JP2000321389A - Subcritical core for accelerator driving system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To equalize heat generation per a unit volume of a fuel placed next to a target, thereby increase a tolerance for integrity of the fuel in a subcritical core for an accelerator driving system. SOLUTION: A subcritical reactor comprises a rod like target 1 receiving a charged particle beam irradiated from the outside of the nuclear reactor and generating a neutron radiation, fuel rods 2, 3, 4 and cooling material placed around the target, and irradiates the neutral radiation to the fuel rods 2, 3, 4. Enrichment of a fissile material in an axial direction of the fuel rods 2 placed closed to the target 1 is decreased at a side receiving the irradiation of the charged particle beam.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a subcritical core of an accelerator driving system for irradiating a target with a charged particle beam, and more particularly to a subcritical core of an accelerator driving system having a flat power distribution in a core region adjacent to a target. .

[0002]

2. Description of the Related Art FIG. 14 is a view showing a subcritical core of a conventional accelerator driving system described in Japanese Patent Publication No. 9-506171. The subcritical furnace is divided into zones, the innermost zone 127 being filled with molten lead,
Acts as a target for the incident beam. The next region 128 is the main core, which contains fuel rods of appropriate geometry and dimensions, the fuel rods being formed by cladding the fuel oxide with thin stainless steel walls. A relaxation zone 129 filled only with molten lead is formed outside the main core region 128, and a breeder reactor region 130 formed of rods or pins having a structure similar to that of the core is formed on the outermost region.

FIG. 15 is a view showing the structure of a fuel rod and a partial assembly of the fuel rod housed in the main core region 128. As shown in FIG. As shown in FIG. 15A, the fuel is a fuel rod 75
A thorium metal fuel element 74 stacked to form a zircaloy sheet 73 to prevent corrosion. Each fuel rod also has an end cap 71 with a spring 72 to hold a fuel element 74. FIG. 15B is a view showing the subassembly, in which each fuel rod 75 is connected to the subassembly 7.
The fuel rods, which are grouped into six, form a rigid unit for easy handling.

[0004]

FIG. 16 is a diagram schematically showing a subcritical core of an accelerator driving system, and FIG.
FIG. 16A is a conceptual diagram of a subcritical core, FIG. 16B is a diagram showing a distribution of the number of neutrons incident on a main core from a target, and FIG.
FIG. 6 (c) is a diagram showing a neutron flux distribution in the main core.

In these figures, 101 is a proton accelerator, 102 is a proton beam irradiated from the proton accelerator, 1
Numeral 03 denotes a target. The proton beam is incident from above the target and causes a spallation reaction with a substance constituting the target to generate neutrons. Reference numeral 104 denotes a main core, in which fuel rods (not shown) are loaded in the main core, and a coolant is disposed in a gap between the fuel rods. Numeral 105 denotes the neutrons incident on the main core, and numeral 106 denotes the neutrons leaking from the main core.

[0006] The subcritical reactor is characterized by high safety because nuclear fission stops when the supply of the proton beam from the accelerator 101 is stopped because the nuclear reactor alone is subcritical.

The target 103 is formed of the same low melting point metal or alloy as the core coolant. For this reason, the target and the coolant are heated by the proton beam and the reaction heat from the core and are in a liquid state, and the axial density of the material constituting the target, that is, the density in the height direction is changed to an axial temperature distribution. It is almost constant when the volume difference based is ignored.

On the other hand, the proton beam 102 accelerated by the proton accelerator 101 enters from above the target, causes a spallation reaction with the material constituting the target 103, and proceeds downward in the target while generating neutrons. For this reason, the intensity of the proton beam gradually attenuates downward, and the number of neutrons generated by the spallation reaction between the proton beam and the target decreases with the attenuation of the proton beam at the lower portion of the target.

FIG. 16B is a diagram showing the relationship between the number of neutrons entering the main core from the target and the target axial height. As shown in the figure, the number of incident neutrons increases in the upper part of the main core. That is, since the number of neutrons generated is almost proportional to the intensity of the proton beam, the total number of neutrons generated from the target is larger at the upper part of the target. However, the maximum number of neutrons actually incident on the main core occurs at a position slightly lower than the top end of the target. Instead of all the neutrons generated at the target entering the main core,
There are neutrons leaking above or below the main core,
At the top end of the main core, even though the neutrons generated by spallation are large, the number of leaked neutrons is large.

In FIG. 16B, the maximum value of neutrons incident on the main core is 0.7% of the target height in the axial direction.
It occurs at about twice the position. The figure is a diagram exemplarily shown, and the position where the maximum value occurs varies depending on the height of the main core or the material constituting the target. However, if the height of the main core is several tens cm or more, the maximum value occurs in the upper region of the main core. Therefore, in the region of the main core adjacent to the target, the fission reaction in the upper region becomes larger than that in the lower region, and the calorific value per unit volume increases in proportion to the number of fission reactions.

FIG. 16C is a diagram showing the relationship between the neutron flux and the distance from the target center. As shown in FIG. 3C, the neutron flux in the radial direction of the main core becomes higher in a region closer to the target. Therefore, the fission reaction in the region closer to the target is larger than that in the region far from the target, and the calorific value per unit volume increases in proportion to the number of fission reactions.

That is, the calorific value per unit volume is larger in the fuel rods arranged in the region adjacent to the target than in the region distant from the target and in the region where the height of the target in the axial direction is low. The fuel region arranged in the region where the height of the target in the axial direction is higher is larger than the arranged fuel region.

As described above, when the heat value per unit volume of the fuel rod locally increases, the possibility that the fuel melts due to the temperature rise at the time of abnormality increases, and the margin regarding the fuel soundness decreases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has been made in consideration of the problem described above. To provide a subcritical core of the system.

[0015]

The present invention employs the following means in order to solve the above-mentioned problems.

A subcritical core comprising a columnar target for generating a neutron beam upon receiving a charged particle beam irradiated from the outside, a fuel rod and a coolant disposed on the periphery of the target, and irradiating the fuel rod with the neutron beam , Wherein the enrichment of fissile material in the axial direction of the fuel rod disposed close to the target is heterogeneous.

Further, in the subcritical reactor, the enrichment of the fissile material in the axial direction of the fuel rod is low on the side receiving the irradiation of the charged particle beam.

Further, it comprises a columnar target for generating a neutron beam upon receiving a charged particle beam irradiated from the outside, a fuel rod and a coolant disposed on the periphery of the target,
In the subcritical core for irradiating the neutron beam to the fuel rod, the enrichment of the fissile material in the radial direction between the plurality of fuel rods arranged close to the target is heterogeneous. .

In the subcritical reactor, the enrichment of the fissile material in the radial direction between the plurality of fuel rods is low toward the target.

Further, in the subcritical furnace, the fissile material is at least one of uranium 235, plutonium, americium, and curium.

Further, in the subcritical furnace, the target is made of a low melting point metal.

In the subcritical furnace, a plurality of the fuel rods are bundled to form a fuel assembly.

[0023]

1 to 6 show a first embodiment of the present invention.
FIG. 1 is a diagram showing a subcritical core of an acceleration drive system according to an embodiment of the present invention, and FIG. 1 is a sectional view of a subcritical reactor main core.

In the drawing, 1 is a target region, 2 is an axially non-homogeneous fuel rod whose plutonium enrichment varies axially, 3 is an axially homogeneous fuel rod of medium plutonium enrichment, 4
Is an axially homogeneous fuel rod with high plutonium enrichment. Note that a fissile material and a nitride fuel pellet of uranium 238 can be used as the fuel. The target region 1 is formed of a lead-bismuth alloy. Although the target region is illustrated in a cylindrical shape in the figure, the target region is formed of the same material as the coolant flowing between the fuel rods 2 to 4, and there is no partition in this portion.

The fuel rods 2 to 4 are composed of an axially non-homogeneous fuel rod 2, a medium plutonium-rich axial homogeneous fuel rod 3, and a high plutonium-rich Of the fuel rods 4 in the axial direction.

FIG. 2 is a longitudinal sectional view of the non-homogeneous fuel rod 2 in the axial direction. In the figure, 5 is a low plutonium enriched nitride fuel pellet, 6 is a medium plutonium enriched nitride fuel pellet, 7 is a high plutonium enriched nitride fuel pellet, and 8 is a compression for pressing the fuel pellet. Spring, 9
Is a cladding tube for containing fuel pellets, and 10 is a lower end plug of the cladding tube. The axially non-homogeneous fuel rod 2 is set so that the enrichment of plutonium, which is a fissile material, is lower as the nitride fuel pellets loaded in the upper region of the fuel rod.

FIG. 3 is a longitudinal sectional view of the axially homogeneous fuel rod 3 having the medium plutonium enrichment. In the drawing, reference numeral 11 denotes a medium plutonium-rich nitride fuel pellet. In the figure, the same portions as those shown in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 4 is a longitudinal sectional view of the axially homogeneous fuel rod 4 having a high plutonium enrichment. In the figure, reference numeral 12 denotes a high plutonium-rich nitride fuel pellet. In addition,
In the figure, the same portions as those shown in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted.

When the plutonium enrichment of the fuel rods arranged in the main core region of the subcritical reactor is set as described above, that is, when the fuel of lower enrichment is arranged in the region where the neutron flux density is higher. In addition, the calorific value per unit volume in the main core is equalized, and the margin for fuel integrity can be increased.

FIG. 5 is a diagram showing the axial height of the main core and the amount of heat generated per unit volume in a region of the main core adjacent to the target. In the figure, a curve 107 shows a calorific value of a conventional example in which fuel pellets with high plutonium enrichment are used for all fuel rods. Curve 108 shows the calorific value when the heterogeneous fuel rod 2 according to the present embodiment is used. Part a of the curve 108 is a calorific value due to the low plutonium-rich nitride fuel pellets 5, and part b is a medium plutonium-rich. The calorific value of the high-nitride fuel pellet 6 and the portion c indicate the calorific value of the high-plutonium-rich nitride fuel pellet 7, respectively.

As shown in the figure, by arranging the non-homogeneous fuel rods 2 in the area adjacent to the target, the maximum value of the calorific value can be suppressed to about 70% as compared with the conventional example. Fuel safety margin can be increased.

FIG. 6 is a diagram showing the heat value per unit volume in the upper region of the main core and the distance from the target center. In the figure, a curve 109 indicates a calorific value of a conventional example in which fuel pellets having a high plutonium enrichment are used for all fuel rods, and a curve 110 indicates a non-homogeneous fuel rod 2 according to this embodiment.
And the calorific value when the axially homogeneous fuel rod 3 of medium plutonium enrichment is used, and the portion a of the curve 110 is the calorific value by the low plutonium enriched nitride fuel pellet 5,
Part b is medium plutonium enriched nitride fuel pellet 11
And the portion c indicates the calorific value of the high plutonium-rich nitride fuel pellets 12, respectively.

As shown in the drawing, by arranging the non-homogeneous fuel rods 2 in the region adjacent to the target, the maximum value of the heat generation can be suppressed as compared with the conventional example.
In addition, high plutonium enriched nitride fuel pellets 12
On the other hand, the calorific value is higher in the outer area where
The calorific value per unit volume is equalized over the entire area of the main core, and the safety margin of the fuel is increased.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 7 is a longitudinal sectional view of the axially heterogeneous fuel rod 2a according to the present embodiment. In the figure, reference numeral 11 denotes a medium plutonium-rich nitride fuel pellet. In the figure, the same portions as those shown in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted.

The non-homogeneous fuel rods 2a in the axial direction are arranged in a region close to the target region similarly to the non-uniform fuel rods 2 in the first embodiment. The fuel rod 2a according to the present embodiment is characterized in that a nitride fuel pellet 11 with a medium plutonium enrichment is arranged on the upper part of the fuel rod.

As shown in FIG. 5 in relation to the first embodiment of the present invention, the calorific value per unit volume is small in the uppermost region of the main core. Accordingly, a medium plutonium-rich nitride fuel pellet 11 having a higher heating value than the low plutonium-rich nitride fuel pellet 5 is arranged at a position corresponding to the vicinity of the uppermost portion of the main core of the fuel rod. Thereby, the calorific value near the uppermost region of the main core can be increased, and the calorific value per unit volume can be further equalized.

Next, a third embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a sectional view of the main core according to the present embodiment. In the figure, 13 is a target region, 14 is an axially heterogeneous fuel assembly whose plutonium enrichment varies axially, 15 is an axially homogeneous fuel assembly with medium plutonium enrichment, and 16 is a high plutonium enrichment Is an axial homogeneous fuel assembly. The present embodiment is characterized in that a fuel assembly in which a plurality of fuel rods are accommodated in a wrapper tube is used as fuel. Although the figure shows the target region in a cylindrical shape, the target region is formed of the same material as the coolant flowing between the fuel assemblies 14, 15, and 16, and there is no partition in this portion.

FIG. 9 shows the axially heterogeneous fuel assembly 14.
9A is a schematic external view, and FIG. 9B is FIG. 9A.
It is AA sectional drawing of. In the figure, reference numeral 17 denotes a handling head attached to a wrapper tube, 18 denotes a wrapper tube for housing a fuel rod, and 19 denotes an axially non-homogeneous fuel rod for a fuel assembly housed in the wrapper tube 18. The axial non-homogeneous fuel rod 19 for the fuel assembly has the same configuration as the axial non-homogeneous fuel rod 2 or the axial non-homogeneous fuel rod 2a described above.

FIG. 10 shows the axial homogeneous fuel assembly 15.
10a is a longitudinal sectional view, and FIG.
It is BB sectional drawing of 0a. In the figure, reference numeral 20 denotes an axially homogeneous fuel rod for a fuel assembly with a medium plutonium enrichment contained in a wrapper tube 18. The axially homogeneous fuel rod 20 for a fuel assembly with a medium plutonium enrichment has the same configuration as the above-mentioned axially homogeneous fuel rod 3 with a medium enrichment. In the drawing, the same portions as those shown in FIG. 9 are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 11 shows the axial homogeneous fuel assembly 16.
11a is a longitudinal sectional view, and FIG.
It is CC sectional drawing of 1a. In the drawing, reference numeral 21 denotes an axially homogeneous fuel rod for a fuel assembly having a high plutonium enrichment and housed in a wrapper tube 18. The axially homogeneous fuel rod 21 for a fuel assembly with a high plutonium enrichment has the same configuration as the above-described axially homogeneous fuel rod 4 with a high enrichment. In the drawing, the same portions as those shown in FIG. 9 are denoted by the same reference numerals, and description thereof will be omitted.

Also in this embodiment, a fuel with a lower enrichment is arranged in a region where the neutron flux is higher in the main core region,
Since the fuel with high enrichment is arranged in the region where the neutron flux is low, the calorific value per unit volume in the main core is equalized, and the margin for fuel integrity is increased.

Next, a fourth embodiment of the present invention will be described with reference to FIGS. FIG. 12 is a sectional view of the main core according to the present embodiment. In the figure, reference numeral 22 denotes a radially heterogeneous fuel assembly whose plutonium enrichment changes radially. In the drawing, the same portions as those shown in FIG. 8 are denoted by the same reference numerals, and description thereof will be omitted.

The fuel assemblies 15, 16 and 22
From the region close to the target region to the outer periphery, radially heterogeneous fuel assemblies 22, medium plutonium-rich axial homogeneous fuel assemblies 15, and high plutonium-rich axial homogeneous fuel assemblies 16 Arrange them in order.

FIG. 13 shows the radially inhomogeneous fuel assembly 2.
FIG. 13A is a schematic external view, and FIG. 13B is a sectional view taken along line DD of FIG. 13A. Note that FIG.
10 to the same parts as those shown in FIG. 10 are denoted by the same reference numerals and description thereof will be omitted.

The radially inhomogeneous fuel assembly 22 accommodates an axially inhomogeneous fuel rod 19 for fuel assembly and an axially homogeneous fuel rod 20 for fuel assembly of medium plutonium enrichment in a wrapper tube 18. Form. The wrapper tube 18 containing the fuel rods 19 and 20 has the side containing the fuel assembly non-homogeneous fuel rod 19 for fuel assembly disposed on the inner peripheral side of the main core.

Also in the present embodiment, the fuel with a low enrichment is arranged in a region where the neutron flux is high in the main core region, and the fuel with a high enrichment is arranged in a region where the neutron flux is low. The calorific value per unit volume is equalized, and the margin for fuel integrity is increased.

In the above description, nitride fuel pellets of fissile material and uranium 238 are used as fuel, but oxide fuel pellets or carbide fuel pellets can be used. Although plutonium is used as the fission material, one or more of uranium 235, americium, and curium can be used. Although a lead-bismuth alloy is used as the target material, at least one of lead, bismuth, mercury, and lithium can be used.

[0048]

As described above, according to the present invention, since the calorific value per unit volume of fuel is equalized, it is possible to provide a subcritical core of an accelerator drive system with an increased margin regarding fuel integrity. .

[Brief description of the drawings]

FIG. 1 is a sectional view of a subcritical reactor main core of an accelerator driving system according to a first embodiment of the present invention.

FIG. 2 is a longitudinal sectional view of an axially heterogeneous fuel rod.

FIG. 3 is a longitudinal sectional view of an axially homogeneous fuel rod of medium plutonium enrichment.

FIG. 4 is a longitudinal sectional view of an axially homogeneous fuel rod with a high plutonium enrichment.

FIG. 5 is a diagram showing an axial height of a main core and a calorific value per unit volume in a region of the main core adjacent to a target.

FIG. 6 is a diagram showing a heat value per unit volume in an upper region of a main core and a distance from a target center.

FIG. 7 is a longitudinal sectional view of an axially heterogeneous fuel rod according to a second embodiment of the present invention.

FIG. 8 is a sectional view of a main core according to a third embodiment of the present invention.

FIG. 9 illustrates an axially heterogeneous fuel assembly.

FIG. 10 is a view showing an axial homogeneous fuel assembly.

FIG. 11 shows an axial homogeneous fuel assembly.

FIG. 12 is a sectional view of a main core according to a fourth embodiment of the present invention.

FIG. 13 illustrates a radially heterogeneous fuel assembly.

FIG. 14 is a diagram showing a subcritical core of a conventional accelerator driving system.

FIG. 15 is a view showing the structure of a fuel rod and a partial assembly of the fuel rod.

FIG. 16 is a diagram schematically illustrating a subcritical core of the accelerator driving system.

[Explanation of symbols]

1,13 Target area 2 Axial non-homogeneous fuel rod 3 Medium plutonium enriched axial homogeneous fuel rod 4 High plutonium enriched axial homogeneous fuel rod 5 Low plutonium enriched nitride fuel pellets 6,11 Medium plutonium-rich nitride fuel pellets 7,12 High plutonium-rich nitride fuel pellets 14 Axial heterogeneous fuel assemblies 15 Medium plutonium-rich axial homogeneous fuel assemblies 16 High plutonium richness Axial homogeneous fuel assembly 17 Handling head 18 Wrapper tube 19 Axial non-homogeneous fuel rod for fuel assembly 20 Axial homogeneous fuel rod for plutonium medium-enrichment fuel assembly 21 Plutonium-enriched fuel assembly axle Unidirectional fuel rod 22 Radial heterogeneous fuel assembly

Continuing from the front page (72) Inventor Shusaku Sawada 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi Plant (72) Inventor Masahisa Ohashi 3-1-1, Sachimachi, Hitachi-shi, Ibaraki Hitachi, Ltd.Hitachi Plant

Claims (7)

[Claims] 1. A target for receiving a charged particle beam irradiated from the outside to generate a neutron beam, comprising: a columnar target; and a fuel rod and a coolant disposed on the periphery of the target, wherein the fuel rod is irradiated with the neutron beam. The subcritical core of an accelerator driving system, wherein the enrichment of fissile material in the axial direction of a fuel rod disposed close to the target in the critical core is heterogeneous. 2. The accelerator driving system according to claim 1, wherein the enrichment of the fissile material in the axial direction of the fuel rod is low on the side receiving the irradiation of the charged particle beam. Subcritical core. 3. A target for generating a neutron beam by receiving a charged particle beam radiated from the outside, comprising a fuel rod and a coolant disposed on the periphery of the target, wherein the fuel rod is irradiated with the neutron beam. A subcritical core for an accelerator driving system, wherein the enrichment of fissile material in a radial direction among a plurality of fuel rods disposed in close proximity to the target is non-uniform. 4. The accelerator driving system according to claim 3, wherein the enrichment of fissile material in the radial direction between the plurality of fuel rods is low on the target side. Subcritical core. 5. The subcriticality of an accelerator driving system according to claim 1, wherein the fissile material is at least one of uranium 235, plutonium, americium, and curium. Core. 6. The subcritical core of an accelerator driving system according to claim 1, wherein the target is made of a low melting point metal. 7. The subcritical core of an accelerator driving system according to claim 1, wherein a plurality of the fuel rods are bundled to form a fuel assembly.