JPH0464092A - Nuclear reactor fuel assembly - Google Patents

Abstract

PURPOSE:To convert a passage shape to contrive the improvement of thermal limiting output by making the inner surface of a fuel spacer the form of a truncated cone for narrowing at the inlet part thereof and expanding at the outlet part. CONSTITUTION:When the inner surface of a cylindrical cell 20 of a fuel spacer is constituted so as to become the form of a truncated cone for narrowing at the inlet part thereof and expanding at the outlet, a gas phase velocity is increased to narrow the diameter of droplets because of increasing a droplet velocity and relative velocity and, when the droplet diameter becomes small, droplets are accelerated to increase their mass velocity, the large state of droplet concentration is maintained in a specified interval of the downstream part because a quantity of the droplet deposition per unit length at the just downstream part of the spacer is decreased to increase the quantity of the droplet deposition in the neighborhood of the spacer of the downstream part to increase channel output for producing liquid film dry-out so as to increase thermal limiting output.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a fuel assembly for a water-moderated nuclear reactor.

(Prior art) Recent water-moderated nuclear reactors, especially light water-moderated nuclear reactors, which are the mainstream of water-moderated nuclear reactors, are designed to reduce operating costs of nuclear couplants, achieve long-term cycle operation, and economically burn fuel assemblies to achieve this. , it has been attempted to introduce a non-uniform shape in the radial direction.

Figure 4 shows an example of a fuel assembly used in a boiling water reactor (BWR), in which a large-diameter water rod 1 corresponding to four fuel rods is arranged in the center of a channel box 3, and around it A fuel assembly is shown in which fuel rods 2 are arranged in a generally square shape. Here, the water rod 1 increases the thermal neutron utilization rate to increase the effective multiplication factor of the core given by the following equation, and contributes to economical combustion of fuel.

k = ε・η・f−p−PL (1) lf where, kel['Core effective multiplication factor ε: Nuclear fission effect of fast neutrons η: Regeneration rate f: Thermal neutron utilization rate p: Resonance Escape probability PL: Rate at which neutrons leak from the core When water rods with such a large cross-sectional area are introduced, the distribution of coolant within the cross-section differs around the fuel rods and the water rods.

Therefore, as mentioned above, it has good characteristics from the viewpoint of efficient fuel combustion, but from the viewpoint of efficient fuel cooling, the coolant tends to collect around the water rod, which limits the operating range of the assembly. It cannot be said that this contributes to improving the thermal limit output, which is one of the design conditions.

On the other hand, assembly components that affect the thermal limit output of the fuel assembly include fuel spacers in addition to the assembly cross-sectional shape.

Fuel spacers are installed at seven locations in a normal fuel assembly to maintain the gap between fuel rods and maintain the shape of the assembly. This is shown in Figure 5. In this diagram,
2 is a fuel rod, 3 is a channel box, 4 is a fuel spacer, 5 is an upper tie plate, 6 is a lower tie plate, and 7 is a fuel effective length.

Points (1) to (10) below are generally considered in fuel spacer design. In other words, 1) seismic resistance of the assembly 2) maintenance of fuel rod spacing 3) suppression of fuel rod vibration 4) allowance for thermal expansion of fuel rods 5) ease of assembling the fuel assembly 6) minimum contact area with fuel rods 7) Maximization of the thermal limit power 8) Minimization of the assembly pressure loss 9) Minimization of parasitic neutron absorption IO) Minimization of the number of parts By the way, the fuel spacer has a significant effect on the thermal limit power. The boiling transition start point position at the thermal limit output is limited to within 1 cm immediately upstream of the fuel spacer. This is thought to be because in the flow mode of the two-phase annular flow, a vortex is created in the coolant on the upstream side by the fuel spacer, and the vortex peels off the liquid film that has adhered to the fuel rod surface, causing a rapid deterioration of heat transfer efficiency. It is being FIG. 6 shows a conceptual diagram explaining the state of the coolant flow. In FIG. 6, 8 is a liquid film, and 9 is an obstacle such as a spacer. The wide range on the upstream side of the spacer 9 is the position where boiling transition starts.

The following two points (a) and (b) are mainly considered as mechanisms for the start of boiling transition exerted by the fuel spacer.

(a) Effect of supplying droplets to the liquid film on the fuel rod surface by mixing coolant (mixing effect) (b) Effect of suppressing breakage of the liquid film covering the fuel rod surface by reducing flow obstruction (scrubbing effect) From what we know, spacer design elements that correspond to the above (a) include the mixing tab 11 provided on the fuel spacer 10 and an increase in its wall thickness, as shown in FIG. Also, corresponding to the above (b), there is edge-shaped processing of the end face (see Fig. 6).

Furthermore, assuming annular flow dryout, the spacer shape to improve the thermal limit output includes (c) an increase in the liquid film flow rate immediately upstream of the spacer, and (d) an increase in the liquid film flow rate immediately upstream of the spacer. This may be due to an increase in the amount of droplet deposition.

From this point of view, as shown in FIG. 8, a fuel assembly constructed as shown in FIG. 9 using cylindrical spacer elements 12 having a uniform thickness has been proposed. In this figure, 13 is a water rod and 14 is a channel box.

However, it has been proposed to introduce a spacer having a non-uniform shape in the radial direction in order to reduce the operating cost of an atomic coupler or to achieve economical combustion of a fuel assembly for long-term cycle operation.

(Problems to be Solved by the Invention) The present invention was made in order to suppress a decrease in the thermal limit output due to a radially non-uniform shape and an increase in local peaking, or to improve the limit output. The purpose of this invention is to improve the flow path shape inside the fuel space to improve the thermal limit output.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a structure in which a plurality of fuel spacers having a cylindrical outer shape are installed in the axial direction to surround the fuel rods. In the fuel assembly in which mutual spacing is maintained, the inner surface of the spacer has a truncated conical shape that narrows at the inlet and a truncated conical shape that widens at the outlet.

(Function) Next, the function of the fuel assembly according to the present invention will be explained.

When a flow path is formed between the fuel spacer portions, the gas phase flow velocity increases, and the relative velocity with respect to the droplet flow velocity increases, resulting in a break-up of the droplet. Normally, when liquid is blown out into the gas phase by a jet stream, the droplet diameter is as follows (2
) of the gas-liquid relative velocity defined by the equation
) is known to be reduced in diameter to a constant value.

We= (ug-uIri 2pgd(na!/a...
(2) Here, U: gas phase flow rate, U: liquid phase flow rate.

ρ: gas phase density, d: droplet diameter g maxσ: surface tension Although droplets coalesce in the widening flow channel at the fuel spacer outlet, the rate of coalescence is considered to be small because the section is short.

As the droplet diameter decreases, the droplet is accelerated by friction with the gas phase, the mass velocity of the droplet increases, and the amount of droplet deposited per unit length immediately downstream of the fuel spacer decreases. A state in which the droplet concentration C is large as defined by equation (3) is maintained for a certain period in the downstream region.

C-GE/GG/ρ2...(3) Here, G
E: Droplet mass velocity G6: Gas phase mass velocity When the amount of droplet deposition near the fuel spacer downstream increases and the liquid film flow rate increases, the channel output where liquid film dryout occurs increases.

By the way, the above-mentioned effect is the same even if a straight pipe section is provided between the sandwiched part and the widened part, but since the straight pipe part has a rectifying effect, turbulence is reduced and local pressure loss due to the fuel spacer is reduced. . Such a structure is used as a venturi tube as a current meter, and is known as a shape with little irreversible pressure loss.

Such a mechanism can provide a reactor fuel assembly with increased thermal critical power.

(Example) Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a cylindrical cell according to an embodiment of the present invention, and FIG. 1(a) is a perspective view of the cylindrical cell of this embodiment, and FIG. It is a sectional view seen from the II direction.

The inner surface of the cylindrical cell 20 shown in FIG. 1 is configured to have a truncated conical shape narrowing at the inlet portion 21 and widening at the outlet portion 22. Therefore, the amount of droplets deposited in the vicinity immediately upstream of the spacer increases, so that, as shown in FIG. 3, the thermal limit output of this embodiment is increased compared to the conventional example.

FIG. 2 is a configuration diagram of a cylindrical cell according to another embodiment of the present invention, and FIG. 2(a) is a perspective view of the cylindrical cell of this embodiment;
Figure (b) is a sectional view taken from the nn direction of figure (a).

As shown in FIG. 2, the inner surface of the cylindrical cell 23 of this embodiment has a truncated conical shape that narrows at the inlet portion 24 and widens at the outlet portion 25, and the upper and lower truncated conical central portions 26 have straight pipes. It is constructed so that it has a section. Therefore, in this embodiment as well, the amount of droplets deposited immediately upstream of the spacer is increased as in the above embodiment, and the thermal limit output is increased compared to the conventional example.

In any of the fuel spacers of the above embodiments, thermal limit output characteristics similar to those of a normal cylindrical cell having a wall thickness similar to that of the thickest part can be obtained, and the local pressure loss at the spacer portion can be reduced. reduce

[Effects of the Invention] As explained above, according to the present invention, since the amount of droplet deposition is increased immediately upstream of the spacer, an improvement in the limit output characteristics can be expected, and the local This has an excellent effect of reducing pressure loss compared to a normal cylindrical cell having a wall thickness similar to that of the thickest part.

[Brief explanation of drawings]

FIG. 1(a) is a perspective view of a cylindrical cell according to an embodiment of the present invention, FIG. 1(b) is a sectional view taken from the direction II in FIG. 1(a), and FIG. A perspective view of a cylindrical cell according to another embodiment of the present invention, FIG. 3(b) is a sectional view seen from the nm direction of FIG. Fig. 4 is a schematic plan view of a conventional fuel assembly, Fig. 5 is a partially cutaway perspective view of a conventional fuel assembly, and Fig. 6 explains the state of coolant flow. 7 is a perspective view of a conventional fuel spacer, FIG. 8 is a perspective view of a conventional cylindrical spacer element, and FIG. 9 is a schematic diagram of a fuel assembly using the cylindrical spacer element of FIG. 8. FIG. 1... Large water diameter rod 2... Fuel rod 3.14...
・Channel box 4, lO...Fuel spacer 5...Upper tie plate 6...Lower tie plate 11...Mixing tab 12.20.23...Cylindrical cell 13...Water rondo (8733 ) Agent: Yoshiaki Inomata, patent attorney (and others)
1 person) 2nd instigation and destruction diagram 1st mu

Claims (1)

[Claims] In a fuel assembly in which a distance between the fuel rods is maintained by installing a plurality of fuel spacers having a cylindrical outer shape surrounding the fuel rods in the axial direction, the inner surface of the spacer has a truncated conical shape narrowing at the inlet part, and an outlet part. A nuclear reactor fuel assembly characterized by having a truncated conical shape that widens in width.