JPH0650354B2 - Control rod for fast breeder reactor - Google Patents

Description

The present invention relates to a control rod for a fast breeder reactor, and more particularly to a lower vent type control rod for a fast breeder reactor.

[Conventional technology]

In general, the control rod for fast breeder reactor is used as a neutron absorber.
B ₄ C concentrated to a concentration of ¹⁰ B is used as a pellet. ¹⁰ B, which is effective for neutron absorption, absorbs neutrons by the (n, α) reaction and releases helium gas.

In sealed control rods, the pressure inside the cladding that houses the B ₄ C pellets increases with neutron absorption, and the life determined by the pressure limit of the cladding is faster than the life determined by the neutron absorption capacity. In order to solve this problem, a vent type control rod that automatically releases helium gas generated by the ¹⁰ B (n, α) reaction into sodium as a coolant was devised. It is used.

As a conventional example of a vent-type control rod, the mark of the fast breeder experimental reactor "Joyo" Two (MK-II) core control rods are included. The vent structure of the "Joyo" MK-II core control rod is an upper vent type, and its vent mechanism is arranged above the effective core of the core. Therefore, the length above the effective core becomes long, and as a result, fuel There was no choice but to increase the length of the assembly. A vent-type control rod of this kind is disclosed in Power Reactor Technical Report, No. 41 (1982.3), 73-82p, for example, regarding transfer of the irradiation core (MK-II core) of Hirotaka Yoshimi and "Joyo". Has been done.

FIG. 3 is a cross-sectional view of a main part of an example of an upper vent type control rod,
Reference numeral 1 is a control rod element, 11 is a B ₄ C pellet, 2 is a vent mechanism, 21 is a vent pipe, 22 is a vent hole, 2a and 2b are sodium liquid level during reactor operation and sodium level during reactor shutdown, respectively. ing. In this upper vent type control rod, the helium gas generated by the absorption of neutrons by the B ₄ C pellet 11 rises in the vent pipe 21 and becomes higher than the pressure of sodium as a coolant, It is released into the sodium through the holes 22. On the other hand, the sodium temperature during operation in the fast breeder reactor is about 450 ° C around the control rod and about 200 ° C when the control rod is stopped.
The temperature difference between reactor operation and shutdown is large. for that reason,
In this upper vent type control rod, when the reactor is shut down, sodium does not reach the upper end of the vent pipe 21 even when the surrounding sodium temperature is low, that is, 2b.
The upper end portion of the vent pipe 21 and the position of the vent hole 22 are configured with a sufficient distance so that the position shown in FIG.

FIG. 4 is an explanatory view showing the relationship between the upper vent type control rod and the fuel assembly. The same parts as in FIG. 3 are designated by the same reference numerals, 3 is a fuel assembly, and 4 is an effective core part. Reference numeral 5 denotes a control rod assembly, 6 denotes a control rod, (a) shows a state in which the control rod 6 is inserted into the effective core portion 4 of the fuel assembly 3, (b) shows a state in which it is pulled out, and (c) ) Indicates the position of the core effective portion 4 of the fuel assembly 3. Then, for the fuel assembly 3,
A control rod assembly 5 having the same axial length is provided, and a control rod 6 composed of a plurality of control rod elements is axially pulled out and inserted by a control rod drive mechanism (not shown). Start up, shut down, and adjust the output of the reactor.

Also, in response to the demand for core length reduction for upper vent type control rods, research on lower vent type control rods has also been made,
Lower vent type control rods are disclosed in the IAA-IWGFB (IAEA-IWGFB) Fast Reactor Control Rod Material Experts Conference Proceedings, or in Japanese Utility Model Publication No. 51-17700. . However, in these proposals, the function of preventing sodium intrusion on the neutron absorber side was not devised, or the vent structure itself had to be relatively long because of that.

[Problems to be Solved by the Invention] FIG. 5 shows a cross section of an example of a lower vent type control rod, and FIG.
The same parts as those in the figure are denoted by the same reference numerals, 12 is an insulator pellet, 23 is a vent chamber, and 24 is a filter. In this lower vent type control rod, the vent chamber of the vent mechanism 2 and 23 occupy most of the control rod element 1.
Since it communicates with the storage portion of the B ₄ C pellet 11, it is necessary to prevent sodium from entering the inside of the control rod element 1 even during the sodium temperature difference between when the reactor is operating and when it is shut down. A relatively large volume is required, and the required length of the vent mechanism 2 is long.

That is, in these conventional techniques, a vent type control rod is adopted from the viewpoint of extending the life of the control rod.
No consideration was given to shortening the core length including the fuel assembly, and there was a problem in that the core length (including the fuel assembly) was relatively long.

It is an object of the present invention to provide a lower vent type control rod that can shorten the core length without impairing the life of the control rod.

[Means for solving problems]

The structure of the present invention used to achieve the above-mentioned object is a vent chamber having a control rod element disposed in the upper portion and helium gas generated from the control rod element introduced in the lower portion and having its discharge port at the lower end. In the fast breeder reactor control rod provided, after introducing the helium gas to a position lower than the upper end of the vent chamber by the downward flow passage, the helium gas is passed upward through the flow passage and then the vent. The flow path of the helium gas that leads to the upper end of the chamber and the flow path that faces upward, the porous material that allows only the helium gas to pass through is arranged in the lower part, and the upper part melts during operation of the reactor. However, a liquid metal invasion prevention part in which a low melting point alloy that solidifies when the nuclear reactor is stopped is provided.

[Action]

In the present invention, the vent mechanism is arranged in the lower part of the control rod element in order to enable the life extension, and further, in order to shorten the core length, the middle of the vent passage is once guided downward and then upward. A porous substance 26 (which will be described using the reference numerals used in the example column in this column) which allows passage of helium gas is arranged in the middle of the introduction, and the low melting point alloy 2 is provided above the porous substance 26.
7 is provided with a liquid metal invasion preventing portion 25 having a check valve function. When the liquid metal is sodium, for example, the low melting point alloy 27 is melted when the reactor is operating, that is, when the temperature around the control rod is about 450 ° C., and allows helium gas to pass upward from below. When the temperature around the control rod is about 200 ° C, it solidifies and the check valve function functions as a sealing end plug.

As a result, the changes in the sodium liquid levels 6a and 6b at the time of reactor operation and at the time of reactor shutdown are taken into account in the vent mechanism 2 considering the volume change from above the low melting point alloy 27 to the vent hole 22. The height can be determined so that sodium does not enter the interior, and the required length of the Ben mechanism 2 can be reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 is a cross-sectional view of an essential part of a fast breeder reactor control rod of one embodiment,
The same parts as those in FIG. 5 are designated by the same reference numerals. In this figure, 25 is a liquid metal invasion prevention part provided on the upstream side of the vent chamber 23, 26 and 27 are a porous material and a low melting point alloy constituting the liquid metal invasion prevention part 25, 28
Indicates a plug made of a low melting point material attached to the vent hole 22.

In this embodiment, the case where sodium is used as the liquid metal will be described. For the low melting point alloy 27,
That is, when the surrounding sodium temperature is about 450 ° C, it becomes liquid, and helium gas can flow upward from below, and when the reactor is shut down, that is, when the sodium temperature is about 2 ° C.
A substance that solidifies at a low temperature of 00 ° C and blocks the flow of helium gas, for example, a lead-bismuth (Pb-Bi) alloy is used. Further, the porous substance 26 has fine pores and can pass helium gas but cannot pass a low melting point alloy in a molten state, for example, a sintered alloy of SUS410 or the like is used.

In the control rod of this embodiment, B ₄ and C Peretsuto 11 absorbs neutrons, (n, alpha) of ¹⁰ B in the B ₄ C Peretsuto 11 helium gas generated by the reaction, houses the B ₄ C Peretsuto 11 It is accumulated in the covering pipe 13 and is guided to the vent mechanism 2 provided below the control rod element 1. In the vent mechanism 2, the helium gas that is first guided downward is redirected upward in the passage to be guided to the liquid metal invasion prevention unit 25. In the liquid metal invasion prevention part 25, the porous substance 26 and the low melting point alloy 27 in a molten state are passed through, and after passing through the low melting point alloy 27, the passage of the helium gas is converted downward again to pass through the vent chamber 23. , And is released into sodium through a vent hole 22 provided at the lower end thereof. In addition, when the vent hole 22 is manufactured, when the control rod element 1 is filled with helium gas,
A stopper 28 made of a low melting point substance is attached to keep the inside of the control rod element 1 sealed. After the reactor is started, the plug 28 elutes into sodium at the rated operating state, that is, when the temperature of the surrounding sodium reaches approximately 450 ° C., and thereafter acts as a vent hole 22 for releasing helium gas.

In the vent mechanism 2 according to the present embodiment, even when the sodium temperature in the rated operation state, that is, the surrounding sodium temperature changes from about 450 ° C. to the reactor shutdown state, that is, when the surrounding sodium temperature changes to about 200 ° C., the low melting point alloy is sodium. The required length for not invading the 27 side is as follows.

The dimension l of the sodium liquid level 6a during reactor operation and the sodium liquid level 6b during reactor shutdown in FIG. 1 is expressed by the following equation.

Here, V ₁ : effective volume in the vent mechanism passage portion at the sodium temperature T ₁ .

V ₂ : Effective volume in the vent mechanism passage portion at the sodium temperature T ₂ .

P ₁ : Sodium pressure at sodium temperature T ₁ .

P ₂ : Sodium pressure at sodium temperature T ₂ .

 C: Volume per unit length of the vent mechanism passage.

Note that V ₁ and V ₂ are the sums of V _a , V _b , and V _c shown in FIG.
The subscripts "1" and "2" in V, T and P indicate the states before and after the change, respectively.

In Equation (1), assuming that the pressure, temperature, and dimensions that can be practically used as a vent mechanism are as follows, T ₁ : 473 ° K (200 ° C) T ₂ : 723 ° K (450 ° C) P ₁ : _{1.8ata P 2: 2.3ata h: 20mm} m: 30mm dimensions l is estimated that (h, the value of m is expected that structural and margin) was can be realized at about 70 mm, very It can be a compact vent mechanism.

FIG. 2 is an explanatory view showing the relationship between the control rod for a fast breeder reactor and the fuel assembly of this embodiment, in which parts corresponding to those in FIG. 4 are designated by the same reference numerals, and (a) (b) ( c) shows the same state as in FIG. As is clear from the comparison with FIG. 4, the core axis direction can be made shorter in the case of this embodiment.

FIG. 6 is a sectional view of a main part of a fast breeder reactor control rod according to another embodiment. The same parts as those in FIG. 1 are designated by the same reference numerals. The difference of this embodiment from the embodiment of FIG. 1 is that the lower part of the vent chamber 23 is extended and expanded to the liquid metal invasion prevention part 25 side, and by adopting such a structure, the length of the vent chamber 23 can be reduced. It is possible to shorten the core length and further shorten the core length.

As described above, according to these examples, since it is possible to provide a control rod assembly having a shortened core axial direction without impairing the life extension of the control rod, a fuel assembly is included. The core components can be shortened, which has the effect of improving the economic efficiency of the entire reactor.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY The control rod for a fast breeder reactor of the present invention can provide a lower vent type control rod capable of shortening the core length without impairing the life extension of the control rod, which is a great industrial effect. .

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an essential part of an embodiment of a control rod for a fast breeder reactor of the present invention, and FIG. 2 is an explanatory view showing the relationship between the control rod for a fast breeder reactor and a fuel assembly, and FIG. Is a cross-sectional view of a main part of an example of a conventional upper vent type control rod, FIG. 4 is an explanatory view showing the relationship between the upper vent type control rod and the fuel assembly, and FIG. 5 is a conventional lower vent type control rod. An example of a principal part sectional view, 6th
FIG. 3 is a sectional view of the essential parts of another embodiment of the control rod for a fast breeder reactor of the present invention. 1 ... Control rod element, 11 ... B ₄ C pellet, 2 ... Vent mechanism, 22 ... Vent hole, 23 ... Vent chamber, 25 ...
… Liquid metal invasion prevention part, 26 …… Porous substance, 27 ……
Low melting point alloy.

Claims (2)

[Claims] 1. A fast breeder reactor control rod comprising a control rod element provided in an upper portion thereof, a helium gas generated from the control rod element being introduced into the lower portion thereof, and a vent chamber provided at a lower end of a discharge port thereof is provided. After the helium gas is once introduced to a position lower than the upper end portion of the vent chamber by the downward flow path, the helium gas flow is guided to the upper end portion of the vent chamber through the upward flow path of the helium gas. A low melting point alloy which is inserted in a channel and the above-mentioned flow path facing upward, a porous material which can pass only the helium gas is disposed in the lower part, and which melts in the upper part when the reactor is operating and solidifies when the reactor is stopped. A control rod for a fast breeder reactor, comprising: 2. The control rod for a fast breeder reactor according to claim 1, wherein the low melting point metal is a lead-bismuth alloy.