JPH02268293A - Natural circulation type boiling water nuclear reactor - Google Patents

Abstract

PURPOSE:To prevent effectively a damage of a turbine and a down-grading of an operating efficiency by expanding an upper part of a reactor core shroud toward the surface of a boiling water and by forming an increasing cross-sectional area of a flow passage, therewith. CONSTITUTION:An upper part of a reactor core shroud 3a expands toward a surface of a boiling water and is constituted by forming an increasing cross- sectional are of a flow passage. A coolant passing through a reactor core 2 forms a two-phase flow of a steam and a water and flows upwardly in a straight tube part of the reactor core shroud 3a at a rather high velocity. However, as the cross-sectional area of the flow passage is increasing according to a height to which the flow reaches above the reactor core shroud 3a, the flow velocity gradually decreases. Finally, the flow velocity of the two phase flow at a periphery of the surface 11 of the boiling water, decreases to one in several or one tenth of the flow velocity just after passing through the reactor core. Therefore, a vapor liquid separation can be easily conducted, a carry-over and a carry-under diminish substantially. Consequently, a damage of a turbine B and a down-grading of an operating efficiency can be effectively prevented.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a natural circulation boiling water nuclear reactor, and in particular improves the separation efficiency of steam and water, and improves the efficiency of auxiliary equipment such as a turbine. This invention relates to a natural circulation boiling water reactor that can improve the operational efficiency and heat transfer efficiency in the reactor core.

(Prior Art) A conventional boiling water nuclear reactor is generally configured as shown in FIG. That is, in the figure, a reactor core 2 is housed in the center of a reactor pressure vessel 1, and a cylindrical core shroud 3 is disposed to cover this reactor core 2. A plurality of recirculation pumps 4 are disposed below the gap between the core shroud 3 and the reactor pressure vessel 1.

The cooling water obtains thermal energy generated by the nuclear reaction in the reactor core 2, turns into high-temperature, high-pressure steam, and flows upward in the core shroud 3. After the water and steam are separated by the separator 5, the mixed flow of water and steam is further introduced into the steam dryer 6, where the moisture is sufficiently removed, and then passed through the main steam pipe 7, etc., to the turbine 8. guided by. The steam that drove the turbine 8 is condensed in a condenser 9,
This condensate passes through the water supply pipe 10 and returns to the core shroud 3.
into the outer annular channel.

By the way, in a forced circulation boiling water reactor as shown in Fig. 2, it is necessary to equip a large recirculation pump 4, an auxiliary power generation equipment attached to it, a head restraint, etc., so the system configuration of the reactor is but? ! There are problems in that the process becomes complicated and equipment costs and operating costs rise.

In recent years, development of a natural circulation boiling water reactor that is not equipped with a recirculation pump 4 has been progressing in order to simplify the system and reduce equipment costs and operating costs. This type of boiling water reactor does not have any equipment for forcibly circulating the coolant, but instead circulates the coolant by natural circulation force based on the density difference between the coolant flowing inside and outside the core shroud 3.

In addition, in the boiling water reactor mentioned above, in order to ensure a large natural circulation force, the flow resistance of the cold parts in the reactor is reduced as much as possible, and for example, the separator 5, which has a large resistance, is also eliminated. It is often done.

(Problems to be Solved by the Invention) However, in the conventional natural circulation type Ni l water reactor as described above, since a separator is excluded,
The separation efficiency between the moisture mixed in the generated steam and the steam is low, and droplets often flow out to the steam dryer side, so-called carryover. Therefore, there is a problem in that the bad air conditions supplied to the turbine deteriorate, promoting corrosion damage to the turbine and reducing its operating efficiency.

On the other hand, a decrease in gas-liquid separation efficiency inevitably induces an increase in so-called carry-under, in which steam is drawn into the coolant descending in the annular flow path between the reactor core shroud and the reactor pressure vessel. As a result, the enthalpy of the coolant at the reactor core inlet changes, reducing the heat transfer efficiency in the reactor core, which in turn causes a reduction in the output of the reactor.

The present invention has been made in order to solve the above problems, and provides a natural circulation boiling water nuclear reactor that can maintain high separation efficiency between steam and water even if a separator is eliminated. With the goal.

[Structure of the invention]

(Means for Solving the Problems) The present invention includes a core shroud that is arranged concentrically and isolated from the inner wall of the reactor pressure vessel, and a coolant rise path that guides the coolant that has passed through the reactor core toward the boiling water surface. In a natural VIU ring boiling water reactor in which a coolant descending passage is formed between the reactor pressure vessel inner wall and the reactor core shroud,
It is characterized in that the upper part of the core shroud is expanded in the direction of the boiling water surface to increase the cross-sectional area of the flow passage.

(Function) According to the natural circulation boiling water reactor with the above configuration, the upper part of the core shroud is expanded in the direction of the boiling water surface to increase the cross-sectional area of the flow passage, so that the flow passage through the core is expanded. The rising speed of the coolant, which has become a gas-liquid two-phase flow, is significantly reduced near the boiling water surface. Therefore, droplets (carryover) entrained in the steam flowing from the boiling water surface toward the side of the steam dryer are significantly reduced.

Therefore, fewer droplets W are fed to the turbine, and damage to the turbine and reduction in operating efficiency are effectively prevented.

On the other hand, since the flow velocity of the coolant flowing into the annular flow path from the boiling water surface is also reduced, the amount of carry-under of steam entrained in the coolant is also reduced, and a decrease in heat transfer efficiency in the core can also be prevented.

(Example) An example of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic vertical sectional view showing an embodiment of a natural circulation boiling water nuclear reactor according to the present invention. Note that the same elements as those in the conventional example shown in FIG. 2 are given the same reference numerals, and detailed explanation thereof will be omitted.

In the natural circulation boiling water reactor according to this embodiment, a core shroud 3a is installed concentrically and isolated from the inner wall of the reactor pressure vessel 1.
is arranged to form a coolant ascending passage 12 that guides the cold En material that has passed through the reactor core 2 in the direction of the boiling water surface 11, and a cooling pipe having an annular cross section between the inner wall of the reactor pressure vessel 1 and the core shroud 3a. In a natural circulation boiling water nuclear reactor in which a material descending passage 13 is formed, the upper part of the core shroud 3a is expanded in the direction of the boiling water surface to increase the cross-sectional area of the passage.

The coolant that has passed through the core 2 forms a gas-liquid two-phase flow of steam and water, and flows at a high velocity in the direction of the steam dryer 6 within the straight cylindrical portion of the core shroud 3a.

However, since the cross-sectional area of the flow path increases as it flows above the core shroud 3a, the flow rate gradually decreases. And the flow velocity of the two-phase flow near the boiling water surface 11 is about 1/10 to 1/10 of the flow velocity immediately after passing through the core? U
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Therefore, gas-liquid separation is easily performed, and carryover and carryunder are significantly reduced.

Therefore, the amount of droplets entrained in the main steam and fed to the turbine 8 is small, and damage to the turbine 8 and reduction in operating efficiency are effectively prevented. .

On the other hand, since the flow velocity of the coolant flowing from the boiling water surface 11 into the coolant descending path 13 having an annular cross section formed on the outer surface of the core shroud 3a is also reduced, the amount of steam mixed into the coolant is also reduced, and the transmission in the core is reduced. 91 degrees which impair thermal properties are also significantly reduced.

〔Effect of the invention〕

As explained above, according to the natural circulation boiling water reactor according to the present invention, the upper part of the core shroud is expanded in the direction of the boiling water surface to increase the cross-sectional area of the flow passage, so that the reactor core The upper limit velocity of the coolant that has passed through the gas-liquid two-phase flow is significantly reduced near the boiling water surface. Therefore, droplets (carryover) entrained in the steam flowing from the boiling water surface toward the steam dryer are significantly reduced. Therefore, the amount of droplets fed to the turbine is also small, and damage to the turbine and reduction in operating efficiency are effectively prevented.

On the other hand, since the flow velocity of the coolant flowing from the boiling water surface into the annular flow path is reduced, the carry-under of steam entrained in the coolant is also reduced, and it is also possible to prevent a decrease in heat transfer efficiency into the reactor core. can.

[Brief explanation of drawings]

FIG. 1 is a schematic longitudinal sectional view of an embodiment of the present invention, and FIG. 2 is a schematic diagram of a conventional boiling water type light water reactor. 1... Reactor pressure vessel, 2... Reactor core, 3, 3a...
・Core shroud, 4... Recirculation pump, 5... Separator, 6... Steam dryer, 7... Main steam pipe, 8
...Turbine, 9...Condenser, 10...Water supply piping, 11...Boiling water surface, 12...Coolant upper boundary path,
13... Coolant descending path.

Claims (1)

[Claims] The core shroud is arranged concentrically and isolated from the reactor pressure vessel inner wall to form a coolant ascending path that guides the coolant that has passed through the reactor core toward the boiling water surface. A natural circulation boiling water reactor in which a coolant descending path is formed between the reactor and the reactor, characterized in that the upper part of the core shroud is expanded in the direction of the boiling water surface to increase the cross-sectional area of the flow path. Natural circulation boiling water reactor.