JPS6262284A - Fast breeder reactor and fuel charging method thereof - 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 Application of the Invention] The present invention relates to a fast breeder reactor core and a fuel loading method thereof suitable for increasing the thermal margin of the reactor core and upper core mechanism.

[Background of the invention]

A fast breeder reactor is a reactor in which neutrons generated by the fission reaction of fissile material within the reactor core are absorbed into the parent fuel material, thereby producing new fissile material within the reactor core.

Nuclear fuel can be used effectively by producing this new fissile material, that is, by breeding it.

The core of a fast breeder reactor is generally cylindrically shaped and is formed from a core region consisting of a core fuel assembly enriched with fissile material and a proprietary socket region characterized by fuel parent material. Additionally, axial and radial blankets are provided around the core. The reactor core is loaded with enriched uranium or plutonium-enriched uranium as fuel, and the blanket is loaded with, for example, natural uranium or depleted uranium as a fuel parent material. This fuel parent material then captures the neutrons leaking from the core, producing useful fissile material.

Even in fast breeder reactors, during power operation, reactivity is lost due to the combustion of fissile material in the core region.

In order to compensate for this reactivity loss, consideration is given to increasing the reactivity at the initial stage of combustion so that the fast breeder reactor can maintain a predetermined output for a predetermined period of time. The reactivity added to the fast breeder reactor in advance is called surplus reactivity. Once the fuel has been burned for a period commensurate with the excess reactivity, a portion of the fuel (eg, the total IA) is replaced with fresh fuel. This is called a partially dispersed batch exchange, and it consists of one fuel exchange and three
In the case of the patch type, the IA fuel assembly in the core is replaced with new fuel. The period from one fuel change to the next is called the combustion cycle length, and the fuel stays in the furnace for (number of patches) x (combustion cycle length). Therefore, it is important that there are as many types of fuel in the core as there are patches, which have different stay periods in the reactor, that is, fuels with different burn-ups. Conventionally, fuel loaded into a reactor core is normally made to stay at the same location within the reactor core for its lifetime (number of patches x fuel cycle length).

In order to reduce nuclear fuel cycle costs, it is necessary to lengthen the stay period of core fuel in the reactor and increase the average fuel extraction burnup. In this case, the variation in the in-core residence period among the fuel assemblies in the reactor core increases, so the variation in the output of the fuel assemblies increases (the output mismatch factor increases).

When the output mismatch factor becomes large, the following two problems arise.

(1) Decrease in core thermal margin: Since the power that can be extracted from a nuclear reactor depends on the maximum power density of the fuel, it is important to flatten the power distribution as much as possible. A large power mismatch factor will reduce the thermal margin of the core as the maximum power density approaches the allowable value based on the thermal limitations of the core fuel pellets.

(2) Reducing the lifetime of the core superstructure: The flow rate of coolant through a fuel assembly is determined according to the maximum power that the fuel assembly generates in the reactor. Therefore, when the mismatch factor is large, the difference between the maximum and minimum values of the aggregate output becomes large, causing the aggregate to overcool while the aggregate output is small.
) state, and the outlet temperature of the aggregate becomes low. If one of the adjacent aggregates is supercooled and the other is in a normal cooling state, coolant having different temperatures will flow out from the outlet of each aggregate. These coolants are known to not mix easily and flow in layers (thermal striping).
g), which places repeated thermal stresses on the superstructure of the reactor core. This will shorten the life of the core superstructure.

Conventionally, efforts have been made to increase the thermal margin of the core of a fast breeder reactor. For example, JP-A-58-19591
In order to flatten the power distribution of the core, it is possible to load fuel with a lower mixing rate of fissile material than the core fuel near the axial center of the reactor core, and to make the mixing rate higher at the radial periphery of the core. has been disclosed to be effective. However, this technology has the problem of increasing the number of types of fuel used, and even if we are not aware of measures to prevent an increase in the output mismatch factor due to a prolonged combustion cycle. Also,
There is no disclosure of measures to mitigate thermal swiping, which shortens the life of the core superstructure, during long combustion cycles.

[Purpose of the invention]

The present invention has been made in view of the above, and its purpose is to increase the thermal margin of the reactor core and shorten the life of the core superstructure by reducing the output mismatch factor of fast breeder reactors. The purpose of the present invention is to provide a fast breeder reactor core that can prevent this and a fuel loading method for realizing it.

[Summary of the invention]

According to the invention, a cylindrical fast breeder comprising a bundle of multiple fuel assemblies with axial compartments for forming an internal blanket, thereby comprising a radially extending disc-shaped internal blanket. In a reactor core, fuel assemblies loaded in a region closer to the radial periphery have a higher burnup than fuel assemblies loaded in a radially central region of the core region having an internal blanket. A fast breeder reactor core is provided, characterized in that it has the following features:

Further, according to the present invention, in such a fast breeder reactor core, the fuel assemblies to be loaded into the core region having an internal blanket are loaded in the radially central region of the core region while the burnup is low, and the fuel assemblies are loaded into the radially central region of the core region, and the This can be realized by a fuel loading method characterized by reloading fuel to a region closer to the radial periphery when the fuel temperature becomes higher than a predetermined value.

The principle of the present invention is as follows.

(1) The output of the fuel assembly near the radial center of the core varies greatly depending on the fissile material that accumulates in the internal blanket during combustion. Therefore, instead of having the fuel assemblies initially loaded in the area near the center of the reactor core remain in the same place until the end of their life (number of patches x burning cycle length) as in the past, we It stays there only as long as the temperature is low, and for the rest of the time it is reloaded into an area near the core with an internal blanket. Thereby, it is possible to reduce output fluctuations caused by combustion of the fuel assemblies loaded in the region near the radial center of the core, and to make the output mismatch factor extremely small.

(2) In order to reduce the maximum power density and increase the thermal margin of the core, it is necessary to reduce the out-kerking coefficient. The axial power ♂-King coefficient can be made smaller as in the conventional case by placing an internal plunket with low power density and low reactivity in the axial center of the core. on the other hand,
As explained in (1) above, radial carking can be made smaller than before by loading high-burnup fuel into the area near the periphery of the core, which has an internal blanket. It is possible. This is because there is a large amount of fissile material accumulated in the blanket inside the high-burnup fuel throat, which alleviates the drop in power in the radial direction of the core.

Based on the above principle, the present invention makes it possible to reduce the output mismatch factor and maximum output density.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described in detail. The target reactor core is one that uses a mixed oxide of plutonium and uranium as fuel, depleted uranium oxide as blanket fuel, and liquid sodium as a coolant.

A conceptual longitudinal cross-sectional view of the core of this embodiment is shown in FIG. 2, and a horizontal cross-sectional view of the core is shown in FIG. 3. In FIG. 2 and FIG. 3, a radial blanket 12 mainly composed of a fuel parent substance and top,
A lower axial blanket 13.14 is installed. The reactor core 11 is composed of fuel of one type of enrichment. Inside the core 11, there is provided a disk-shaped internal blanket 15 that is mainly composed of a fuel parent material and that extends in the radial direction of the core. Therefore, there are two types of core fuel assemblies: those with internal blankets and those without internal blankets. That is, in the reactor core 11, the regions 11A and IIB where the internal blanket 15 exists are loaded with fuel assemblies having an axial section mainly composed of the fuel parent material constituting the internal blanket in the axial center, and A fuel assembly that does not have such a front section at the center in the axial direction is loaded in the region 11C where no blanket exists.

The inner blanket thickness is radially constant.

The burnup of the fuel assemblies loaded in the region 11A near the radial center of the core is higher than that in the region 11A nearer to the periphery of the core.
The burnup is lower than that of the fuel assembly loaded in B. That is, the fuel assembly having the internal blanket is first loaded into the core region 11A, and when the burnup reaches a certain level, it is reloaded into the core region 11B. Therefore, the burnup is higher in fillB than in IIA. On the other hand, the peripheral core region 11C is loaded in a normal three-patch distributed manner, and fuel assemblies with different burn-ups are mixed together.

That is, FIG. 1 shows the loading pattern of the fuel assembly of this embodiment. However, this figure shows the IA quadrant of the core and blanket for the sake of simplification, but the same applies to the other 5A quadrants. Each hexagon in the diagram represents a fuel assembly, and the number written inside each hexagon represents the replacement patch number, that is, the number of years the fuel assembly has been in the core. ing. Fuel assemblies with the same replacement patch number are replaced or reloaded at the same time. Also, the black circles indicate control rods.

As can be seen from this figure, only the fuel assemblies from the first and second years of stay exist in the core region 11A, and the core region 11B
Only the fuel assembly from the third year of his stay exists. That is,
The fuel elements that stayed in core region 11A for two years were moved to core region 11B in the third year, and a new first-year fuel assembly was loaded in its place, and the fuel element ended its third year stay in core region 11B. The fuel assembly is taken out of the reactor. On the other hand, in the core region 11C, normal three-patch distributed loading is applied,
There, each fuel assembly remains in the same location for three years.

The core design parameters and operating conditions in this example are as shown in Table 1. The reactor thermal output is approximately 250
0 MW, refueling interval (combustion cycle length) is 1 year, and number of refueling patches is 3. It is assumed that the core fuel smear density is 87 cm theoretical density, and the axial blanket fuel is used as is for the internal blanket fuel.

In contrast, Table 1 shows a conceptual longitudinal cross-sectional view, a horizontal cross-sectional view, and a fuel loading pattern (1/6
quadrants) are shown in FIGS. 4, 5 and 6, respectively. As shown in the figure, the core consists of a region 21A having an internal blanket and a region 21B outside the region having no internal blanket, and three patches are distributed over the entire region of the core, and the core fuel having an internal blanket is The loading location of the assembly does not change even if the burn-up increases, and it remains at the initial location until the end of its life.

In other words, fuels with different burn-ups are mixed in all regions.

A comparison of the fuel exchange methods for fuel assemblies including internal blankets between the embodiment of the present invention and the conventional example is as shown in Table 2 below. The conventional example and the embodiment of the present invention are the same in terms of the strength of the fuel assembly, including the inner blanket.

Table 2 Therefore, in contrast to the conventional example where υ is replaced in three batches over the entire region of the core, in the core of the embodiment of the present invention, for the fuel assembly having an internal blanket, the region 11A near the radial center of the core is replaced. In this case, two patches will be replaced, and in the region 11B near the core, which has an internal blanket, one patch will be replaced. That is, in the reactor core of the embodiment of the present invention, new fuel is first loaded into a region 11A near the radial center of the core, burned there for two years, and then reloaded into a region 11B near the periphery of the core having an internal blanket. After burning there for one year, it is extracted from the core and sent to a reprocessing facility.

The radial distribution of the output in the embodiment of the present invention at the beginning and end of the equilibrium cycle is shown in FIG. 7, and that in the conventional example is shown in FIG. The power was expressed as a patch average value of the axial integral value (corresponding to the average value of the power of fuel assemblies with different burn-ups). The solid line and broken line in the figure represent the beginning and end of the cycle, respectively. Regarding the flatness of the radial distribution of power, the core of the embodiment of the present invention is superior to the conventional example.

Table 3 compares the power peaking coefficient at the beginning of the combustion cycle and the maximum power fluctuation due to combustion of the fuel assembly between the present invention and the conventional example.

Table 3 shows that in the core of the embodiment of the present invention, the power density is maximum in the core region with one patch, whereas in the conventional core, it occurs in the region with three patches, and the patch average power peaking coefficient In the embodiment of the present invention, the total output kerking coefficient is improved (by about 2 inches) over that of the conventional core.Compared to the conventional core, the total output kerking coefficient is about 5 inches smaller in the core of the embodiment of the present invention. There are 9. The region where the power density is maximum varies depending on the design. For designs where the output density is maximized in the area where the number of patches is 3, use Misma.

The chifactor is the same for both cores, but even in this case,
The effect of improving the patch average output peaking coefficient can be obtained. In addition, the maximum value of output fluctuation due to combustion of the fuel assembly is
The core of the embodiment of the present invention, which has fewer patches, is approximately 25% smaller than the core of the conventional example. This is because the output fluctuation due to combustion in the area where the number of patches is 1 is small. These effects can be explained by the principle stated in (2) of the [Summary of the Invention] section.

In addition, in the above embodiment, the number of patches is 3, and as shown in FIG. 9, the number of patches is 4. Alternatively, an embodiment in which the number of patches is two as shown in FIG. 10 may be used.

FIGS. 11 and 12 are conceptual vertical cross-sectional views of a core showing other embodiments of the present invention, respectively. In FIG. 11, the core has an internal blanket region 31A.

31B1 and regions 31C and 31D outside of it, which have inner blankets of smaller thickness. 12th
In the figure, the core consists of regions 41A to 41D having internal blankets similar to those described above and a region 411 outside of the regions 411 without internal blankets.

Regarding the area with internal blanket, in FIG.
The core regions 31A and 31C are loaded with fuel assemblies with low burnup, while the region 31B outside the region 31A and the core region 31D outside the region 31C are loaded with the fuel assemblies having a low burnup.
Fuel assemblies coming from 1A and region 31C are reloaded to form a high burnup region. Similarly in FIG. 12, low burnup fuel assemblies and high burnup fuel assemblies are loaded in the core regions 41A, 41C and the core regions 41B, 41D outside of the core regions 41A, 41C, respectively. Both embodiments are effective in reducing the output mismatch factor and maximum output density, similar to the first embodiment described above.

In addition, even in the core region that does not have an internal blanket, it is possible to reload the aggregate according to the burnup as described above.
The output molding coefficient and mismatch can be reduced.

In both the embodiments shown in Fig. 11 and Fig. 12, the case where the present invention is not applied, that is, the loading location of the core fuel assembly having an internal blanket is fixed at a constant core (ordinary patch Similar to the first embodiment, the output mismatch factor and the maximum output density are reduced compared to the case where a distributed loading method is adopted.゛The relative degree of improvement by applying the present invention (maximum weight 'j density after application/maximum power density before application, etc.) is shown in Figure 11.

The embodiment shown in FIG. 12 can be considered to be almost the same as the first embodiment.

Comparing the first embodiment, the embodiments shown in FIGS. 11 and 12, qualitatively, regarding the maximum output density, the internal blanket thickness is thicker at the center and thinner at the periphery, and Since it is known that it is best to have a core configuration in which the blanket does not extend to the radial blanket, the embodiment shown in FIG. 12 in which the present invention is applied to this is considered to be the best. However, the embodiment shown in FIG. 11 is considered to be equivalent to or even better than the first embodiment in terms of maximum output density. For reference, the correlation between the internal blanket configuration and the maximum output density is described in Japanese Patent Application Laid-open No. 57-119280 (57, 7, 24
) describes the details regarding the patch distributed loading method.

Determining the reloading pattern of fuel assemblies from the viewpoint of minimizing the number of assemblies to be moved is important in improving reactor operating efficiency. One embodiment is shown in FIG. The figure (,) shows a conceptual cross-sectional view of the core, and the figure (b) shows the fuel loading pattern. In this example, the core includes regions 51A, 51B, 51C with internal blankets and
The region 51D has no inner blanket around it. In this core, regions 51A and 510 are respectively the same as regions 11A and IIB in the first embodiment, but a new region 51B is provided between them. This area is 51
This area is a combination of B1 patch numbers 1 to 3, and the fuel assembly is not moved in this area. In this case, compared to the embodiment shown in FIG. 1, the maximum linear power density and mismatch factor are slightly larger due to the presence of the region 51B, but the number of fuel assemblies moving from the region 51A to the region 51C can be reduced. This has the advantage of shortening the period during which the reactor is shut down for fuel exchange.

〔Effect of the invention〕

As explained above, according to the present invention, the maximum power density can be reduced by about 5% compared to the conventional core, and the power fluctuation due to combustion of the fuel assembly can be reduced by about 25 inches. These effects increase the thermal margin of the core and help prevent shortening of the life of the core superstructure. In addition, it becomes possible to further lengthen the combustion cycle, which is one of the keys to improving the economic efficiency of fast breeder reactors.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a fuel loading/gut turn according to an embodiment of the present invention, FIGS. 2 and 3 are a vertical cross-sectional conceptual diagram and a cross-sectional view, respectively, of the core of the same embodiment, and FIGS. Figure 5 is a conceptual vertical cross-sectional view and cross-sectional view of a conventional reactor core, respectively;
Figure 7 shows the fuel loading pattern in the conventional example.
The figure shows the radial distribution of the output at the beginning and end of the equilibrium cycle in the embodiment of the present invention, FIG. 8 is a similar diagram in the conventional example, and FIGS. FIGS. 11 and 12 are diagrams showing different fuel loading patterns according to modified examples, and FIGS. )
2A and 2B are diagrams respectively showing a longitudinal cross-sectional conceptual diagram and a fuel loading pattern of a reactor core according to still another embodiment of the present invention. 11A, 11B, 11C... Core region 12... Radial blanket 13.14... Axial blanket 15... Internal blanket 21A, 21B; 31A to 31D: 41A to 41E: 5
1A to 51D...Core area Fig. 3 Fig. 4 Fig. 5 Fig. 7 Core radial position 1 Core radial direction AΩ position Fig. 11 Fig. 12

Claims (2)

[Claims] (1) Consisting of a bundle of multiple fuel assemblies having axial partitions for forming an internal blanket whose main component is a fuel parent substance, thereby providing a disk-shaped internal blanket that expands in the radial direction. In a cylindrical fast breeder reactor core, among the core fuel assemblies having an internal blanket, the fuel assemblies are loaded in a region closer to the radial periphery than the fuel assemblies loaded in the radially central region. A fast breeder reactor core characterized by having a higher burnup. (2) Consisting of a bundle of multiple fuel assemblies having axial partitions for forming an internal blanket whose main component is a fuel parent substance, thereby providing a disk-shaped internal blanket that expands in the radial direction. A method of loading fuel into a cylindrical fast breeder reactor core, in which fuel assemblies are loaded into a core region having an internal blanket, and are loaded into the radially central region of the core region while the burnup is low. A fast breeder reactor core loading method characterized by reloading fuel to a region closer to the radial periphery when the fuel temperature becomes higher than a predetermined value.