JPS5857718B2 - composite fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an open lattice nuclear fuel assembly loaded with fuel rods, and particularly to a nuclear fuel assembly having fuel rods of different diameters.

A nuclear reactor typically includes a core comprised of a plurality of nuclear fuel assemblies contained within a vessel through which gaseous or liquid cooling 47 flows.

As the coolant flows through and around the fuel assembly, it removes energy in the form of heat.

Particularly in pressurized wood-carved nuclear reactors that circulate liquid coolants such as water, reaching boiling conditions is undesirable, and therefore the reactor core is normally referred to as ``nucleate boiling transition heat flux'' (DNB). It is designed to prevent this from happening.

The presence of vapors along the fuel rods prevents the heating fuel rods from transferring heat to the surrounding coolant, which can lead to damage to the fuel rods due to overheating.

Therefore, the safety limit associated with DNB sets an upper limit on the maximum coolant temperature and thus limits the overall reactor efficiency.

These problems have led to the desire to use mixed oxide or plutonium fuels, which are more expensive to produce and which inherently have larger neutron absorption cross sections and moderator temperature coefficients than the commonly used uranium fuels. This makes things even more complicated.

Also, as the coolant flows upward through the core, it is gradually heated and its density is gradually changed, so that the deceleration at the top of the core becomes progressively smaller compared to the bottom.

As a result, control and design flexibility becomes worse.

Solutions that seek to alleviate these limitations, primarily in response to DNB effects, include increasing the water-to-fuel ratio in the core;
Fuel rods may be used that span the entire length of the core and have small cross-sectional areas that generate less energy per unit length.

However, these solutions are complicated by excessive manufacturing costs, large cores and other factors.

In addition, reactor structures that include redundant safety systems that respond in the unlikely event of a rupture in the reactor coolant system that results in loss of coolant, inject coolant into the core region, It spreads from the bottom to the top.

It would be advantageous to have more flexibility in designing the core to accommodate this condition.

Accordingly, a primary objective of this invention is to provide a nuclear fuel assembly that alleviates temperature and efficiency limitations and is compatible with the use of mixed oxide fuels.

For the above purpose, the present invention provides a composite fuel assembly for use in a thermal neutron reactor having a liquid coolant flowing upwardly through the reactor core containing vertically oriented composite fuel assemblies, the composite fuel assembly being capable of nuclear fission in a hermetically sealed jacket. a plurality of elongated nuclear fuel rod structures containing nuclear fuel and arranged in a regular grid pattern;
a plurality of lattice structures arranged around said fuel rod structure at various preselected height positions to laterally support said fuel rod structure in individual cells; , the fuel rod structure has an upper and lower fuel rod bundle extending a predetermined axial distance from each other, the lower fuel rod bundle having a diameter larger than that of the upper fuel rod bundle. A small number of fuel rods are present, and the lattice structure of the lower fuel rod bundle is characterized in that its circumference is approximately the same size as the upper fuel rod bundle.

This composite fuel assembly can be advantageously adapted to use mixed oxide fuel in the bottom fuel rods and uranium fuel in the top fuel rods.

In this configuration, power peaks do not occur at the interface between the plutonium and uranium fuels because the favorable spacing between the upper and lower fuel rods reduces sharp power peaks.

In a typical nuclear reactor where the coolant flows upward through the core, the coolant is already at a high temperature by the time it reaches the small diameter fuel rods in the upper part of the composite fuel assembly, where the amount of energy generated per unit length of the fuel rod is low. It is.

This allows higher coolant discharge temperatures from the core without the risk of boiling or the need for more efficient systems.

Additionally, the relative cross-sectional dimensions of the individual upper and lower fuel rods can be varied as directed by the designer in response to changes in deceleration with changes in coolant temperature.

Additionally, a plenum is typically constructed in the upper portion of most fuel rods to contain gaseous fission products released during reactor operation.

Due to the lower incidence in the upper fuel rod, this upper plenum can be dimensioned smaller compared to previous fuel rods.

The more active lower fuel rods also have a plenum located at the bottom of the lower fuel rods to reduce neutron flux peaking in the center of the core.

Additionally, neutron absorbing material may be placed at the top of the lower fuel rods and at the bottom of the upper fuel rods to limit moderation in the region between the two fuel rod arrays.

The invention will become more readily apparent from the following description of a preferred embodiment, shown by way of example in the accompanying drawings.

Referring to FIG. 1, a composite fuel assembly 1 according to the present invention
A pressurized watercolor reactor using 0 is illustrated as an example.

This nuclear reactor has a core 12 made up of fuel assemblies 10 arranged in a nearly right cylindrical shape.

The fuel assembly 10 is supported within the vessel 14 between an upper core plate 16 and a lower core plate 18, both of which are perforated to allow the flow of waste material therethrough.

Reactor coolant fluid, preferably a liquid such as water, enters vessel 14 through inlet 20, passes downwardly through annulus 22, is diverted at plenum 24, and passes upwardly through core 12.

In various similar reactor configurations, coolant enters vessel 14 from below the core and passes upwardly.

The coolant absorbs energy as it flows through and around the fuel assembly 10 and exits the vessel 14 through the outlet 26, ultimately delivering energy to a device (not shown), such as for power generation. discharge.

The power generated within the reactor core 12 can be controlled in a variety of known ways, such as using a neutron absorbing control element 28 in conjunction with a neutron absorbing material, such as boron, flowing with the coolant.

According to the invention, the control element 28 is provided on the top and includes: 1. It can be reciprocated and positioned within or around the fuel assembly 10 by the drive device 30.

Although there are many structures for the reactor core and fuel assembly, this invention
It is particularly useful in thermal neutron reactors having vertically arranged elongated fuel assemblies through which the coolant flows generally upwardly.

A preferred fuel assembly 10 according to the invention is shown in FIG.

The fuel assembly includes a plurality of upper fuel rods 40 and a plurality of lower fuel rods 42 arranged in regular, preferably rectangular rows.

The fuel rods 40,42 are preferably cylindrical, and the upper fuel rods 40 have a smaller diameter than the lower fuel rods 42.

Therefore, the number of upper fuel rods is greater than the number of lower fuel rods.

Each fuel rod 40.42 has a closed metal cladding 44 containing fissile fuel, preferably in the form of cylindrical pellets 46.

This fuel may be of various types known in the art, such as enriched uranium, and in one embodiment advantageously includes enriched uranium in the upper fuel rods 40 and lower fuel rods 42.
contains plutonium fuel.

Although different enrichments can be employed, for example, if both the upper and lower fuel rods contain the same enrichment of uranium, the power produced per unit length of each fuel rod will be less than that of the lower fuel rod 42.
The upper part is larger than the upper fuel rod 40.

Thus, the coolant flowing upwardly within and around the fuel assembly 10 is heated first in the lower portion of the fuel assembly and further heated to a higher temperature in the upper portion.

However, the problem of nucleate boiling transition heat flux (1) NB) is alleviated and the coolant exiting the core as a result of the lower energy output per unit length of the upper fuel rods, measured for example in KW/ft. The temperature can be increased.

Table 1 shows the upper fuel rod 40, which is approximately 1.83 m (6 ft) long, and the upper fuel rod 40, which is also approximately 1.83 m (6 ft) long.
Typical parameters obtained by using the fuel assembly of the present invention, which uses enriched uranium fuel in the lower fuel rod 42 of the reactor, in the core of a pressurized water wet reactor are shown.

The reference for comparison is a core of a fuel assembly having 3.4 m (12 ft) long fuel rods in an array of 15 rows by 15 columns; Bar 42, 20 rows x 20 columns and 30 rows
The comparison is made with a composite fuel assembly having 30 rows of upper fuel rods 40 in an array.

As can be seen from Table 1, when the number of upper fuel rods is large, the temperature rise of the coolant increases.

Also, the number of fuel rods is less than that of a complete array as a result of the assembly of fuel assembly 10 and the addition of additional components such as guide tubes, which will be discussed below.

For example, a complete array of 15 rows by 15 columns has 225 fuel rods, but only 204 fuel rods are utilized, with the remaining positions occupied by guide tubes.

In addition to the fuel rods 40, 42, the fuel assembly 10 includes guide tubes 52.
It has a top nozzle 48 and a bottom nozzle 50 attached to it, forming a load transfer skeletal unit.

The guide tube 52 may simply support the fuel assembly or guide the control element rod 54 (FIGS. 1 and 6) into and out of the fuel assembly;
Alternatively, it can be used for positioning other core components.

Upper and lower lattice structures 5.6, 57, shaped roughly like an "egg cram", are attached to the guide tube 52 at each selected position.

As best shown in FIGS. 3-5, the lattice structure 56,
57 forms a cell 58 surrounding each bale or guide tube.

Cells 58 in fuel rods 40,42 provide lateral support while allowing axial expansion of the rods.

The grid structure 56,57 typically includes repelling flow mixing vanes 59.
and a spring-loaded support 61, it has an outer band 60 forming a surrounding boundary around the array of rods and an inner band 62 which together form the individual cells 58.

Upper and lower lattice structures 56 defined by outer strips 60.
The outer dimensions of 57 are the same.

The lattice structure 56, 57 not only provides support for fuel rods 40, 42 of different diameters, but also cells 58a, 58btm, which in the embodiment can be attached to a guide tube 52 of uniform and preferably circular cross section. must be arranged as follows.

Since the cells of the upper and lower lattice structures have different dimensions, provision must be made for installing guide tubes.

Therefore, the lower fuel rods 42 preferably have the same cross-section so that the cells 58 and 58a of the lower lattice structure 57 are all of the same size.

However, the cells 58 of the upper lattice structure 56 have guide tubes 52
The upper fuel rod 40 includes a cell 58b that receives the upper fuel rod and is larger than the support cell 58 of the upper fuel rod 40.

As shown in FIG. 3, in the illustrated composite fuel assembly 10, each cell 58b is a four cell combination that receives a fuel rod in an upper lattice structure.

In a modified example, since the guide tube 52 can also be made larger than the lower fuel rods 42, cells larger than those that accommodate the fuel rods 42 are also formed in the lower lattice structure 57.

Furthermore, as can be seen by comparing FIGS. 3 and 4, the cell 58b that receives the guide tube is the cell 5 of the lower lattice structure 57.
Thicker than 8.

Since the lattice structure is attached to a guide tube within the cell, means must be provided for attaching the guide tube to both the upper and lower lattice structures.

Means for such attachment is shown in FIG.

Here, the sleeves 64 provided are attached to the strips of the cells 58b of the upper grid structure 56 by brazing, welding or otherwise, for example at four locations indicated at 66.

To maintain alignment of the guide tubes, the inner diameter of the sleeve 64 may be the same as the cross-planar dimensions of the cells 58.58a of the lower lattice structure.

In this case, the guide tubes can be brazed or otherwise attached directly to the strips of the lower lattice structure.

Alternatively, thicker brazes 66a (FIG. 3), welds, or other attachment points may be created between the inner strip 62 and the guide tube outer periphery to eliminate the use of the sleeve 64. .

Additionally, the guide tube can bulge or expand above and below the sleeve 64 for attachment.

Alternatively, the cell 58b can include a strut 68 for attaching a guide tube.

Further, the guide tube 52cj has a cross section smaller than the lower fuel rods 42, and therefore the cells of the lower lattice structure, and smaller than the large cells 58b of the upper lattice structure.

With this construction, the sleeves 64 can be fitted one by one into both the cells of the upper and lower lattice structures, where the sleeves have the same inner diameter for receiving the guide tubes, and where the individual sleeves The cells 58b of the lattice structure and the cells 58a of the lower lattice structure have different outer diameters to match the respective sizes.

Furthermore, as shown in FIG. 7, the guide tube 52 has its hole 53
has a diameter that differs along its length, insofar as the vine is large enough over its entire length to allow control element rod 54 to pass therethrough.

The holes 53 may also vary in size, for example larger in the upper part and smaller in the lower part, which can provide an additional damping effect for the control element rod 54 in the lower part of the guide tube 52.

The outer cross-section of the guide tube, which varies in size, can be sized to best fit within the cells of the lattice structure, with or without the sleeve.

A space 70 (FIGS. 2 and 5) is preferably provided between the bottom of the upper fuel rod 40 and the top of the lower fuel rod 42.

Space 70 is particularly suitable when using mixed oxide fuels.
The lack of this space reduces the significant changes in power distribution that may occur at the upper and lower fuel rod boundaries.

The space is approximately 2% of the total length of the upper and lower fuel rods.
Preferably no longer.

If the spacing is too large, there will be an excess region of moderator coolant in the center of the core, causing undesirable neutron flux peaking.

However, this effect can be counteracted by assembling or attaching a fuel rod sealing end plug in the interface region of a material and size that absorbs excess neutrons.

This effect can also be counteracted by placing a plenum at the bottom of the lower fuel rods and at the top of the upper fuel rods to account for the production of fission product gases.

Springs or other support means supporting the fuel pellets in the upper fuel rods can be used to maintain the lower plenum of the fuel rods.

Similarly, depleted fuel in the form of pellets or other forms may be placed at the bottom of the upper fuel rods and at the top of the lower fuel rods, or inert ceramic spacers may be provided at these locations.

At each end of the spacer, the lowermost upper grid structure 56 is preferably secured to the uppermost lower grid structure 57 via their outer strips 60.

This installation is carried out by thin plates 72 as shown alternately in the fifth country.
or using a large plate 74.

Plates 72, 74 are located outside the fuel rod array;
The structure may include flow mixing vanes 59, support blades 61, flow openings 80, etc., as used in typical lattice structures.

The lowermost upper lattice structure is clamped to the uppermost lower lattice structure in order to strengthen the fuel assembly in the central region where relatively large stresses and deformations occur in the event of an earthquake.

The space 70 between the upper and lower fuel rod arrays is
When plutonium fuel is used in the lower fuel rod and uranium is used in the upper fuel rod, the power peak that occurs at the boundary when the two fuels are used in anything other than a homogeneous mixture can be reduced.
Particularly effective.

Additionally, composite fuel assembly 10 is suitable for use with mixed oxide fuels.

For example, the relative amounts of uranium and plutonium fuel can be adjusted by changing the lengths of the upper fuel rod 40 and the lower fuel rod 42.

Preferably, the lower fuel rods containing plutonium are arranged over 1/2 to 2/3 of the length of the fuel assembly, and the fuel rods containing uranium are arranged over the remaining 1/2 to 1/3 of the length of the fuel assembly. .

For example, plutonium fuel in the form of plutonium oxide
The composite fuel assembly of the present invention is economically advantageous in terms of plutonium use, since the high toxicity of plutonium, which requires remote production, makes it significantly more expensive than producing uranium, such as uranium dioxide. It is.

Placing plutonium in larger diameter fuel rods reduces the number of plutonium-loaded fuel rods per fuel assembly, thus lowering cost.

Also, the placement of plutonium in the lower part of the reactor core is advantageous in terms of nuclear control characteristics.

By positioning the plutonium near the bottom of the core where the top-mounted control elements (shown in Figure 1) are weakened, the effect of plutonium's large neutron capture cross section is reduced.

Additionally, by reducing full-load to no-load coolant temperature fluctuations in the plutonium region where the coolant enters the core 12, the high moderator temperature coefficient of plutonium reactivity that contributes to control requirements is reduced.

The disclosed composite fuel assembly also has the advantage of responding under hypothetical accident conditions in which core coolant is temporarily released.

During such an accident condition, multiple redundant systems are activated to refill the reactor vessel from bottom to top.

Therefore, the more active lower fuel rods are flooded with coolant earlier than the less active upper fuel rods, increasing the safety margin compared to a core that is uniform along its length.

Therefore, the above-described composite fuel assembly is useful in increasing the temperature of the reactor coolant and increasing the overall efficiency of the nuclear reactor plant.

Additionally, the composite fuel assembly is useful for the use of mixed oxide fuels while providing increased flexibility in all types of core designs.

[Brief explanation of the drawing]

FIG. 1 is an elevational cross-sectional view of a nuclear reactor including a composite fuel assembly according to the present invention, FIG. 2 is a schematic elevational view of a composite fuel assembly according to the present invention, and FIG. 3 is an embodiment of the present invention. 4 is a simplified plan view of the fuel assembly lower lattice structure which can be used in common with the embodiment of FIG. 3; FIG. 5 is a simplified plan view of the fuel assembly lower lattice structure according to the embodiment of the present invention. FIG. 6 is a perspective view of one cell of a composite fuel assembly according to an embodiment of the present invention, and FIG. 7 is a perspective view of an example of a fuel assembly guide tube according to the present invention. It is. In the figure, 10 is a composite fuel assembly, 12 is a core, 40 is an upper fuel rod, 42 is a lower fuel rod, 44 is a sealing cladding, 46
is the nuclear fuel, 56 is the upper lattice structure, 57 is the lower lattice structure,
58, 58a and 58b are cells, and 70 is a space (predetermined distance in the axial direction).

Claims (1)

[Claims] 1 A composite fuel assembly for use in a thermal neutron reactor having a liquid coolant flowing upwardly through the core containing the vertically oriented composite fuel assembly, comprising fissionable nuclear fuel in a hermetic cladding and generally containing a plurality of elongated nuclear fuel rod structures arranged in a regular grid and surrounding said fuel rod structures at various preselected height positions to provide lateral support for said fuel rod structures in individual cells; In a composite fuel assembly comprising a plurality of lattice structures arranged, the fuel rod structure has upper and lower fuel rod bundles spaced apart from each other by a predetermined distance in the axial direction, and the lower fuel rod bundle There are a small number of fuel rods with a larger diameter than those in the upper fuel rod bundle, and the lattice structure of the lower fuel rod bundle is made to have a circumference approximately the same size as the upper fuel rod bundle. A composite fuel assembly characterized by: