JPS58186080A - 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] [Technical Field of the Invention] The present invention relates to a reactor fuel assembly, such as a blanket fuel assembly for a fast breeder reactor, in which the flow rate of coolant can be automatically adjusted according to the calorific value. .

[Technical background of the invention]

A blanket fuel assembly is constructed by regularly arranging a plurality of fuel elements in which depleted uranium (UOt) fuel pellets are housed in a cladding tube within a coolant flow path tube.

Depleted uranium (
Uranium-238 (UOt) absorbs neutrons and undergoes several stages of nuclear decay to become plutonium.

In fast breeder reactors, blanket fuel assemblies are loaded into the reactor in order to produce more plutonium than is consumed, but these blanket fuel assemblies contain less than 0.3% of fissile uranium-235. Since it uses depleted uranium, it generates almost no heat at the beginning of its life, when not much plutonium is produced.

When a blanket fuel assembly is placed in a reactor for 2 to 5 years, it gradually begins to generate heat due to plutonium that is gradually generated from uranium-238, and at the end of its life, the average fuel element power reaches about 100 W/CIL. .

The coolant sodium necessary to remove the generated heat is supplied from the orifice hole of the entrance nozzle installed at the bottom of the fuel assembly, but in the conventional technology, the coolant flow rate is During its life, it is maintained at a constant value determined by the orifice hole in the entrance nozzle section.

[Problems with background technology]

In conventional blanket fuel assemblies, as described above, the orifice holes are fixed, so the coolant flow rate is constant throughout the period from the beginning when there is little heat generation to the end when there is much heat generation.

Therefore, in the early stage of use, the coolant flowing in through the orifice hole is hardly heated and flows out of the fuel assembly at a low temperature, which is disadvantageous in that the coolant outlet temperature of the entire reactor is lowered.

Also, when trying to keep the reactor outlet temperature constant,
In the early stages of life, the outlet temperature of the core fuel assembly must be increased by that amount, placing an extra burden on the core fuel elements, which are normally under severe conditions, and in any case This is undesirable in terms of furnace performance.

[Purpose of the invention]

The present invention has been made in response to such conventional circumstances, and provides a nuclear reactor fuel assembly having a flow rate adjustment mechanism that variably adjusts the flow rate as heat generation increases during the life of the blanket fuel assembly. It is intended to.

[Structure of the invention]

That is, the reactor fuel assembly of the present invention is a reactor fuel assembly in which a plurality of fuel elements are arranged and housed in a flow path pipe, and a flow path adjustment device is installed at the upper end of each of the fuel elements via a metal bellows. and connecting the upper end of the flow path adjustment device to a partition plate installed in the sleeve tube through the outer bellows of the double bellows, airtightly blocking the space between the sleeve tube and the flow path tube with an upper support plate, A low melting point metal is disposed between the outer bellows and the inner bellows provided inside the outer bellows and melts when the temperature of the coolant flowing outside the outer bellows increases and flows into the inner bellows through the through hole provided in the inner bellows. A flow path is provided in the flow path adjustment device and the side surface of the sleeve, respectively, so that when the double bellows is retracted due to the low melting point metal flowing into the inner bellows, the flow path area of the coolant increases. It is configured with a restriction orifice.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail with reference to the drawings.

In FIG. 1, a plurality of fuel elements 1 are housed in a coolant flow pipe 2 in regular alignment parallel to the flow path, and the lower part of each fuel element 1 is a support that protrudes from an end plug 6. The strip 4 is fixed to the flow pipe 2 by passing through the fixing rod 5.

At the top of each fuel element 1, a flow rate adjustment mechanism 7 having a variable ogifice 6 is mounted.

These flow rate adjustment mechanisms 7 are connected to each flow rate adjustment mechanism and the flow path pipe 2.
It is fixed to the inner surface of the flow path tube 2 via an upper support plate 8 arranged so as to close the flow path between them.

An entrance nozzle 9 is installed at the lower end of the flow pipe 2, and a handling head 10 is provided at the upper end of the flow pipe.

The coolant flowing into the flow pipe 2 from the entrance nozzle 9 flows into the adjacent fuel element 1 and the flow pipe 2 as shown by the arrow.
After rising through the gap formed between the inner walls of the fluid, the fluid enters the flow regulating mechanism through the variable orifice and exits through the handling head 10 through the upper end through hole.

FIG. 2 is an enlarged longitudinal sectional view of the vicinity of the flow rate adjustment mechanism 7.

The fuel element 1 contains depleted uranium pellets 12 in a cladding tube 11, and a metal bellows 16 is welded to the upper end of the cladding tube.

A flow path adjustment device 14 that moves up and down in conjunction with the metal bellows is attached to the upper end of the metal bellows.

This flow-path regulating device is provided with a flow-path restricting orifice 15, and the lower end surface of an outer bellows 17 of the double bellows is connected to the upper end of the orifice 15 via a connecting pin 16.

The upper ends of the outer bellows 17 and inner bellows 18 of the double bellows are welded to a partition plate 19, and a low melting point metal 20 is filled between them.

A molten low melting point metal 20 is attached to the lower end surface of the inner bellows 18.
A through hole 21 is provided through which water can flow.

Furthermore, on the upper part of the flow path adjustment device 14 and the partition plate 19,
Through holes 22, 23 are provided for coolant flow.

The bellows 16, the flow path adjustment device 14, the connecting bottle 16.
The double bellows 17, 18 and the partition plate 19 are housed within the jacket 24, and the peripheral edge of the partition plate 19 is fixed to the inner surface of the jacket 24.

A flow path restriction orifice 25 that forms a variable orifice 6 together with the flow path restriction orifice 15 of the flow path adjustment device 14 is transparently provided on the side surface of the mantle 24, and a through hole 26 is formed in the upper end surface of the mantle. .

The periphery of the mantle 24 is hermetically fixed to the support plate 8, completely blocking the flow paths other than the variable orifice 6.

Note that an inert gas 2 such as helium gas is contained in the fuel element 1.
7 is filled at a pressure similar to atmospheric pressure.

In the above-described nuclear reactor fuel assembly of the present invention, after the flow rate adjustment mechanism 7 is attached to the fuel element 1, this is fixed to the upper support plate 8, which is inserted into the flow path pipe 2, and then fixed with the fixing rod 5. Support strip 4
If it is fixed, it can be assembled on Yongjima.

Next, the present invention will be explained in detail.

The nuclear reactor fuel assembly of the present invention is normally used by being arranged around the core of a fast breeder reactor.

The period of use is usually 2 to 5 years.

In the early stage of combustion, the fuel element 1 is composed of depleted uranium, and the rate at which nuclear fission reactions occur is low, so the amount of gaseous fission products produced is small, and at the same time, at low burnup, the gaseous fission products are contained in the uranium pellet. Because it is difficult to release because it is confined in Since the internal pressure is low, the metal bellows 16 is contracted as shown in FIG. 25 has a small overlapping area, and the flow path area of the variable orifice 6 is small.

Therefore, at the initial stage of use of a nuclear reactor fuel assembly, the amount of coolant flowing through this fuel assembly is small.

In this case, if the elasticity of the metal bellows 16 and the positional relationship of the flow path limiting orifice 15.25 are appropriately determined, the variable orifice 6 can reduce the amount of heat generated in the fuel element to at least the coolant outlet of this fuel assembly. The temperature can be made comparable to the coolant exit temperature from the core fuel assembly.

As the combustion of the fuel element 1 gradually progresses and plutonium accumulates, the amount of gaseous fission products produced by 1) fission 1m of plutonium increases, and at the same time, the temperature of the coolant flowing in the flow pipe increases. do.

Due to this temperature rise of the coolant, the double bellows becomes 17°18
The low melting point metal 20 between them melts and flows into the inner bellows 18 through the through hole 21 .

As a result, the metal bellows 13 is expanded as shown in FIG.
5 and 25 increases, and the flow path area of the variable orifice 6 increases.

Therefore, the coolant flow rate increases and the cooling effect increases.

Therefore, even if the amount of heat generated within the fuel element 1 increases compared to the initial stage, the coolant temperature does not rise.

When the coolant flow rate further increases, the coolant temperature decreases, the low melting point metal 20 is also cooled, and the position of the variable orifice is fixed.

As mentioned above, in the nuclear reactor fuel assembly of the present invention,
As combustion progresses, the coolant temperature and the amount of fission products rise to melt the low melting point metal, and by increasing the flow area of the variable orifice, the coolant temperature is moderated and the low melting point metal is solidified. and keep the coolant flow constant.

If the dimensions of the variable orifice 6 and the melting point of the low melting point metal 20 are set appropriately, the coolant outlet temperature can be maintained at the same level as that of the core fuel assembly over the entire life of the fuel assembly. .

In the above description, an example in which the shape of the variable orifice 6 is rectangular has been described, but FIG. 4a shows an example.

It can also be of any circular, non-circular, polygonal shape, and combinations thereof, as shown in b l e l d l e, in which case the variable orifice 6 produced by the same stroke of the metal bellows Since the rate of increase in the flow path area can be adjusted, the degree of freedom in design increases.

Further, it is also possible to use a combination of a flow-path regulating device and a flow-path restricting orifice of a mantle having different shapes.

〔Effect of the invention〕

As mentioned above, the reactor fuel assembly of the present invention has a flow rate adjustment mechanism, so that the coolant outlet temperature of the fuel assembly is kept constant throughout the reactor during its life of 2 to 5 years. Therefore, even if the cooling temperature within the core fuel assembly is not so high, the coolant outlet temperature of the reactor as a whole can be made sufficiently high.

Therefore, the integrity of the core fuel elements can be improved and the economic efficiency of the fast breeder reactor can be improved.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view showing an embodiment of the nuclear reactor fuel assembly of the present invention, and FIG. 2 is a vertical cross-sectional view showing an enlarged view of the vicinity of the flow rate adjustment mechanism in the early stage of combustion of the nuclear reactor fuel assembly of the present invention. , FIG. 3 is an enlarged vertical cross-sectional view showing the vicinity of the flow rate adjustment mechanism at the final stage of combustion of the reactor fuel assembly of the present invention, and FIG.
FIG. 3 is a front view showing a modified example of the shape of the flow path restriction orifice. 1 ...... Fuel element 2 ...... Flow pipe 6 ...... End plug 4 ...... Support strip 5 ...... Fixed rod 6 ...... Variable orifice 7 ...
...... Flow rate adjustment mechanism 8 ...... Upper support plate 9 ...... Entrance nozzle 10 ・
・・・・・・・・・ Han F! 1 inch head 11...
...... Cladding tube 12 ...... Depleted uranium pellets 16
・・・・・・・・・ Metal bellows 14 ・・・・・・
... Flow path adjustment device 15.25 ... Flow path restriction orifice 16 ...... Connection bottle 17 ...... Outer bellows 18 ...
...... Inner bellows 13-19 ...... Partition plate 20 ...... Low melting point metal 21.22.
26.26...Through hole 24...Cover 27...Inert gas agent patent attorney
Suyama Sa - 14- Figure 1 Figure 2 Figure 3 Figure 4 Cde

Claims (1)

[Claims] 1. In a reactor fuel assembly arranged and housed in a plurality of fuel element flow path pipes, a flow path adjustment device is provided at the upper end of each of the fuel elements via a metal bellows; The upper end of the flow path adjustment device is connected to a partition plate installed in the sleeve tube through the outer bellows of the double bellows, and the space between the sleeve tube and the flow path tube is airtightly isolated by the upper support plate, and the outer bellows and an inner bellows provided on the inside thereof, filled with a low melting point metal that melts when the temperature of the coolant flowing outside the outer bellows rises and flows into the inner side through the through hole provided in the inner bellows. In order to increase the flow path area of the coolant when the double bellows contracts due to the flow of the low melting point metal into the inner bellows, a flow path restriction orifice is provided on the side surface of the flow path adjusting device and the sleeve, respectively. A nuclear reactor fuel assembly characterized by being provided with. 2. The reactor fuel assembly according to claim 1, wherein each of the flow path restriction orifices is rectangular. 6. The reactor fuel assembly according to claim 1, wherein the flow path restriction orifice is composed of a combination of irregularly shaped orifices.