JPS591996B2 - nuclear reactor fuel elements - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear reactor fuel element, and more particularly to a nuclear reactor fuel element in which the effective volume of a gas plenum containing nuclear fission product gas is boosted.

The reactor fuel element (hereinafter simply referred to as fuel element) of a liquid metal cooled fast breeder reactor (hereinafter simply referred to as fast reactor) is provided with a core fuel section, lower and upper blanket fuel sections, and a gas plenum inside. ing.

In a fast reactor, the fuel burnup is 100,000 MW.
day/ton or more, with a maximum linear heat output density of approximately 400 W/c.
Since the rIl is set high, a large amount of fission gas is released.

A gas plenum is provided within the fuel element to contain the fission gases to prevent them from accumulating within the fuel element and increasing the pressure within the fuel element to an extremely high level.

The gas plenum within the fuel element used in a typical fast reactor is longer than the core fuel section, and is about 40% of the total length of the fuel element.

Since such a long gas plenum is provided within the fuel element, a mechanism is required to suppress movement and vibration of core fuel pellets, blanket fuel pellets, etc. during fuel element transportation.

This mechanism is typically located within the gas plenum.

Figure 1 shows details of the fuel element near the gas plenum.

The fuel element 1 includes a cladding tube 2, core fuel pellets 3, blanket fuel pellets 4, end plugs 5, and the like.

Both ends of the cladding tube 2 are sealed with end plugs 5.

Although not shown, several blanket fuel pellets 4 are loaded into the lower part of the cladding tube 2 to form a lower blanket fuel section.

The upper part of the lower blanket fuel section is loaded with several core fuel pellets 3 to form a core fuel section.

Furthermore, several blanket fuel pellets 4 are loaded into the upper part of the burn-up fuel section to form an upper blanket burn-out section.

Blanket fuel pellets 4 contain less fissile material (for example, uranium-235, plutonium-239) and more fissile parent material (for example, uranium-238) than core fuel pellets 3.

A casplenum 6 is formed above the upper blanket fuel section.

A coil spring 7 is arranged within this gas plenum 6.

The lower end of the coil spring 7 is in contact with the upper end surface of the blanket fuel pellet 4 present at the top of the upper blanket fuel section.

Blanket fuel pellets 4 and core fuel pellets 3
is pressed down by the action of the coil spring 7.

These fuel pellets therefore do not move and vibrate during the transport of the fuel element 1.

In addition, the expansion of pellets that occurs during reactor operation is also prevented by the coil spring 7.
This reduces the impact on surrounding structures.

Another example is shown in FIG. A press plate 8 is placed on top of the uppermost blanket fuel pellet 4, and a sleeve 9 is provided on top of the press plate 8.

The sleeve 9 is pressed against the presser plate 8 via the presser plate 10 by the action of a coil spring 11 located above the sleeve.

That is, the presser plate 10, the sleeve 9, and the presser plate 8 are
It has the role of transmitting the force of the coil spring 11 to each fuel pellet.

Several through holes are provided in the side walls of the holding plate 10 and the sleeve 9.

Placing the members shown in FIGS. 1 and 2 within a gas plenum reduces the effective volume within the gas plenum by the volume each member occupies.

The ratio is about 20-30% in the case of FIG. 1 and about 10-20% in the case of FIG.

The lifetime of the fuel element is limited due to the stress in the cladding, which increases under the influence of the increased pressure of fission product gases that accumulate within the fuel element.

That is, when the stress generated in the cladding tube due to an increase in the internal pressure of the fuel element due to the accumulation of nuclear fission product gas exceeds a permissible value, the fuel element reaches the end of its life.

To extend the life of the fuel element, the volume of the gas plenum can be increased.

However, increasing the volume of the gas plenum increases the overall length of the fuel element, requiring a larger reactor vessel.

Further, since the pressure loss between the lower end and the upper end of the fuel element becomes large, it is necessary to increase the capacity of the coolant circulation pump that supplies the coolant.

On the other hand, increasing the wall thickness of the cladding tube also extends the life of the fuel element.

However, the amount of neutrons absorbed by the cladding increases.

The present invention aims to eliminate the above-mentioned drawbacks of the prior art and to increase the effective volume of the plenum for containing fission products.

A feature of the present invention is that the plenum spacer, which is made of a linear thin wire portion that bends in the radial direction of the sealed container due to the expansion of the fuel pellets, and loop portions formed at both ends of the thin wire portion, is installed in the plenum spacer in the sealed container. This is because it was set up inside.

A preferred embodiment of the invention will be described below with reference to FIGS. 3 and 4.

Components that are the same as those of the conventional embodiment are designated by the same reference numerals.

In the gas plenum 6 provided in the fuel element 39, a plurality of plenum spacers 15 as shown in FIG. 4 are arranged in a stacked manner.

The plenum spacer 15 includes a loop portion 16 and a straight portion 17, and the loop portion 16 is formed at both ends of the straight portion 17.

Plenum spacer 15 may have the shape shown in FIG.

Although the loop portion 16 shown in FIG. 4 has only one loop in the axial direction, the loop may be wound multiple times, that is, in the form of a densely wound coil.

This plenum spacer 15 is made of a single thin stainless steel wire with a diameter of approximately 11nr/L.
The height H is about 10 cIn, and as will be described later, the straight part 17 does not bend due to the pellet load during transportation, etc., and the straight part 17 bends to absorb the pellet expansion during reactor operation. It is a thin line.

There is a plenum spacer 15 like this in the gas plenum.
are stacked with their loop portions 16 in contact.

The diameter of the loop portion 16 needs to be equal to the inner diameter of the cladding tube 2, or smaller than the inner diameter of the cladding tube 2 and larger than the inner diameter of the cladding tube 2 minus the diameter of the thin wire.

When ten such plenum spacers 15 are stacked in the gas plenum 6, the effective volume of the gas plenum 6 increases by about 6% compared to the effective volume of the gas plenum 6 of the embodiment shown in FIG.

Therefore, the capacity for fission product gases is increased and the life of the fuel element can be extended.

Furthermore, it is also possible to shorten the length of the gas plenum 6 and downsize the nuclear reactor.

A coil spring 11 is placed between the top plenum spacer 15 and the end plug 5.

The coil spring 11 may be placed between the bottom plenum spacer 15 and the blanket fuel pellets 4 present at the top of the upper blanket combustion slope, or between the plenum spacers 15.

The coil spring 11 may not be arranged in the gas plenum 6, and only the plenum spacer 15 may be arranged.

The plenum spacer 15 disposed within the gas plenum 6 is subjected to a load of approximately 2 kg during fuel element transportation.

FIG. 6 shows the relationship between the load applied in the axial direction of the plenum spacer 15 and the compression ratio of the plenum spacer 15.

As is clear from the figure, the load and the compression ratio of the plenum spacer 15 are in a proportional relationship, and the plenum spacer 15 has a diameter of 1
．． Although it is made of mm-thin wire, there is no risk of buckling during transportation.

Therefore, since the fuel pellets are always held down by the plenum spacer 15, movement and vibration of the fuel pellets during fuel element transportation can be suppressed.

Various shapes of plenum spacers are possible.

Modifications thereof are shown in FIGS. 7 to 10. The plenum spacer 20 shown in FIG.
3 is formed, the thin wire is bent upward to form a straight portion 24, and the other end of the thin wire is further bent along the outer circumference or inner circumference of the loop portion 21.

Since two straight parts are formed in this plenum spacer 20, the diameter of the thin wire can be made thinner.

The plenum spacer 25 shown in FIG. 8 has a plurality of straight portions 28 and one or more loop portions 29 formed between a loop portion 26 and a loop portion 27 formed at both ends of a thin wire. be.

A plenum spacer 30 shown in FIG. 9 has a portion of the loop portion shaped like a coil spring.

That is, coil spring portions 31 and 32 are formed at both ends of the plenum spacer 30.

In the example shown in FIG. 9, a loop portion 33 is formed at the center of the plenum spacer 30, and straight portions 34, 35 are present above and below the loop portion 33.

The coil spring portion 31, the straight portion 34, the loop portion 33, the straight portion 35, and the coil spring portion 32 are made from a single thin wire.

With this configuration, as in the embodiment shown in FIG. 3, only a deviation occurs between the coil spring and the plenum spacer, and the force of pressing the fuel pellets by the coil spring is not diminished.

In other words, the fuel pellets are more effectively held down by the coil spring.

When a deviation occurs between the coil spring and the plenum spacer, the loop part of the plenum spacer or a part of the inner surface of the cladding tube comes into contact, and part of the force exerted by the coil spring is transmitted locally to the contact part, causing the fuel pellets to There is a possibility that the pressing force will be slightly reduced.

This also applies when a plurality of plenum spacers as shown in FIGS. By arranging a plenum spacer (as shown in the figure) in the gas plenum, fuel pellets can be held down effectively.

The number of straight sections and loop sections in the embodiment shown in FIG. 9 can be arbitrarily determined depending on the length of the gas plenum.

A coil spring portion may be formed only at one end of the plenum spacer 30, and a loop portion may be formed at the other end of the plenum spacer 30.

The coil spring section can also be said to be a modification of the loop section.

Furthermore, as shown in FIG. 10, a coil spring portion 37 may be formed in the center of the plenum spacer 36.

Loop portions 38 are formed at both ends of the plenum spacer 36.

The straight sections between the loop sections of the plenum spacer may be somewhat serpentine.

As is clear from the above explanation, since the loop part forms a contact part with the coil spring receiver or the pellet, it is not necessary to form it integrally with the straight part into a circular shape as in the above-mentioned embodiment. It may be a separate body, for example a disk.

According to the present invention, the effective volume of the plenum can be increased without impairing the function of the spacer.

The present invention can also be applied to blanket fuel elements loaded with blanket fuel pellets and to reactor fuel elements of other nuclear reactors such as boiling water reactors, pressure water reactors, etc. It can also be applied.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of the vicinity of the gas plenum of a nuclear reactor fuel element according to one conventional embodiment, FIG. 2 is a vertical cross-sectional view of the vicinity of the gas plenum of a nuclear reactor fuel element according to another conventional embodiment, and FIG.
The figure is a longitudinal sectional view of the vicinity of the gas plenum of a nuclear reactor fuel element according to an embodiment of the present invention, and FIGS. 4 and 5 are detailed views of a plenum spacer applied to the reactor fuel element of the present invention.
FIG. 6 is a characteristic diagram showing the compression ratio of the plenum spacer shown in FIG. 4, and FIGS. 7 to 10 are detailed views of other plenum spacers applied to the reactor fuel element of the present invention. Explanation of symbols, 2... Cladding tube, 4... Plunket fuel pellets, 6... Gas plenum, 11... Coil blade, 15...・Plenum spacer, 16...Loop part, 17...
...straight line section.

Claims (1)

[Scope of Claims] 1. In a nuclear reactor fuel element in which fuel pellets containing the fissile material are filled in a sealed container, and a plenum for storing the fission products is formed in the sealed container, A plenum spacer including a thin wire portion that bends in the radial direction of the sealed container due to the expansion of the fuel pellets and loop portions formed at both ends of the thin wire portion is configured to suppress vibration of the fuel pellets within the sealed container. a nuclear reactor fuel element located within said plenum; 2. The nuclear reactor fuel element according to item 1, wherein at least one of the loop portions is formed in the shape of a coil spring integrally formed with the straight portion.