measure to improve the elongation of the fuel assembly, in addition to being a structural member,
It is possible to reduce the height dimension of the lower nozzle, but this method reduces the strength, so it is necessary to consider carrying out detailed stress analysis or using a material with high strength. In any case, there is a limit to how much the size can be reduced, so it is not a good idea.

On the other hand, in a type in which the nozzle plate of the upper nozzle is movable, a spring is inserted around the upper part of the control rod guide tube so that the entire upper nozzle and the control rod guide tube can be moved relative to each other. (GE), but it is difficult to adopt this for fuel types that have many control rod guide tubes. This is because the diameter of the springs becomes large and if there are many springs, they will interfere with each other.

Thus, the present invention focuses on the above-mentioned points and aims to further improve them, and provides a hold-down spring type in which the hold-down function of the hold-down spring is unlikely to be impaired by the elongation of the fuel assembly in a high-burnup core. The purpose is to provide fuel assemblies.

(Means for Solving the Problem) That is, in order to meet the above object, the fuel assembly of the present invention is characterized in that the plate and nozzle frame of the upper nozzle of the nuclear fuel assembly are separated, and the plate is oriented upward. In addition, instead of installing a hold-down spring at the top of the upper nozzle, a hold-down spring is built into the frame of the upper nozzle to increase the spring margin of the upper nozzle. This ensures a large elastic range.

(Function) By configuring as above, the entire upper nozzle moves upward as the fuel assembly expands, as before, but the plate moves further upwards, pressing the spring and increasing the The overall movable dimension is larger than that of conventional leaf springs, and therefore, even in high burn-up applications, damage to the springs and, ultimately, to the fuel assembly is prevented.

(Example) Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

Figures 1 to 3 show the structure of the upper nozzle (4) of the fuel assembly according to the present invention. has a water flow hole that connects to the upper part of the control rod guide tube (2), and is similar to a conventional nozzle plate in that it serves as a flow path for the coolant, but in the present invention, the nozzle frame (2) and The plate QD is formed as a separate body, so that the plate QD is movable only upwardly with respect to the upper nozzle frame (2).

The upper nozzle frame (2) has a locking part (12a) at the top where a hanging tool (not shown) locks when lifting the assembly, and the upper nozzle has a locking part (12a) that intersects with each other on two opposing sides. The spring a1 is assembled so that the lower part thereof is engaged with the spring stopper aS, and the upper part thereof is engaged with the engaging part (12a).

Therefore, when lifting the assembly, the above-mentioned locking part (12a) is used to lift the assembly, but the plate α
Since D is lowered (up to the lowest end of the upper nozzle), there is no adverse effect on the spring a3.

As shown in the figure, a leaf spring is preferable as the spring αj, but a coil spring may be attached to the corner of the upper nozzle. It is essential to bond.

Therefore, in this case, the structure of the conventional leaf spring installation in the upper nozzle has a movable dimension in the range (A1) in Fig. 5, but the movable dimension is basically in the range A++Az). (the dimension D in the figure approximately corresponds to this), the absorption of changes in the elongation of the aggregate becomes greater.

The above description is just one example of the essential parts of the present invention, and it goes without saying that modifications can be made as appropriate without departing from the purpose thereof.

(Effects of the Invention) As described above, the present invention provides an upper nozzle that is a component of a nuclear fuel assembly by separating the plate and the nozzle frame so that the plate is movable only upwardly with respect to the nozzle frame. This structure incorporates a hold-down spring between the nozzle frame and the nozzle frame. By making the nozzle plate movable relative to the nozzle frame, the overall movable range is expanded, thereby securing a larger elastic range, and it is effective in absorbing changes in the elongation of the fuel assembly, resulting in a high burnup core. It corresponds to the required allowable displacement of the spring and has a remarkable effect of ensuring mechanical soundness at high burnup.

Moreover, since the locking portion at the upper part of the upper nozzle frame for lifting the assembly can be the same as before, there is no need to change the hanging tool, which is extremely practical.

[Brief explanation of the drawing]

FIG. 1 is a plan view of the upper nozzle related to the main part of the fuel assembly of the present invention, FIG. 2 is a front view of the upper nozzle shown in FIG. 1, and FIG. 3 is a right side view of the upper nozzle shown in FIG. 1. 5 is a partially omitted overall schematic view of the fuel assembly, and FIG. 5 is an enlarged view of the upper nozzle portion of FIG. 4. (11...Fuel rod, (2)...Control rod guide tube, (3)...Support grid, (4)...Upper nozzle, (5)...Lower nozzle, αυ... Plate, (2)... Upper nozzle frame, Q3)... Hold down spring.

Claims (1)

[Claims] 1. In a nuclear fuel assembly consisting of a fuel support section in which a large number of fuel rods are arranged in parallel and control rod guide tubes are mixed in and supported by a plurality of support grids, and upper and lower nozzles that fix it, the above-mentioned upper The nozzle plate and nozzle frame are formed separately, the plate is movable only upwardly relative to the nozzle frame, and a hold-down spring is incorporated between the plate and the nozzle frame. nuclear fuel assembly.