JPS6262309B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a nuclear fuel assembly for use in a fast breeder reactor.

(Prior Art) Conventional core fuel assemblies and blanket fuel assemblies for fast breeder reactors are constructed by regularly arranging fuel elements in which fuel pellets are housed in cladding tubes in a coolant flow pipe. The coolant flows through the flow tubes and transports the thermal energy generated by the fuel element while simultaneously cooling the fuel element and preventing excessive increases in temperature of the cladding containing the fuel pellets. The flow rate of the coolant is determined by the size of the orifice in the entrance nozzle of the fuel assembly.

Particularly in blanket fuel assemblies, the fuel pellets are made from depleted uranium containing only about 0.3% fissile uranium-235. Therefore, at the beginning of combustion, the calorific value is small, but at the end of combustion, plutonium produced from uranium-238 accumulates, and the calorific value reaches approximately 100 W/cm on average for the fuel elements. Therefore, in order to ensure sufficient coolant flow, the size and number of orifices in the blanket fuel assembly should be
It must be determined according to the calorific value at the final stage of combustion.

(Problem to be solved by the invention) However, in the case of this design, the temperature of the coolant does not rise much during the early stages of combustion when the calorific value is low, so the low-temperature coolant is directly transferred to the reactor coolant outlet. It ends up flowing into the reactor core and mixing with the coolant heated in the reactor core. Therefore, it acts to lower the coolant outlet temperature of the reactor as a whole, which is unfavorable in terms of reactor performance. Furthermore, in order to keep the coolant outlet temperature constant, the outlet temperature of the core fuel assembly must be increased excessively in the initial stage of combustion, which is undesirable in terms of maintaining the integrity of the fuel elements. In addition, in the case of a core fuel assembly, if the calorific value of the core fuel assembly increases due to some abnormality, the coolant temperature will rise and the cladding will increase because the coolant flow rate is constant depending on the size of the orifice. There is a risk of damage.

The present invention has been made in consideration of the above-mentioned drawbacks when determining the flow rate based on the size of the orifice. The object is to provide a nuclear fuel assembly that automatically controls the assembly outlet temperature.

[Structure of the invention]

(Means for Solving the Problems) The nuclear fuel assembly of the present invention is a nuclear fuel assembly formed by arranging a plurality of fuel elements in a flow pipe, and includes a gas tank filled with an inert gas, and a gas tank filled with an inert gas. a connected metal bellows, a flow path adjustment device that is movable in conjunction with the expansion and contraction of the metal bellows and has a flow path restriction orifice, and a flow path adjustment device that surrounds the flow path adjustment device and that restricts the flow path in accordance with the stroke of the bellows. A mantle having an orifice communicating with the orifice.

(Function) During normal operation, the outlet temperature of the coolant flowing out from the flow pipe is approximately 550°C, so the temperature of the inert gas in the gas tank is also approximately 550°C. At that temperature, the pressure of the inert gas increases and the metal bellows expands, causing the flow regulating device to stroke overcoming the fluid force of the coolant, and the flow restricting orifice and the orifice provided in the jacket communicate with each other to allow the coolant to flow. Keep the road area at an appropriate value. As the temperature of the inert gas increases, the area facing each orifice increases, and as a result, the flow rate of the coolant increases and the temperature of the coolant can be lowered. If the output of the fuel element subsequently decreases, the flow area of the coolant decreases due to the opposite phenomenon, and as a result, it is possible to prevent the temperature of the coolant from decreasing.

(Example) Hereinafter, an example of the present invention will be described with reference to FIGS. 1 to 3.

FIG. 1 shows a first embodiment of a nuclear fuel assembly according to the present invention, and a vertical sectional view thereof is shown in FIG. A plurality of fuel elements 1 are regularly housed in a coolant flow pipe 2 parallel to the flow path, and the lower part of the fuel element 1 is connected to the support strip by a pin 5 with an end plug 3 fitted into a support strip 4. It is fixed to piece 4. The plurality of supporting strips 4 are further fixed to the flow pipe 2 by means of fixing rods 6 passing through them at right angles. With this structure, one end of the plurality of fuel elements 1 is fixed to the flow path pipe 2, but this fixing means is not limited to this, and the end plug 3 is directly fixed to the end plug 3 by the fixing rod 6.
It is also possible to fix it by penetrating it.
In the nuclear fuel assembly according to the present invention, coolant sodium is supplied from an entrance nozzle 7 installed at the bottom of the flow pipe 2, passes between fixed rods 6, is superheated by the fuel element 1, and is heated by the fuel element 1. The water flows out from a handling head 8 mounted on the upper part of the pipe 2. Inside the handling head 8, a variable flow adjustment mechanism 10 having a variable flow orifice 9 is fixed to the handling head 8 by a plurality of supports 11. The variable flow rate adjustment mechanism 10 includes a gas tank 13 in which an inert gas 12 such as helium gas is sealed, a metal bellows 14 that contracts and expands according to changes in the volume of the inert gas 12, and a vertical stroke in conjunction with the gas tank 13. It is composed of a flow path adjustment device 15 that allows the flow path adjustment device 15 to move, and a jacket 16 that has a structure that ensures the stroke of the flow path adjustment device 15. The flow path regulating device 15 is provided with a flow path restriction orifice 17, and the mantle 16 is provided with an orifice 18 at a position overlapping with the flow path restriction orifice 17. The positional relationship between the flow path restriction orifice 17 and the orifice 18 changes depending on the vertical stroke of the flow path adjustment device 15, and as a result, the flow path area created by the overlap between the flow path restriction orifice 17 and the orifice 18 changes. so that a variable flow orifice 9 is formed. Also,
The upper end of the mantle 16 is fixed to the gas tank 13, and the lower end is fixed to the inner wall of the handling head 8. The structure is such that all flow paths other than the variable flow rate orifice 9 are cut off by this jacket. Further, the protrusion 19 is for restricting the downward stroke of the flow path adjustment device 15.

Next, the operation of the nuclear fuel assembly according to the present invention will be explained.

Sodium as a coolant is supplied into the flow pipe 2 from the entrance nozzle 7, and as it rises inside the flow pipe 2, it is heated by the thermal energy generated in the fuel element 1 and its temperature rises, and the sodium coolant is attached to the handling head 8. The liquid flows out from the flow pipe 2 through the variable flow orifice 9 of the variable flow rate adjustment mechanism 10. During normal operation, the outlet temperature is about 550°C, so the temperature of the inert gas sealed in the gas tank 13 is also about 550°C. metal bellows 14
The elastic force causes the metal bellows 14 to expand due to the pressure generated at this time, and stroke the flow path adjustment device 15 to overcome the fluid force of the coolant, thereby adjusting the flow path area of the variable flow rate orifice 9 to an appropriate value. It is set to .

When the coolant temperature rises due to heat generation in the blanket fuel assembly or an abnormal increase in the output of the core fuel assembly, the temperature of the inert gas 12 in the gas tank 13 rises, and due to the thermal expansion of the gas, the metal The bellows 14 is further expanded. When this happens, the flow path adjustment device 15 strokes further downward, increasing the flow path area of the variable flow orifice 9, and as a result, the coolant flow rate increases, so that the coolant temperature can be lowered. Thereafter, when the output of the fuel element decreases, the flow path area of the variable flow orifice 9 decreases due to the opposite phenomenon, and as a result, it is possible to prevent the coolant temperature from decreasing. Further, since the diameter of the metal bellows 14 is smaller than the diameter of the gas tank 13, a large stroke of the metal bellows 14 can be caused by a slight change in the volume of the gas in the gas tank 13.

In this embodiment, the shapes of the flow path restricting orifice 17 and the orifice 18 of the mantle 16 may be circular a, non-circular b, polygonal c, d, e as shown in FIG. As shown by number e, the downwardly expanding shape is superior in that the area of the variable flow rate orifice 9 increases gradually with each stroke.

FIG. 3 shows a second embodiment of the present invention, and the same parts as those in FIG. In this embodiment, the stroke of the flow path adjustment device 15 causes the flow path restriction orifice 17 to be blocked by a portion of the mantle 16.
The structure is such that the flow rate is controlled by the representation of the flow rate. When obtaining a large flow rate of coolant, such as when used in a core fuel assembly, the structures shown in FIGS. 1 and 3 may be used together.
Of course it is possible.

4 and 5 show the characteristics of the coolant for the nuclear fuel assembly according to the present invention. In the conventional example, the coolant flow rate remains constant even if the output of the fuel element increases, as shown by number B in FIG. Flow rate increases. As a result, the coolant outlet temperature is constant regardless of the output of the fuel element, as indicated by D in FIG. On the other hand, in the conventional example indicated by number C in FIG. 5, the coolant outlet temperature increases significantly because the coolant flow rate is constant. Furthermore, at low output, the coolant outlet temperature cannot be increased. Therefore, if the present invention is applied to a blanket fuel assembly of a fast breeder reactor, the coolant outlet temperature can be kept constant even when the output increases as combustion progresses, so the outlet temperature of the blanket fuel assembly can be kept constant. can be made the same throughout the combustion,
It is possible to increase the coolant outlet temperature of the entire reactor and improve the economic efficiency of the reactor. In addition, when used in a core fuel assembly, even in the event of an abnormal increase in output, the coolant flow path is automatically widened without the need for external operation or power, and the coolant reaches a high temperature and fuel It has an inherent safety that prevents the element from being damaged. Further, even during low output operation, the coolant flow path can be narrowed to prevent the coolant temperature from dropping.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to maintain the coolant outlet temperature constant even if the calorific value of the fuel element changes during combustion, thereby improving the health of the nuclear fuel assembly and the economic efficiency of the reactor. can.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal sectional view showing one embodiment of the present invention, Fig. 2 is a front view showing an example of the shape of the orifice used in Fig. 1, and Fig. 3 is a longitudinal sectional view showing another embodiment of the invention. The top view, FIGS. 4 and 5 are diagrams for explaining the effects of the present invention. DESCRIPTION OF SYMBOLS 1...Fuel element, 2...Flow path pipe, 9...Variable flow rate orifice, 10...Variable flow rate adjustment mechanism, 12...Inert gas, 13...Gas tank, 14...Metal bellows,
15... Flow path adjustment device, 16... Mantle, 17... Flow path restriction orifice, 18... Orifice.

Claims (1)

[Claims] 1 In a nuclear fuel assembly consisting of a plurality of fuel elements arranged in a flow pipe, there is a gas tank filled with an inert gas inside, a metal bellows connected to this gas tank, and a stroke that is linked to the expansion and contraction of the metal bellows. and a mantle surrounding the flow regulating device and having an orifice that is interlocked with the flow limiting orifice in response to the stroke of the bellows. Characteristic nuclear fuel assembly.