JPS6282393A - Fuel rod for boiling water type reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel rod for a boiling water nuclear reactor (hereinafter referred to as BWR).

[Technical background of the invention and its problems] Generally, a boiling water reactor has a plurality of fuel assemblies loaded in its core, and each fuel assembly has a cross-shaped space for inserting a control rod between them. The settings are arranged in a grid. Conventionally, this type of fuel assembly has been constructed as shown in FIG. That is, a plurality of fuel rods 21 and the like are accommodated in the square tubular fuel channel box 20. These fuel rods? The first class are arranged upright, parallel, and at equal intervals by spacers 22, and are instructed to be arranged in a grid pattern. The upper end of the fuel rod 21 is supported by an upper tie plate 24 via an upper end plug 23, and the lower end thereof is supported by a lower tie plate 26 via a lower end plug 25. Cooling holes 2 are provided at the lower end of the lower tie plate 26 for introducing reactor water, which is a coolant, into the fuel channel box 20.
7 is drilled. Reactor water flowing into the fuel channel box 20 from this cooling hole 27 is configured to be heated by 7 Jll by the heat-generating fuel rods 21 as it rises toward the upper end, and to boil and generate steam. . That is, the reactor water flows through the cooling holes 27 at the lower end of the fuel assembly.
As well as flowing into the fuel channel box 20 from
The reactor water flows into the spaces between adjacent fuel channel boxes 20, which are control rod insertion spaces, and rises upward, and these flows of reactor water merge above the fuel assembly. The reactor water rising inside the fuel channel box 20 is heated by the fuel rods 21 to a boiling point to form a gas-liquid two-phase flow. On the other hand, the reactor water that has flowed up through the space between the fuel and channel boxes 21 remains in a liquid state. Therefore, above the fuel assembly, the gas-liquid two-phase flow and the liquid flow merge to form a new gas-liquid two-phase flow.

The reactor water flowing into the fuel assembly from the lower part thereof is in a subcooled state, but due to heating by the fuel rods 21, it reaches a saturation temperature and becomes a gas-liquid two-phase flow. Heat transfer by this gas-liquid two-phase flow is very good, and the surface temperature of the fuel rod 21 is only slightly higher than the fluid temperature, and a liquid film is formed on the surface of the fuel rod 21. However, as the heat generation m of the fuel rod 21 increases, a region occurs where this liquid film disappears and a boiling transition occurs. In this region, the surface temperature of the fuel rod 21 rises rapidly, and the temperature difference between the fluid and the reactor water becomes very large, which is unfavorable for the health of the fuel rod 21.

Further, when operating a nuclear reactor, when the output is too high compared to the flow rate, there is a possibility that the same mgi transfer as described above will occur in the event of an abnormal transient or an accident.

A fuel rod has pellets (for example, uranium oxide) that serve as fuel inside, and is covered with a cladding tube, and a gas such as a radioactive product is necessarily present between the pellets and the cladding tube. Heat is generated in the pellet portion inside the fuel rod, and the heat is removed by the fluid outside the cladding tube, so the pellet portion is at a high temperature. Therefore, in order to keep the cladding temperature as low as possible when boiling transition occurs, it is desirable to improve the heat transfer between the pellets and the cladding.

[Object of the Invention] The present invention has been made in view of the above circumstances, and its purpose is to provide a fuel rod for a boiling water reactor that improves heat transfer between a fuel rod cladding tube and pellets. be.

[Summary of the Invention] In order to achieve the above object, the present invention provides a fuel rod for a boiling water reactor in which a low melting point metal is sealed in the space between fuel pellets and a fuel cladding tube that houses the fuel pellets. It is.

[Embodiments of the invention] An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a part of an embodiment of the present invention. As shown in the figure, the fuel of this embodiment (1) is covered with a fuel cladding tube 2 on the outside, and fuel pellets 3 are provided inside the fuel cladding tube 2. It has a structure in which a low melting point metal 5 is sealed in a space 4.

Next, FIG. 2 shows a BWR in which a fuel assembly 6 composed of the fuel rods 1 described above is loaded into the core.

In FIG. 2, a plurality of fuel assemblies 6 are arranged in a lattice pattern within a core shroud 8 disposed approximately in the center of a reactor pressure vessel 7. As shown in FIG. A lower plenum 9 is formed below these fuel assemblies 6, and a cross-shaped control rod 10 disposed in this lower plenum 9 is inserted into a cross-shaped space between adjacent fuel assemblies 6. It is configured. The space between these six fuel assembly phases forms a bypass section 11 that becomes a bypass flow path for the reactor core. An upper plenum 12 is formed above the fuel assembly 6, and a steam separator 13 and a dryer 14 are provided above the plenum 12, respectively.

In addition, a recirculation pump 15 for forcibly circulating reactor water in the reactor pressure vessel Y is provided outside the reactor pressure vessel 7, and the recirculation pump 15 forcibly discharges reactor water to the lower plenum 9. The water rises upward and separates into the inside and outside of the fuel assembly 6, ie, the bypass portion 11, at the lower end of the fuel assembly 6. The reactor water that flows into the fuel assembly 6 and rises therein is heated by the fuel rods 1 which generate heat during rising, and boils and becomes steam.

In such a situation, the temperature at each part of the fuel rod 1 during BWR steady output operation is shown in FIG.

In FIG. 4, the broken line a represents the temperature when there is gas (thermal conductivity ki of gas) between the fuel cladding tube 2 and the fuel pellets 3 as in the conventional example, and the temperature when the low melting point metal 5 as in the present invention is The solid line already shows the temperature in the case of (its thermal conductivity kz).

Since kl<k2, it can be seen that when the output of the fuel rod is the same, the outer surface temperature of the cladding tube 2 is the same, but the temperature of the fuel pellet 3 is lower when the low melting point metal 5 is present.

Roughly speaking, the heat flux on the surface of the fuel rod during transient BWR operation is shown in Figure 5. The broken line C shows the change in heat flux over time when the heat flux is present, and the solid line d shows the case where the low melting point metal 5 (its thermal conductivity kz) according to the present invention is present.As shown in FIG. The time change in the fuel rod surface heat flux at is smaller for the solid line d.In other words, there is an object with high thermal conductivity between the fuel rod cladding tube 2 and the pellet 3 as in the present invention, and the thermal resistance therebetween is small. The surface temperature of the fuel rod 1 will be lower to a certain extent, which is good from the standpoint of maintaining the integrity of the fuel rod.Also, the temperature difference between the broken line C and the solid line d regarding the outer surface temperature of the fuel rod is This becomes more noticeable when a boiling transition occurs.

Next, as an example of a low melting point metal, sodium (Na
), its melting point is about 98°C and its boiling point is about 881°C.
°C (all under atmospheric pressure), is solid at constant temperature, and B
It becomes a liquid at approximately 300°C as during normal WR operation. Therefore, when manufacturing fuel rods, the sodium metal is solid because it is in a very bad state.

Therefore, when fuel pellets are placed inside the fuel cladding tube, a sodium metal column may be placed above the fuel pellets in the fuel cladding tube or in the gap between the fuel cladding tube and the fuel pellets, and then the fuel rod may be sealed. After loading several fuel pellets into the cladding tube, a piece of sodium metal may be placed in the cladding tube, and then fuel pellets 1 to 1 are placed in the cladding tube before the fuel rod is sealed.

In fuel rods manufactured in this way, the sodium metal within the fuel rod is solid at the lowest temperatures, but during BWR operation, Na melts and the space between the fuel rod cladding tube and the fuel pellets is filled with Na. become. Therefore, since an object with good thermal conductivity exists in the space between the fuel rod cladding tube and the fuel pellet, the fuel assembly according to the present invention is It is advantageous compared to the

Next, the temperature effect of the fuel rod of the present invention will be explained using a formula.

In a conventional fuel rod, the distance between the inner wall of the cladding tube and the fuel pellet is 2, and the gap thermal conductivity is hl, and the temperature difference is ΔT.
, the following equation (1) can be obtained by shifting the heat flux from ql.

qT = h・△T1 ...(1) Also, sodium metal is used as the low melting point metal sealed in the fuel rod of the present invention, and its thermal conductivity is λ2, and the inner surface of the cladding tube and the outer peripheral part of the fuel pellet are If the temperature difference is ΔT2 and the heat flux is q2, the following equation (2) can be obtained.

q2-λ, aT2 ... (2) See Therefore, if ql = (12) △Tz=h ΔT1 (λ/2) becomes.

Next, 9. = 0.12x 10-3 mh=270
0Kcal/m2 h'Cλ-115Kcal/
If mh'c, then ALL=0.0028 ΔT1.

Also, (h = q2 = 3.4 xt05 Kcal
/m2 h, △T+=125.9°C2ΔT2 =0.4°C2△T
1-ΔT2 = 125.5°C.

According to this trial calculation, when the fuel rod according to the present invention has a poor heat transfer condition on its surface, the temperature of the fuel cladding surface can be lowered by 120''C or more compared to the conventional one.

Although the above is just an example of a trial calculation, the fuel according to the present invention allows the fuel rod surface temperature to be considerably lowered.

[Effects of the Invention] As explained above, according to the present invention, the space between the fuel cladding tube and the fuel pellets is filled with a material with good thermal conductivity, so the temperature inside the fuel rod can be lowered, and the BWR operation can be improved. When the temperature changes over time, the heat flux on the surface of the fuel rod is smaller than that of conventional fuel rods, and boiling transitions are less likely to occur on the outer surface of the fuel rod. Even if a rough boiling B transition occurs, the maximum temperature of the fuel cladding tube can be kept low, so the excellent effect of improving the integrity of the fuel rods is achieved.

[Brief explanation of drawings]

FIG. 1 is a side view showing a part of an embodiment of the present invention in cross section, and FIG. 2 is a longitudinal sectional view of a boiling water reactor pressure vessel loaded with a fuel assembly made of the fuel rods shown in FIG. 1. , Fig. 3 is a partially cutaway perspective view of a conventional fuel assembly, Fig. 4 is a diagram showing the temperature distribution inside the fuel rod during steady operation of BWR, and Fig. 5 is a diagram showing the temperature distribution in the fuel rod during steady operation of BWR.
This is a diagram showing the temporal change in heat flux on the surface of the fuel rod during transient operation. 1... Fuel rod, 2... Fuel cladding tube 3...
Fuel pellets, 5...Low melting point metal 6...Fuel assembly, 7...Reactor pressure vessel agent Patent attorney
Noriyuki Noriyuki Yudo Hirofumi Mitsumata Figure 1 Figure 2 Figure 3 Figure 4 Figure 5

Claims (3)

[Claims] (1) A boiling water reactor characterized in that, in the fuel rods constituting the reactor fuel assembly, a low melting point metal is sealed in the space between the fuel pellets and the fuel cladding tube that houses the fuel pellets. fuel rods. (2) The fuel rod for a boiling water reactor according to claim 1, wherein the melting temperature of the low melting point metal is below the normal operating temperature of the fuel cladding. (3) The fuel rod for a boiling water reactor according to claim 1, wherein the low melting point metal is sodium metal.