JPH0254518B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control rod for a nuclear reactor, and in particular to a control rod for a nuclear reactor that can effectively improve the reactor shutdown margin of a boiling water reactor and can safely and reliably shut down a nuclear reactor. Regarding.

A nuclear reactor control rod is constructed by loading a large number of neutron absorption rods into a plurality of wings formed by attaching an elongated U-shaped sheath to a central structural member. The above-mentioned neutron absorption rod generally has a cladding tube made of stainless steel (SUS) filled with B ₄ C powder as a neutron absorbing material in a uniform density distribution, and partition balls are placed at regular intervals in the cladding tube to prevent powder movement. arranged and configured.

The above-mentioned nuclear reactor control rods are arranged so as to be freely inserted into and removed from a reactor core loaded with a large number of fuel assemblies, and are used to control the output of the nuclear reactor.

By the way, in the fuel assembly of a boiling water nuclear reactor (hereinafter referred to as BWR), voids occur due to evaporation of coolant during operation. The incidence of voids is small in the lower and central parts of the core, and larger in the upper part. Also, the burnup of the fuel is large at low void ratios and small at high void ratios. However, since neutron leakage is large at the upper and lower ends of the core, the axial distribution of burnup is smaller in the lower part of the core than in the center of the core, and smaller in the upper part of the core than in the lower part.

In other words, when the axial length of the nuclear reactor core is L, in the upper end region from 3/4L from the lower end to the upper end, the air bubble ratio (void ratio) during operation is
uranium (U-235) with a mass number of 235, which is a fissile material,
The remaining amount of neutron is relatively large, and the generated bubbles (voids) cause a phenomenon of hardening of the neutron spectrum. As a result, the plutonium production reaction (neutron absorption reaction) is promoted, so the concentration of fission nuclides in the upper part of the reactor core increases after the reactor is operated, and the reactor shutdown margin (subcriticality) in that region decreases.

Here, the reactor shutdown margin Δk is the reactivity added to the reactor core when the control rods are fully inserted from the position where the reactor is in a critical state, and the effective multiplication factor of neutrons in the reactor when the reactor is shut down k _eff Then, the following
Defined by equation (1).

Δk=1− _keff ...(1) This reactor shutdown margin is extremely important for management purposes as a measure of the reactor's shutdown ability. In a typical nuclear reactor, control rods are inserted into the reactor core as part or all of the means to shut down the reactor. In this case, as a standard for the reactor shutdown margin, even if one control rod with the largest reactivity effect (also called reactivity value) becomes unable to be inserted, the reactor can be shut down with sufficient margin. It is required that there be sufficient margin for reactor shutdown.

On the other hand, the degree of subcriticality ρ is generally defined by the following equation (2) using the effective multiplication factor k _eff of neutrons in the core.

ρ=(1−k _eff )/k _eff ……(2) This subcriticality ρ varies depending on the distribution composition and position of the fuel loaded in the reactor core, and the reactor shutdown margin depends on the control rod core It changes depending on the insertion position and changes in the core composition at that position, and both values have location dependence. This degree of subcriticality ρ and the reactor shutdown margin Δk do not necessarily correspond numerically accurately. However, when the reactor is stopped due to control rod insertion, the degree of subcriticality ρ increases as the reactivity value of the control rod increases.
In addition, since there is an almost proportional correlation that the reactor shutdown margin is large, in the explanation of the present invention,
Reactor shutdown margin and subcriticality ρ are equally used as a guideline for measuring the reactor shutdown ability.

When a boiling water reactor is operated for a specified period of time, U-
The amount of Pu produced as a result of the combustion of 235 is lower in the lower part of the core than in the middle, and more in the upper part of the core than in the lower part. Therefore, as shown in Figure 1A, the reactor shutdown margin (subcriticality) is approximately 1/1/1 of the total core length L from the top of the core.
The minimum value is reached at position X, which is approximately in the center of the four areas.

However, conventional control rods act as neutron absorbers over their entire effective length and have normal reactivity.
It is formed by filling a cladding tube with B ₄ C powder at a uniform density, and the neutron absorption capacity distribution in the axial direction, that is, the reactivity, is adjusted to be approximately equal, so it is possible to receive high-intensity neutron irradiation after operation. receive,
Since the reactivity of the neutron absorbing material may partially decrease, even if control rods are inserted to shut down the reactor, there is a risk that the upper part of the core, which has a low degree of subcriticality, will exceed criticality. There is a possibility that it may not be able to stop safely.

The present invention was made based on the above circumstances, and
The aim is to obtain a control rod for a nuclear reactor that can safely and reliably shut down a BWR.

In the present invention, when a control rod is fully inserted into a core region where burnup is low and subcriticality is small, the reactivity of the portion of the control rod facing the core region is high to achieve the above object.

High reactivity here means that the control rod has a large neutron absorption effect, and in particular in the present invention,
Compared to conventional reactor control rods, which are constructed by arranging neutron absorption rods filled with B ₄ C powder in a stainless steel cladding tube, which has been commonly used in the past, each sheath has a greater neutron absorption effect. It means big.

That is, the control rod for a nuclear reactor according to the present invention has a plurality of wings formed by attaching an elongated U-shaped sheath to a central structural member, and a neutron absorbing rod formed by filling a neutron absorbing material in a cladding tube to form a plurality of wings. In the control rod for a nuclear reactor, the B ₄ C powder has a neutron absorption effect inside the cladding tube located at the tip region, which is about 1/4 of the total effective length of the control rod, excluding the slightly longer tip. The cladding tube is filled with a highly reactive material having a large amount of reactivity, while the inside of the cladding tube located in an area other than the above-mentioned tip area is filled with B ₄ C powder.

According to the above configuration, when a reactor control rod is fully inserted into the reactor core, the tip region of the control rod filled with a highly reactive substance faces the upper region of the core where the degree of subcriticality is originally particularly small. Therefore, the degree of subcriticality in the upper region of the reactor core, that is, the reactor shutdown margin increases, making it possible to safely and reliably shut down the reactor, and further improving the safety of the operation management of the nuclear reactor Plato.

Next, an embodiment of the present invention will be described with reference to the accompanying drawings. In FIG. 2, the nuclear reactor control rod 1 having a cross-shaped cross section according to the present embodiment has four sheaths 3 each having an elongated U-shaped cross section attached to each protruding leg of a central structural member 2 having a cross-shaped cross section. neutron absorbing rods 4 are formed in each wing.
It is formed by loading.

Then, a tip region 6 having a length of about 1/4 of the total effective length L of the control rod, that is, L/4, is relative to the other regions from a position 5 a little distance l from the tip of the control rod. It has a high reactivity.

That is, the tip region 6 is made to have a high reactivity by filling the inside of the cladding tube of the neutron absorption rod 4 located in the tip region 6 with Eu ₂ O ₃ pellets or concentrated ¹⁰ B ₄ C pellets obtained by concentrating B-10. On the other hand, the inside of the cladding tube located in a region other than the tip region 6 is filled with B ₄ C powder having a normal reactivity.

When the reactor control rod 1 according to this embodiment is fully inserted into the reactor core, the subcriticality distribution of the reactor core is as shown by the solid line in FIG. 1B.

That is, in Fig. 2, Eu ₂ O ₃ or enriched
The tip region 6 of the reactor control rod 1 is filled with a highly reactive substance such as ¹⁰ B ₄ C pellets to have a high reactivity.
Since it is opposed to the upper core region where the frequency of subcriticality is lower, the neutron absorption capacity, that is, the reactivity in the upper core region is increased. Therefore, the subcriticality of the upper core region, that is, the margin for reactor shutdown, is improved,
It becomes possible to shut down the nuclear reactor more safely and reliably.

Note that the present invention is not limited to the above embodiments. For example, Eu ₂ O ₃ and concentrated ¹⁰ B ₄ C pellets to be filled in the cladding tube of the neutron absorption rod 4 located in the tip region 6 are filled in a small-walled Hf tube, and the neutron absorption rod is attached together with the Hf tube. It may be placed inside the cladding tube.

Here, even if Hf absorbs neutrons multiple times, it still forms an isotope with _a large neutron absorption cross section. Ability lasts for a long time.

In this case, not only can the nuclear life of the control rod in the tip region 6 be extended;
Swelling due to changes in properties due to neutron absorption of Eu ₂ O ₃ and enriched ¹⁰ B ₄ C pellets can be sufficiently absorbed by the tiny gaps between the Hf tube and the inner circumferential surface of the cladding tube, thereby improving the mechanical life of the control rods. Can be done. In addition, in the present invention, expensive and large specific gravity
Since Eu ₂ O ₃ , Hf, etc. are only partially used in the tip region 6 of the control rod, the cost, weight, etc. of the control rod will not increase significantly.

Therefore, it is easy to apply to existing nuclear reactors designed and built under conventional control rod loading conditions, and replacement costs are low.

[Brief explanation of drawings]

Figure 1A is an axial subcriticality distribution diagram at the time of shutdown of the BWR core when conventional control rods are used, and Figure 1B is the operation of a BWR using the reactor control rods according to the present invention. The axial subcriticality distribution diagram at the time of shutdown, FIG. 2 is a schematic diagram showing an embodiment of the nuclear reactor control rod according to the present invention. 1...Reactor control rod, 2...Central structural member, 3
...Sheath, 4...Neutron absorption rod, 6...Tip region.

Claims (1)

[Claims] 1. A plurality of wings are formed by attaching an elongated U-shaped sheath to the central structural member, and a neutron absorbing rod formed by filling a cladding tube with a neutron absorbing material is loaded into the wing. In a nuclear reactor control rod consisting of
The inside of the cladding tube located at the tip area, which is about 1/4 of the total effective length of the control rod, excluding the slightly longer tip, is filled with a highly reactive material that has a greater neutron absorption effect than B ₄ C powder. A control rod for a nuclear reactor, characterized in that the inside of a cladding tube located in an area other than the area is filled with B ₄ C powder. 2 Highly reactive substances are Eu ₂ O ₃ pellets or B-
The control rod for a nuclear reactor according to claim 1, wherein the control rod is a concentrated ¹⁰ B ₄ C pellet. Claim 2, characterized in that the 3 Eu ₂ O ₃ pellets or the ¹⁰ B ₄ C pellets are inserted into an Hf tube with a small wall thickness, and are placed together with the Hf tube in a cladding tube of a neutron absorption rod. Control rods for nuclear reactors.