JPH0342637B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an emergency core cooling system (ECCS) that cools the reactor core in the event of a loss of coolant accident (LOCA) in a boiling water reactor. The present invention relates to a fuel assembly suitable for more reliably and efficiently utilizing water flowing into a support structure for core cooling.

[Background of the invention]

Figure 1 shows the ECCS injection system and fuel support structure of a current boiling water reactor.

There is a shroud 2 within a reactor pressure vessel 1, and a fuel assembly box 4 surrounding fuel rods 3 is provided inside the shroud 2. A lower tie plate 5 is inserted into the lower part of the fuel assembly box 4, and this lower tie plate 5 is supported by a fuel support fitting 6 fitted into a lower core support plate 7. On the side surface of the lower tie plate 5 are a bypass section 8, which is an outer area of the fuel assembly box 4, and a lower tie plate.
A cooling hole 9 is provided that connects the inside of the plate 5. Further, at the lower side of the fuel support fitting 6, there is an opening 10 that communicates with the pressure vessel lower plenus 15, and an orifice 11 that opens laterally is provided here.

Here, the opening 10 in which the orifice 11 is installed is oriented horizontally because the control rod guide pipe is connected below the fuel support fitting 6.

During normal operation, cooling water inserted into the lower plenum 15 by the jet pump 12 flows through the opening 10.
lower tie plate 5 through orifice 11 of
The heat generated by the fuel rods 3 is absorbed by the fuel rods 3. On the other hand, a portion of the cooling water that has flowed into the lower tie plate 5 flows out from the cooling hole 9 to the bypass section 8 . The water flow in the bypass section 8 is required to be such that no voids are generated in this region (approximately 10% of the reactor core), and the water in this region serves as a moderator for neutrons. (See Figure 2 A) To prevent the cooling water in the reactor pressure vessel 1 from flowing out during LOCA, exposing the fuel rods 3, and causing the fuel rods 3 to become hot due to decay heat and cause damage.
ECCS is provided. 13 is the water injection nozzle of the water injection system, which is one of the ECCS, and 14 is the spray header of the core spray system. Water injection nozzle 13
The injected water flows into the bypass section 8 which is the area between the fuel assembly boxes 4, and floods from the bottom of the bypass section 8.

The water collected in the bypass section 8 flows back into the lower tie plate 5 through the cooling holes 9 and falls through the orifice 11 of the aperture 10 into the lower plenum 15. In addition, some of the water uniformly sprayed from the core spray header 14 onto the top of each fuel assembly box 4 passes through the fuel assembly box 4 and falls while cooling the fuel rods, and some of this water becomes steam. It slips upwards. Further, since the other spray water falls into the bypass section 8 outside the fuel assembly box 4, it has the same function as the water in the water injection system. (Second
(See Figure B) The water falling into the lower plenum 15 causes the water level in the lower plenum 15 to rise, and this reaches the fuel area beyond the lower core support plate 7, thereby achieving final core cooling (core re-flooding). .

However, there are the following problems regarding the above-mentioned core cooling.

(1) With cooling by the water injection system, the core cooling effect cannot be expected until the lower plenum 15 is filled with water, so it takes time to cool the core, and the heat-up of the fuel rods 3 cannot be kept low.

(2) In cooling the core spray system, the technology for spray nozzle shape and arrangement is complex and difficult in order to spray water evenly over the entire core at all times even when the core spray system flow rate changes due to pressure fluctuations in the reactor pressure vessel 1. That's funny. Therefore, it is necessary to increase the spray flow rate, which increases the system capacity.

FIG. 3 shows a conventional example disclosed in Japanese Patent Application Laid-Open No. 100092/1983.

The orifice installed at the bottom opening of the fuel support fitting of current boiling water reactors has a horizontal opening direction (flow direction) as shown in Figure 1, so ECCS injection during LOCA is difficult. The structure is such that water flows below the orifice, and steam blown up from the lower plenum flows above the orifice, making it easy for gas-liquid phase separation to occur.

On the other hand, if the orifice is opened vertically, as shown in A in Figure 3, the above-mentioned gas-liquid phase separation phenomenon will be less likely to occur, and the cooling water falling into the orifice from above will flow into the lower plenum. The cooling water fall suppression phenomenon (CCFL phenomenon), in which the steam blown up from the cooling water prevents it from flowing below the orifice, is likely to occur.

However, in the orifice structure shown in A in Figure 3, the blow-up steam flow is greatly disturbed at the orifice, and the balance of the CCFL phenomenon is easily destroyed due to the disturbance of the blow-up steam flow. However, it is not suitable for increasing the steam flow rate at the orifice.

At B in Figure 3, there is little resistance to the falling water flow from above and it flows easily, but there is great resistance to the rising water flow from below (normally), and the rising steam flow rate does not increase. , in this case, the CCFL phenomenon does not occur here.

As mentioned above, although both A and B in Figure 3 have a removable structure, they have a structure that makes it difficult for the CCFL phenomenon to occur, resulting in increased water flow resistance during normal operation, which ultimately hinders the most important normal operation. This leads to a decrease in the efficiency of the entire reactor.

FIG. 4 shows a conventional example shown in Japanese Patent Application Laid-Open No. 58-7594.

A in FIG. 4 shows a cross section of a fuel assembly in which an orifice with several holes is installed in the lower tie plate. B is a view of A from below, and it can be clearly seen that there are three holes in the orifice.

What is distinctive about Fig. 4 is that the orifice is installed on the lower tie plate, which is closer to the fuel than in Fig. 3, but the water flow resistance under normal conditions is still large (Fig. 3). (becomes larger), leading to a decrease in the overall efficiency of the reactor during normal operation.

Additionally, because a multi-hole orifice is used, separation flows are likely to occur, where water falls from one hole and steam blows up from another hole.

[Purpose of the invention]

It is an object of the present invention to prevent ECCS injection water flowing into the lower part of the fuel assembly via the fuel bypass section during LOCA from the steam flow from the lower plenum without affecting the reactor efficiency during normal operation. The purpose of this invention is to provide a fuel assembly that more effectively generates the CCFL phenomenon (cooling water fall suppression phenomenon) at the bottom of the fuel assembly and significantly improves core fuel cooling from below.

[Summary of the invention]

The present invention limits the flow rate of cooling water from the lower plenum into the lower tie plate to a desired flow rate, and has a throat whose diameter decreases upward and whose upper end protrudes into the lower tie plate. The present invention is characterized in that a small hole is provided in the lower tie plate and is bored at a side circumference of the lower tie plate and below the upper end of the throat. In addition, it is preferable that a plurality of the small holes are formed at approximately equal intervals on the side circumference of the lower tie plate.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIGS. 5 to 8.

As shown in Fig. 5, a lower tie plate 5 is inserted into the lower part of the fuel assembly box 4, and a throat 16 is installed in the lower flow path of this lower tie plate 5 so that the upper part becomes narrower. did. In addition,
A plurality of cooling holes 9 are provided on the side surface of the lower tie plate 5 to connect the bypass section 8, which is the outside area of the fuel assembly box 4, to the inside of the lower tie plate 5, but these holes are similar to the conventional one. If there is only one, the water flowing from the bypass section 8 through the cooling holes 9 will be locally concentrated in one place at the top of the throat 16, resulting in two parts: the part where steam blows up and the part where water falls. There is a possibility that it will split into phases and the CCFL phenomenon will break. For this reason, in this embodiment, four cooling holes 9 are uniformly arranged in the side wall of the lower tie plate 5 below the upper opening position of the throat 16. Note that the total flow area of this cooling hole 9 is the same as that of the conventional cooling hole 9.

Further, there is an opening 10 on the lower side of the fuel support fitting 6 that communicates with the pressure vessel lower plenum 15.
The flow path area other than the above-mentioned throat 16 is widened to reduce the flow path resistance and is desirable from the viewpoint of securing the blow-up steam flow rate, so the protrusion below the throat 16 (for installing the conventional inlet orifice 11) is The flow path area of the opening 10 is increased.

As shown in FIG. 6a, during normal operation, cooling water introduced into the lower plenum 15 by the jet pump 12 rises through the openings 10 into the fuel support fitting 6 and flows into the lower tie. - Passes through the throat 16 installed on the plate 5 and ascends between the fuel rods 3. Further, a portion of the cooling water that has flowed into the lower tie plate 5 flows out from the cooling hole 9 to the bypass section 8 . The water flow to this bypass section 8 needs to be of the same level as the conventional one in order to suppress the generation of voids in this region.

In this way, during normal operation, in order to supply sufficient cooling water to the reactor core, it is better to have a smaller flow path resistance in the lower part of the fuel, and to maintain the balance for each fuel assembly in the entire reactor core. In order to ensure the appropriate flow distribution, it is necessary to standardize the flow path at the bottom of each fuel assembly. Conventionally, this flow rate distribution was achieved using the orifice 11 shown in Fig. 1, but the same flow rate distribution can be easily obtained with the throat 16 shape as in the embodiment of the present invention, and furthermore, the throat 16 shape allows for higher water flow. Since it has low resistance to water flow and high resistance to falling water flow (backflow), it improves the natural circulation ability of the reactor, which is advantageous. Here, the reason why the resistance to the falling water flow is large is because the upper end of the throat 16 is located above the cooling hole 9. In other words, the water flowing in from the cooling holes 9 does not immediately flow downward, but the upper end of the throat 16 acts as a resistance, and once flows upward, the flow rate of the falling water flow is suppressed.

Generally speaking, the flow distribution to each fuel assembly is divided into two areas, the central part of the core and the peripheral part, and the distribution is different. , the fuel in the central core region is always replaced in the central core region,
Since fuel in the peripheral region is always replaced within the core peripheral region, problems with the throat 16 size will not occur during fuel replacement, and even if such a situation occurs, the throat 16
This can be easily solved by making it removable.

In FIG. 5, the throat 16 and the lower tie plate 5 are integrally constructed, but if there is a problem with manufacturing costs or other aspects, the throat 16 can be made separately and fitted into the lower tie plate 5. Alternatively, this can be solved by using a screw-in type. In this case, even if the size of the throat 16 changes, it is necessary to keep the size of the fitting or threaded portion the same. Figure 8 shows an example of a screw-in type.

As shown in Figure 6b, in the unlikely event that a major rupture occurs in the recirculation piping and the cooling water in the core decreases significantly, the ECCS injection water will flow into the bypass section 8 between the fuel assembly boxes 4 and Water begins to flood from the bottom of the bypass section 8. The water accumulated in the bypass section 8 flows back into the lower tie plate 5 through the cooling holes 9. The water that has flowed into the lower tie plate 5 is caused by steam generated by boiling under reduced pressure, etc., which blows up from the lower plenum 15.
Since the upper part of the throat 16 prevents it from falling downward, it accumulates at the upper part of the throat 16. Furthermore, this throat 1
The water accumulated in the upper part of the fuel rod 6 is transported in the form of water droplets by the steam blown up from the throat 16, and greatly contributes to the cooling of the fuel rod 3 from below.

As shown in FIG. 6b, in the present invention, it is important that the opening of the throat 16 be vertical in order to promote the CCFL phenomenon at the throat 16 portion.

In the case of an orifice with multiple holes shown in Figure 4, the blow-up steam flow is greatly disturbed at the orifice part, and the balance of the CCFL phenomenon is easily destroyed due to the disturbance of the blow-up steam flow, and the orifice structure is not suitable for flow control. However, it is not suitable for increasing the steam flow rate at the orifice. This also shows that the throat structure of the present invention is an essential condition for stably and more effectively generating the CCFL phenomenon.

FIG. 7 shows the effects of the present invention. The upper diagram in Figure 7 shows the change in the reactor water level after LOCA, and the lower diagram shows the fuel rod surface temperature in response to this water level change. Since the surface temperature of the fuel rods is highest at the center of the core, the temperature at this center is used as the representative temperature.

Assuming a major rupture in the recirculation piping, etc., the water level inside the reactor would drop due to blowdown of the coolant inside the reactor after the accident, and in the conventional example (dashed line in Figure 7), the reactor core would be exposed. The reactor water level reaches the lower plenum 15. Furthermore, the water level gradually recovers due to water injection into the reactor by ECCS, and when it reaches the bottom of the core, there is flashing due to rapid heat input into the water from the core heat-up area, and the structure of the core is smaller than that of the lower plenum. Due to the small space for water other than wood, the water level rises faster and the core is re-flooded. The surface temperature of the fuel rods begins to rise once the core is exposed and is cooled by re-flooding.

According to the present invention (solid line in FIG. 7), [A] the throat 1
Since the drop in the reactor water level in the 6th section is greatly suppressed and the drop in the core water level is delayed, the start of fuel rod heat-up is also delayed. In order to suppress the fall of the reactor water level, as mentioned above, by using the throat 16 whose flow path area gradually becomes smaller as it goes upward, there is little resistance to the steam blowing up from the lower plenum 15. Since the upper end of the throat 16 is located above the cooling hole 9, the flow velocity of the falling water flow is directly suppressed;
This is largely due to the fact that the passage direction of the throat 16 is vertical and a plurality of cooling holes 9 are provided, making it difficult for gas-liquid separation to occur. [B] After that, steam continues to blow up from the lower plenum due to reduced pressure boiling and heat from the structural materials, but the lower plenum water level decreases accordingly.
However, this blown-up steam continues to prevent water from falling at the throat 16 at the bottom of the fuel assembly, so water injected into the reactor by the ECCS flowing into the lower tie plate 5 from the bypass section 8 flows into the lower plenum. The amount of water falling is small and accumulates in the core (bypass area and fuel area), so the core water level rises quickly. At this time, the water that has flowed into the lower part of the fuel assembly is turned into water droplets and transported by the steam blown up from the throat 16, and the fuel rods 3 are cooled from below, thereby suppressing fuel rod heat-up. . [C]
Furthermore, as the core water level rises more quickly as described above, the time for re-flooding the core is greatly shortened, and fuel rod heat-up is greatly suppressed.

As described above, although this example improves the natural circulation capacity of the reactor during normal operation,
There are no other factors that impede performance, and heat-up of the fuel rods can be greatly suppressed during LOCA.

〔Effect of the invention〕

Implementation of the present invention has the following effects.

(a) It improves the natural circulation capacity of the reactor during normal operation, and greatly suppresses fuel rod heat-up during LOCA. (driving performance,
(improved safety)

[Brief explanation of drawings]

Fig. 1 is a longitudinal cross-sectional view of a conventional example, Fig. 2 is an explanatory diagram of the conventional example, Figs. 3 and 4 are longitudinal cross-sectional views of other conventional examples, and Fig. 5 is a longitudinal cross-section of an embodiment of the present invention. Figure, 6th
7 is an explanatory diagram of an embodiment of the present invention, FIG. 7 is an explanatory diagram of phenomena in the embodiment of the present invention, and FIG. 8 is a longitudinal sectional view of a modification of the present invention. DESCRIPTION OF SYMBOLS 1... Reactor pressure vessel, 2... Shroud, 3... Fuel rod, 4... Fuel assembly box, 5... Lower tie plate, 6... Fuel support fitting, 7... Lower core support plate, 8... Bypass section, 9...Cooling hole, 10...
Opening portion, 11... Orifice, 12... Jet pump, 13... Water injection system nozzle, 14... Core spray header, 15... Lower plenum, 16... Throat, 17... Threaded portion.

Claims (1)

[Claims] 1. The flow rate of cooling water from the lower plenum into the lower tie plate is limited to a desired flow rate, and the diameter decreases as the penetration direction is vertical and goes upward, and the upper end thereof is connected to the lower tie plate. A fuel assembly comprising: a throat protruding into a tie plate; and a plurality of small holes formed at a side circumference of the lower tie plate and below an upper end of the throat. 2. The fuel assembly according to claim 1, wherein the plurality of small holes are formed at approximately equal intervals on the side circumference of the lower tie plate.