JPH08110389A - Fuel assembly and core for pressurized water reactor - Google Patents

Abstract

PURPOSE: To mitigate the departure from nucleate boiling point (DNB) penalty without changing the basic design of fuel assembly with high pressure loss grids by providing path holes for connecting to core space in the middle of control rod guide tubes. CONSTITUTION: A fuel assembly is constituted by bundling fuel rods 11 and control rod guide tubes 14 with grids 15. The flow velocity in path A passing in the fuel rods 11, the guide tubes 14 and the high pressure loss grids 15a is smaller than the flow velocity in path B passing in the fuel rods 11, guide tubes 14 and low pressure loss grids 15b. In the fuel assembly (a), bypass paths C are formed. This bypass path C is formed with a path hole 10 connecting to the core space in the middle of the guide tube 14. By providing bypass paths C in this manner, average pressure loss in the high pressure loss grid assembly is reduced and the difference in coolant flow rate from the low pressure loss grid assembly can be reduced. By this with small design change of assembly, the DNB penalty can be mitigated in mixed core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a pressurized water nuclear reactor having different pressure losses of a coolant, which alleviates high pressure loss, and a core loaded with the fuel assembly. is there.

[0002]

2. Description of the Related Art FIG. 3 is a diagram showing a boiling heat transfer curve,
In the figure, the vertical axis represents the logarithm of the heat flux q ″, and the horizontal axis represents the logarithm of the heat transfer surface overheating temperature (difference between the heat transfer surface temperature and the saturated liquid temperature) ΔT _SAT . Light water reactor (pressurized water type and boiling water type) In order to maintain the smooth cooling of the fuel by the coolant and to ensure the integrity of the fuel, the design and management are performed so that the film boiling state does not occur on the surface of the fuel rods, as shown in FIG. In the film boiling state, the heat flux from the fuel rod surface is significantly reduced,
This is because the fuel rod surface temperature rises sharply.

In a pressurized water reactor, such a transition to the film boiling region is performed by the nucleate boiling departure point (DNB (Departure) in FIG.
From Nucleate Boiling)) is used as an index for operation and monitoring. Specifically, the nucleate boiling departure point (hereinafter referred to as DNB
The heat flux on the surface of the fuel rod (so-called “critical heat flux q” _DNB ”) is reached by predicting and evaluating it by the following correlation equation so that each fuel rod surface heat flux does not exceed q” _DNB. ing.

Q " _DNB = f (P, G, D, X ...) (1) P: pressure, G: mass velocity, D: fuel rod diameter, X: local quality

Incidentally, DNB means that the heat load is gradually increased from the state of nucleate boiling, and when it reaches a certain point, the coalescence of bubbles becomes remarkable and the behavior of foaming is also the foaming from the independent foaming nuclei in nucleate boiling. The situation is far from. The point at which the deviation from such nucleate boiling begins.

The "critical heat flux q" _DNB "is the relationship between the heat flux of heat transfer in the boiling state and the heat transfer surface superheat degree (difference between heat transfer surface temperature and saturated liquid temperature) as shown in the figure. Although it follows a simple boiling heat transfer curve, the heat flux has a limit value at the boundary between nucleate boiling and film boiling, and the heat flux at this point is called the critical heat flux. This is the limit in thermal design because burnout may occur when the heat load reaches the critical heat flux.

[0007]

FIG. 4 is an explanatory diagram showing the structure of a fuel assembly for a pressurized water reactor. As shown in FIG. 4, the fuel assembly for a pressurized water reactor has a plurality of fuel rods (4
1) and the control rod guide pipe (44) which is passed from the upper nozzle (42) to the lower nozzle (43), are provided with seven support grids (4).
According to 5), it is configured by bundling in a bundle.

In the 14 × 14 fuel assembly shown in the figure,
Sixteen control rod guide tubes (44) are arranged together with one in-core instrumentation guide tube in the central portion. In this fuel assembly,
(41) (Fuel cladding tube), Upper nozzle (42), Lower nozzle (4
3) and control rod guide tube (44) are the same, grid
Due to the difference in the design of (45), the pressure loss of the obtained fuel assembly may be different.

FIG. 5 is an explanatory view showing a flow path when two types of fuel assemblies having different pressure loss characteristics are adjacent to each other from the grid design. As shown in the figure, of the two types of fuel assemblies, the fuel assembly a is a high pressure loss grid with a high pressure loss.
(45a), the fuel assembly b is a low pressure loss grid with a lower pressure loss than the fuel assembly a.
It is a fuel assembly b having (45b). The flow velocity of the flow passage A passing through the fuel assembly a is smaller than the flow velocity of the flow passage B of the low pressure loss grid.

By the way, as shown in FIG. 5, when, for example, two types of fuel assemblies having different pressure loss (hereinafter referred to as "pressure loss") characteristics from the grid design are mixed, a set of high pressure loss grids is mixed. Coolant is less likely to flow in the body than in low pressure loss grids. Therefore, according to Eq. (1), in the high pressure loss grid assembly where the mass velocity G of the coolant is small, q " _DNB is also small and DNB is more likely to occur. This determines the fuel loading position of the replacement core. Narrowing the conditions for replacement core design,
The design flexibility is reduced.

Therefore, in the pressurized water reactor, when two kinds of fuels having different pressure loss characteristics are mixed from the grid (supporting grid) design, the fuel assembly of the high pressure loss grid has a relatively low flow rate, "DNBR" becomes more severe (so-called DNB penalty). In addition, "DNB
“R” is an index obtained by dividing the value of the limit heat load on the surface of the fuel rod, which is called DNB heat load, where a film of vapor is formed on the surface of the fuel rod and rapid heat transfer deterioration starts, by the value of the operating heat load. .

According to the present invention, there is no need to change the basic design (grid, upper and lower nozzles, etc.) of the fuel assembly for such a high-pressure loss grid, and the fuel assembly and core for a pressurized water reactor that alleviates this DNB penalty. Aim to get.

[0013]

In the fuel assembly for a pressurized water reactor according to the invention described in claim 1, a fuel rod and a control rod guide pipe extending from the upper nozzle to the lower nozzle are provided. In a fuel assembly for a pressurized water nuclear reactor bundled in a bundle, a core space is formed in an axially central region of a control rod guide tube having a communication hole that penetrates the lower nozzle and communicates with the space below the nozzle in a lower end. A core communication hole communicating with the core is provided, and a bypass flow path from the space below the nozzle to the core space is formed.

In the core of the pressurized water reactor according to the invention described in claim 2, several fuel assemblies having the same pressure loss due to the cooling water circulated from inside to above are loaded. In the core of the pressurized water reactor described above, some of the fuel assemblies are communicated from the lower space through the lower nozzle at the lower end of the control rod guide pipe to the lower space, and the control rod guide pipe. By virtue of the bypass passage that reaches the core space through the core communication hole that communicates with the core space formed in the axially central region of the fuel assembly, the fuel assembly is made to have the same pressure loss as other fuel assemblies. It is a waste.

[0015]

According to the present invention, the core communicating hole communicating with the core space is provided in the axially central region of the control rod guide tube having the communicating hole communicating with the lower space of the nozzle at the lower end thereof. Since the bypass passage extending from the nozzle lower space to the core space is formed, the pressure loss of the fuel assembly can be reduced. Further, the DNB penalty caused by the decrease in the flow rate of the coolant can be mitigated.

That is, by forming the bypass flow passage which does not pass through the grid portion which causes a large local pressure loss in all or a part of the control rod guide tube of the fuel assembly for a pressurized water reactor, the bypass flow passage is formed. The pressure loss of the fuel assembly can be reduced according to the size and number.

Therefore, for example, by selecting the size and number of the bypass passages, the average pressure drop of the fuel assembly having the grid having the high pressure loss is reduced, and the fuel assembly having the grid having the low pressure loss is reduced. The difference in the coolant flow rate between and can be reduced or made equivalent.

By the way, the coolant passing through such a bypass passage does not contribute to the cooling of the fuel rods at all. For this reason,
Even if another flow path that does not pass through the grid portion is simply formed to secure the flow rate of the coolant as the fuel assembly,
It does not help to mitigate the B penalty. Therefore, it is necessary to devise so that the coolant flows out of the control rod guide pipe in the upper region of the assembly where the DNB is usually more severe.

Therefore, in the present invention, since the core communicating hole is provided in the axially central region of the control rod guide pipe, the coolant flows out of the control rod guide pipe in the upper region of the assembly where the DNB is usually more severe. The DNB penalty caused by the decrease in the flow rate of the coolant can be mitigated. For this reason, it is preferable that the axially central region where the core communication hole is provided is 40 to 70% from the bottom of the overall core length.

Considering the pressure difference between the inside and the outside of the control rod guide tube in the core communication hole, the inside of the coolant, which does not have a high pressure loss, has a higher pressure. Smoothly flows out of the control rod guide pipe through the core communication hole, and improves DNB in the upper region of the fuel assembly.

Further, in the present invention, some fuel assemblies loaded in the core of the pressurized water reactor communicate with the lower space from the lower space through the lower nozzle at the lower end of the control rod guide tube. By the bypass passage that reaches the core space through the communication hole and the core communication hole that communicates with the core space formed in the axially central region of the control rod guide tube, the pressure of the same degree as other fuel assemblies is obtained. Since it includes the lost fuel assemblies, the basic design of the fuel assembly of the high pressure loss grid (grid, upper and lower nozzles, etc.) can be performed even when two types of fuel assemblies with different pressure loss characteristics are mixed in the core. The DNB penalty can be mitigated without modification.

In the present invention, the bypass flow passage is formed in a communication hole that penetrates the lower nozzle at the lower end of the control rod guide pipe and communicates with the lower space, and an axially central region of the control rod guide pipe. It is a flow path that reaches the core space through a core communication hole that communicates with the core space. This is because the pressurized water type fuel assembly has no room to form another bypass flow path, and therefore uses a control rod guide tube through which water is transmitted for cooling such as γ heat generation. The instrumentation tube provided at the center of the assembly of is not included.

[0023]

1 is an explanatory view showing the structure of a control rod guide tube of a fuel assembly showing the structure of an embodiment of the present invention. FIG. 2 is an explanatory diagram showing a flow path when the fuel assembly having the high pressure loss grid loaded with the control rod guide tube shown in FIG. 1 and the fuel assembly having the low pressure loss grid are adjacent to each other.

As shown in FIGS. 1 and 2, in this embodiment, a control rod guide tube (14) having a total length of about 4000 mm is connected to the lower nozzle (13) through the lower end plug (16) of the guide tube. A fuel assembly having a high pressure loss grid (15a) with a control rod guide tube (14) having core communication holes (10) arranged in three rows on the circumference in the axial center (about 2000 mm from the lower end). It is used for a.

As shown in FIG. 2, fuel rods are attached to the grid (15).
(11) and a control rod guide pipe (14) extending from an upper nozzle (not shown) to a lower nozzle (13) are bundled in a fuel assembly, the fuel rod (11) and the control rod guide pipe ( 14), the flow velocity of the flow path A passing between the high pressure loss grid (15a) is the same as the fuel rod (1
1) It is smaller than the flow velocity of the flow path B passing between the control rod guide pipe (14) and the low pressure loss grid (15b).

However, the bypass channel C is formed in the fuel assembly a. This bypass flow path C has a lower end portion which penetrates the lower nozzle (13) and has a communication hole (17) which communicates with the space below the nozzle, in the axially central region of the control rod guide pipe (14). It is provided with a core communication hole (10) communicating with.

By providing a new bypass flow path C which does not pass through the grid portion which causes such a large local pressure loss, the average pressure loss of the high pressure loss grid assembly is reduced and cooling with the low pressure loss grid assembly is achieved. The difference in material flow rate can be reduced or made equivalent.

By the way, since the coolant simply passing through the flow path C does not contribute to the cooling of the fuel rods (11) at all, securing the flow rate as an assembly does not lead to alleviating the above-mentioned DNB penalty. Therefore, it is necessary to devise so that the coolant flows out of the control rod guide tube in the upper region of the assembly where the DNB is usually more severe. That is why the core communicating hole (10) is provided in the central region of the control rod guide tube.

Looking at the pressure difference between the inside and the outside of the control rod guide pipe in the core communication hole, the pressure inside the coolant is higher than that through the high pressure loss grid portion. Therefore, the coolant is the core communication hole. It will flow out of the guide tube more smoothly.

As described above, in the present invention, the DNB penalty can be reduced or eliminated by reducing or eliminating the flow rate difference between different kinds of fuel in the mixed core by slightly changing the design of the assembly.

By the way, the present invention has another effect of improving the DNB characteristic. In FIG. 2, the coolant in the flow path C added to the flow path A in the upper part of the high pressure loss assembly remains at a relatively low temperature because it does not go through the heat generating part (fuel rod) from the inlet of the lower nozzle of the assembly. By decreasing the local quality X in the equation, q " _DNB increases, and as a result, DNB
Is less likely to occur.

Even if the above-mentioned effects occur on the high pressure loss grid assembly side, the flow rate of the coolant in the low pressure loss grid assembly does not become particularly smaller than in the conventional case, and there is no influence in terms of DNB. Further, a core communication hole is provided in the upper part of the control rod guide tube for cooling the γ heat generation and the like.
Another effect.

As described above, in the fuel assembly provided with the control rod guide tube in which the bypass flow passage C is formed, in the core where two kinds of fuel assemblies having different pressure losses are mixed due to the difference in grid design, (1) The flow rate difference between different kinds of fuels is mitigated or eliminated, and the DNB penalty conventionally generated on the high pressure loss grid fuel assembly side is mitigated or eliminated. (2) In addition to the effect of (1), DNB of high pressure loss grid aggregate
Further improve the characteristics. And so on.

[0034]

As described above, the present invention communicates with the core space in the axially central region of the control rod guide tube having the communication hole which penetrates the lower nozzle at the lower end and communicates with this nozzle lower space. Since the core communication hole is provided and the bypass flow path from the space below the nozzle to the core space is formed, the pressure loss of the fuel assembly can be reduced.
Further, the DNB penalty caused by the decrease in the flow rate of the coolant can be mitigated.

Further, in the present invention, some fuel assemblies loaded in the core of the pressurized water reactor communicate with the lower space through the lower nozzle at the lower end of the control rod guide tube from the lower space. By the bypass passage that reaches the core space through the communication hole and the core communication hole that communicates with the core space formed in the axially central region of the control rod guide tube, the pressure loss of other fuel assemblies Since the fuel assembly has a reduced difference, the basic design of the fuel assembly of the high-pressure loss grid (grid, upper and lower nozzles, etc.) even when two types of fuel assemblies with different pressure loss characteristics are mixed in the core There is an effect that the DNB penalty can be alleviated without changing the.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of a control rod guide tube of a fuel assembly showing a configuration of an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a flow path when a fuel assembly having a high pressure loss grid loaded with the control rod guide tube shown in FIG. 1 and a fuel assembly having a low pressure loss grid are adjacent to each other.

FIG. 3 is a diagram showing a boiling heat transfer curve.
The vertical axis represents the logarithm of the heat flux q ″, and the horizontal axis represents the logarithm of the heat transfer surface overheating temperature (difference between the heat transfer surface temperature and the saturated liquid temperature) ΔT _SAT .

FIG. 4 is an explanatory view showing the structure of a pressurized water reactor fuel assembly.

FIG. 5 is an explanatory diagram showing a flow path when two types of fuel assemblies having different pressure loss characteristics are adjacent to each other from a grid design.

[Explanation of symbols]

 (10) ... Core communication hole, (11) ... fuel rod, (13) ... lower nozzle, (14) ... control rod guide tube, (15) ... grid, (15a) ... high pressure loss grid, (15b) ... low pressure Loss grid, (16)… lower end plug of guide tube, (17)… communication hole, C… bypass flow path

Claims (2)

[Claims] 1. A fuel assembly for a pressurized water reactor, comprising a bundle of fuel rods and a control rod guide pipe extending from an upper nozzle to a lower nozzle, wherein a lower end of the fuel assembly penetrates the lower nozzle. A core communication hole that communicates with the core space is provided in an axially central region of the control rod guide tube that has a communication hole that communicates with the space below the nozzle, and a bypass flow path from the space below the nozzle to the core space is formed. A fuel assembly for a pressurized water reactor characterized by the above. 2. In a core of a pressurized water reactor equipped with several fuel assemblies having the same pressure loss due to cooling water circulated from inside to below, the several fuel assemblies are A communication hole that communicates with the lower space from the lower space through the lower nozzle at the lower end of the control rod guide pipe, and a core communication that communicates with the core space formed in the axially central region of the control rod guide pipe. A core of a pressurized water reactor, comprising a fuel assembly that has a pressure loss comparable to that of other fuel assemblies due to a bypass flow path reaching the core space through the holes.