JP2708237B2 - Neutron absorbing element - Google Patents

Description

The present invention relates to a neutron absorbing element for a neutron shield used in a liquid metal-cooled fast breeder reactor, and more particularly to a longer life and a shorter length. The present invention relates to a neutron absorption element for a fast breeder reactor capable of achieving the following.

(Prior Art) FIGS. 5 and 6 show a conventional neutron absorbing element used in a liquid metal cooled fast breeder reactor, respectively.
The structure is such that the B ₄ C pellet 1 as a neutron absorber does not directly contact the liquid metal as the coolant.

That is, in the conventional neutron absorbing element for a fast breeder reactor shown in FIG. 5, the cladding tube 2 has a sealed structure by welding upper and lower end plugs 3 and 4 to upper and lower ends thereof, and a neutron absorbing material therein. And a structure in which a B ₄ C pellet 1 is housed and filled with helium (He) gas.

The conventional neutron absorbing element for a fast breeder reactor shown in FIG. 6 has the following structure. That is, the cladding tube 2 having the upper end plug 3 and the lower end plug 4 welded to both upper and lower ends.
The inside is divided into an upper chamber a and a lower chamber b by an intermediate end plug 5. These two chambers a and b are communicated by a communication pipe 8 attached to the intermediate end plug 5, and the inside of the coating pipe 2 is filled with He gas. Further, a discharge hole 7 is provided in the lower chamber b for accommodating the B ₄ C pellet 1 and communicating from the upper end surface of the intermediate end plug 5 to the peripheral wall, and the peripheral wall surface is sealed by high-temperature solder 6.

When the neutron absorbing element for the fast breeder reactor rises to a predetermined temperature by loading the reactor, the high-temperature solder 6 is melted, and the liquid metal as the primary coolant flows into the upper chamber a from the discharge hole 7. However, since the configuration is such that the liquid metal does not rise above the upper end of the communication tube 8, the B ₄ C pellet 1 and the liquid metal do not come into direct contact.

(Problems to be solved by the invention) In conventional neutron absorption elements for fast breeder reactors, B ₄ C
Pellets 1 were converted to He by the neutron absorption ¹⁰ B (n, α) ⁷ Li reaction.
Generate gas. In the example of FIG. 5, the He gas is sealed in a space surrounded by the cladding tube 2, the upper end plug 3 and the lower end plug 4. However, in order to reduce the internal pressure in the cladding tube 2, it is necessary to increase the space for containing the He gas, and as a result, there is a problem that the length of the neutron absorbing element becomes long. FIG. 6 is an improvement of the example of FIG. 5, in which the He gas in the cladding tube 2 is released to the outside after the high-temperature solder 6 melts during operation in the reactor. Therefore, since the pressure in the cladding tube 2 is balanced with the external pressure, no force due to gas pressure is applied to the cladding tube 2. However, it is conceivable that the coolant may enter the cladding tube 2 from the discharge holes 7 due to the fluctuation of the internal pressure or the external pressure during the transient operation of the reactor. It is necessary to configure the height of the coolant that intrudes against the pressure fluctuation so as not to exceed the height of the communication pipe 8, and there is a problem that the length of the neutron absorbing element becomes considerably longer.

In the conventional neutron absorbing element, in order to shorten the total length, a neutron absorbing element is provided in which a porous plug for preventing, for example, coolant from infiltrating and for passing He gas is provided in the discharge hole 7 or the communication pipe 8 shown in FIG. Can be considered. However, it is necessary to wet the porous plug with the coolant and to combine a plurality of porous plugs in order to obtain the reliability of preventing the coolant from entering by the porous plug.

The present invention has been made to solve the above problems,
An object of the present invention is to provide a neutron absorbing element for a fast breeder reactor, which can have a longer life and a shorter length.

[Means for Solving the Problems] According to the present invention, the inside of a cladding tube whose upper and lower ends are sealed by an upper end plug and a lower end plug is divided into an upper chamber and a lower chamber by an intermediate end plug. The chamber is communicated with a communication pipe attached to the intermediate end plug, a neutron absorbing material is accommodated in one of the upper chamber and the lower chamber, and a cylindrical support is provided in the other chamber with the upper end plug or the A plurality of porous plugs are fixedly provided on an intermediate end plug, a plurality of porous plugs are arranged in series in the cylindrical support, and the upper end plug or the intermediate end side opening of the cylindrical support is connected to the upper end plug or the intermediate end. A metal plug that communicates with the gas discharge hole provided in the plug and melts when the discharge hole is heated to the normal operating temperature of the fast breeder reactor is provided, and the porous plug is melted from the metal breeder reactor from within the fast breeder reactor. The high speed flowing through the discharge hole Characterized by comprising the ones wettable 殖炉 own coolant.

(Operation) During the operation of the reactor, coolant enters the porous plug under limited conditions before starting the reactor, the porous plug gets wet with the coolant, and the temperature rises and ¹⁰ B (n,
α) He gas is released by the generation of He gas by the ⁷ Li reaction. In this process, first, when the gas pressure becomes equal to or higher than the He gas holding differential pressure of the lowermost porous plug, the He gas passes through the lowermost porous plug.

The passed gas is accumulated on the lower surface of the porous plug in the next stage, and when the gas pressure eventually becomes equal to or higher than the holding pressure difference of the porous plug, the He gas passes through the porous plug. The total pressure difference of the holding differential pressures of the respective porous plugs formed by repeating the above process is the pressure difference between the space of the B ₄ C pellet 1 and the outside.

Therefore, by setting the pressure difference so as not to be reversed during the transient period assumed during the operation of the reactor, the backflow of the coolant does not occur and the coolant does not enter the space of the B ₄ C pellets 1. .

(Embodiment) An embodiment of a neutron absorbing element according to the present invention will be described with reference to FIG. 1 to FIG.

FIG. 1 shows an example of a neutron absorbing element for a fast breeder reactor according to the present invention.
Shows the B ₄ C and pellets. The B ₄ C pellet 1 is surrounded by a cladding tube 2, a lower end plug 4, and an intermediate end plug 5. The upper end of the intermediate end plug 4 has an upper chamber a surrounded by the cladding tube 2 and the upper end plug 3, and the upper end plug 3 is provided with a discharge hole 7,
Communicates with the outside world. A communication pipe 8 is provided in the intermediate end plug 5 and communicates with the lower chamber b containing the B ₄ C pellets 1.

On the lower surface of the discharge hole 7 provided in the upper end plug 3, a cylindrical support 11 in which three porous plugs 12 are arranged in series is fixedly provided.

The inside of the cladding tube 2 is kept airtight by these porous plugs 12. The end of the discharge hole 7 is sealed with a high-temperature solder 6 and the inside of the cladding tube 2 is in a He atmosphere. The high-temperature solder 6 is a metal plug that melts when the temperature rises to the normal operation temperature of the fast breeder reactor.

Next, the operation of this embodiment will be described with reference to FIGS.

FIG. 1 shows a state before loading into a fast breeder reactor (hereinafter referred to as a nuclear reactor), and FIG. 2 shows a state in which the neutron absorbing element is loaded into a nuclear reactor and the temperature is raised to a predetermined temperature. FIG. 3 shows the final balance state.

That is, from the state of FIG. 1 to the state of FIG.
Heated in the reactor, the high-temperature solder 6 melts, and the coolant , The porous plug 12 leaks due to the coolant.

In this case the pressure in the lower chamber b (figure P _B) and external pressure (figure P _∞) is balanced equal (this condition can be easily set by the design).

After that, when the reactor operation starts, the temperature rise and ¹⁰ B (n,
α) Pressure P _B shown in Fig. 2 due to B ₄ CHe gas generation by ⁷ Li reaction
Is the coolant that has risen and entered Is extruded to obtain a state as shown in FIG. FIG. 3 shows a final balance state, in which a coolant is present above the polar plug 12 and a He gas space is created below. This state is B ₄ C
The He gas generated from the pellet 1 passes through the communication pipe 8 and becomes bubbles at the lower end of the cylindrical support 11, rises in the cylindrical support 11, and reaches below the porous plug 12 at the lower end. Where the pressure in the figure
P _B = P _V1 . The He gas is a porous plug holding differential pressure 差 P _P (this causes He
If gas passes, H which corresponds to the force to overcome the surface tension of the coolant on the porous flag 12, to maintain a determined by the characteristics of the porous plug), generated from the B ₄ C pellet 1
The e gas is sequentially supplied into the cylindrical support 11 and passes through the porous plug 12, and a He gas layer is formed below the porous plug 12, and a coolant layer is formed thereon. In this state, the following relational expression holds.

_{P V1 -P V2 = △ P P} P V2 -P V3 = △ P P P Vn -P ∞ = △ P P _{Thus, P B = P V1 = P} ∞ + n △ P Incidentally, transients are all contemplated reactor against _{states, P 'B = P' V1} = (P ∞ + n △ P) '> P ∞' no ingress of coolant by Mitsurusu and this condition (n △
The term P) can be easily determined by appropriately selecting the number n of the porous plug stages and the porous plug differential pressure (△ P).

By arranging a plurality of porous plugs 12 at appropriate intervals below the discharge holes (in the direction of gravity) in the series in this manner, the porous plugs 12 are wetted by the coolant of the reactor itself at the normal operating temperature when the reactor is loaded. After the operation of the reactor, the coolant is held on the upper surfaces of the plurality of porous plugs 12, and a large holding pressure difference due to surface tension can be obtained.

FIG. 4 shows the second embodiment, and the same parts as those in FIG. 1 are denoted by the same reference numerals. In this embodiment, the porous plug 12 and the cylindrical support cylinder 11 are provided on the lower surface of the intermediate end plug 5.

The intermediate end plug 5 is provided with a discharge hole 7 provided with a metal plug made of a high-temperature solder 6, and a communication pipe 8 communicating the upper chamber a and the lower chamber b.

Since the operation and effect of this embodiment are almost the same as those of the embodiment of FIG. 1, the description thereof will be omitted.

In each of the above embodiments, even if the B ₄ C pellet 1 is B ₄ C powder, the gist of the present invention is not changed.

[Effects of the Invention] According to the present invention, the holding pressure difference of the porous plug can be increased while discharging the He gas generated from B ₄ C in the cladding tube to the outside, and there is a margin for preventing the coolant from entering the B ₄ C space. And the porous plug can be reliably wetted with the coolant.

Therefore, a highly reliable neutron absorbing element can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a first embodiment of a neutron absorbing element according to the present invention. FIGS. 2 and 3 are longitudinal sectional views for explaining the operation of the neutron absorbing element in FIG. FIG. 4 is a longitudinal sectional view showing a second embodiment of the neutron absorbing element according to the present invention, and FIGS. 5 and 6 are longitudinal sectional views showing a conventional neutron absorbing element. 1 ...... B ₄ C pellet 2 ...... cladding tube 3 ...... upper end plug 4 ...... lower end plug 5 ...... intermediate end plug 6 ...... high temperature solder 7 ...... discharge holes 8 ...... communicating pipe 11 ...... tubular Support 12: Porous plug

Claims (1)

(57) [Claims] 1. A communication pipe in which the inside of a cladding tube whose upper and lower ends are sealed by an upper end plug and a lower end plug is divided into an upper chamber and a lower chamber by an intermediate end plug, and both chambers are attached to the intermediate end plug. The upper chamber or the lower chamber accommodates a neutron absorbing material in one of the upper chamber and the lower chamber, and a cylindrical support is fixed to the upper end plug or the intermediate end plug in the other chamber, and the cylindrical support is provided. A plurality of porous plugs are arranged in series in the support, and the upper end plug or the intermediate end plug side opening of the cylindrical support communicates with a gas discharge hole provided in the upper end plug or the intermediate end plug, The discharge hole is provided with a metal plug that melts when the temperature rises to the normal operating temperature of the fast breeder reactor, and the porous plug is provided with the fast breeder reactor itself flowing through the discharge hole from inside the fast breeder reactor due to melting of the metal plug. It consists of something wetting with coolant Neutron absorbing elements characterized by.