JPS636496A - Fast breeder reactor - 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 [Field of Industrial Application] The present invention relates to a fast breeder reactor, and particularly to a fast breeder reactor suitable for reducing fluctuations in power distribution due to combustion.

[Prior Art] As is well known, a fast breeder reactor causes a fuel parent material to absorb neutrons generated by nuclear fission in the core of a nuclear reactor to produce new fissile material, which is called breeding. It has the characteristic of being able to be used effectively. The core of such a fast breeder reactor is generally formed in a cylindrical shape, and the core is surrounded by axial and radial blankets made of fuel parent material. Such a core configuration can be achieved by arranging a large number of fuel assemblies in a cylindrical shape, in which a plurality of g'' momme fuel rods are bundled together. Alternatively, plutonium-enriched uranium is loaded, and the blanket is loaded with a fuel parent material, such as natural uranium or depleted uranium.This fuel parent material captures neutrons leaking from the reactor, producing useful fissile properties. Substances are produced.

Incidentally, the upper limit of the heat output that can be extracted from the core depends on the thermal limit of the highest temperature point. Therefore, by flattening the power distribution and reducing the maximum linear power density, it is possible to increase the thermal margin of the core. For this reason, the Evaluation of the Parfait Blanket Concept for First Breeder Reactors, COO-2250-5, MITINE-157, Massachusetts. Institute of Technology (1974), (#
Evaluation of the Parfai
tBlanket Concept for
Fast Breeder Reactors,
If Coo-2250-5, MITNE-157,
Massachusetts Institute o
A core concept called a parfait core was devised, as described in F Technology (1974), ). The parfait core has an internal blanket region that spreads out in the radial direction of the core in a disk shape at the axial center of the core to flatten the power distribution in the axial direction.At the same time, the enrichment of fissile substances in the core region is The output is low on the inside and high on the outside to flatten the radial output distribution.

Furthermore, there is a core concept called a fuel volume ratio two-region core as described in Japanese Patent No. 774,013. In a core with two fuel volume ratio regions, the enrichment of fissile material in the core region is made uniform, and the fuel volume ratio is reduced radially inside the core by changing the diameter of the fuel body or the arrangement pitch of the fuel bodies.

It is made larger on the outside to flatten the radial power distribution. In addition, by increasing the core volume to approximately 16 kQ to reduce the leakage of neutrons from the core, and by increasing the enrichment of fissile material in the fuel to approximately 10%, changes in reactivity are suppressed.
The output distribution is made to be approximately constant between burnup levels of 0 to approximately 1000 Wd/l.

[Problems to be Solved by the Invention] However, the above two conventional technologies do not sufficiently consider flattening of the power distribution when the burn-up of the fuel is as high as 1000 Wd/ or more. .

FIG. 2 shows the change in the infinite neutron multiplication factor of each region of a conventional parfait core due to combustion.

In the early stage of combustion, the difference in the effect of neutron leakage in each region is offset by changing the enrichment degree and arranging an internal blanket to make a difference in the infinite neutron multiplication factor, thereby flattening the output distribution. Due to combustion, the density of fissile material increases
It increases in the low enrichment region and the internal blanket region inside the core, and decreases in the high enrichment region outside the core, resulting in, for example, fuel depletion and a reduction in the power distribution flattening function. As a result, the power distribution fluctuated greatly, making it impossible to flatten the power distribution throughout the operation period, and it was necessary to use other means such as control rod insertion. In addition, fluctuations in power distribution create temperature differences between the coolant flowing in adjacent fuel assemblies, subjecting the upper core structure to periodic thermal shocks (thermal striving) and shortening its lifespan. Problems also arise.

Conventional cores with two fuel volume ratio regions also have problems similar to those described above. FIG. 3 shows the change in the infinite neutron multiplication factor of each region of a conventional two-region fuel volume ratio reactor core due to combustion. In the early stage of combustion, the difference in the effect of neutron leakage in each region is offset by changing the fuel volume ratio and making a difference in the infinite neutron multiplication factor, thereby flattening the power distribution. If the power density is constant, the burnup is higher in the low fuel volume ratio region inside the core, so the infinite neutron multiplication factor decreases faster than in the high fuel volume ratio region outside the core.

Therefore, in the final stage of combustion, the difference in the infinite neutron multiplication factor between surface areas became too large, making it impossible to flatten the output distribution throughout the operation period.

One of the determining factors for fuel life is the swelling of materials due to fast neutron irradiation. In a core with a fuel volume ratio of 2, the amount of fuel per unit volume in the center of the core where neutron flux is high is reduced to flatten the power distribution during low burnup.

Conversely, if the power distribution is flat, the neutron flux has a peak at the center of the core where the amount of fuel per unit volume is small, and is not sufficiently flattened. This limits the fuel life and makes it difficult to achieve a high burnup.

The purpose of the present invention is to reduce the maximum neutron flux level and extend the fuel life compared to the above-mentioned conventional technology, and at the same time, reduce fluctuations in the power distribution due to combustion and maintain a flat and stable output distribution throughout the operating period. The objective is to provide a fast breeder reactor with

[Means for solving problems] To achieve this objective, in the present invention.

In addition to reducing the amount of fuel per unit volume of the reactor core on the radially inner side and increasing it on the outer side, a disk-shaped internal blanket area that extends in the radial direction is provided inside the core, and the axial thickness of the internal blanket area is increased. thinner in the inner region of the core;
The structure is made thicker in the outer region.

[Operation Figure 4 shows the change in the infinite neutron multiplication factor of each region of the reactor core based on the present invention due to combustion. The degree of decrease in the infinite neutron multiplication factor in the inner core region (the broken 4 in the figure is similar to the conventional two-fuel volume ratio region core, which is larger than the outer region, but the infinite neutron multiplication factor in the inner blanket region As a whole, the difference in the infinite neutron multiplication factor between the inner region and the outer region is almost constant throughout the operation period.In order to compensate for the difference in the effect of neutron leakage in each region, the power distribution is Fluctuations can be suppressed to a small level.Also, by making the inner blanket thicker in the outer region of the core than in the inner region, a reaction due to combustion can be obtained during the operation period, so the maximum linear power density can be reduced. .

In the core based on the present invention, the internal blanket is placed in the center of the core where the neutron flux is generally high, so the neutron absorption effect is large, and the maximum fast neutron flux is significantly increased compared to the conventional core with a fuel volume ratio of 2. can be reduced and fuel life can be extended.

[Example 1] The present invention will be explained below according to an example. The target reactor core is loaded with core fuel consisting of a mixed oxide of plutonium and uranium and blanket fuel consisting mainly of depleted uranium, and uses sodium as a coolant. However, even if fuels other than those mentioned above are used, such as mixed carbide fuel, mixed nitride fuel, metal fuel, etc., and coolants other than those mentioned above, such as helium, water vapor, carbon dioxide, etc. The invention is applicable.

It consists of an inner core 1 with a small amount of fuel, an outer core 2 on the outer periphery with a large amount of fuel per unit volume, and an inner blanket 3 arranged in a disk shape in the axial center of the inner core 1. Ru. In the figure, 4 and 5 are radial and axial blankets, respectively.

The first difference from the conventional parfait core is that the plutonium enrichment is the same in the inner core 1 and the outer eccentric core 2, but the amount of fuel per unit volume is different. That is, the amount of fuel per unit volume in the inner core 1 is about 80% of that in the outer core 2. In order to achieve this, hollow pellets are used in the inner core 1 here. The second point is to spread the inner blanket to the middle of the outer core 2, and
Its axial thickness is thicker than that of the inner core.

In such a core configuration, the equivalent diameter of the inner core 1 is approximately 0.6 to 0.8 times the equivalent diameter of the core, and the axial thickness of the inner blanket in the outer region is 1.5 times that of the inner region.
By increasing the ratio by approximately 2.5 times, a flattening of the power distribution is achieved throughout the combustion period.6 Figure 5 shows that when the ratio of the thickness of the inner blanket to the outer region of the inner blanket is changed, The change in maximum line power is shown.

Next, the results of calculating the radial power distribution fluctuation characteristics of the core according to the present invention will be explained. The core design parameters and operating conditions are shown in Table 1 on the next page.

That is, the reactor thermal output is approximately 2,600 MW, and the electrical output is approximately 1. OOOMW, equivalent core diameter and core height are each 30
They are 0 cm and 120 cm. The axial and radial blanket thicknesses are 25 cm and 30 cm, respectively.

The refueling interval will be 15 months, the capacity factor will be 80%, and the number of refueling batches will be 3 for both core and blanket.

Table 1 Table 1 (Continued) FIG. 6 shows the radial power distribution of the core of the present invention (FIG. 1) calculated using the above core design parameters at the beginning and end of the equilibrium cycle. For comparison, the results for a conventional parfait core are shown in FIG. 7, and the results for a conventional two-area fuel volume ratio core are shown in FIG. As is clear from these results, in the core of the present invention, the rate of variation in the radial power distribution is reduced by a maximum of 1/2 to 1/3 compared to the conventional core. As a result, thermal striping problems are alleviated. In addition, the maximum linear output density during operation is approximately 1
3% reduction, increasing the thermal margin of the core. or,
If the maximum linear power density is kept constant, the number of core fuel assemblies can be reduced by about 13% compared to conventional reactor cores, and the fuel manufacturing cost can be reduced accordingly. Furthermore, the maximum fast neutron flux can be reduced by about 20% compared to the conventional two-region fuel volume ratio core, and the fuel life can be extended accordingly.

In the second embodiment shown in FIG. 9, the center of the internal blanket 3 is located below the axial center of the reactor core. During reactor operation, control rods are inserted from the upper end of the core, but by placing the internal blanket at the bottom as in this example, distortion of the power distribution due to control rod insertion when operating in a mid-insertion state can be avoided. can be reduced. Also, the first
Instead of locating the center of the inner blanket 3 below the axial center of the reactor core, as in the embodiment shown in FIG.
Also by making the shape of the internal blanket 3 asymmetrical in the axial direction and increasing the volume on the lower side, it is possible to reduce the distortion in the output distribution due to the insertion of the control armor.

In the embodiment shown in FIG. 11, regions having the same large amount of fuel per unit volume as the outer region are provided in the axially upper and lower parts of the inner region of the core. In this embodiment, since the output distribution is further flattened in the axial direction, the effect of reducing the maximum linear output is large.

Furthermore, as shown in FIG. 12, the effects of the present invention can be similarly obtained by forming a low enrichment region 6 in which the inner blanket is slightly enriched with fissile materials such as low enriched uranium and Pu. can. For example, it is possible to emphasize the flattening of the power distribution of the initially loaded core.

In the above example, fuel per unit volume between regions =, -:
In order to vary the amount of fuel in the inner core 1 (:
A hollow pellet was used for the sword. As a method other than the above, it is conceivable to reduce the sintered density of the fuel pellets in the inner core 1. In addition, by using small-diameter fuel rods in the inner core 1 and large-diameter fuel rods in the outer core 2, or by making the fuel rod arrangement pitch larger in the inner core 1 and smaller in the outer core 2, the unit volume It is possible to vary the amount of fuel per area. It is also possible to use oxide fuel in the inner core 1 and to use metal fuel, carbide fuel, or nitride fuel in the outer core 2.

Furthermore, in the inner core 1, a method of sandwiching structural material pellets between each fuel pellet, a method of mixing a substance with low neutron absorption into the fuel pellets, etc. are also considered.

In the above embodiment, the enrichment of fissile material in the inner core 1 and the outer core 2 is uniform, but it is possible to slightly lower the enrichment of the inner core 1 and reduce the thickness of the inner blanket 3. ,
Alternatively, the effects of the present invention can also be obtained by slightly increasing the enrichment of the inner core 1 and increasing the thickness of the inner blanket 3.

In addition, in the above embodiment, the amount of fuel per unit volume is 2
Although the changes were made in one area, it is also possible to implement this in three or more areas. FIG. 13 shows an example in which the core is divided into three regions in the radial direction and the amount of fuel per unit volume of the outer region is increased. The output distribution can be flattened. FIG. 13 shows an example in which there are three regions in both the radial direction and the axial direction.

In current designs, fuel assemblies are configured to load fuel pin bundles into hexagonal wrapper tubes. One of the main functions of the wrapper tube is to distribute the coolant flow rate for each fuel assembly. According to the core based on the present invention,
Since a flat and stable power distribution can be achieved throughout the operating period, flow distribution becomes unnecessary and there is the possibility of eliminating the trumpet tube. In this case, by making the fuel assembly of the inner core 1 have a structure without a trumpet tube and the fuel assembly of the outer core 2 having a structure with a trumpet bone, the amount of fuel per unit volume can be reduced in the inner core. It is conceivable to reduce the amount and increase it in the outer core.

[Effects of the Invention] As explained above, in the core of the present invention, compared to conventional parfait cores and cores with two fuel volume ratio regions, (1) power distribution fluctuations during operation can be reduced, and thermal damage to the upper core mechanism can be reduced.・The effect of striping is alleviated. (2) The maximum linear power density during operation can be reduced, increasing the thermal margin of the reactor core. Or (3) the amount of core fuel can be reduced by approximately 100%, reducing fuel manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of the core showing the first embodiment of the present invention, and FIGS. 2 and 3 are characteristic diagrams showing changes in the infinite neutron multiplication factor due to combustion in various conventional core inner and outer regions. FIG. 4 is a characteristic diagram showing the change in the infinite neutron multiplication factor due to combustion in the core inner region and outer core region according to an embodiment of the present invention, and FIG. 5 is a diagram illustrating the present invention. , FIG. 6 is a characteristic diagram showing the change in maximum linear power depending on the internal blanket shape, and FIG. 6 is a characteristic diagram showing the radial power distribution of the core based on the present invention, and FIGS. 7 and 8 are characteristic diagrams showing the radial power distribution of the conventional core. FIGS. 9, 10, 11, 12, 13, and 14 are furnaces showing various embodiments of the present invention in one diagram. FIG. 2 is a vertical cross-sectional view of the heart. DESCRIPTION OF SYMBOLS 1... Inner core, 2... Outer core, 3... Internal blanket, 4... Radial blanket, 5... Axial planchera b, 6... Low enrichment region, 7... ...Low fuel density region, 8...Medium fuel density region, 9...High fuel density region. Fig. 1 1... Inner core, 2... Outer core, 3... Internal blanket, 4... Radial blanket, 5... Axial blanket. Fig. 2 Fig. 3 (υ) IIJJ) → Operating period → (end 1υ1) Fig. 4 Fig. 5 2+, Figure 8 Figure 6 (furnace center) Strange direction PI Figure 11 Figure 12 Figure 13 Figure 14

Claims (1)

[Scope of Claims] 1. A high-speed reactor having a cylindrical reactor core whose main component is fuel enriched with fissile material, and a disk-shaped internal blanket region whose main component is fuel parent material located inside the core. In a breeder reactor, the amount of fuel per unit volume of the core is reduced on the radially inner side of the core and larger on the outer side, and the axial thickness of the disk-shaped internal blanket region that extends from the radially inner region to the outer region of the core is increased. , a fast breeder reactor characterized by being thinner in the inner region of the core and thicker in the outer region. 2. A fast breeder reactor according to claim 1, characterized in that the axial center of the internal blanket region is located below the axial center of the reactor core. 3. A fast breeder reactor according to claim 1 or 2, characterized in that a region with a large amount of fuel per unit volume, which is the same as that of the outer region, is provided in the upper and lower parts of the core inner region in the axial direction. . 4. Claim 1, 2, or 3 is characterized in that the geometry of the inner blanket region is asymmetric in the axial direction, and the volume of the inner blanket region is increased on the lower side in the axial direction. Fast breeder reactor. 5. A fast breeder reactor according to claim 1, 2, 3, or 4, characterized in that the inner blanket region is enriched with fissile material. 6. In claim 1, 2, 3, 4, or 5, the number of regions in which the amount of fuel per unit volume differs is 3 in the radial direction, axial direction, or both directions of the core. A fast breeder reactor characterized in that it is more than 100% in size. 7. In claim 1 or 2 or 3 or 4 or 5 or 6, the degree of enrichment of fissile material is changed between the inner core region and the outer core region. A fast breeder reactor featuring: