JP2922772B2 - Control float equipment for nuclear reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control float apparatus for automatically controlling a nuclear reaction in a nuclear reactor having a forced circulation cooling system.

[0002]

2. Description of the Related Art In a reactor equipped with a forced circulation cooling system such as a water reactor, a liquid metal cooling furnace, and a gas cooling furnace, when a coolant is lost due to a broken pipe or when a flow rate of the coolant is reduced due to a pump stop (hereinafter, these are referred to as " (Collectively referred to as "abnormal flow rate drop")
A highly safe control float device capable of automatically shutting down a furnace without using an electric circuit has already been applied for a patent by the same applicant as the present application (Japanese Patent Laid-Open No. 3-24).
No. 6489). The control float device 1 illustrated in FIG.
Is a cladding tube 4 filled with a neutron absorber 3 such as B ₄ C inside a guide tube 2 disposed vertically in the core of the reactor.
And a control float 6 comprising a bottomless hollow tube skirt 5 attached to the lower end of the cladding tube. The control float 6 can be raised or lowered depending on the flow state of the coolant flowing in the guide pipe 2.
The position of the neutron absorber filling section 3 rises outside the core region during rated operation of the furnace, and falls into the core region when the abnormal flow rate of the coolant decreases.

As shown in FIGS. 7 and 8, a control float device 1 shown in FIG. 6 in which a control float 6 is inserted inside a guide tube 2 has a plurality of control rods in addition to ordinary control rods. Be attached. FIG. 7 shows a state at the time of rated operation in which the flow rate of the coolant is in the rated state. The control float 6 rises due to the coolant flow in the guide pipe 2 and the neutron absorber filling section 3
Is completely out of the core region 9 and the reactor power is not affected by the neutron absorber. If the pipe breaks or the pump stops, causing an abnormal decrease in the flow rate of the coolant, the coolant in the guide pipe 2 also has an extremely low flow rate, and the control float 6 hardly floats, and as shown in FIG. Therefore, the neutron absorber filling portion 3 is placed in the core region 9,
The nuclear reaction of the core is suppressed by the action of the neutron absorber, and the reactor shutdown is naturally achieved.

The upper and lower ends of the guide tube 2 are provided with an upper stopper 7 and a lower stopper 8 through which a coolant can flow (see FIG. 6). It does not fall out of tube 2.

[0005]

According to the above-mentioned conventional control float device, the control float 6 is automatically lowered in response to the abnormal decrease in the flow rate of the coolant, thereby effectively stopping the furnace. It is not possible to cope with one major abnormal event, that is, an abnormal increase in temperature of the coolant due to furnace overpower or the like.

As a safety device against an abnormal rise in the temperature of the coolant, a self-acting reactor shut-down mechanism has been developed which uses a Curie point electromagnet incorporating a magnetic material having a Curie point in a magnetic circuit to attract and hold control rods. (For example, JP-A-2-243959, JP-A-4-98805)
issue). However, incorporating such a self-acting reactor shutdown mechanism together with the conventional control float device 1 in a reactor is
This is not desirable because a large space must be secured in the core and the mechanism in the furnace is complicated.

Accordingly, the present invention relates to a control float device mounted on a reactor having a forced circulation cooling system for controlling a nuclear reaction, which not only functions to reduce an abnormal flow rate of the coolant, but also functions as a coolant. It is an object of the present invention to provide an improved control float device for a nuclear reactor, which automatically and reliably functions even when the abnormal temperature rises.

[0008]

In other words, a control float apparatus for a nuclear reactor according to the present invention comprises a guide tube vertically disposed in a core portion of a reactor having a forced circulation cooling system.
A control float consisting of a sealed cladding tube that can rise or fall in the guide tube depending on the flow state of the coolant flowing in the guide tube is inserted, and the position of the neutron absorber filling section rises outside the core area during reactor rated operation However, this is substantially the same as the conventional control float device in that it has a configuration in which the coolant is lowered into the core region when the abnormal flow rate of the coolant is lowered.

The control float device of the present invention is different from the conventional device in that a neutron absorber-filled portion is provided downward and a lithium 6 is provided above the control float via a partition plate which melts when the coolant temperature rises abnormally. A [ ⁶ Li] reservoir is provided, a through hole extending in a vertical direction is formed in the neutron absorber filling portion, and a lower cavity is provided below the neutron absorber filling portion and an upper cavity is provided above the lithium 6 reservoir. It is a point.

[0010]

[Function] When the coolant is in a strong flow state during the rated operation of the reactor, the control float in the guide tube rises to the uppermost position, and the neutron absorber filling portion is placed outside the core region.
Smooth reactor operation is performed without being affected by the neutron absorber. On the other hand, when the coolant is weakly flowing due to abnormal coolant flow reduction due to pipe breakage or pump stop, etc., the control float in the guide pipe descends to the lowest position, and the neutron absorber in the control float moves into the core region. As a result, the nuclear reaction of the core is suppressed by the action of the neutron absorber, and the reactor shutdown is automatically achieved.

When the coolant temperature during the furnace operation is maintained, that is, when the control float is located at the uppermost position, an overpower occurs and the coolant temperature rises abnormally.
The partition plate in the control float is melted, and lithium 6 having a large neutron absorption cross section stored in the lithium 6 storage portion above the partition plate (the absorption cross section for thermal neutrons is 945 burn) penetrates the neutron absorber filling portion. It falls through the hole into the lower cavity of the control float. At this time, since the lower cavity is located in the core region, the nuclear reaction of the core is suppressed by the action of the lithium 6, and the shutdown of the furnace is automatically and reliably achieved.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail with reference to the preferred embodiments illustrated below. FIG. 1 shows a control float device 10 according to the present invention, which is constructed by inserting a control float 13 composed of a sealed cladding tube 12 into a guide tube 11 disposed vertically in a core portion of a nuclear reactor. I have. An upper stopper 14 and a lower stopper 15 through which coolant can flow are attached to the upper and lower ends of the guide tube 11 so that the control float 13 does not come out of the guide tube 11 even when the control float 13 is raised or lowered. The cladding tube 12 serving as the control float 13 has a pellet-like neutron absorber filling portion 16 made of B ₄ C or the like disposed substantially at an intermediate portion in the vertical direction, and above the neutron absorber filling portion 16. The lithium 6 reservoir 18 is provided via the partition plate 17. A vertically extending through hole 19 is formed in the center of the neutron absorber filling portion 16. The partition plate 17 is made of a metal having a melting point so as to be surely melted when an abnormal temperature rise of the core coolant and having good compatibility with lithium 6. Further, a lower cavity 20 is provided below the neutron absorber filling portion 16 and an upper cavity 21 is provided above the lithium 6 reservoir 18 in the cladding tube 12.

The neutron absorber filling section 16 and lithium 6
The vertical length L ₁ of the reservoir 18 is the core 30 (see FIG. 2).
Approximately equal to the height of The length of the lower cavity 20 in the vertical direction is also substantially equal to L _{1 so} that the lower cavity 20 has a volume capable of accommodating the entire amount of lithium 6 in the reservoir 18. The upper cavity 21 is for accumulating helium gas generated when lithium 6 absorbs and reacts with neutrons leaking from the reactor core. The pressure of the accumulated helium gas causes the cladding tube 1 to accumulate.
It is sufficient to secure a volume that does not cause damage to 2. Skirt 2 consisting of a hollow tube without a bottom provided below cladding tube 12
Vertical length L ₂ of 2, and (L ₁ + L ₂₎ is substantially equal length such the distance between the lower stop 15 and the core 30 lower end (see FIG. 3).

FIGS. 2, 3 and 4 show a state in which the control float device 10 of FIG. 1 is mounted on a core 30 of a liquid metal cooling fast reactor. Actually, similarly to the mounting of the conventional control float device shown in FIGS. 7 and 8 on a nuclear reactor, the control float device 10 of the present invention also includes a plurality of the control float devices in addition to the normal control rods. In order to easily explain the positional relationship between the control float 13 and the core region 30, in FIG. 2 to FIG. 4, only one control float device 10 and its periphery are enlarged and simplified. Is shown.

In FIG. 2 in which the coolant is in a strong flow state during the rated operation of the furnace, the control float 13 in the guide tube 11 rises to the uppermost position, and the neutron absorber filling portion 16 is placed outside the core region 30. Therefore, a smooth reactor operation is performed without being affected by the neutron absorber. On the other hand, in FIG. 3 in which the coolant is weakly flowing when the abnormal flow rate of the coolant is reduced due to a pipe breakage or pump stop, the control float 13 in the guide pipe 11 is lowered to the lowest position, and the neutrons in the control float are reduced. The absorber filling section 16 is placed in the core region 30, and the nuclear reaction of the core is suppressed by the action of the neutron absorber. These operations of the control float device 30 include:
This is the same as the conventional device shown in FIGS.

Next, a description will be given of a function of stopping the furnace when the temperature of the core coolant is abnormally high, which is a feature of the control float device 10 of the present invention. The state where the coolant flow rate is maintained during the furnace operation as shown in FIG.
When the coolant temperature rises abnormally due to an over-power in a state in which is located at the uppermost position, the partition plate 17 in the control float 13 melts. As a result, as shown in FIG.
The lithium 6 stored in the lithium 6 storage section 18 falls into the lower cavity 20 of the control float through the through hole 19 of the neutron absorber filling section 16. At this time, since the lower cavity 20 is located in the core region 30, the nuclear reaction of the core is suppressed by the action of lithium 6, and the shutdown of the furnace is automatically and reliably achieved.

The pressure in the lower cavity 20 in the control float is
It is desirable that the pressure is reduced to a value close to the lower limit allowed by the strength of the cladding tube 12, for example, about 0.5 atm, so that the pressure difference from the pressure in the upper cavity 21 is increased as much as possible. By increasing the pressure difference, the movement of the lithium 6 from the reservoir 18 to the lower cavity 20 is caused not only by gravity but also by the two cavities 20 and 2.
A pressure difference of 1 also acts and is effected quickly.

The partition plate 17 can be made of a metal having a melting point slightly higher than the coolant core outlet temperature at the time of rated operation, for example, barium in a liquid metal cooling fast reactor.
When the reactor type of the reactor equipped with the control float device 10 of the present invention is a liquid metal-cooled fast reactor, the coolant core outlet temperature during the rated operation is 500 to 650 ° C. Lithium 6
Has a melting point of 180.6.degree. C. and a boiling point of 1342.degree. C. at 1 atm. Thus, it is liquid at the coolant temperature during the rated operation. In contrast, barium has a melting point of 725.
° C. If the coolant temperature rises abnormally at the core outlet and exceeds this temperature, the barium partition plate 17 melts, and the liquid lithium 6 is transferred to the neutron absorbing material filling portion 1.
It falls through the through hole 19 in the lower part 6 to the lower cavity 20, where it can absorb neutrons and suppress the nuclear reaction.

The material of the partition plate 17 can be arbitrarily selected to some extent depending on the coolant core outlet temperature during the rated operation and the allowable value of the temperature rise. Considering only the melting point, antimony at 631 ° C.
Metals such as magnesium at 651 ° C. and strontium at 769 ° C. are mentioned as candidate materials, and if they are alloys, the melting point can be selected more freely. However, when any material is selected, compatibility with lithium 6 for a long period of time It is assumed that there is.

The degree to which the reactor reactivity changes due to the injection of lithium 6 into the core depends on the design of the core. The graph of FIG. 5 shows the calculated results of the effect of lithium 6 injection on a liquid metal-cooled fast reactor of a certain design. This core uses highly enriched uranium fuel, has dimensions of 42 cm in diameter and 40 cm in height, and uses lithium 7 which does not react with neutrons as a coolant. From the graph of FIG. 5, for example, when lithium 6 corresponding to 6% of the coolant by volume mixing ratio is injected into the core, 5% dk / k
It can be seen that a negative reactivity of k 'is injected.

In order to suppress the reactivity using the control float device of the present invention, the coolant core outlet temperature at the time of rated operation is higher than the melting point of lithium 6 (about 181 ° C.), and the coolant core outlet temperature and the cooling If a partition plate 17 of a material having a melting point between the material and the boiling point can be prepared, it can be applied regardless of the type of the coolant. Generally speaking, it is easy to apply to a liquid metal-cooled fast reactor in which the temperature difference between the coolant core outlet temperature at the rated operation and the boiling point of the coolant is large, but it cools liquids such as molten salts and organic substances. It is also possible to apply it to a nuclear reactor. Further, the present invention can also be applied to a gas cooling furnace by selecting a partition plate material having a melting point that is appropriately higher than the coolant core outlet temperature during rated operation.

[0022]

As can be seen from the above description, the control float device of the present invention can be installed in a reactor having a forced circulation cooling system to prevent abnormal cooling due to unexpected accidents such as pipe breakage or pump stoppage. When the flow rate decreases, the control float in the guide tube naturally descends to the lower position and the neutron absorber filling portion is placed in the core region, so that the nuclear reaction of the core can be reliably suppressed, When an abnormal temperature rise of the coolant occurs due to the overpower operation of the furnace, the partition plate in the guide tube is melted, lithium 6 moves down by gravity and is injected into the core region, resulting in a neutron absorption cross-sectional area. The nuclear reaction of the reactor core can be effectively and reliably suppressed by the action of lithium 6 having a large value.

As described above, according to the control float device of the present invention, not only the abnormal flow rate of the coolant but also the abnormal phenomenon of the abnormal temperature rise of the coolant can be prevented without a driving mechanism such as an electric circuit. Since it is possible to automatically and surely take measures using only natural phenomena due to the characteristics of the material, the inherent safety of the nuclear reactor can be improved.

It has already been proposed to mix lithium 6 into the coolant in the emergency of the liquid metal cooling furnace to suppress the nuclear reaction, but as a subsequent process, the lithium 6 is separated from the coolant and recovered. Furnace operation must be resumed,
The separation and recovery of lithium 6 from the coolant is not easy.
On the other hand, according to the present invention, even when the partition plate is melted and the lithium 6 falls into the lower cavity of the control float, the lithium 6 does not leak to the outside of the sealed cladding tube and therefore does not mix with the coolant. . Therefore, the operation of separating and recovering the lithium 6 from the coolant is not required, and the operation of the furnace can be restarted only by pulling out the control float device together with the sealed cladding tube from the core and replacing it with a new control float device.

Further, the pressure in the lower cavity of the control float is reduced to a value close to the lower limit of the cladding tube strength to increase the pressure difference from the pressure in the upper cavity. The falling movement speed to the lower cavity can be further increased, and the response to the abnormal coolant temperature rise can be made faster.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a part of a control float device of the present invention in a cutaway manner.

FIG. 2 is an explanatory diagram showing a partially enlarged state of a rated operation when the control float device of FIG. 1 is mounted on a nuclear reactor.

FIG. 3 is a partially enlarged explanatory view showing a state when an abnormal flow rate is reduced when the control float device of FIG. 1 is mounted on a nuclear reactor.

FIG. 4 is an explanatory diagram showing a partially enlarged state of an abnormal temperature rise when the control float device of FIG. 1 is mounted on a nuclear reactor.

FIG. 5 is a graph showing a calculation result of a relationship between a mixing ratio of lithium 6 in a coolant and a change in reactor reactivity.

FIG. 6 is a partially cutaway perspective view of a conventional control float device.

FIG. 7 is an explanatory diagram showing a state at the time of rated operation when the conventional control float device of FIG. 6 is mounted on a nuclear reactor.

FIG. 8 is an explanatory diagram showing a state when an abnormal flow rate is reduced when the conventional control float device of FIG. 6 is mounted on a nuclear reactor.

[Explanation of symbols]

 10: Control float device 11: Guide tube 12: Cladding tube 13: Control float 16: Neutron absorber filling portion 17: Partition plate 18: Lithium 6 reservoir 19: Through hole 20: Lower cavity 21: Upper cavity 22: Skirt 30 ： Core

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G21C 9/033 GDF G21C 7/08 G21C 7/22

Claims (4)

(57) [Claims] 1. A hermetic seal capable of ascending or descending inside a guide tube according to a flow state of a coolant flowing through the guide tube inside a guide tube vertically disposed in a core portion of a nuclear reactor having a forced circulation cooling system. A control float consisting of a cladding tube is inserted, and a neutron absorber filling portion is provided below and a lithium 6 reservoir is provided above the control float inside the control float through a partition plate which melts when the temperature of the coolant rises abnormally. The neutron absorber filling portion is provided with a vertically extending through hole, and a lower cavity is provided below the neutron absorber filling portion and an upper cavity is provided above the lithium 6 reservoir. The control float device for a nuclear reactor, wherein the position of the part rises outside the core region during the rated operation of the reactor and falls into the core region when the abnormal coolant flow rate decreases. 2. The vertical length of the neutron absorber filling portion, the lithium 6 reservoir and the lower cavity in the control float,
2. The control float apparatus for a nuclear reactor according to claim 1, wherein the length is substantially equal to the vertical length of the core. 3. The control float device for a nuclear reactor according to claim 1, wherein the partition plate is made of barium. 4. The control float apparatus for a nuclear reactor according to claim 1, wherein the pressure in the lower cavity is reduced to a value close to a lower limit value of the strength of the cladding tube.