JPH0456955B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a fast breeder reactor, and particularly to a fast breeder reactor suitable for improving safety against reactivity injection events during earthquakes. [Background of the Invention] FIG. 1 shows a schematic structure of a typical tank-type fast breeder reactor that uses liquid metal sodium as a coolant. This tank-type fast breeder reactor 1 has a reactor core 4 disposed within a reactor vessel 2, and an upper reactor mechanism 6 within this reactor core 4.
A plurality of control rods 8 supported by the control rods 8 are inserted from above. Note that the reference numeral 10 is a roof slab that supports the furnace upper mechanism 6. The control rod 8 has a neutron absorbing material stored inside it, and has functions such as absorbing excess reactivity of the reactor and flattening the power distribution during operation of the reactor. As shown in the example column, the output of the reactor is controlled by moving all the control rods 8 in and out of the reactor core 4. The control rods 8 are ultimately supported by a roof slab 10 provided on the ceiling via the reactor upper mechanism 6, but this is different from the supporting portion of the reactor core 4. Therefore, when an earthquake occurs, the control rods 8 and the reactor core 4 will undergo relative vertical displacement, and especially if the control rods 8 are pulled out of the reactor core 4, there is a risk of immediate criticality or fuel failure, resulting in a major accident. is high. Therefore, in order to be safe against reactivity events such as earthquakes, the structure is designed to increase the rigidity of the roof slab 10 and the strength of its strength members. Generally, the maximum input reactivity amount is calculated by multiplying the maximum reactivity value per unit stroke of the control rod 8 partially inserted into the reactor core 4 by the vertical relative displacement amount, that is, when the control rod 8 is partially inserted into the reactor core 4. control rod 8
are all calculated assuming that the reactivity value per unit stroke is at the maximum position in the reactor core 4, and the reactor structure is designed to satisfy the allowable relative vertical displacement of the control rods and the core based on this value. It was necessary to do so. Note that partial insertion here refers to the range in which a minute relative displacement of the control rod causes a significant change in reactivity according to the present invention (for example, the reactivity value per unit stroke is about 1/10 or more of the maximum reactivity value). A control rod that is not partially inserted is hereinafter referred to as fully inserted or fully withdrawn. For example, this allowable value in a fast breeder reactor with an electrical output of 1 million KW class is approximately 30 mm, the diameter of the roof slab 10 is approximately 20 m, and the diameter of the core 4 and the roof slab 10 are approximately 30 mm.
This is very small compared to the vertical distance of about 10m. In order to satisfy this allowable value of approximately 30 mm, the girder height of the strength members of the roof slab 10 must be at least 3 m, and in order to further increase its rigidity, it is necessary to make efforts in the seismic design of the reactor building and reactor structure. However, the problem was that the reactor structure became more complex and larger, which made it uneconomical. [Object of the Invention] The purpose of the present invention is to reduce the number of control rods that are partially inserted into the reactor core during operation of a nuclear reactor, thereby relaxing the allowable relative vertical displacement between the control rods and the reactor core. [Summary of the Invention] The fast breeder reactor according to the present invention includes control rods that control the output by moving them in and out of the reactor core, and a first group that mainly has a reactor shutdown margin function and from which withdrawal is started first; A second group mainly has an output temperature compensation function and is extracted together with the first group, and a second group mainly has an operation maintenance function and is started after the first group is extracted, and the second group
The third group is extracted together with the first group, and the third group is extracted together with the second group. [Embodiments of the Invention] Table 1 shows reactivity values required for control performance and examples of their breakdown in a typical fast breeder reactor with an electrical output of 1 million KW class.

【table】

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the permissible value of the relative vertical displacement between the control rods and the reactor core is relaxed, so that the seismic safety of the fast breeder reactor is improved. In other words, since the rigidity of the roof slab and its strength members can be relaxed, this has the effect of reducing the size of the nuclear reactor, which in turn leads to lower manufacturing costs.

[Brief explanation of drawings]

Fig. 1 is a schematic structural diagram of a tank-type fast breeder reactor, Fig. 2 is a plan layout showing the positional relationship of fuel, main system control rods, and standby system control rods in the reactor core according to the first embodiment of the present invention; FIG. 3 is a diagram showing the relationship between the position of the control rod in the reactor core and the operating state of the reactor. 4... Core, 10... Roof slab, 12...
Core plate, 14... fuel, 16A... first group control rod, 16B... second group control rod, 16C... third group control rod, 18... backup system control rod.

Claims (1)

[Scope of Claims] 1. In a fast breeder reactor in which the reactivity of the reactor is controlled by moving a control rod containing a neutron absorbing material into and out of the reactor core, the control rod mainly has a reactor shutdown margin function. A first group whose extraction is started first, a second group which mainly has an output temperature compensation function and is extracted together with the first group, and a second group which mainly has an operation maintenance function and which is the first group. After the drawing is completed, the drawing is started, and the second
A fast breeder reactor characterized in that it is divided into three groups: a third group that is extracted together with the group; and a third group that is extracted together with the group. 2. The fast breeder reactor according to claim 1, wherein the average reactivity value of each first group control rod is greater than the average reactivity value of each third group control rod. . 3. The maximum value of the sum of all reactivity values of the control rods in the first group and the second group is set to be greater than the maximum value of the sum of all the reactivity values of the control rods in the second group and the third group. A fast breeder reactor according to claim 1 or 2, characterized in that: 4. The second group control rods and the third group control rods have the same structure containing the same neutron absorbing material, and the loading amount of the neutron absorbing material per control rod is the same. A fast breeder reactor according to claim 1 or 2. 5 the first group control rods and the third group control rods,
They are characterized in that they have the same structure housing the same neutron absorbing material, and the amount of neutron absorbing material loaded per first group control rod is greater than the amount of neutron absorbing material loaded per third group control rod. Claim 1
The fast breeder reactor according to paragraph 2 or paragraph 2.