JPS6016598B2 - nuclear fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel assembly, and more particularly to an improved nuclear fuel assembly with a large thermal margin.

Nuclear fuel assemblies are constructed by watertightly sealing both ends of a cladding tube loaded with a large number of cylindrical enriched uranium pellets with end plugs, so that the enriched uranium pellets and fission products produced by the combustion of uranium are directly atomically A nuclear fuel rod is made up of multiple nuclear fuel rods that are configured so that they do not come into contact with the reactor coolant, and are bundled together in a lattice shape (usually a square lattice in light water reactors) using spacers.

By the way, nuclear fuel rods must be operated with a certain margin from the thermal loss limit of the cladding in order to completely retain the enriched uranium pellets and fission products inside the cladding during normal operation and during single transient changes. is being considered.

The thermal loss limit of a nuclear fuel assembly is called the burnout point A shown in Figure 1, and the heat flow east when the boiling curve L transitions from the nucleate boiling region to the transition boiling region ratio is defined as the limit heat flow east. .

The boiling mode during normal operation is in the nucleate boiling region, which is a stable state, and the cladding tube surface temperature is kept constant near the saturation temperature of the coolant.

On the other hand, when the burnout point A is exceeded, the difference between the cladding tube surface temperature and the coolant saturation temperature gradually increases, resulting in an unstable boiling state. This burnout point A is not a critical point that directly leads to breakage of the cladding tube. However, this is a boiling region that is not permissible for nuclear fuel even during normal operation and single transient changes. In order to determine this burnout point A, numerous experiments have been conducted in various countries, and various factors such as pressure, coolant flow rate, coolant inlet temperature, shape of nuclear fuel assembly, axial power distribution, power distribution of each nuclear fuel rod, etc. have been conducted in various countries. It has been found that it depends on the parameters of Furthermore, experiments have shown that the location where burnout occurs is near the bottom of the spacer. The reason for this is that the spacer that supports the nuclear fuel rods becomes a flow path resistance, and flow stagnation occurs below the spacer, deteriorating the effectiveness of heat removal from the cladding tube. In conventional nuclear fuel design, many efforts have been made to flatten the power distribution as a measure to improve burnout output in order to ensure a certain level of thermal margin for nuclear fuel assemblies during operation.

For example, typical examples include flattening the power distribution in the radial direction of the nuclear reactor by grading the enrichment of the nuclear fuel rods 35, and flattening the power distribution in all axial directions by mixing burnable poisons. In the case of conventional nuclear fuel assemblies, the output of the nuclear fuel assembly as a whole is reduced due to operating constraints due to locally poor thermal conditions, such as the vicinity of the lower side of the spacer. The present invention has been made in consideration of the above-mentioned circumstances, and aims to obtain a nuclear fuel assembly with a large thermal margin by providing one or more nuclear fuel rods having a low-power portion near the lower side of the spacer. be.

An embodiment of the present invention will be described below with reference to the drawings. Second
The figure shows a nuclear fuel assembly 10 according to the invention. The nuclear fuel assembly 10 is constructed by bundling a plurality of fuel rods 1 into a lattice shape using spacers 2. Nuclear fuel rod 1 has a cladding tube loaded with a large number of cylindrical enriched uranium pellets, and both ends of the cladding tube are watertightly sealed with end plugs to prevent fission products from coming into direct contact with the reactor coolant. . FIG. 3 is a sectional view of a main part showing an embodiment of the nuclear fuel assembly 10 of the present invention. In FIG. 3, the vicinity of the lower side of the spacer 2 (
A neutron absorber 8 is provided on the inner or outer surface of the cladding tube 7 of three (3) enriched uranium pellets. With the above configuration, in this nuclear fuel assembly, the portion where the neutron absorber 8 is provided near the lower side of the spacer has a low output. Comparing the average power distribution of a nuclear fuel assembly with the conventional one, as shown in Figure 4, a low power part B appears near the smoother position S, and as a result, the thermal margin from the burnout point is as shown in Figure 5. As shown in FIG. 2, the width becomes larger near the baser position S than before. In FIGS. 4 and 5, the broken line shows the conventional nuclear fuel assembly, and the solid line shows the nuclear fuel assembly of the present invention. Fifth
As shown in the figure, the nuclear fuel assembly of the present invention has an increased thermal margin, so that the probability that a nuclear fuel rod will undergo transition boiling during a transient change is less than 10% compared to the conventional nuclear fuel assembly. Thermal limit values during reactor operation can be easily met. As mentioned above, the nuclear fuel assembly of the present invention includes:
Since the thermal margin is large, a nuclear reactor using the nuclear fuel assembly of the present invention has the following effects.

That is, by being able to easily meet the thermal limit value, the operating rate of the reactor increases by several percent per year. Furthermore, a highly efficient nuclear reactor with a high power density can be realized, and a reactor with a power density as high as about 55 KW from the power density of a conventional boiling water reactor of about 50 KW' can be realized. As described above, according to the present invention, the thermal margin is likely to be impaired due to the output drop near the lower side of the spacer where the heat removal effect is poor, and the area is reduced and the thermal margin is increased.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing a boiling curve, Fig. 2 is a perspective view showing a nuclear fuel assembly of the present invention, Fig. 3 is a sectional view of essential parts showing an embodiment of the invention, and Fig. 4 is a nuclear fuel assembly. FIG. 5 is an explanatory diagram showing the thermal margin at the position of the nuclear fuel assembly in the mar direction. 1... fuel rod, 2... baser, 7
......Claying tube, 8...Neutron absorber. Figure 1 Figure 5 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. In a nuclear fuel assembly in which a plurality of nuclear fuel rods are arranged and supported in a lattice shape with spacers, each of which is formed by sealing both ends of a cladding tube loaded with a large number of enriched uranium pellets with end plugs, a portion of three enriched uranium pellets below the lower end of the spacer is used. A nuclear fuel assembly characterized in that a neutron absorber is provided on at least one of the inner or outer surfaces of the cladding tube.