JPH06331767A - Fuel assembly and core of nuclear reactor - Google Patents

Abstract

PURPOSE:To realize the high safety peculiar to a transitive event such as the decrease in coolant flow rate in a liquid metal cooling fast breed reactor. CONSTITUTION:This device consists of fuel element bundles which are formed by bundling fuel elements with fuel materials enclosed and where openings between fuel elements 1 are passages for cooling water, a neutron-reflection area 2 equipped with neutron-scattering materials and passages for coolant, a wrapper tube 3 surrounding the fuel element bundles and the neutron-scattering area, a coolant entrance 4 at both ends of the wrapper tube 3, a coolant outlet 5 and an exothermic material 6 installed on the inner surface of the wrapper tube 3 in the neutron-scattering area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid metal cooled fast breeder reactor, and more particularly to a fuel assembly suitable for improving the reactivity controllability of the core when the temperature of the coolant rises, and a core composed of the fuel assembly.

[0002]

2. Description of the Related Art The structure of a fuel assembly for a fast breeder reactor is discussed, for example, in a known example (1) Ryohei Miki "Fast breeder reactor", Chapter 6 of Nikkan Kogyo Shimbun. Regarding the configuration of the core used, the publicly known example (2) Alice Steamers Atomic Energy Division Etoual Large Fast Reactor Design Study (Allis-Chalmers atomic energy division, et a
l., Large Fast ReactorDesign study, 5th chapter. C
hicago Operations Office US Atomic Energy Commisi
on, ACNP-64503 TID4500 [26th Edition] UC-80 reac
tor Technology).

Conventionally, a fuel assembly of a fast breeder reactor is located above a fuel element bundle in which a nuclear fuel material is enclosed in a cladding tube and bundled in a large number, a trumpet tube surrounding the fuel element bundle, and a fuel element bundle to scatter neutron scattering materials. The neutron reflection area and the coolant outflow portion,
And a coolant inlet below the fuel element bundle.
The fuel element was produced by a cladding tube with plugs at the upper and lower ends, a core fuel pellet enriched with fissile material, or a blanket fuel pellet containing fuel parent material as a main component, and a fission reaction. It consists of a gas plenum for containing gas. Liquid metal sodium or the like is used as the coolant.

The core comprises a cylindrical core region in which a plurality of core fuel assemblies loaded with core fuel pellets are bundled, and a radial blanket in which a plurality of blanket fuel assemblies loaded with blanket fuel pellets are surrounded and which surround the core region. Area and.

[0005]

Generally, in a liquid metal-cooled fast breeder reactor, the reactivity of the core changes as the temperature of liquid sodium as a coolant rises during a transient event such as a decrease in the flow rate of the coolant. . When the temperature of liquid sodium as a coolant rises, the density of sodium decreases due to thermal expansion. Therefore, neutrons are less likely to collide with sodium atoms, and the average energy of neutrons in the core increases. In a fast reactor, this effect acts on the positive side, so the reactivity of the core increases.

In order to mitigate this tendency of increase in reactivity,
In the known example (2), it is proposed to provide a liquid sodium region as a neutron reflection region above the core fuel region. However, in the conventional fuel assembly, the temperature near the trumpet tube surface in the neutron reflection region is lower than the temperature in the center of the neutron reflection region due to the heat removal effect of the coolant flowing outside the region. There was a problem in that the density of liquid sodium in could not be efficiently reduced.

The object of the present invention is to install a heat generating substance on the inner surface of the trumpet tube so as to cancel the heat removal effect of the above-mentioned coolant, and to raise the average temperature of liquid sodium in the neutron reflection region, and at the same time, to increase the temperature in the radial direction. By flattening the distribution, when the temperature of liquid sodium rises during a transient event of the reactor, the density of sodium in the neutron reflection region is efficiently reduced, and by promoting the neutron leakage effect from the core region, It is intended to provide a fuel assembly capable of mitigating an increase in the reactivity of the core and a core of a nuclear reactor constituted by the fuel assembly.

[0008]

In order to achieve the above object, in the present invention, a plurality of fuel elements containing a core fuel enriched with fissile material and a blanket fuel containing a fuel parent material as main components are bundled. A fuel element bundle having a flow path of a coolant as a gap between fuel elements, a neutron reflection area having a flow path of a neutron scattering substance and a coolant, and a wrapper surrounding the fuel element bundle and the neutron reflection area In a fuel assembly including a pipe and a coolant inflow portion and a coolant outflow portion at both ends of the trumpet tube, a heat generating substance is installed on the inner surface of the trumpet tube in the neutron reflection region.

Further, in the fuel assembly, the upper end of the outermost fuel element in the fuel element bundle is extended to the upper end of the neutron reflection region, and the fuel is enclosed in this extended portion.

Further, in the fuel assembly, the upper end of the outermost fuel element in the fuel element bundle is extended to the upper end of the neutron reflection region, and the exothermic substance is enclosed in this extended portion.

Further, in the present invention, a core is constructed by using the above fuel assembly.

[0012]

The fuel assembly according to the present invention increases the average temperature of liquid sodium in the neutron reflection region by providing the heating portion near the trumpet tube surface in the neutron reflection region having a negative temperature reactivity coefficient. Moreover, the temperature distribution in the radial direction is kept flat. Therefore, in a reactor using a fuel assembly, when the temperature of liquid sodium as a coolant rises during a transient event such as a coolant flow rate decrease, the density of sodium is efficiently increased by thermal expansion in the neutron reflection region. As the neutron leakage effect from the core fuel region is reduced, the increase in reactivity is mitigated.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 shows a vertical cross section of a fuel assembly of a liquid metal cooled fast breeder reactor according to the present invention. A blanket mainly composed of a core fuel enriched with fissile material and a fuel parent material. Bundling a plurality of fuel elements 1 containing fuel,
A fuel element bundle having a gap between the fuel elements as a coolant passage, a neutron reflection region 2 having a passage for a neutron scattering substance and a coolant, and a trumpet tube 3 surrounding the fuel element bundle and the neutron reflection region, And a coolant outflow portion 4, a coolant outflow portion 5 at both ends of the trumpet tube, and a heating material 6 installed on the trumpet tube surface in the neutron reflection region.

The present embodiment is characterized in that a heat-generating substance is installed on the surface of the trumpet tube in the neutron reflection region, and as the heat-generating substance, hafnium Hf or the like which generates heat by the neutron capture reaction is used. The diameter of the fuel element is 0.75 cm, the total length is 280 cm, the neutron reflection area above the fuel element is 80 cm, the facing distance of the fuel assembly is 16 cm, and the total length is 4 cm.
It is 50 cm.

According to the present invention, when the temperature of liquid sodium as a coolant rises during a transient event such as a decrease in the coolant flow rate, the average temperature of the neutron reflection region having a negative temperature reactivity coefficient is improved and the diameter is increased. By flattening the directional temperature distribution, the density of liquid sodium can be efficiently reduced, and the effect of neutron leakage from the core fuel region can be promoted, so the increase in reactivity of the core can be mitigated and safety can be improved. it can.

Next, a second embodiment will be described with reference to FIG. In the fuel assembly shown in FIG. 1, instead of the exothermic material installed on the trumpet tube surface in the neutron reflection region, the fuel element at the outermost periphery of the fuel element bundle is made long, and the upper end reaches the neutron reflection region. It corresponds to the one in which the fuel substance is enclosed in the portion corresponding to the neutron reflection region, instead of the shaku fuel element 7.

The effects of the present invention are equivalent to those described with reference to FIG. Furthermore, an increase in the fuel loading amount enables an increase in the combustion reactivity to extend the operating period, thereby improving the operating rate.

Further, a third embodiment will be described with reference to FIG. In the fuel assembly described in FIG. 2, a long fuel element in the outermost periphery of the fuel element bundle is filled with a heat generating substance such as hafnium Hf instead of the fuel substance sealed in the neutron reflection region. This corresponds to the element 8 replaced.

The effects of the present invention are equivalent to those described with reference to FIG.

Finally, a fourth embodiment will be described with reference to FIG.

FIG. 4 shows a vertical cross section of a cylindrical core constructed by using the fuel assemblies shown in Examples 1 to 3, and the inside of a core fuel region 9 having a diameter of 330 cm and a height of 100 cm. , Inner diameter 230 cm, thickness 20 cm, inner blanket region 10, diameter 330 cm above core fuel region, thickness 4
A 0 cm sodium region 11 is provided, and an upper axial blanket region 12, a lower axial blanket region 13 and a radial blanket region 14 having a thickness of 40 cm are provided surrounding the core fuel region and the sodium region. The upper axial blanket is obtained by replacing a part of the shielding material in the neutron reflection region shown in FIG. 1 with blanket fuel.

The effects of this embodiment are the same as those described with reference to FIG.

[0024]

According to the present invention, in the liquid metal cooling type fast breeder reactor, it is possible to mitigate the increase in the reactivity of the core due to the temperature rise of liquid sodium as a coolant at the time of a transient event such as a decrease in the coolant flow rate. , High inherent safety against reactor transients can be achieved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of a fuel assembly of the present invention.

FIG. 2 is an explanatory view of a second embodiment of the fuel assembly of the present invention.

FIG. 3 is an explanatory view of a third embodiment of the fuel assembly of the present invention.

FIG. 4 is a vertical sectional view of a core.

[Explanation of symbols]

1 ... Fuel element, 2 ... Neutron reflection region, 3 ... Trumpet tube, 4
... Coolant inflow part, 5 ... Coolant outflow part, 6 ... Exothermic substance.

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location 9216-2G G21C 3/30 GDF A

Claims (4)

[Claims] 1. A core fuel enriched with fissile material, a blanket fuel containing a fuel parent material as a main component, a fuel element having the fuel enclosed in a cladding tube, a bundle of the fuel elements, and a space between the fuel elements. A fuel element bundle having a gap as a coolant passage,
A neutron reflection region provided above the fuel element bundle and having a neutron scattering substance and a coolant flow path, a trumpet tube surrounding the fuel element bundle and the neutron reflection area, and cooling at both ends of the trumpet tube. A fuel assembly comprising a material inflow part and a coolant outflow part, wherein a heat generating substance is installed on the inner surface of the trumpet tube in the neutron reflection region. 2. The fuel assembly according to claim 1, wherein an upper end of the outermost fuel element in the fuel element bundle is extended to an upper end of the neutron reflection region, and the extended portion is filled with fuel. 3. The fuel assembly according to claim 1, wherein an upper end of the outermost fuel element in the fuel element bundle is extended to an upper end of the neutron reflection region, and an exothermic substance is enclosed in the extended portion. 4. A reactor core constructed according to claim 1, 2, or 3.