JPS6046488A - Control rod for nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a control rod of a nuclear reactor, and in particular to a control rod filled with pelletized material obtained by sintering polon carbide (B4C) as an absorbent material. Regarding.

[Background of the invention]

FIG. 1 is a perspective view of a so-called cruciform control rod (generally designated by the reference numeral 1) commonly used in boiling water reactors, which contains poron isotope BIO.
A large number of neutron absorbing materials 22 made of B4C) powder are sealed in a stainless steel tube 21, and a large number of bubble rods 2 are arranged in a cross shape. Covered by sheath 3.

The tie rod 4 connects four sheaths 3 forming cross-shaped wings at the center. A handle 5 is attached to the upper part of the sheath 3, so that the control rod can be grasped and handled from the upper part. A roller 6 is attached to the handle 7, and the control rod moves within the reactor core (
This is intended to reduce friction between the control rods and the side walls of the fuel channels (see FIG. 2) that form the path through which the control rods pass during insertion into and withdrawal from the reactor core. Further, a fall limiter 7 is attached to the lower part of the sheath 3 to regulate the falling speed of the control rod. The lower part of the fall limiter 7 can be connected to the drive system of the control rod.

FIG. 2 is a cross-sectional view showing the mutual positional relationship with fuel assemblies when the control rods shown in the first section are inserted into the reactor core, and is a unit formed by four fuel assemblies 8. One control rod 1 is arranged for each cell.

The control rods 1 are inserted into passages formed between the side walls of the fuel channels of the fuel assembly 8 from below the reactor core.

The lower half of FIG. 2 shows a typical example of the distribution of neutron flux when the control rods described above absorb neutrons within the reactor core and control the output of the reactor.

That is, the neutron flux is low in the center of the monocrystalline cell and high in the outer part of the cell. Also, normally the tip of the control rod receives a large amount of neutron irradiation.

As a natural result of these, the amount of neutron absorption by the control rod tends to increase upward from the outer side of the cruciform wing.

By the way, the life of a control rod is determined by a first-order factor. In other words, (by the phantom neutron absorbing material absorbing neutrons, it nuclearly transforms into a substance that does not absorb neutrons and no longer has sufficient neutron absorption ability, and (2) the neutron absorbing material and control rod structural members By absorbing neutrons, the material deteriorates and becomes unable to maintain the necessary strength and performance.

In the control rod described above, the neutron absorbing material boroylO(
B”)U, B10(n.

α) A reaction called I, i7 occurs, transmuting into lithium α particles that do not have the ability to absorb neutrons. This α particle collides with other atoms and causes an event in which the B4C particles are bonded to each other. Furthermore, alpha particles change into helium by capturing electrons.

When this helium stays within the B4C particles bound by the above action, it expands and comes into contact with the stainless steel tube 21, thereby applying a mechanical load thereto. On the other hand, the generated helium remains in the stainless steel tube 21 as a gas, increasing the internal pressure and applying a mechanical load to the tube 21.

Due to these nuclear and material changes, the conventional control rods suffer from relatively short lifetimes.

In order to suppress the expansion of the absorbent material and the increase in helium pressure, control rods made of pelletized B4C particles are known, but there is still no regulation of the gap between the pellets and the tube.

[Purpose of the invention]

The object of the present invention is to define the size of the gap between the B and C pellets and the tube as a function of the B10 concentration and the sintered density of the B and C pellets. The object of the present invention is to provide a control rod that has a control rod value and nuclear life equal to or higher than that of a control rod, and has a long mechanical life by eliminating the mechanical load on the tube due to pellet expansion.

[Summary of the invention]

The size of the gap between the pellets B and C and the tube is %.The outer diameter of the pellet B and C is Dp.

When the inner diameter of the tube is Di, it is expressed as δ=(1−(Dp/Di))×Di/2, and the size δ is determined by the following three points.

(1) The initial gap size should be large enough to absorb the decrease in the gap between the pellet and the tube due to the expansion of B4C.

(2) In order to have a nuclear lifespan equal to or longer than that of conventional control rods, the number of Blo in the Apunbar rod must be equal to or greater than that of conventional control rods.

(3) In order to have a control rod value equal to or higher than that of conventional control rods, the surface density of B10 must be equal to or higher than that of conventional control rods.

In the present invention, considering that the above three points differ depending on the concentration of Bto and the sintering density of pellets B and C, the gap between pellets B and C and the tube during control rod manufacturing is This is characterized by defining the size δ of KJ:J), and is intended to extend the life of the control rod by preventing mechanical load from being applied to the tube due to expansion of B4C. be.

[Embodiments of the invention]

An embodiment of the invention is shown in FIG. Here, the size δ of the gap between the B4C pellet 9 and the tube 21 is defined as follows.

(1) The volume expansion coefficient of B and C pellets is p0.3% per 1% combustion of BIG. On the other hand, if we take the time when the control rod value deteriorates by 10% as the nuclear life, then Bto is 42
% combustion, the volumetric expansion is considered to be 12.6%.

Therefore, in order for the bB4c pellet 9 not to contact the tube 21 when 42% of Bto burns and reaches its nuclear life, the relationship Dp (1+0.126)l/3 <Di is required, and in that case, the relationship between the pellet 9 and the tube 21 is required. The initial gap δ that should be provided between the tubes 21 is:

δ>0.019 D i That is, at least 1.9% of the tube inner diameter is required.

(2) In order to make the nuclear life equivalent to or higher than that of conventional control rods, it is necessary to make the BIG bush in the absorber rod 2 equal to or higher than that of conventional control rods. If the abundance ratio of BIG in pellet 9 is ε and the sintered density is ρ, the abundance ratio of BIG in the conventional control rod is 0.19.
8, and the packing density is 70% T, D, so 1) p 2 Xε×ρ〉Di2xO, 198X0.7
It must be. At this time, the bellet 9 and tube 2
The initial gap δ that should be set between 10 and 10 is.

It is necessary that δ((l-0,373(gsu)-'/'')Di/2.

(3) Since BIO has a large neutron absorption cross section, the control rod value is considered to be proportional to the number of BIGs on the surface of the absorber. In order for the control rod value of B4C Bellet 9 to be equal to or higher than that of conventional control rods, Dp(ε×ρ)2/3≧Di (0,1,98X0.7
) must be 2/3. At this time, B4C Beret 9
The gap δ between and the tube 21 is δ (1-0,
268 (ε x ρ 3) Di/2.

Figure 4 is a graph showing the results of (1), (2), and (3) for one or more cases. From this figure, the gap δ between the 134c pellet 9 and the tube 21 is 0.01
9Dikuδku(1-0,373(ε×ρ) -1/2)
It is necessary that it be in the range of D l/2.

[Effects of the Invention] According to the present invention, even when B4C, which is a neutron absorbing material, is irradiated with neutrons, it does not come into contact with the tube and apply mechanical load, so the life of the control rod can be extended. .

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a conventional control rod and absorber rod, FIG. 2 is a schematic diagram showing the arrangement of the control rod and its neutron flux distribution, and FIG. 3 is a sectional view of the absorber rod in an embodiment of the present invention. FIG. 4 is a diagram showing the relationship between the gap, the sintered density, and the BIG abundance ratio, showing the specified range of the gap between the B and C pellets and the tube in the present invention. 1... Control rod, 2... Absorber rod, 21...
・Stainless steel tube, 22...B4Cfi,
3... Sheath, 4... Tie rod, 5... Handle, 6... Roller, 7... Falling speed limiter, 8...
...Fuel assembly, 9th figure 2nd figure 3I21 4th port and xf

Claims (1)

[Claims] 1. A control rod in which a tube is filled with a pellet made by sintering B4C (poron carbide) as a neutron absorbing material, and the sintered density of the pellet is -76% or more. Characteristic reactor control rod. 2. Between a B4C pellet having an abundance ratio ε of B14 in B4C and a sintered density ρ and a tube having an inner diameter Di, there is
×ρ)-V2) A nuclear reactor control rod according to claim 1, characterized in that it has a gap of not more than Di/2.