JPH0644058B2 - Fast breeder reactor - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a fast breeder reactor, and more particularly to a fast breeder reactor having a neutron shielding structure that protects a structure around a core from irradiation damage caused by neutrons.

[Background of the Invention]

For the neutron shielding structure of fast breeder reactors, stainless steel was conventionally used as a neutron shielding material, and a structure in which this was placed around the core was used, but the shielding effect is insufficient, so the structure receives The neutron dose exceeds the allowable value,
The thick shield made the entire reactor large, and the large weight made the support structure large.

Regarding such a neutron shield, for example, JP-A-48-28888, JP-A-50-66691, and JP-A-51 are available.
The disclosure is found in JP-A-130786 and JP-A-52-137588.

[Object of the Invention]

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned problems of the prior art and to provide a fast breeder reactor that can be downsized and simplified.

[Outline of Invention]

The present invention comprises a plurality of fuel assemblies, a core tank surrounding the outer periphery of the fuel assemblies, a core support structure member supporting the fuel assemblies, and a core upper structure member provided on the fuel assembly upper part. In a fast breeder reactor having, a first neutron shielding portion containing a fast neutron moderator made of graphite or beryllium is provided in a portion located between the core fuel portion inside and the core supporting structural member and used at low temperature. Have,
A second neutron shield containing a thermal neutron absorber made of any one of europium oxide, boron carbide, hafnium, and tantalum in a portion located between the core fuel portion and the core upper structural member and exposed to high temperature. Fast neutron moderator made of graphite or beryllium, which has a neutron shielding part-containing fuel assembly or has the neutron shielding part-containing fuel assembly having the first and second neutron shielding parts On the inner peripheral side of the neutron shielding part containing europium oxide, boron carbide, hafnium, having a neutron shielding part containing the thermal neutron absorber made of any of tantalum on the outer peripheral side, adjacent to the inner periphery of the reactor core A neutron shield that is installed and is supported by the core support structure member and is installed in a portion that is exposed to temperatures ranging from low temperature to high temperature. A.

That is, the present invention, as a shielding material around the core, fast neutron moderator made of graphite or beryllium and europium oxide, boron carbide, hafnium, a thermal neutron absorber made of any of tantalum, suitable for shielding purpose It is possible to place it in the machine and achieve the intended purpose.

Example of Invention

FIG. 1 is a side view showing a structure of a core part of an embodiment of a fast breeder reactor of the present invention, 1 is a core, 2 is a blanket, 3 is a gas plenum, 4 is a neutron shielding part having a neutron moderator, 5 Is a neutron shield having a thermal neutron absorber, 6 is a neutron shield having a neutron moderator and a thermal neutron absorber,
Reference numeral 7 indicates a core supporting structural member, 8 indicates a core upper structural member, and 9 indicates a core tank. In this fast breeder reactor, the outer periphery of a core 1 is covered with a blanket 2 and a gas plenum is provided above it. This core part is configured by bundling a large number of core constituent elements such as fuel assemblies, and these core constituent elements are supported by a core support structural member 7 and are laterally supported by a core tank 9 surrounding the outer periphery and at the upper part. The core upper structural member 8 provided is provided with a grid for rectifying the coolant at the outlet. A neutron shield 4 having a neutron moderator,
The neutron shielding portion 5 having the thermal neutron absorbing material and the neutron shielding portion 6 having the neutron moderating material and the thermal neutron absorbing material are provided to protect the in-core structure from neutron irradiation damage.

Here, the mechanism of damage by neutrons will be described with reference to FIG. There are two types of damage to the core structure, which are the atom ejection b by neutron irradiation a and the nuclear transmutation c. Atomic bounce is generated by fast neutrons,
As a result, lattice defects are generated d, the generated lattice defects form dislocation loops or voids e, f, and as a result, embrittlement and irradiation-induced creep occur in crystal grains g, h, mechanical strength and soundness. Jeopardize. On the other hand, transmutation c is generated mainly by thermal neutrons, and as a result, He is generated and i generated He forms bubbles and precipitates at the grain boundaries of j, resulting in embrittlement of the grain boundaries and k The mechanical strength is compromised. A typical transmutation c reaction is shown in Table 1.
Those contributing to the generation are ⁵⁸ Ni and ¹⁰ B, both of which are generated by thermal neutrons.

At temperatures higher than 500 ° C., the tendency of recombination and disappearance of pairs of vacancies and interstitial atoms, which generate lattice defects, is enhanced, and thus radiation damage due to atom ejection is a problem only at low temperatures of 500 ° C. or lower. On the other hand, at a low temperature of 500 ° C. or lower, He generated by transmutation cannot move to the grain boundary, so transmutation becomes a problem only at high temperature.

Since the core support structure member 7 is used at a low temperature of 350 to 400 ° C., it is necessary to shield it to prevent atoms from being ejected by fast neutrons. The core upper structural member 8 is 600-
Since it is exposed to a high temperature of 650 ° C., shielding is required to prevent transmutation by thermal neutrons. Further, since the core tub 9 is exposed to temperatures ranging from low temperature to high temperature, it is necessary to shield both.

Therefore, in this embodiment, the neutron shielding portion 4 containing the neutron moderator is provided inside the fuel assembly between the fuel portion of the core 1 of the fuel assembly and the core supporting structure member 8, and the fuel portion of the core 1 of the fuel assembly is provided. Neutron shield 5 containing a thermal neutron absorber inside the fuel assembly between the core core structural member 8 and the neutron moderator and thermal neutron absorber between the fuel assembly tank and the core tank 9. The neutron shielding section 6 is provided. Here, it is preferable to use graphite as the neutron moderator and surround it with stainless steel to form a shield. Beryllium may be used instead of graphite. Further, it is preferable that europium oxide is used as the thermal neutron absorber and it is surrounded by stainless steel to form a shield. Boron carbide or hafnium or tantalum may be used instead of europium oxide. When using boron carbide, it is desirable to provide a vent mechanism for the generated helium gas.

FIG. 3 is a partial cross-sectional perspective view of a specific example of a fuel assembly with a neutron shield part, in which a neutron shield part 4 containing a neutron moderator and a neutron shield part 5 containing a thermal neutron absorber are provided in the fuel assembly. The figure shows a cutaway sectional view of the main part. In these figures, the same parts as those in FIG. 1 are designated by the same reference numerals, 10 is a fuel assembly, 11 is a fuel element, 12, 13,
Reference numeral 14 is a support rod with a neutron shield that constitutes a neutron shield, 15 is a thin container, 16 is a support rod, a hole through which 12 passes, 17 is a vent mechanism, and 18 is a thermal neutron absorber.

As shown in FIG. 4, the neutron shielding portion 5 including the thermal neutron absorber is configured by filling the thin neutron absorber 15 made of stainless steel with the thermal neutron absorber 18, and the support rod 12 is passed through the hole 16 to make the fuel. It is fixed to the assembly 10. The vent mechanism 17 generates helium gas when the thermal neutron absorber 18 is boron carbide, and is necessary for releasing this. This figure shows the case where the dubbing bell type is adopted.

FIG. 5 shows a cross-sectional perspective view of a main part of a specific example of the neutron shield 6 including the neutron moderator and the thermal neutron absorber, which is provided between the fuel assembly 10 group and the core tub 5. Indicates an element including a shield. Element 1 that has the same external shape as the fuel assembly but includes a shield instead of a fuel element
9 is incorporated. In the element 19 including the shield, a neutron moderator or a thermal neutron absorber is filled in a stainless steel cladding tube and end plugs are welded to both ends. When boron carbide is used as the thermal neutron absorber, it produces helium gas, so a vent mechanism for releasing it is required, but the upper end plug has the same vent mechanism as shown in FIG. Use one.

When Graphite is used as the neutron moderator, the shield length can be set to 60% compared to the case where all stainless steel is used, and when beryllium is used, the shield length is 30%.
Can be

When europium oxide is used as the thermal neutron moderator, the shield length is 40% as compared with the case of all stainless steel.
%, And when boron carbide, hafnium, or tantalum is used, 40% and 90%, respectively.
%, 40%.

In the fast breeder reactor of this embodiment, since the neutron shielding portion 4 containing the neutron moderator is provided between the core supporting structure member 7 used at a low temperature of 350 to 400 ° C. and the fuel portion of the core 1, high speed is achieved. Since the ejection of atoms due to neutrons can be prevented, and the neutron shielding portion 5 containing a thermal neutron absorber is provided between the core upper structural member 8 and the fuel portion of the core 1 which have a high temperature of 600 to 650 ° C. The reactor core 9 that can prevent transmutation by thermal neutrons and is exposed to temperatures from low temperature to high temperature has a neutron shielding portion including a neutron moderator on the inner peripheral side and a neutron shielding portion including a thermal neutron absorbing material. Since this is provided on the outer peripheral side, it is possible to prevent atoms from being ejected by fast neutrons and transmutation by thermal neutrons.
Therefore, since a sufficient shielding effect can be obtained, it is possible to prevent the neutron irradiation dose from exceeding the allowable value, so that it is not necessary to thicken the shielding body, and it is possible to avoid increasing the size of the entire reactor and supporting structure. Things do not have to be particularly large.

In the above examples, the fast breeder reactor having a neutron shielding part containing a neutron moderator, a neutron shielding part containing a thermal neutron absorber, and a neutron shielding part containing a neutron moderator and a thermal neutron absorber has been described. , In some cases, depending on the purpose and application, a neutron shielding part containing a neutron moderator and a thermal neutron absorber may not be provided, and even in such a fast breeder reactor, a neutron shielding part containing a neutron moderator and a thermal neutron absorber The function and effect of the neutron shielding portion containing the material are the same as those in the above embodiments.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY The present invention makes it possible to provide a fast breeder reactor that can be miniaturized and simplified, and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a side view showing the structure of the core of an embodiment of the fast breeder reactor of the present invention, FIG. 2 is an explanatory view of the mechanism of damage by neutrons, and FIG. 3 is a specific example of the main part of FIG. FIG. 4 is a partial cross-sectional perspective view of a constituent fuel assembly with a neutron shielding portion, FIG. 4 is a cutaway sectional view of a principal portion of FIG. 3, and FIG. 5 is a neutron shielding constituent example of the principal portion of FIG. It is a partial cross-sectional perspective view of a body. 1 ... Core, 2 ... Blanket, 3 ... Gas plenum, 4,
5, 6 ... Neutron shielding part, 7 ... Core support structural member, 8 ... Core upper structural member, 9 ... Core tank, 10 ... Fuel assembly, 11
... Fuel element, 12, 13, 14 ... Support rod with neutron shield, 15 ... Thin container, 16 ... Hole (through which the support rod penetrates),
17 ... Vent mechanism, 18 ... Thermal neutron absorber, 19 ... Elements including a shield.

Claims (1)

[Claims] 1. A plurality of fuel assemblies, a reactor core surrounding the outer periphery of the fuel assemblies, a core support structure member supporting the fuel assemblies, and a core upper structure member provided on the fuel assemblies. In a fast breeder reactor having a first neutron shielding portion including a fast neutron moderator made of graphite or beryllium in a portion located between the core fuel portion and the core supporting structural member and used at a low temperature. Having a thermal neutron absorber made of any one of europium oxide, boron carbide, hafnium, and tantalum in a portion which is located between the core fuel portion and the core upper structural member and exposed to high temperature. A fuel assembly with a neutron shielding portion having two neutron shielding portions, or the fuel assembly with a neutron shielding portion having the first and second neutron shielding portions, and a graph On the inner peripheral side of the neutron shield portion containing a fast neutron moderator made of Ito or beryllium, europium oxide, boron carbide, hafnium, having a neutron shield portion containing a thermal neutron absorber made of tantalum on the outer peripheral side, A fast breeder reactor having a neutron shield disposed adjacent to the inner circumference of the core core, supported by the core support structural member, and arranged in a portion exposed to a temperature ranging from low temperature to high temperature. .