JPS6361184A - Fast breeder reactor - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to fast breeder reactors, which extend the operating period and reduce fuel cycle costs without increasing the enrichment of fuel loaded into the reactor core. The present invention relates to a fast breeder reactor suitable for.

[Conventional technology]

Fast breeder reactors produce new fissile material within the reactor core by absorbing fast neutrons generated by the fission reaction of fissile material within the reactor core into the parent fuel material.

The core of a fast breeder reactor is generally formed into a columnar shape and consists of a core region consisting of driver fuel assemblies enriched with fissile material and a blanket region consisting of blanket fuel assemblies mainly composed of fuel parent material. has been done. It is also equipped with control rods containing neutron-absorbing materials that can be inserted into the reactor core at any time. The blanket region is divided into a radial blanket region surrounding the outer periphery of the core region and an axial blanket region located at both ends in the axial direction of the core. The fissile material in the core region is mainly plutonium-239, and the parent fuel material in the blanket region is mainly uranium-238. Uranium-238 absorbs neutrons generated by nuclear fission of plutonium-239 and changes into plutonium-239.

Fast breeder reactors also lose reactivity during power operation due to consumption of fissile material in the core region.

In order to compensate for this reactivity loss, the reactivity at the initial stage of combustion in the fast breeder reactor is made larger than the reactivity during operation, so that the fast breeder reactor can maintain a predetermined output for a predetermined period. The reactivity added to the breeder reactor in advance is called surplus reactivity. Control Ja inserted into the core of a fast breeder reactor
n controls excess reactivity to prevent nuclear runaway.

In order to reduce fuel cycle costs, it is important to lengthen the stay period of core fuel in the reactor and increase the average burnup after fuel removal. As a conventional technique for this purpose, it has been considered to increase the enrichment of fuel loaded into the reactor core.

[Problem that the invention seeks to solve]

However, this method has a first-order problem.

(1) Increase in control rod manufacturing costs As fuel enrichment increases, surplus reactivity increases, and in order to control this surplus reactivity, it is necessary to increase the worth and number of control rods. Therefore, the cost of manufacturing control rods increases.

(2) Increased fuel cycle cost The strong absorption of neutrons by the control rods significantly reduces the power density of the fuel region adjacent to the control rods and increases the power density of the fuel regions distant from the control rods. The degree of distortion in the power distribution due to control rod insertion depends on the control rod worth, and the more a control rod with a higher worth is inserted into the reactor core, the greater the distortion in the power distribution becomes. This increases the maximum value of the burnup in the loaded fuel, and the average burnup within the allowable maximum burnup of the fuel decreases compared to a core where the distortion of the outlet distribution is small. therefore.

Fuel cycle costs increase.

In addition, related fast breeder reactors include, for example,
49 (1984, 3) page 45 [There is a Joyo JMK-II reactor core. However, the MK-II core has a core structure with a fixed reflector without an external blanket region, and has a different structure and purpose of use from the fast breeder reactor targeted by the present invention.

An object of the present invention is to provide a fast breeder reactor that can increase the average burnup of fuel within the allowable maximum burnup and reduce fuel cycle costs and control rod manufacturing costs.

[Means for solving problems]

In order to achieve the above object, the present invention provides a material between a reactor core region having fuel enriched with fissile material and an outer blanket region or shield region surrounding the outside of this core region and mainly consisting of fuel parent material. The present invention provides a fast breeder reactor having a structure in which a neutron reflector is inserted into the region at an appropriate time during operation.

[Effect]

A neutron reflector inserted between the neutron region and the outer blanket or shield reflects neutrons flowing from the core region into the outer blanket or shield, increasing the neutron density in the core region.

Thereby, the reactivity loss due to neutron leakage from the core region is compensated for, and the surplus reactivity of the target fast breeder reactor is increased. In addition, the core before the reflector is inserted requires less control rod insertion than the conventional core during the same operation period, and the power distribution is less distorted and the power distribution is flat. This has the effect of lowering the maximum burnup of the fuel and increasing the average burnup of the entire core. On the other hand, in the core after the reflector is inserted, the region where the neutron density increases is outside the core region where the neutron density is low. It averages the power distribution within the furnace. Therefore, the average burn-up taken out after the reflector is inserted increases.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to Examples 6. The target reactor core is loaded with core fuel containing a mixed oxide of plutonium and uranium, mainly blanket fuel containing depleted uranium, and liquid sodium is used as a coolant. Although it was used.

The present invention is also applicable to cases where fuels and coolants other than those described above are used.

The first embodiment of the fast breeder reactor of the present invention is shown in FIG.
The horizontal view of the 6-core core is shown in (b) as a vertical cross-sectional view of the core. In the horizontal cross-sectional view of FIG. 1(a), the cylindrical core 1
A radial blanket 2 and a radial shield 3 are provided around the reactor core, and a control rod 4 is inserted into the reactor core.

A feature of this embodiment is that a reflector guide tube 5 is newly provided between the core 1 and the radial blanket. Figure 1 (b
), the movable reflector 6 is axially movable inside the reflector guide tube 5 and extends across the core 1.0, which corresponds to the axial length of the fuel assembly. axial blanket 7,
It can be installed at a position facing the structural member 8 of the reactor core. Note that the interior of the reflector guide tube 5 in the portion where the reflector 6 is not present is filled with a coolant. Table 1 shows the main design parameters of the reactor used in this example.

FIG. 2 is a vertical cross-sectional view of the movable reflector.

In this example, a circular stainless steel FF4 is used as one reflector.
A reflector rod 9 made of Both ends of the reflector rod 9 are connected to an upper tie plate 1o and a lower tie plate 11.
is a reflector rod 9 connected by a cylindrical cover 12.
is arranged within the cylindrical cover 12. The upper tie plate 10 has a connecting part 13 at its upper end, and the reflector drive device, which is connected to a reflector drive device (not shown), inserts the movable reflector 6 into the reactor core 1 using the rotational force of a motor. or
Or pull it out from the reactor core 1. Note that 14 in Figure 2
15 indicates an upper plenum, 15 indicates a lower plenum, and 16 indicates a coolant orifice.

Figures 3(a) and 3(b) show vertical cross-sections of the reactor core state during each operating period of the fast breeder reactor of this example.
a) shows the state of the core during the first year of the combustion cycle, and the movable reflector 6 is placed at the bottom in the axial direction. Core status (
The core a) exhibits the same characteristics as a general fast breeder reactor without the guide tube 5.

FIG. 3(b) shows that when the surplus reactivity reaches O in the core in the operating state (a), a movable reflector 6 is inserted into the guide tube 5 between the core 1 and the radial blanket 2. Realize. With the transition from operating state (a) to (b), the surplus reactivity increases, and the operating period is extended by operating in state (b).

Next, the effects of this embodiment will be explained. Figure 4 is similar to Figure 3 (b
) is a transition diagram of surplus reactivity during operation. The reactor was in the state shown in Figure 3(a) before reaching Figure 3(b).
It has been operated for a year and the surplus and attitude are 0. Seventy-two movable reflectors 6 are inserted into the reactor core to realize the reactor core configuration shown in FIG. 3(b).

By inserting the reflector 6, the surplus reactivity changes from 0 to a positive value, allowing operation. Case 1 in Figure 4 is a transition diagram when the composition volume ratio of the stainless steel of the movable reflector to the coolant is 70% to 30%, and Case 2 is a transition diagram when the composition volume ratio is 100% to 0%. This is a transition.

The number of days of extension of the operation period due to the insertion of a movable reflector is case 1.
55 days (15% increase over normal operating period) in case 2 and 76 days (21% increase over normal operation period) in case 2. Next, the conventional method of increasing fuel enrichment improves the performance of the engine and its performance. Compare. The operating period is the case of case 2 of the embodiment, which is a case of 441 days of operation (76 days extension). In order to achieve this number of operating days, the enrichment of the conventional core was increased by 0.3%. Table 2 shows a comparison of characteristics.

Table 2 Comparison of characteristics of embodiments The power peaking of the power distribution of the core before inserting the movable reflector in this embodiment is approximately 4% lower than the power peaking of the conventional core. This is because the amount of control rod insertion in this embodiment is significantly smaller than in the conventional core, and the distortion in the power distribution is small. FIG. 5 shows the radial relative output distribution before and after inserting the movable reflector. By inserting the movable reflector, the power of the radially outermost aggregate of the core increases, and the power distribution within the core becomes flat. This increase in burnup of the outermost peripheral assembly corresponds to about 1% of the average burnup taken out per operating cycle compared to the conventional core.

Next, other embodiments of the present invention will be described.

FIG. 6 is a vertical sectional view of the reactor core state corresponding to FIG. 3 of the first embodiment, in which the movable reflector 6 is installed at the upper part in the axial direction. In this case as well, the same effects as in the first embodiment can be obtained.

FIG. 7 shows an embodiment in which a movable reflector 6 is installed on the outer periphery of the radial shield 3. As shown in FIG. In this case as well, the same effects as in the first embodiment can be obtained.

In the above embodiments, stainless steel is used as the material of the reflector, but other materials such as beryllium, zirconium, and carbon can also be used as the material of the reflector.

〔Effect of the invention〕

According to Akira Mototo, in the case of a fast breeder reactor with an electrical output of 1000 Mw class, the amount of control rod insertion is reduced by about 2% compared to a conventional reactor core.
Since only a small amount is required, control rod manufacturing costs can be reduced. Also,
Since the average fuel extraction burnup can be increased by about 4%, the operating cycle cost can also be reduced.

[Brief explanation of drawings]

FIG. 1(a) is a horizontal sectional view of a core showing an embodiment of a fast breeder reactor according to the present invention, FIG. 1(b) is a vertical sectional view of the core, and FIG. 2 is a vertical sectional view of a movable reflector. Figure 3 is a vertical cross-sectional view of the core state during each period of the combustion cycle, Figure 4 is a diagram showing the transition of surplus reactivity when the reflector is inserted, Figure 5 is a radial power distribution diagram, and Figure 6 is and FIG. 7 are vertical sectional views (corresponding to FIG. 3) showing other embodiments of the present invention. 1... Core, 2... Radial blanket, 3...
Radial shield, 4... Control rod, 5... Reflector guide tube, 6... Movable guide tube, 7... Axial blanket, 8... Structural material, 9... Reflector Body Rondo, 10...
Upper tie plate, 11...lower tie plate, 12
...Cylindrical cover. 13... Connecting portion, 14... Upper plenum, 15...
-Lower plenum, 16...Coolant orifice.

Claims (1)

[Claims] 1. A reactor core region having fuel enriched with fissile material;
In a fast breeder reactor comprising a blanket region surrounding the core region and having fuel mainly made of a fuel parent material, and a shielding region surrounding the blanket region and having a neutron shielding material, a material is inserted outside the core region. A fast breeder reactor, characterized in that a neutron reflector made of a material that reflects neutrons is removably disposed in the material insertable region. 2. A fast breeder reactor according to claim 1, wherein the region into which material can be inserted is a neutron reflector guide tube installed between the core region and the blanket region. 3. The fast breeder reactor according to claim 1, wherein the material insertion possible region is a neutron reflector guide tube installed between the blanket region and the shielding region. 4. The fast breeder reactor according to claim 1, wherein the region into which a substance can be inserted is a neutron reflector guide tube installed within the blanket region. 5. The fast breeder reactor according to claim 1, wherein the material insertion possible region is a neutron reflector guide tube installed within the shielding region. 6. The fast breeder reactor according to claim 1, wherein the material insertion possible region is a neutron reflector guide tube installed outside the shielding region in the radial direction. 7. The fast breeder reactor according to any one of claims 1 to 6, wherein the neutron reflector is housed in the axially lower part of the insertable region at the time of escape. . 8. A fast breeder reactor according to any one of claims 1 to 6, characterized in that the neutron reflector is housed in an axially upper portion of the insertable region at the time of escape. .