JPH0580179A - Natural circulation type nuclear reactor - Google Patents

Abstract

PURPOSE:To increase the natural circulation amount by precluding carry-under of bubbles in a downcomer of a nuclear reactor of natural circulation type. CONSTITUTION:Along the periphery of the top of a riser 3, a coolant sump 9 is installed to stagnate temporarily the coolant 5 overflowing from the riser 3. The coolant in carrying-under of the bubbles having flowed into the coolant sump is free of being influenced by the driving force given as the sum of the elevating force in the riser and the water head pressure in a downcomer, so that the bubbles can be separated irrespective of the driving force.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a natural circulation type nuclear reactor, and more particularly to a nuclear reactor which prevents entrainment of bubbles (carry under) in downcomers of the natural circulation type nuclear reactor and increases natural circulation power.

[0002]

2. Description of the Related Art As shown in FIG. 5, a natural circulation reactor has a core 2, which is formed by loading a large number of fuel assemblies inside a pressure vessel 1, and a cylindrical riser 3 which is provided above the core. It consists of Coolant 5a supplied from the water supply pipe 4
Is mixed with the coolant 5b descending the downcomer 6 and introduced into the core. In the core, the coolant that has received the heat generated by the fission reaction of the fuel boils, becomes a two-phase flow state of water and steam, and rises in the riser. The steam 7 is separated on the liquid surface above the riser and is guided to a turbine (not shown) from the main steam pipe 8.

The driving force for circulating the coolant is that the coolant in the downcomer (single-phase flow of water only) and the mixed flow of water and steam in the riser (two-phase flow). ) Is the density difference. In downcomer, water head pressure works downwards, and in the riser, two-phase flow buoyancy works upwards,
A natural circulation flow is formed as a whole. In order to keep the driving force large, it is desirable to completely separate water and steam at the liquid surface. However, in reality, due to the fluctuation of the liquid surface and the inertia of the coolant, minute vapor bubbles are mixed into the water, and the bubbles are entrained in the water and descend in the downcomer, so-called carry under occurs, The density difference between the downcomer and the coolant inside the riser becomes smaller. Therefore, there is a problem that the driving force for natural circulation is reduced.

The prior art devised for the method of increasing the coolant flow rate in a natural circulation reactor is the one described above as the known example closest to the present invention. All of them adopt a structure that prevents carry-under inside the downcomer and enhances natural circulation power by improving the water-water separation efficiency. As a way to do that,
In (1), the baffle plate is provided over the entire circumference of the upper end of the riser to increase the flow passage area in the upper region of the downcomer and reduce the flow velocity of the coolant to separate the entrained bubbles. Further, in (2), a method is used in which the bubble guide plate provided in the downcomer guides the bubbles to the stagnation area and separates the bubbles through the bubble removal channel. Furthermore, in (3), by providing a baffle plate, a bubble collection space, and a bubble removal flow path above the downcomer,
Prevents air bubbles from flowing into the downcomer.
In (4), the inside of the riser is divided into a number of sections having a height higher toward the center to increase the flow path length required for air-water separation,
It has a structure that improves air-water separation efficiency and reduces the inflow of air bubbles into the downcomer.

[0005]

The above-mentioned conventional technique is insufficient from the viewpoint of water-water separation efficiency for the following reasons. That is, assuming that minute bubbles are present in the upper part of the downcomer, the flow velocity of the coolant flowing with the bubbles is given by the sum of the above-mentioned rising force in the riser and the hydraulic head pressure in the downcomer as the driving force. It will be. In a natural circulation reactor, the larger the driving force, the larger the circulation flow rate of the coolant. However, on the other hand, there is a problem that the larger the driving force, the larger the amount of bubbles entrained. Therefore, even if a stagnation area is provided as in the prior art, if this stagnation area is directly connected to the downcomer,
Since it is affected by the downward flow, it is essentially difficult to form a complete stagnation region, and the separation of bubbles is not perfect even in the known example.

An object of the present invention is to solve the contradictory problem that the carry under of bubbles increases as the driving force of the coolant increases in the natural circulation type nuclear reactor and to increase the natural circulation flow rate.

[0007]

The above object can be achieved by the following means. That is, the outer peripheral portion of the upper end of the riser may be provided with a space (hereinafter referred to as a coolant reservoir) having only an open upper portion for temporarily storing the coolant overflowing from the riser.

[0008]

The operation of the present invention will be described below.

FIG. 2 is an enlarged view showing only the upper left side of the riser and the downcomer of the natural circulation type reactor in which the coolant reservoir according to the present invention is installed. The coolant reservoir 9 is installed on the entire outer circumference of the upper end of the riser 3. The stagnation area 12 in the coolant reservoir has a structure not directly connected to the downcomer. With such a structure, the coolant 5c containing vapor bubbles rising in the riser changes its flow direction to the horizontal direction at the upper end of the riser wall and flows into the coolant reservoir. In the coolant reservoir, the momentum possessed by the coolant is lost and the flow becomes stagnant, so that the bubbles of the vapor 7 are easily released to the upper space. The coolant 5d from which the air bubbles have separated further overflows and descends in the downcomer.

With such a structure, the coolant entrained with the bubbles flowing into the coolant reservoir is not affected by the driving force given by the sum of the ascending force in the riser and the head pressure in the downcomer. Therefore, the bubbles can be separated regardless of the magnitude of the driving force, and the problem of the prior art that the carry-under increases when the driving force increases can be solved.

[0011]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 is a cross-sectional view of a natural circulation type nuclear reactor showing a basic embodiment of the present invention. In the figure, a coolant reservoir 9 is installed along the outer periphery of the upper end of the riser 3. With such a structure, the vapor bubbles can be efficiently separated, and the carry-down of the coolant descending in the downcomer can be prevented, so that the natural circulation flow rate can be increased.

FIG. 3 is a diagram showing another embodiment of the present invention. The difference from the embodiment shown in FIG. 1 is that a downcomer pipe 10 communicating with the downcomer 6 is installed on the bottom surface of the coolant reservoir 9. With such a structure, the coolant from which bubbles have been separated in the coolant reservoir is guided to the downcomer through the downcomer pipe. At this time, the coolant flowing through the downcomer flows only due to the water head difference between the liquid surface in the coolant reservoir and the downcomer, and is not affected by the drive force of the riser and downcomer coolant. It is possible to solve the problem of the prior art that the value becomes large.
Further, in the embodiment shown in FIG. 1, some bubbles may be entrained when overflowing from the coolant reservoir to the downcomer, but in this embodiment, this is a structure that does not overflow, so prevent this. You can

FIG. 4 is a diagram showing another embodiment of the present invention. A difference from the embodiment shown in FIG. 3 is that a disc-shaped collar 11 is provided from the riser 3 toward the stagnation region 12 of the coolant reservoir 9. With such a structure, the coolant containing the vapor bubbles rising in the riser changes the flow direction downward at the upper end of the riser wall, and when flowing into the coolant reservoir, the flow is along the brim. Since it flows in, the ripples are reduced and the entrainment of bubbles can be reduced. For this reason, it becomes possible to further improve the steam-water separation efficiency in the coolant reservoir.

[0015]

According to the first aspect of the present invention, the coolant entrained with the bubbles flowing into the coolant reservoir can be separated in the stagnation region. Also, in this stagnation area, the coolant is not affected by the driving force given by the sum of the rising force in the riser and the head pressure in the downcomer, so bubbles can be separated regardless of the magnitude of the driving force. However, it is possible to solve the problem of the prior art that the carry-under becomes large when the driving force becomes large.

According to the second aspect of the invention, the coolant that has flowed into the coolant reservoir and separated the bubbles flows in the downcomer pipe only due to the water head difference between the coolant reservoir and the downcomer, thereby cooling the riser and the downcomer. It is not affected by the driving force of the material. Furthermore, since the coolant can be guided to the downcomer without depending on the overflow from the coolant reservoir, carry under due to the inclusion of bubbles at the time of overflow can be further reduced.

According to the third aspect of the present invention, it is possible to prevent bubbles from being entrained when overflowing from the riser to the coolant reservoir, so that the efficiency of separating water and water in the stagnation region is improved, and the cooling for descending the downcomer pipe is improved. It is possible to prevent bubbles entrained in the material.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a natural circulation type nuclear reactor to which a coolant reservoir according to the present invention is applied.

FIG. 2 is an explanatory view showing the function of the coolant reservoir according to the present invention.

FIG. 3 is a sectional view of a natural circulation reactor according to another embodiment of the present invention.

FIG. 4 is a cross-sectional view of a natural circulation reactor according to another embodiment of the present invention.

FIG. 5 is a cross-sectional view showing the structure of a conventional natural circulation reactor.

[Explanation of symbols]

1 ... Reactor pressure vessel, 2 ... Reactor core, 3 ... Riser, 4 ... Water supply pipe, 5 ... Coolant, 6 ... Downcomer, 7 ... Steam, 8 ... Main steam pipe, 9 ... Coolant reservoir, 10 ... Downcomer pipe, 12 ... Stagnation area.

Claims (3)

[Claims] 1. A reactor pressure vessel is surrounded by a core constituted by a fuel assembly, a cylindrical riser provided above the core, an outer peripheral portion of the riser and the pressure vessel,
A natural circulation type atom which has a downcomer which serves as a flow path of water as a coolant, and which circulates the coolant by using ascending force by the buoyancy of water and steam in the riser and water head pressure by the coolant of the downcomer as a driving force. In the furnace, a coolant reservoir for temporarily storing the coolant overflowing from the riser is provided along an outer periphery of an upper end portion of the riser, and a natural circulation type nuclear reactor is provided. 2. The natural circulation reactor according to claim 1, wherein a downcomer pipe for guiding the coolant to a downcomer is provided at the bottom of the coolant reservoir. 3. A natural circulation type atom according to claim 2, wherein a collar is provided at an upper end portion of the riser, and an outer peripheral portion of the collar is lower than a liquid surface formed in the coolant reservoir. Furnace.