JPS5844385A - Nuclear fuel assembly - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel assembly suitable for fast breeding f using liquid metal coolant.

As shown in Fig. 1, the conventional structure of a nuclear fuel assembly is that a plurality of pipes are installed on a fixing member 4 in a two-A pipe 3 with a hand cylinder head P1 attached to the upper part and an entrance nozzle 2 connected to the lower part. A fuel pin 5 is installed.

A large number of these nuclear fuel assemblies are loaded inside the reactor vessel to form a reactor core. It is used while being cooled with a liquid metal coolant, but since the calorific value per unit fuel element is large, it is necessary to circulate the liquid metal coolant even within the nuclear fuel assembly. A large amount of coolant flows from the cooling medium to the outlet 6 of the handling head 1.

Since individual nuclear fuel assemblies have different calorific values, the flow rate of the liquid metal coolant at the thermal efficiency upstream outlet 6 is distributed.

By the way, since the upper end of the fuel pin 5 is a free end trap as described above, it is conceivable that the fuel pin 5 may vibrate due to the flow K of the coolant. If the amplitude of this vibration ′ increases, it will collide with the adjacent fuel pin or ratpin wire, and if this is repeated, there is a risk of damage to the cladding tube surface, and in the worst case, cracks will occur in the fuel pin cladding tube. It is possible.

In order to prevent the above concerns, an object of the present invention is to provide a nuclear fuel assembly that suppresses the upper ends of fuel pins to reduce the amplitude of vibration and prevent collisions between adjacent fuel bins.

In order to achieve this object, the present invention provides a nuclear fuel assembly in which a plurality of fuel pins are implanted and arranged by a rattan tube internal pressure fixing member having a handling head attached to the upper part and an entrance nozzle connected to the lower part. The fuel pin is fixed in the casing with axial tension applied to the fuel pin by a tension applying means such as a screw.

Embodiments of the nuclear fuel assembly according to the present invention will be described below with reference to the accompanying drawings.

FIG. 2 is a longitudinal sectional view of a nuclear fuel assembly showing one embodiment of the present invention. Components that are the same as those in FIG. 1 are designated by the same reference numerals, and explanations thereof will be omitted. In FIG. 2, a tension spring 10 as a tension applying means is attached to the upper part of the fuel pin 5, and an upper fixing member 11 is fixed to the inner upper part of the rack A pipe 3 constituting the casing.
The upper end of the tension spring 1o is locked, and the fuel pin 5 is fixed in a vertically pulled state.

During reactor operation, when the fuel pin continues to undergo minute fluctuations due to the flow of coolant, the fluctuating pressure of the fluid is applied to the fuel pin as an external force, and this external force can be considered to be constant. Since the excitation blade of the fuel pin and the external balance are in balance, the relational expression (1) is established: amplitude x (vibration) constant number.

The frequency of vibration when a tension S is applied to the fuel pin with a length of t as a string can be expressed by the following equation.

n: Order of vibration S: Tension applied to the fuel pin r8: Unit weight of the fuel pin From equation 2), when the tension S is increased, the vibration frequency δ□ increases in proportion to the 1/2 power. In other words, the tension S is m
When doubled, the frequency δ1 becomes fi times. According to equation (1), when the frequency is multiplied by /m, the amplitude becomes 1/m,
It can be explained that if the tension is multiplied by m, the amplitude will be multiplied by IAn.

The bending natural frequency 12 of the fuel pin is expressed by the following equation.

λ: Frequency coefficient I: Moment of inertia of area A: Cross-sectional area E: Longitudinal elastic modulus An existing fuel pin has the following 9 main dimensions.

Length j = 191 (cil) Outer diameter d = 0.65 (cIL) Cross-sectional area A = 0.33 (m”) Moment of inertia I = 8.7 Xl0-3 (cs+
') Weight of unit volume 11 = 1.23 Xl0-"(
Kpf/m”) Weight per unit length rz=3-70X10
(?f/m) Calculating the tension of the string that gives a frequency equal to the primary bending natural frequency of this fuel pin, it becomes 8o = 0.53 (Ilf), which is 8°010 times the current fuel pin. Tension, S=5.
When pulled at 3 (4f), the amplitude becomes 1/10.

Maximum allowable tension for creep failure of fuel pin (Capital)
Since it is possible to pull up to 8 = 30 (Kpf), the amplitude can be reduced to about 1/60 by applying tension to the fuel pin.

In this way, by increasing the axial tension S of the fuel bottle 5, it is possible to reduce the amplitude (displacement X) due to fluid vibration and prevent collision with the adjacent fuel bottle 5. If collisions between the fuel bottles 5 can be prevented, damage such as cracks on the surface of the cladding tube due to collisions can be prevented, and the integrity of the fuel pins 5 can be improved.

FIG. 3 shows another embodiment of the present invention, in which the upper end of the fuel pin 5 is fixed to a locking member 12, a tension spring 1o constituting tension applying means is attached to the locking member 12, and a trumpet tube (
By locking the upper end of the tension spring 10 to the upper fixing member 11 fixed to the casing 3, a required tension (axial tension) is applied to the fuel pin 5.

FIG. 4 shows still another embodiment of the present invention, in which a bottle upper part 15 is provided in the upper part of the fuel tube 5 and is fastened with a nut 14 to an upper fixing member 13 fixed to the trumpet pipe 3. This applies tension to the fuel pin 5 in the axial direction.

In this case, it is conceivable that the fuel pin 5 will extend and the tension will decrease due to the rise in temperature, so it is necessary to take countermeasures to keep the fuel pin 5 under the required tension even in the high temperature state during reactor operation. As a countermeasure against this problem, as shown in FIG. 5, a compression spring 16 is installed between the nut 14 and the upper fixing member 13.
Tighten by intervening.

In the case shown in FIG. 6, a bolt 17 having a larger coefficient of thermal expansion than the fuel pin 5 is interposed between the nut 14 and the upper fixing member 13 and tightened.

Furthermore, by using a memory alloy that has the characteristic of shrinking when the temperature rises for the tension springs, it is possible to apply a constant tension to the fuel pins even when the reactor is starting up and during high-power operation.

Other modified embodiments of the invention are listed below.

The structure shown in Figures 4 to 6 is a structure in which tension is applied to each fuel bottle, but as shown in Figure 3, one row or the entire fuel bottle is tightened with darts. A compression spring or a cylinder may be interposed between the parts. Furthermore, the structure is not limited to the structure in which the tension member is attached only to the upper end of the fuel bottle, but it is also possible to attach the tension member to the lower part or middle part of the fuel pin. It is also conceivable to make the trumpet tube or a portion thereof of a material having a larger coefficient of thermal expansion than the fuel pin, and to apply tension in the axial direction to the fuel pin as the temperature rises. Furthermore, it is also effective to arrange a memory alloy at the upper end or intermediate portion of the fuel pin so as to apply tension to the fuel pin when the fuel pin becomes hot during operation of the nuclear reactor.

As described above, the nuclear fuel assembly of the present invention does not have the upper end of the fuel pin as a free end, but is fixed or locked while applying tension, thereby reducing vibrations caused by the fluid force of the liquid metal coolant flowing around the fuel pin. suppress. In particular, by applying tension, the amplitude can be reduced by several tens of minutes by IK, completely eliminating the fear that fuel bottles may collide with each other due to vibration, which may cause cracks in the cladding over a long period of time. This is a unique nuclear fuel assembly that is extremely effective for use in fast breeder reactors that use liquid metal coolant.

Although the nuclear fuel assembly has been described using a wrapping wire type to maintain the spacing between the fuel pins, the present invention can also be carried out using a grid spacer type nuclear fuel assembly. Furthermore, it is also possible to consider a method of securing the coolant passage around the fuel pins only by applying tension to the fuel pins without using wrapping wires or grid spacers.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of a conventional nuclear fuel assembly, FIG. 2 is a vertical cross-sectional view showing one embodiment of the nuclear fuel assembly of the present invention, and FIGS. 3 to 6 are respective views of other nuclear fuel assemblies of the present invention. It is a vertical sectional view of a part showing an embodiment. 1... Handling head, 2... Entrance nozzle, 3... Lanose pipe (
casing), 4... Upper fixing member, 5... Fuel pin, 6... Coolant outlet, 7... Channel hole, 8...
Wrapping wire, 1o... tension spring, 11...
Upper fixing member, 12... Locking member, 13... Upper fixing member, 14... Nut, 15... Pin upper bolt, 16... Compression spring, 17... Cylinder. Applicant's agent Kiyoshi Inomata 1st vote 2nd closing Figure 3 '! Figure 64 Figure 5 Figure 5

Claims (1)

[Scope of Claims] 1. In a nuclear fuel assembly in which a plurality of fuel pins are installed by fixing members in a casing with a handling head attached to the upper part and an entrance nozzle connected to the lower part, springs, screws, etc. A nuclear fuel assembly characterized in that a fuel pin is fixed in a casing with tension in the axial direction applied to the fuel e7 by a tension applying means. 2. The nuclear fuel assembly according to claim 1, wherein all or part of the springs, screws, etc. used as the tensioning means are made of a memory alloy that has the property of shrinking when the temperature rises.