JPH083542B2 - Nuclear fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear fuel assembly of a boiling water reactor, and more particularly to a spacer structure for increasing a thermal margin for a boiling transition of a fuel rod.

[Background of the Invention]

As shown in Fig. 3, in a nuclear fuel assembly of a boiling water reactor, about 64 fuel rods 2 are regularly arranged in an 8 × 8 square lattice, and a coolant flows from the lower part to the upper part between them, The heat is removed by the heat transfer phenomenon that accompanies boiling. The spacers 1 support the vertical axes of the fuel rods in parallel and prevent the fuel rods from coming into contact with each other. About seven spacers are attached to the fuel rod effective heat generating portion of about 3.6 m at intervals of about 0.5 m. Has been. These fuel rods 2 are held by the spacer 1 at predetermined intervals, and the channel box 20
It is stored inside and can be put in and taken out of the furnace as a unit. Reference numeral 13 is an orifice at the bottom of the assembly. The maximum heat output of the nuclear fuel assembly is defined by the heat removal capacity of the coolant. When heat output exceeding this heat removal capacity is generated, the fuel rod surface is covered with a vapor film (boiling transition), the temperature rises rapidly and burns out, and fission products with high radioactivity flow out, The output of the nuclear fuel assembly must be kept below a certain level.

FIG. 4 shows an example of a nuclear fuel assembly using a conventional spacer. In the figure, 1 represents the entire spacer. Two
Are fuel rods arranged regularly in an 8 × 8 square lattice, 3
Is a thick water rod in the center thereof, 4 is a cell for accommodating individual fuel rods 2, 5 is a fixed holding material for the fuel rods, 6 is a spring-like holding material, 7 is a fixed holding material for the water rods 3, and 8 is It is a spring-like holding material. An embossment 9 is formed on the outermost peripheral portion of the spacer 1 so as to keep a constant distance from the inner surface of the channel box 20. The protrusion 11 at the upper end of the outermost peripheral portion is for keeping the cell 4 in a predetermined position.

In these conventional examples, no particular attention was paid to the fact that the structure of the spacer affects the heat removal of the nuclear fuel assembly. Further, in general, the flow of the coolant is resisted by the spacer, the coolant is less likely to flow in the lower part of the spacer, the heat removal effect is deteriorated, and the boiling transition is likely to occur.

In order to solve this drawback, for example, JP-A-53-6209
In No. 3, a method of reducing the uranium enrichment of the fuel pellets at the spacer position was proposed. But with this technology,
There is a drawback that the fuel pellet manufacturing process and the uranium enrichment process are complicated.

[Object of the Invention]

An object of the present invention is to provide a nuclear fuel assembly provided with a spacer having a structure capable of increasing the thermal margin for boiling transition.

[Outline of Invention]

Generally, in the existing boiling water reactor operated at a pressure of 70 atm and an output density of 50 kW /, the coolant has a subcooling degree of about 15
It flows at kcal / kg and at an average velocity of 2 m / s. The coolant flowing from the lower inlet orifice 13 with a slight subcooling degree takes heat from the fuel rod while flowing through the fuel assembly, boils, bubbly flow, slag flow, and annular flow from the fuel assembly. leak. The annular flow is a state in which a liquid film is flowing on the surface of the fuel rod and vapor is flowing in other portions, and droplets are mixed in the liquid film. The liquid film on the surface of the fuel rod becomes thinner toward the upper part of the assembly due to heat removal from the fuel rod. If this liquid film disappears, the heat removal from the fuel rods will not be carried out sufficiently, and the surface temperature will rise sharply and burn out. Therefore, in order to prevent burnout, it is necessary to always have a liquid film on the surface of the fuel rod.

On the other hand, there is a liquid film of the coolant on the wall surface of the channel box surrounding the fuel rods, but the channel box does not generate heat and therefore does not serve for heat removal. If the coolant liquid film on the wall surface of the channel box can be peeled off and made to flow toward the fuel rods, the heat removal effect should be improved.

Therefore, in the present invention, since the coolant liquid film of the fuel rod becomes thin in the uppermost and next spacers and boiling transition is likely to occur due to the annular flow, the lowermost outermost spacer of the uppermost spacer is A coolant that rises above the wall surface of the channel box by forming a plurality of outward projections that face the inner surface of the channel box but do not contact the inner surface, and form a plurality of inward projections on the outermost peripheral part of the same spacer. We propose a spacer structure that peels off the liquid film and directs it toward the outermost fuel rod.

[Action]

The boiling transition occurs at the place where the coolant in the upper part of the assembly flows in an annular flow state.Therefore, in order to suppress the boiling transition, it is necessary to provide a protrusion on the uppermost group of the upper part of the assembly and the outermost peripheral part of the next spacer. Good. Providing protrusions on spacers smaller than that is not very effective because the annular flow state has not been reached at those places. Rather, by not forming protrusions on the spacers below the uppermost and next spacers, it is possible to suppress the pressure loss due to the disturbance of the coolant flow by the protrusions as much as possible. In addition, it is possible to form the outward projection so as to contact the channel box wall surface, and further peel off the coolant liquid film that rises on the channel box wall surface, but this increases the turbulence of the coolant flow due to the projection. Then
This will increase the pressure loss. Forming the outward protrusion so as not to contact the wall surface of the channel box not only facilitates attachment of the protrusion, but also sufficiently peels off the coolant liquid film and suppresses pressure loss generated from the protrusion as much as possible. That is.

Example of Invention

Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 (A) is a cross-sectional view of a nuclear fuel assembly including a spacer according to the present invention, and FIG. 1 (B) is a side view of the spacer. The parts having the same functions as those of the conventional example shown in FIG.

As is apparent from the comparison between FIG. 1 and FIG. 4, the feature of the present embodiment is that the uppermost and the next spacers are provided under the outermost peripheral structural material and the plurality of outward projections 10 that do not contact the surface of the channel box wall. And the plurality of inward projections 11 are formed together on the upper part. The structure is shown in more detail in the cross-section enclosed by the dotted circle C.

Here, the difference between the present invention and the conventional example will be described with reference to FIG. 2 which shows the flow state of the coolant and the thickness of the liquid film on the surface of the fuel rods in the vicinity of the uppermost and next spacers above the assembly. The liquid film becomes thinner as the coolant moves toward the upper part of the assembly, forming an annular flow near the upper and lower spacers, and the coolant flows in a liquid film state on the surface of the fuel rods 2, and vapors when it separates from it. Is flowing, and droplets are mixed in it. Further, the coolant also flows in a liquid film state on the surface of the channel box 20. When the spacer 1 of the present invention is used, the liquid film on the surface of the channel box 20 is stripped off by the plurality of outward projections 10 provided on the lower portion of the outermost peripheral portion of the structural material and not in contact with the surface of the channel box 20. The liquid film is made to flow inward by a plurality of inward projections 11 provided on the upper part of the spacer, and the liquid film on the surface of the fuel rod is thickened. The inward projections 11 attached to the upper part of the spacer also have a role of adhering the droplets in the vapor to the surface of the fuel rod. That is, the liquid film thickness on the surface of the fuel rod becomes thicker as shown by the solid line 15 as compared with the case where the conventional spacer is used (dotted line 14). The liquid film on the surface of the fuel rod becomes thinner as it goes to the upper part of the assembly due to the heat removal of the fuel rod, and a boiling transition occurs in the state where the liquid film disappears.However, in the case of the present invention, the liquid film on the surface of the fuel rod is As it becomes thicker, boiling transition is less likely to occur. Further, since the outward protrusion 10 does not contact the surface of the channel box 20, the pressure loss caused by the protrusion can be reduced as much as possible.

The effect of thickening the liquid film on the surface of the fuel rods by the projections 10 and 11 attached to the upper and lower portions of the outermost peripheral structure material of the uppermost stage and the next stage spacer according to the present invention is not limited to the outermost peripheral fuel rods adjacent to it. No, it extends to the fuel rods inside. However, the effect becomes smaller toward the inner side, but since the liquid film inside the fuel rod is originally thick and the situation is not as severe as the outermost periphery, it is easy to prepare for the boiling transition.

Table 1 shows the amount of increase in the assembly output of the nuclear fuel assembly using the spacer of this example until boiling transition occurs, in comparison with the nuclear fuel assembly using the conventional spacer.

(Reference for nuclear fuel assembly using conventional spacers) Spacer outermost part Due to the difference in the range of the effect of thickening the liquid film on the fuel rod surface by the projections 10 and 11 mounted on the upper and lower parts of the structural material, the outermost part of the fuel When the amount of heat generated by the rod is large, the aggregate output until the boiling transition occurs is increased by 4%. When the calorific value of the fuel rod located further inside from the outermost circumference is large, it increases by 2%.

 Next, this embodiment will be described more specifically.

In the nuclear fuel assembly of this embodiment, the spacer 1 shown in FIG. 1 (a spacer having an outward protrusion 10 at the lower portion of the outermost peripheral portion of the spacer structural material and a spacer having an inward protrusion 11 at the upper portion) of the fuel rod effective heat generating portion is used. Two of them are installed only about 1m above the 3.6m length.
At the mounting position of the spacer 1, the coolant flows in an annular flow under normal operating conditions of the boiling water reactor. Below this, the conventional spacer shown in FIG. 4 is mounted.

The nuclear fuel assembly of this example is made based on the following consideration.

(1) The boiling transition occurs where the coolant in the upper part of the assembly flows in an annular flow state.

(2) Providing protrusions 10 and 11 on the upper and lower parts of the outermost peripheral structure material of the spacer and using the liquid film on the channel box wall surface for cooling the fuel rod is effective because the coolant is an annular flow. It is only due to the spacer attached at a certain place, and it is not effective when attached below it.

(3) When the spacer 1 of the present invention is attached to the lower part of the assembly, the projections 10 and 11 disturb the flow of the coolant, resulting in a slight increase in pressure loss in the assembly.

(4) When the projections 10 and 11 are brought into contact with the surface of the inner wall of the channel box, the turbulence of the coolant flow is increased and the pressure loss in the assembly is increased.

That is, in the embodiment, a conventional spacer is attached to the lower part of the assembly near the coolant inlet to reduce the pressure loss, and it is effective to change the flow of the coolant in the upper part of the assembly near the coolant outlet. The output of the aggregate is increased until a boiling transition occurs by adding a spacer.

By forming a plurality of protrusions that do not contact the inner wall surface of the channel box in the outermost peripheral portions of the spacers only in the uppermost and next stages of this embodiment, compared to the case where the same protrusions are provided in the outermost peripheral portions of all the spacers, The pressure loss can be reduced by about 0.3 psi (2%). When the nuclear fuel assembly of the present embodiment is loaded into the core, the pressure loss is small, so that it becomes possible to operate in a more stable state by nuclear thermohydraulics, and the reduction ratio, which is a measure of stability, is improved by about 3%. To be done.

Next, still another embodiment will be described. For the effect of stripping off the liquid film on the surface of the channel box and thickening the liquid film on the surface of the fuel rod, it is also desirable to have a projection that goes to the wall surface of the channel box from a certain temperature or higher. Therefore, the spacer shown in FIG. 1 is made of a shape memory alloy so that the lower protrusion is not directed to the outside up to 250 ° C., for example. It goes without saying that it should be turned outward in the production state. By doing this,
The pressure loss can be further reduced.

In this embodiment, by forming a plurality of protrusions that do not contact the surface of the channel box on the outermost peripheral portions of the uppermost and next spacers, the thermal margin for the boiling transition of the fuel assembly can be increased and the pressure loss due to the protrusions can be minimized. Can be suppressed to

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, the thermal margin with respect to the boiling transition of a nuclear fuel assembly can be enlarged, and the pressure loss by a protrusion can be reduced. Therefore, the power density of the nuclear reactor can be increased, which is effective for downsizing or increasing the capacity of the nuclear reactor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view and a side view showing an embodiment of a nuclear fuel assembly provided with a spacer according to the present invention, and FIG. 2 is a view showing the thickness of a liquid film on the surface of a fuel rod. FIG. 3 is a perspective view showing the whole nuclear fuel assembly, and FIG. 4 is a cross-sectional view and a side view showing an example of a conventional nuclear fuel assembly. 1 ... Spacer, 2 ... Fuel rod, 3 ... Water rod, 4 ...
... Cell, 5 ... Fixed storage material, 6 ... Spring-shaped holding material, 7 ...
… Fixed holding material, 8 …… Spring-shaped holding material, 9… Embossing,
10 …… Outward protrusion, 11 …… Inward protrusion, 13 …… Lower stage inlet orifice, 14 …… Conventional liquid film surface, 15 …… First embodiment liquid film surface, 20 …… Channel box.

Claims (2)

[Claims] 1. A plurality of spacers are arranged along the vertical direction of a fuel rod effective heat generating portion so that a plurality of fuel rods are arrayed and the vertical axes of the fuel rods are supported in parallel to prevent mutual contact of the fuel rods. In the fuel assembly housed in the channel box, a plurality of outward projections that cause the coolant liquid film rising along the inner surface of the channel box to flow toward the fuel rod are provided with the uppermost and the next spacers. It is arranged so as not to contact the channel box wall surface at the bottom of the outermost periphery of the above, and a plurality of inward projections are arranged at the top of the outermost periphery of the same spacer so as not to contact the channel box wall surface. A nuclear fuel assembly having no protrusions. 2. The outermost periphery of the spacer according to claim 1, wherein a protrusion made of a shape memory alloy does not face outward at a certain temperature or less and faces outward when the temperature exceeds a certain temperature. A nuclear fuel assembly characterized by being provided at the bottom.