JPH06186373A - Control rod for fast breeder reactor - Google Patents

Abstract

PURPOSE:To obtain a safety regulation control rod by covering pellets of neutron absorbing material with a metal having large neutron absortion area thereby preventing relocation of boron-carbide pellets. CONSTITUTION:A control rod pin 7 is prepared by loading boron-carbide pellets 3 into a stainless steel coating pipe 2. Outer periphery of the pellet 3 is protected by a niobium tube 8 having large cross-section for absorbing neutron. The pellet 3 is supported, at the upper and lower ends thereof, by means of springs 4 and the pin 7 is sealed hermetically by end plugs 5 and constructed into unbent type. Even when the pellet 3 is cracked, the niobium tube 8 blocks the pellet chips and prevents the gap between the coating pipe 2 and the pellet 3 from narrowing thus blocking occurrence of mechanical interaction. A metal having large cross-section for absorbing neutron, e.g. rhenium(Re), europium(Eu), lithium(Li), niobium(Nb), is suitably employed as the metal covering the pellet 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod for a fast breeder reactor which is used for insertion and withdrawal into a core to adjust the power.

[0002]

2. Description of the Related Art There are two types of control rods for fast breeder reactors: safety rods and adjusting rods. The safety rod is for inserting the reactor into the core during a scram of the reactor to stop the reactor, while the adjusting rod is for inserting into and extracting from the core to adjust the power output of the reactor. is there.

FIG. 5 shows a control rod pin housed in a conventional control rod for a fast breeder reactor for power adjustment. Control rod pin 1
Is a stainless steel cladding tube 2 with boron carbide (B
₄ C) A plurality of pellets 3 are laminated. Both upper and lower ends of the boron carbide pellet 3 are elastically supported by springs 4, and upper and lower ends of the control rod pin 1 are sealed by end plugs 5. The space (gas plenum) 6 inside the control rod pin 1 is boron B ¹⁰ of boron carbide pellets.
The helium (He) gas generated by the (n, α) reaction is stored.

By the way, during the operation of the nuclear reactor, an adjusting control rod is inserted in the core for adjusting the output. Since a plurality of boron / carbite pellets are loaded in the adjusting control rod as described above, the boron B ¹⁰ (n,
Helium gas is generated by the α) reaction, and one (n, α) reaction is accompanied by a heating value of about 28 MeV. Since this reaction occurs in the entire pellet, a temperature distribution having a high central portion and a low peripheral portion is generated in the boron carbide pellet, and tensile stress is generated in the peripheral portion of the pellet. When the tensile stress in the periphery of the pellet exceeds the pellet breaking stress, the boron carbide pellet 3 is cracked, and at that time,
Broken boron carbide pellets scatter (relocate).

By the relocation of the boron carbide pellets, the gap width between the stainless steel cladding tube and the boron carbide pellets is narrowed, and the stainless steel cladding tube and the boron carbide pellets come into partial contact with each other. . When the adjusting control rod is used in such a state, helium gas is successively produced in the pellet by the (n, α) reaction of boron B ¹⁰ of the boron-carbite pellet. A part of this helium gas is discharged from the pellet and stored in the gas plenum portion 6, but the remaining helium gas is confined in the pellet 3. Therefore, helium gas accumulates in the pellets as the boron B ¹⁰ is burned by the (n, α) reaction, and expansion of the pellets (gas swelling of the boron carbide pellets by the helium gas) occurs.

Since the coefficient of thermal expansion of stainless steel used as a cladding tube for control rods is several times larger than that of boron carbide, the gap between the stainless steel cladding tube and the boron carbide pellets at this time. The width is wider than when it was manufactured. For this reason, the contact between the stainless steel cladding and the boron carbide pellets by relocation and swelling is not so strong. However, when the reactor is shut down and the temperature of the coolant is lowered, the thermal contraction of the stainless steel cladding is several times larger than that of boron carbide pellets, so the stainless steel cladding is subject to relocation and swelling. Receive excessive stress. When this stress exceeds the breaking stress of the stainless steel cladding tube, the adjusting control rod is damaged.

However, the conventional control rods for fast breeder reactors have no means for preventing the relocation of the neutron absorbing material pellets.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a safe adjusting control rod by preventing relocation of boron carbide pellets in a fast breeder reactor control rod. It is intended.

[0009]

The above object can be achieved by covering a part or all of the side surface of the neutron absorbing material pellet in the control rod pin with a metal having a large neutron absorbing cross section. As a metal for covering the boron carbide pellets, a neutron absorber metal having a large neutron absorption cross-section is preferable so as not to impair the reactivity control ability, and particularly, rhenium (Re), europium (Eu), lithium (L)
i), tantalum (Ta), tungsten (W), niobium (Nb), molybdenum (Mo), indium (In),
Cadmium (Cd) and silver (Ag) are preferable.

[0010]

In the present invention, since a part or all of the side surface of the neutron absorbing material pellet is covered with a metal having a large neutron absorption cross-sectional area, even if a crack occurs in the pellet, the broken pellet piece scatters. It does not result in a narrow gap between the cladding and the boron carbide pellets. Therefore, the gap width between the cladding tube and the boron carbide pellets is secured, and the mechanical interaction between the pellets and the cladding tube can be prevented.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. In the figure, the control rod pin 7 is a boron-carbide pellet 3 having an outer diameter of 16 mm and containing boron-10 (B ¹⁰ ) concentrated on a stainless steel cladding tube 2 having an outer diameter of 18.5 mm and a wall thickness of 0.8 mm. It is loaded. The outer periphery of the boron carbide pellets 3 is protected by a niobium cylindrical tube 8 having a large neutron absorption cross-sectional area of 0.2 mm. The upper and lower ends of the boron carbide pellet 3 are elastically supported by springs 4, respectively, and the control rod pin 7 is of a non-vent type in which the upper and lower ends are sealed by end plugs 5.

Since the control rod pin of the present invention is constructed as described above, even if a boron carbide pellet is cracked, it is hindered by this niobium cylindrical tube 8 and the broken pellet piece causes the cracked pellet piece to be covered. Therefore, the mechanical interaction between the cladding tube 2 and the pellet 3 can be prevented without narrowing the gap width between the pellet 3 and the pellet 3.

FIGS. 2, 3 and 4 show other embodiments of the present invention, respectively, and are views showing the cross section of a boron carbide pellet whose outer periphery is covered with a metal having a large neutron absorption cross section. A metal having a large neutron absorption cross-section does not necessarily have to cover the entire length of the boron carbide pellet stack as shown in FIG. 1, and as shown in FIG. 2, for example, a pellet stack part with a high burnup (the pellet part at the tip of the control rod pin). ), Only the pellets may be individually covered with a metal cylindrical tube, or a part of the outer periphery of each pellet may be covered with a metal cylindrical tube as shown in FIG. 3, or as shown in FIG. Only the upper and lower end surface portions of the vertically adjacent pellets may be covered with the metal cylindrical tube.

Further, the shape of the metal covering the outer peripheral portion of the boron carbide pellets may be, for example, a mesh-shaped metal or a spiral shape in addition to the cylindrical tube. Further, even if a cylindrical tube having a slit in the vertical direction is attached to the pellet, relocation of the pellet can be prevented.

As a metal for covering the outer periphery of the boron carbide pellets, a metal having a large neutron absorption cross section, for example, rhenium (, in order to prevent relocation of the boron carbide pellets, without impairing the reactivity control capability. Re), europium (Eu), lithium (Li), tantalum (Ta), tungsten (W), niobium (Nb), molybdenum (Mo), indium (I).
n), cadmium (Cd), and silver (Ag) are preferable, but metals produced by adding these metals having a large neutron absorption cross-section to other metals may also be used due to manufacturing problems.

[0016]

As described above, the control rod for a fast breeder reactor according to the present invention can reduce the relocation of boron / carbite pellets and prevent the mechanical interaction between the cladding tube and boron / carbite pellets. .

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of a control rod pin for a fast breeder reactor which is an embodiment of the present invention.

FIG. 2 is a view showing a cross section of a boron carbide pellet according to another embodiment of the present invention.

FIG. 3 is a view showing a cross section of a boron carbide pellet according to another embodiment of the present invention.

FIG. 4 is a view showing a cross section of a boron carbide pellet according to another embodiment of the present invention.

FIG. 5 is a vertical cross-sectional view of a conventional control rod pin for a fast breeder reactor.

[Explanation of symbols]

2 ... Cladding pipe, 3 ... Boron carbide pellets, 4 ... Spring, 5 ... End plug, 6 ... Gas plenum part, 7 ... Control rod pin for fast breeder reactor, 8 ... Neutron absorbing material metal cylindrical pipe.

Claims (2)

[Claims] 1. A control rod for a fast breeder reactor in which a control rod pin having a neutron absorbing material pellet loaded in a cladding pipe is housed in a control rod protecting pipe, wherein one side surface of the neutron absorbing material pellet in the control rod pin. A control rod for a fast breeder reactor, characterized in that a part or the whole is covered with a metal having a large neutron absorption cross-section, and a gap is provided between the metal and the cladding tube. 2. A metal having a large neutron absorption cross section is rhenium (Re), europium (Eu), lithium (L).
i), tantalum (Ta), tungsten (W), niobium (Nb), molybdenum (Mo), indium (In),
The control rod for a fast breeder reactor according to claim 1, which is cadmium (Cd), silver (Ag), or a metal containing these metals as a main component.