JPH0564757B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to fast breeder reactors, particularly fast breeder reactors having an axially heterogeneous core.

[Background of the Invention] The core of a fast breeder reactor is generally formed in a cylindrical shape, and consists of a core region in which fissile material is present, and a blanket region surrounding the core region mainly consisting of fuel parent material. The blanket region includes a radial blanket region surrounding the outer periphery of the core region and an axial blanket region located at both axial ends of the core region. The fissile material present in the core region is primarily protonium-239, and the fuel parent material present in the blanket region is primarily uranium-238. Uranium-238 absorbs fast neutrons generated by nuclear fission of plutonium-239 to form plutonium-239.
Changes to

A common fast breeder reactor core is called a homogeneous core, in which the core region is divided into an inner core region and an outer core region in a circular manner. When the core region is divided in this way, the enrichment of fissile material (amount of fissile material/(amount of fissile material + amount of parent fuel material)) is higher in the outer core region. That is, the radial power distribution is flattened by having two types of enrichment in the core region.

An axially non-homogeneous core is a fast breeder reactor core (K. Inoue et al. (K.
Inowe, et al): “An Axially Heterogeneous Core Concept of a Large Liquid Metal Cooled Fast Breeder Reactor and its Behavior in a Hypothetical Core Collapse Accident.”
Core Concept for Large LMFBRs and Its
HCDA Behavior”), Nucl.Tech., 63.215 (1983)
Figure 2 shows a vertical cross-section of this core, with 1 being the core region, 2 being the radial blanket, and 3 being the core area.
4 indicates the shaft blanket, and 4 indicates the internal blanket. In this core, an internal blanket 4 is disposed at the axial center of a core region 1 surrounded by a radial blanket 2 and an axial blanket 3, and the axial thickness of the peripheral portion of the internal blanket 4 is determined by the inner blanket 4 region. It is thinner than the axial thickness at the center. Internal blanket 4 of this shape
By arranging the core, the power distribution in the radial and axial directions is flattened at the same time, and a maximum linear power output lower than that of a homogeneous core is achieved while the degree of enrichment in the core region remains the same. In this way, the thermal margin increases,
In addition to being able to achieve core miniaturization, this core has low combustion reactivity, which allows for a reduction in the number of control rods, low fast neutron flux levels, which extends fuel life, and increases the breeding rate. It has many advantages in improving economic efficiency, multiplication performance, and safety, such as shortening fuel doubling time, reducing the permissible vertical vibration fluctuation range of control rods during earthquakes, and reducing core release energy in the event of a hypothetical accident. It has certain characteristics.

However, although the shape of the internal blanket of the axially non-homogeneous core has been optimized to simultaneously improve many core properties as described above, there is still room for further improvement in terms of flattening the power distribution. is left behind.

[Purpose of the invention]

The purpose of the present invention is to improve the multiplication property of an axially non-homogeneous core,
The object of the present invention is to provide a fast breeder reactor that can flatten the power distribution and increase the thermal margin of the core or downsize the core while maintaining high performance in terms of safety and the like.

[Summary of the invention]

The present invention provides a reactor core region having fissile material;
an outer blanket region that surrounds the outside of the early core region and is mainly composed of a fuel parent material and is composed of a radial blanket region and an axial blanket region; and an inner blanket that is disposed within the core region and has a fuel parent material as a main component. The fast breeder reactor is characterized in that the axial height of the core region including the internal blanket varies in the radial direction of the core, and is higher in the outer region.

In the present invention, by making the axial height of the outer core region of an axially non-homogeneous reactor core higher than the axial height of the core inner region, the power sharing ratio of the core inner region where power peaks occur is reduced. This makes it possible to flatten the output distribution by evenly sharing the output with the outer region.

[Embodiments of the invention]

Examples will be described below.

Figures 1a and b respectively show a horizontal cross section and a vertical cross section of a core of a fast breeder reactor according to an embodiment, and the same parts or corresponding parts as in Figure 2 are given the same reference numerals, and 5 is a control rod. , 6 indicates a fuel assembly. As shown in Figure 1a, this core is loaded with a large number of fuel assemblies 5 and control rods 6 with hexagonal cross-sections, and as shown in Figure 1b, the core region 1 is surrounded by radial blankets. 2 and a shaft blanket 3, and an internal blanket 4 is located in the axial center of the core region.
The axial thickness of the core is thick at the center of the core and thinner at the periphery of the core, and the axial thickness of the upper and lower axial blankets 3 and 3 differs in the radial direction of the core and becomes thinner at the outer region. ing.

Figure 3 shows the peak line power and area average power of each region of the core of this example in the core radial direction, and is compared with the conventional case shown in Figure 2. , the horizontal axis shows the core radius (cm), the vertical axis shows the peak linear power (W/cm) and the area average power (relative value), and the core radius shows the position of the internal blanket. , C show the case of this embodiment, and B and D show the conventional case. In the conventional axially heterogeneous core shown in Fig. 2, the area average power decreases toward the outside of the core, as shown by the broken line D, and is extremely small in the outer area where the internal blanket 4 does not exist. It's summery. This is because in a fast breeder reactor, the average energy of neutrons is high and the average travel distance is large, so in the outer region where the effect of neutron leakage from the core region is large, the neutron flux decreases significantly and the output also decreases. be. Therefore, the broken line B indicating the peak line output
has a maximum value in the inner region where the inner blanket exists, and the peak line output in the outer region is relatively small. In contrast, in this embodiment, in order to solve this problem, the height of the outer region in the core axis direction is increased, and as shown by solid line C, the average output of the outer region is increased, and the average output of the inner region is increased. It is relatively low. As a result, the peak linear power in the inner region can be reduced as shown by the solid line A, and the maximum linear power of the core is reduced. In other words, in a conventional axially non-homogeneous core, the power peaks that occurred due to uneven power sharing in each core region can be reduced to a uniform power output throughout the core by increasing the power sharing in the outer regions. It is intended to reduce the

As described above, in this embodiment, the output peak in the outer region where the height in the axial direction of the core is increased hardly increases, and the output peak in the inner region significantly decreases. Next, this point will be explained in detail. Figure 4 shows the axial power distribution of the innermost fuel assembly, which has the largest fuel assembly power in the outer region of the core, and is shown overlapping the conventional case, with the horizontal axis representing the core height and the vertical axis. The axial output is set at , and E and F indicate the case of this embodiment and the conventional case, respectively.
The core height is indicated by the core and the blanket, respectively. As can be seen from this figure, the axial center part where the power peak occurs in this fuel assembly is adjacent to the internal blanket in the inner core region, so the power peak is reduced due to the effect of neutron leakage into the inner blanket region. has been done. Therefore, even if the height in the axial direction of the core is increased as in this example,
The power peak at the axial center hardly increases, and the power at the upper and lower parts of the core increases, forming a flat axial distribution and sharing the power in this region. Obtaining such an effect shown in this embodiment is one of the advantages of the present invention. For example, if the power sharing ratio is increased by increasing the enrichment of fissile material in the outer core area without changing the axial height of the core, as in the case of the homogeneous core described above, the fifth
The result will be as shown in the figure. In this figure, the horizontal axis shows the core height, and the vertical axis shows the axial power (relative value), and the core height is shown by the core and the blanket. In this figure, G indicates the case of high enrichment, and H indicates the case of the conventional example. As indicated by G in this figure, in the case of high enrichment, the output increases uniformly in the axial direction, so the output peak also increases significantly. This is because the heat output generated by nuclear fission increases at the axial center where the power peak occurs, as in other locations, as the content of fissile material increases, and the present invention as described above increases. Such an effect cannot be obtained.

In addition, as shown in FIG. 6, a core region 1, a radial blanket region 2, an axial blanket region 3
H. Spenke:
“Fast-breeder nuclear”
reactor”), US Patent 820038 (1969). This core was devised primarily for the purpose of reducing the sodium void reactivity coefficient, and the effect of neutron leakage from the core region was reduced by reducing the core diameter. In order to make the neutrons uniform in the direction, the axial height of the core region is lowered in the center of the core, where the contribution of leakage is smaller, to increase the leakage of neutrons.In this way, neutrons leak from the core region. Although equalizing the effect of leakage is effective in flattening the output distribution, the effect of the present invention, which effectively utilizes the output peak reduction effect due to the adjacency with the internal blanket, cannot be expected.Also, It goes without saying that the above-mentioned high performance in terms of multiplication performance, safety, etc. cannot be achieved in an axially non-homogeneous core by arranging an internal blanket.

Next, a method for determining the boundary between the core inner region and the outer core region in this embodiment will be described. FIG. 7 shows the maximum fuel assembly output and the outermost fuel assembly output in each core region when the boundaries between the inner and outer regions are changed. The horizontal and vertical axes of this figure are core inner region radius/core outer region radius (outer circumference) and fuel assembly output (relative value), respectively.
, and I, J, and K respectively indicate the cases of the highest aggregate in the inner region, the highest aggregate in the outer region, and the outermost periphery aggregate. Since the output of the outermost fuel assembly corresponds to the minimum assembly output of the core, the variation range of the assembly output within the reactor can be determined from the difference from the maximum fuel assembly output. When determining the flow rate distribution of sodium, which is a coolant, to each assembly, if the range of fluctuation in the output of the assembly increases, the range of flow rate distribution also expands, which complicates the core or assembly substructure and increases manufacturing costs. do. Therefore, if the boundary is stabilized under the condition that the fluctuation width of the aggregate output does not increase,
As can be seen from Figure 7, it is outside 70% of the radius of the core region. Therefore, in order to achieve the flattening of the power distribution, which is the effect of the present invention, without sacrificing economic efficiency, the height in the axial direction of the core should be constant up to a region exceeding 70% of the radius of the core region, and the height should be increased in the outer region. Just do it.

In an axially heterogeneous core of a large fast breeder reactor with an electrical output of 1000 MWe, 336 fuel assemblies, and a core axial height of 1 m, the present invention was applied to increase the core axial height of the outer region by 30 cm to 1.3 m. In this case, the maximum line power was reduced by 7%. Therefore. Thermal margin is 7
%, which makes core operation easier and improves fuel integrity. In addition, if this power distribution flattening effect is used to downsize the reactor core, that is, to reduce the number of fuel assemblies, it is possible to reduce the number of fuel assemblies by 7% of the total core, reducing the construction cost of fast breeder reactors and the fuel cycle. It is highly effective in reducing costs.

Other effects of the present invention include a reduction in the number of control rods due to a reduction in combustion reactivity, an increase in Doppler coefficient, and an improvement in safety due to a decrease in sodium void coefficient.

Note that the effects of the present invention described above are effects obtained in comparison with conventional axially non-homogeneous cores,
The high performance in terms of multiplication, safety, etc. possessed by the axially non-homogeneous core described above can be similarly achieved by the core of the present invention. That is, in the present invention, in addition to the characteristics of an axially non-homogeneous core that achieves high multiplication performance, high safety, etc., the above-mentioned improved effects can be obtained.

Next, a description will be given with reference to FIGS. 8 to 13 showing vertical cross-sectional views of the core of a fast breeder reactor according to another embodiment. In both figures, the same or corresponding parts as in FIG. 1 are given the same reference numerals.

The embodiment shown in FIG. 8 is a modification of the embodiment shown in FIG. 1, in which the thin part in the axial direction around the inner blanket 4 is expanded to fill the entire core area. The embodiment shown is a modification of the embodiment shown in FIG. 1, in which the thickness of the inner blanket 4 in the axial direction is made uniform in the radial direction.
By optimizing the inner blanket thickness and the boundary of the outer region, the same effect as the previous embodiment can be obtained in either case.

In the embodiment shown in FIG. 10, the radius of the inner blanket 4 is made smaller than that in the embodiment shown in FIG. 1, and a region is provided outside of the inner blanket 4 in which the height in the axial direction of the core remains unchanged. In this case as well, the radial power distribution is composed of three regions: the central region where the internal blanket exists, the region where there is no internal blanket and the height in the core axial direction is the same, and the region where the height in the core axial direction is high. Flattening of the directional power distribution can be achieved in the same manner as in the embodiment shown in FIG. Since the outer region is no longer adjacent to the inner blanket, the ability to reduce the axial output peak due to the adjacency effect described above will be reduced, but if the boundary of the outer region is slightly moved to the outside, it is possible to prevent the maximum linear output from occurring in this region. can. By optimizing the core configuration as described above, it is possible to achieve a flattened power distribution equivalent to that of the embodiment shown in FIG. 1 in this embodiment as well. The embodiment shown in Fig. 11 is a modification of the embodiment shown in Fig. 10, in which the height in the axial direction of the core in the outer region is changed in two stages, and the number of types of fuel assemblies increases by one, but the output The flattening rate of the distribution is further improved.

Furthermore, changing the height of the outer region by three or more steps is also effective in flattening the output distribution. Further, this structure can be applied to the embodiment shown in FIG. 1 as well as the embodiment shown in FIG. 12, which will be described later.

The embodiment shown in FIG. 12 is effective in flattening the power distribution when the control rods inserted from the upper part of the core are operated in a half-inserted state. When the control rods are half-inserted, the axial power distribution produces a power peak at the bottom of the core, so the internal blanket is placed slightly lower than the axial center position to reduce the power peak. The embodiment shown in FIG. 13 is a modification of the embodiment shown in FIG. 12, and similar to the embodiment shown in FIG. 12, flattening the power distribution when the control rod is partially inserted is realized. The inner blanket 4 is not lowered from the center in the axial direction, but only increases the height in the axial direction of the upper part of the core in the outer region, and a similar power distribution flattening effect can be obtained.

In each of the above-mentioned embodiments, by increasing the height in the core axial direction of the outer region of the core region, the power sharing ratio of this region is relatively increased, and the power sharing with the inner core region is equalized and the output is increased. Peaks are reduced. In this way, in order to flatten the power distribution using the simple principle of changing the axial height of the core region, the types of fuel assemblies and enrichment are the same as in conventional axially non-homogeneous cores, and the fuel Manufacturing costs will also remain unchanged. Furthermore, the effects of the present invention can be added without impairing the high multiplication ability, high economic efficiency, and high safety that are the major features of an axially non-homogeneous core.

Therefore, according to the fast breeder reactor of the present invention, the power distribution can be flattened by a simple change in the core configuration, so in addition to high breeding performance, high economy, and high safety, the thermal margin of the core can be improved. Improvements can improve fuel integrity and operational flexibility, or reduce plant costs and fuel cycle costs by downsizing the core by reducing the number of fuel assemblies.

〔Effect of the invention〕

The present invention is a fast breeder that can flatten the power distribution and increase the thermal margin of the core or downsize the core while maintaining high performance in terms of breeding performance and safety of an axially non-homogeneous core. This makes it possible to provide furnaces and has great industrial effects.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the core configuration of an embodiment of the fast breeder reactor of the present invention, Figure 2 is a vertical sectional view of the core of a conventional fast breeder reactor, and Figure 3 is the core radius of the core of Figure 1. Figure 4 shows the relationship between the core height of the reactor core in Figure 1 and the axial power in comparison with that of the conventional core in Figure 2. Figure 5 is a diagram showing the relationship between the core height and axial output of a core with a high enrichment degree in the outer core region in comparison with that of the conventional core shown in Figure 2. FIG. 7 is a vertical cross-sectional view of the core of another conventional fast breeder reactor; FIG. 1 to 13 are vertical sectional views of cores of other different embodiments of the fast breeder reactor of the present invention. DESCRIPTION OF SYMBOLS 1... Core region, 2... Radial blanket region, 3... Axial blanket region, 4... Internal blanket region, 5... Fuel assembly, 6... Control rod.

Claims (1)

[Scope of Claims] 1. A core region having fissile material, an outer blanket region surrounding the outside of the core region and consisting of a radial blanket region and an axial blanket region, the main component of which is a fuel parent material; In a fast breeder reactor having a built-in reactor core, which is disposed within a reactor core region and consists of an inner blanket region containing a fuel parent substance as a main component, the axial height of the core region including the inner blanket varies in the radial direction of the core. A fast breeder reactor that is characterized by being as high as the area. 2. The axial blanket region has an equal axial height until the fuel assembly outputs of the inner region highest assembly and the outer region highest assembly become equal in the radial direction of the inner region. Fast breeder reactor according to item 1.