JPH02154195A - Incore insert assembly - Google Patents

Abstract

PURPOSE:To simplify operation control by incorporating a reactivity control member contg. a material which is variable in neutron absorptivity during fuel cycle into an intermediate part and incorporating a material which hardly absorbs neutrons and converts neutron spectra to heat into the upper and lower parts. CONSTITUTION:The hollow cladding pipe 3 made of zircaloy of the reactivity control member 2 is formed of the upper cladding pipe part 3a, the intermediate cladding pipe part 3b, and the lower cladding pipe 3c. The primary coolant in a reactor vessel flows via a slit 7 into the part 3b but the concn. of the boric acid in the primary coolant is high and the thermal neutron absorption of the boric acid is large; therefore, the nuclear fission reaction of the fuel and neutrons in the adjacent regions of the part 3b is suppressed and the relative output in the axial intermediate part of the fuel assembly is lowered. The thermal neutrons are hardly absorbed in the parts 3a, 3c and since the pellets 6 of the zirconium halide which is converted to the thermal neutron exist, the nuclear fission reaction of the fuel and thermal neutrons in the adjacent regions thereof becomes active and the degradation in the output of the part 3b is compensated. The output distribution in the axial direction is thus flattened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a core insert assembly for power distribution control used for the purpose of flattening the power distribution of a nuclear reactor core.

[Prior Art] The conditions for economical and stable operation of a nuclear reactor core are to suppress excessive reactivity at the beginning of the fuel life and to flatten the power distribution of the core. This is why so-called insert assemblies have traditionally been used, which are appropriately inserted into the fuel assemblies.

The general form of a conventional insert assembly is as shown in FIGS. 6 and 7, in which a plurality of long reactivity control members 12 and a plurality of short plugs 13 are arranged in parallel. When such an insert assembly 11 is inserted into a guide pipe (not shown) of a fuel assembly, the reactivity control member 12 controls the reaction of the fuel assembly, which is the main factor influencing the power distribution in the core. reduce the degree of Moreover, the plug 13 plugs the guide tube in which the reactivity control member 12 is not inserted.

As can be understood from FIG. 8, the plug 13 is usually made of a round bar made of stainless steel, and the reactivity control member 1
2 is a burner-pull poison rod, and as shown in FIG. 9, a neutron absorbing substance 15 such as 10B is housed in the form of glass in the annular space of a cladding tube 14 made of a double tube made of stainless steel. has been done. One insert assembly 11 normally includes 8 to 20 (12 in the example of FIG. 7) reactivity control members 12, and these reactivity control members 12 are inserted into the fuel assembly. As a result, the reactivity of the fuel assembly decreases to about 10-20% at the beginning of its life.

In addition, as for other insert assemblies, the external shape is the same as that described above, but the reactivity control member is made of stainless steel (Japanese Patent Application Laid-open No. 111491/1982) or Zircaloy (Japanese Patent Laid-Open No. 111491/1983). JP-A No. 61-122595) is known.

[Problems to be Solved by the Invention] However, with the conventional interpolation assembly as described above, it is possible to flatten the lateral power distribution of the reactor core, but it is possible to flatten the lateral power distribution of the reactor core. The power distribution was substantially evenly distributed over the entire effective length of the control member and did not take into account the flattening of the axial power distribution, resulting in thermal limitations of the fuel early in the life of the fuel cycle. Create a situation that you cannot afford. Additionally, since the axial power distribution varies widely over the life of the fuel cycle, using an interpolation assembly such as the one described above will help ensure that the axial power distribution is as constant as possible. It is necessary to periodically update the setting of the axial neutron flux deviation target value, which is an operating index, which leads to complicated core operation management.

Therefore, an object of the present invention is to provide a core insert assembly capable of flattening the axial power distribution.

[Means for Solving the Problem] In order to achieve this object, according to the present invention, a core insert assembly for power distribution control inserted into a fuel assembly includes a plurality of reactivity control members, At least a predetermined one of the reactivity control members may include a material having a variable neutron absorption rate during the fuel cycle in its axially intermediate portion, and may contain a substance having a variable neutron absorption rate during the fuel cycle in its axially upper and lower portions. It is characterized by containing a substance that absorbs little and heats up the neutron spectrum.

[Function] Boric acid is injected into the primary coolant (substance with variable neutron absorption rate) for the purpose of reactivity control, and its concentration is, for example, about 1200 ppm at the beginning of the cycle life and about 10 ppm at the end of the cycle life. be adjusted.

When the insert assembly is inserted into the fuel assembly, the primary coolant in the reactor vessel flows into the middle part of the reactivity control member during reactor operation, but at the beginning of the cycle life, the primary coolant in the reactor vessel flows into the middle part of the reactivity control member. Due to the high concentration of boric acid and the large absorption of thermal neutrons by boric acid, the fuel 23 in the adjacent region and peripheral region of the middle portion
The nuclear fission reaction between 5U and thermal neutrons is suppressed, and the relative output of the axially intermediate portion of the fuel assembly is reduced. On the other hand, in the upper and lower parts of the reactivity control member, there is a substance that absorbs almost no thermal neutrons and converts them into thermal neutrons, that is, zirconium hydride pellets, so that the fuel 215U and thermal neutrons in the adjacent and surrounding areas are present. The nuclear fission reaction becomes active, compensating for the relative power drop in the axially intermediate portion of the fuel assembly, and flattening the axial power distribution.

In addition, at the end of the cycle life when the concentration of boric acid in the secondary coolant is low, thermal neutrons, which are generated in large quantities in the water region, are By limiting the number to an appropriate number, the fission reaction between the fuel 235U and thermal neutrons in the adjacent and surrounding areas of the upper and lower parts of the reactivity control member is suppressed, and its compensation is carried out in the axially intermediate part of the fuel assembly. This is done by increasing the relative output of , thereby achieving a flattened axial output distribution.

[Embodiments] Next, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which the same reference numerals indicate the same or corresponding parts.

In FIG. 1, a core insert assembly 1 according to the present invention includes:
Similar to the conventional one described in connection with FIGS. 6 and 7, a plurality of reactivity control members 2 and a plurality of plugs 13 are arranged in parallel so as to hang down from a support 8. be. The support 8 is connected to a drive device 9 (only part of which is shown in the figure). The support 8 and drive 9 may be of any known type, and the plug 13 may be a short solid piece of stainless steel, similar to the conventional one described in connection with FIGS. 6 and 8. A round bar is used, but it may also be made of Jirshikaloy.

In a preferred embodiment, each insert assembly 1 has 12 reactivity control members 2 and 12 plugs 13, but these numbers can be changed as appropriate depending on the core design. Further, it is not necessary to configure all the reactivity control members of each interpolated material assembly 1 according to the present invention, and a predetermined one is used as the reactivity control member 2.
However, other components may be used as the conventional reactivity control member 12.

According to a preferred embodiment of the present invention, the Zircaloy hollow cladding tube 3 of each reactivity control member 2 includes an upper cladding tube section (upper part) 3a, an intermediate cladding tube section 3b, and a lower cladding tube section 3c.
It consists of. A top end plug 4a and a bottom end plug 4b are attached to the top and bottom ends of the upper cladding tube section 3a, respectively, by means such as welding, and these end plugs 4a and 4b allow the upper cladding tube section 3a to is sealed, and
A top end plug 4c and a bottom end plug 4d are attached to the top and bottom ends of the lower cladding tube portion 3c, respectively, and the lower cladding tube portion 3c is sealed by these end plugs 4c and 4d. The end plugs 4b and 4c are also inserted into the top and bottom ends of the intermediate cladding tube section 3b, respectively, to connect the upper cladding tube section 3a, the intermediate cladding tube section 3b, and the lower cladding tube section 3c in alignment with each other. However, one hollow cladding tube 3 is formed. Preferably, each end plug is also made of Zircaloy.

Further, according to the present invention, in the above-mentioned upper cladding tube section 3a and lower cladding tube section 3c, a substance that absorbs most of the neutrons and thermalizes the neutron spectrum, for example, a zirconium compound such as zirconium hydride (Zrll□) is included. are stacked and loaded preferably in the form of pellets 6 while being pressed from above by a spring 5,
In addition, the intermediate cladding tube portion 3b has a structure in which the inside thereof is communicated with the outside, that is, when the insert assembly 1 of the present invention is inserted into the fuel assembly, the primary coolant is supplied to the intermediate cladding tube portion 3b during operation of the reactor. In the embodiment, eight slits 7 extending in the axial direction are formed so that the fluid can flow into and out of the cladding tube portion 3b. The ratio of the lengths of the upper cladding tube section 3a, the intermediate cladding tube section 3b, and the lower cladding tube section 3c is (for the upper and lower cladding tube sections, strictly speaking, the axial length of the stack of pellets 6 inside them) ), approximately 1 considering the reactivity balance within the core:
Although the ratio is set to 2+1, other ratios can be set as appropriate depending on the core design.

An insert assembly 1 having the reactivity control member 2 as described above
The axial power distribution at the beginning and end of the cycle life when inserted into a fuel assembly (not shown) is shown in FIGS. 4 and 5. The axial power distribution when a conventional insert assembly is inserted is shown in FIGS. Shown as a solid line compared to a dashed line. As can be seen from these graphs, the use of the interpolator assembly 1 according to the present invention flattens the axial power distribution both at the beginning and end of the cycle life.

That is, as is well known, boric acid is injected into the primary coolant for the purpose of reactivity control, and its concentration in the secondary coolant is about 1200 ppm at the beginning of the cycle life and about 10 ppm at the end of the cycle life. It is adjusted to variably control the absorption effect of thermal neutrons. That is, the coolant of the reactor sub-cooling system is a material whose neutron absorption rate or cross-sectional area is variable.

Therefore, when the insert assembly 1 according to the present invention is inserted into a fuel assembly, the primary coolant in the reactor vessel will flow into the intermediate cladding tube section 3b through the slit 7 during operation of the nuclear reactor. , at the beginning of the cycle life, the concentration of boric acid in the primary coolant is high and absorption of thermal neutrons by boric acid is large;
The nuclear fission reaction between 35U and thermal neutrons is suppressed, and the relative output of the axially intermediate portion of the fuel assembly decreases.

On the other hand, in the upper and lower cladding tube sections 3a and 3c,
Because there are zirconium hydride pellets 6 that absorb almost no thermal neutrons and convert into thermal neutrons, the nuclear fission reaction between the fuel 235U and thermal neutrons becomes active in the adjacent and surrounding areas, and the fission reaction in the axial direction of the fuel f1 assembly becomes active. By compensating for the relative output decrease in the intermediate portion, the axial output distribution can be flattened. In this way, as can be seen from the thermal limitation envelope shown by the dashed line in FIG. 4, the margin of heat flux, which is the thermal limitation of the fuel, with respect to the hydrothermal channel coefficient is improved.

In addition, at the end of the cycle life when the concentration of boric acid in the secondary coolant is low, thermal neutrons, which are generated in large quantities in the water region, are By limiting the number to an appropriate number, the fission reaction between the fuel 2FFSIJ and thermal neutrons in the adjacent and surrounding areas of the upper and lower cladding tube sections 3a and 3c is suppressed, and the compensation is performed in the axial direction of the F4 assembly. This is done by increasing the relative output in the middle section, with the aim of flattening the axial output distribution.

[Effects of the Invention] As is clear from the above description, the core insert assembly according to the present invention can contain a substance whose neutron absorption rate during the fuel cycle is variable in the middle part, and In addition, it has a reactivity control member containing a substance that absorbs almost no neutrons and heats the neutron spectrum, so changes in the axial power distribution are extremely small throughout the life of the fuel cycle, and the axial power distribution, which is an indicator of core operation, is There is no need to update the setting of the directional neutron flux deviation target value, and operation management can be simplified.

In a preferred embodiment, the core insert assembly also includes:
It is composed of materials or substances that have been used in nuclear reactors for a long time, such as zirconium compounds, water coolants, stainless steel, etc., so it has been proven that it can be used for a long time and can be used repeatedly. Because you can
Unlike conventional insert assemblies, there is no need to generate a large amount of used inserts, and the problem of radioactive waste accumulation can be solved.

[Brief explanation of the drawing]

FIG. 1 is a schematic side view showing a preferred embodiment of the core insert assembly according to the present invention, and FIG. 2 shows a reactivity control member used in the core insert assembly of FIG. 38 is a sectional view taken along line 3^-38 in FIG. 2, FIG. 3B is a sectional view taken along line 3B-3B in FIG. 2, and FIGS. The figure shows the axial power distribution of the fuel assembly at the beginning and end of the life of the fuel cycle when a conventional interpolant assembly is used (dashed line) and when the interpolator assembly of the present invention is used (actual t FIG. 6 is a schematic diagram of a conventional interpolation assembly, and FIGS. 7, 8, and 9 are lines 7-7, 8-8, and 8-8 in FIG. 6, respectively. It is a sectional view taken along line 9-9. 1... Core insert assembly 2... Reactivity control member 3... Cladding tube 3a... Upper cladding tube portion (upper part) 3b...・Middle cladding tube part (middle part) 3C...Lower cladding tube part (lower part)
＞ Applicant Mitsubishi Atomic Crab Industry Co., Ltd. Agent Teru So Gado□゛1

Claims (1)

[Claims] A core insert assembly for power distribution control inserted into a fuel assembly, comprising a plurality of reactivity control members, at least a predetermined one of the reactivity control members being inserted into the axially intermediate portion thereof. It is possible to include materials with a variable neutron absorption rate during the fuel cycle, and in the upper and lower parts of the core, there are materials that absorb few neutrons and thermalize the neutron spectrum. collection of objects.