JPS626194A - Fuel aggregate for nuclear 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] [Industrial Application Field] The present invention provides a method for controlling the power increase rate at the initial stage of combustion of a newly loaded fuel assembly during reactor power operation.
This invention relates to a nuclear reactor fuel assembly in which a burnable poison is dispersed at a low concentration in most of the fuel pellets 1 in the fuel assembly, so that fuel can be loaded during power operation.

[Prior Art] In general, a pressure tube power reactor has a major feature in that fuel can be replaced even during output operation without shutting down the reactor. However, in reality, when fuel is exchanged during power operation, the output of the newly loaded new fuel assembly and the surrounding fuel assemblies increases rapidly (in only about 2 hours) as the new fuel assembly is loaded. This increase in output increases as the difference in infinite multiplication factor between the new fuel assembly and the removed spent fuel assembly increases.

When the output of the fuel assembly suddenly increases, the fuel pellets may come into contact with the cladding tube due to the difference in thermal expansion between the fuel pellets and the cladding tube, and may further push the cladding tube open and cause damage.

Therefore, in the conventional technology, a bunch method is adopted in which fuel is exchanged while the reactor is stopped, and when the reactor is started up, a method is adopted in which the rate of increase in the output of the reactor is adjusted to reduce the rate of increase in the output of the fuel assembly. ing.

[Problems to be solved by the invention] If the fuel is exchanged in a batch manner while the reactor is stopped, and the rate of increase in the output of the reactor is adjusted at startup to reduce the rate of increase in the output of the fuel assembly, the fuel pellets can be reduced. Since the high-temperature center of the fuel cell creep deforms due to compressive stress, and the cladding tube creep deforms due to tensile stress, the stress generated in the cladding tube is alleviated and the integrity of the fuel assembly is ensured.

However, this type of fuel exchange method loses one of the major features of pressure tube reactors, which is that fuel can be exchanged while the reactor is operating at its output, resulting in not only a decrease in operating availability but also a decrease in fuel utilization. This results in a major drawback.

An object of the present invention is to eliminate the drawbacks of the prior art as described above, and to suppress the increase in output of new fuel assemblies and fuel assemblies around the loading channel even if fuel is loaded during power operation of a nuclear reactor. , it is possible to control the rate of increase in output in the initial stage of combustion of the new fuel assembly, ensuring the integrity of the fuel,
An object of the present invention is to provide an improved nuclear reactor fuel assembly that can significantly improve fuel utilization and operational availability.

[Means for Solving the Problems] The present invention, which can achieve the above-mentioned objects, contains a burnable poison in most of the fuel pellets in a fuel assembly, the combustion rate of which changes over time at the initial stage of combustion. It is a reactor fuel assembly that is dispersed at a low concentration such as burnable poison, and its major feature is that the output increase rate of the fuel assembly is controlled by the burnable poison, and fuel can be loaded during output operation. It is something.

Here, "most of the fuel pellets in the fuel assembly" refers to approximately half or more of the fuel pellets present in the fuel assembly, and more preferably approximately 80% or more. A fuel assembly is composed of a large number of fuel rods loaded with a large number of fuel pellets arranged in a row.The power distribution is not uniform for each fuel pellet, and the fuel rods arranged on the outside are generally higher in power. Also, looking at each fuel rod, the center is higher than both ends. Therefore, if the proportion of fuel pellets containing burnable poison is as small as about half, it is sufficient to select fuel pellets in a region where it is particularly desired to control the increase in output and insert the burnable poison.

For example, gadolinia (Gd, O3) can be used as the burnable poison dispersed within the fuel assembly, and after being uniformly mixed into uranium dioxide powder, which is the reactor fuel, sintered oxide fuel pellets are used. It's good. In this case, the concentration of gadolinia is about 0.2% or less, preferably 0.
．． The concentration should be as low as 0.05%.

In addition to the above gadolinium, burnable poisons include boron,
Cadmium, dysprosium, etc. can be used.

Incidentally, fuel assemblies using fuel pellets mixed with burnable poisons are conventionally known, and are intended to flatten the power distribution of the reactor core. Excess reactivity and output are suppressed by burnable poisons and set to appropriate surplus reactivity and output, and in order to maintain this constant level, high degree burnable poisons are applied locally within the fuel assembly ( This is a technology that is completely different from the control of the rate of increase in output power of a fuel assembly as intended by the present invention. .

[Effect] The burnable poison dispersed at a low concentration in the fuel pellets of most of the fuel assemblies has a fuel rate that changes over time at the initial stage of combustion, and a change in reactivity due to the combustion of the burnable poison. By using the output change, it is possible to control the output increase rate so that the output of the loaded fuel assembly reaches approximately a predetermined output state in about 15 days.

[Example] Hereinafter, one embodiment of the present invention will be described in more detail.

The basic structure of the fuel assembly is the same as the conventional one. Figure 1 shows a fuel pellet 10 containing burnable poison.
This fuel pellet 10 is made by uniformly mixing gadolinia (Ca, O,) as a burnable poison with uranium dioxide powder as a nuclear fuel, and then forming the mixture into a cylindrical shape and sintering it. However, in the present invention, gadolinia is mixed in at such a low concentration that its combustion rate changes rapidly at the beginning of combustion.

As shown in FIG. 2, a large number of such fuel pellets 10 are filled in a cladding tube 12 made of Zircaloy or the like, and both ends of the cladding tube 12 are sealed with end plugs 14 to form a fuel rod 16.
is configured. The fuel assembly 20, as shown in FIGS. 3 and 4, is a cluster type in which 28 fuel rods 16 are bundled. Since the fuel rods 16 are loaded into the circular pressure tube, they are arranged in three layers of concentric circles, fixed by an upper tie plate 22 and a lower tie plate 24, and spacers 26 are attached to ensure horizontal structural stability. Note that the number 2 indicates a spacer support rod.

As mentioned above, except for the difference in fuel pellet IO, the other configurations are the same as the conventional technology, so it can be used as is.The present invention is significantly different from the conventional technology. The difference is in the composition of the fuel pellets, and the second difference is in the state of dispersion of the burnable poison within the fuel assembly. In other words, in the present invention, the concentration of the burnable poison is low, and the burnable poison is present in most of the fuel pellets in the fuel assembly.

FIG. 5 is a graph showing the relationship between the infinite multiplication factor and the burnup of a nuclear reactor fuel assembly in which 0.05% gadolinia is uniformly dispersed. In the figure, the solid line shows the case where the present invention is applied, and the broken line shows the case where no burnable poison is used. Even if a nuclear reactor fuel assembly with burnable poison evenly dispersed is loaded into a power reactor that is operating at high output, the new nuclear reactor fuel assembly loaded will not be able to achieve high output due to the neutron absorption effect of this burnable poison. Must not be. Since the infinite multiplication factor of the new fuel assembly is smaller than that of the replaced spent fuel assembly, the output in the replacement channel and the channels around it is smaller than the output before replacement, and the infinite multiplication factor immediately after loading is approximately 0.5
It is.

As the burnable poison burns, the infinite multiplication factor of this fuel assembly is restored, and since it normally burns about 20 MWd/day, the infinite multiplication factor becomes almost the same as that of the spent fuel assembly that was replaced after about 5 days. , the replaced channel and the channels around it are restored to almost their pre-replacement output state. When about 15 days have passed after loading, the burnable poison disappears, the infinite multiplication factor becomes about 1.2, and a predetermined output state is reached.

Therefore, this blood formation period is approximately 300 MWd/.

The power increase rate of this reactor fuel assembly is approximately 0.1% power/hour, and the power increase rate of the fuel assemblies around the exchange channel is less than that, so this is carried out at reactor startup. It can be made sufficiently lower than the output increase speed limit.

In the above example, the pellets containing one type of burnable poison are arranged almost uniformly in the reactor fuel assembly, but the burnable poison By changing the concentration of fuel, it is also possible to adjust the rate of increase in output power for each fuel rod.

It is also possible to combine the present invention with a technique for containing a highly concentrated burnable poison in order to flatten the power distribution, and in that case, the rate of increase in power of the fuel assembly can be adjusted in the same way as in the above embodiment. I can do it. An example thereof is shown in FIG. In the figure, the solid line indicates the case when the method of the present invention is applied (Gd
25 fuel rods with -0.08% Gd and 4 fuel rods with -1.0% Gd), and the dashed line shows the case without the conventional burnable poison and the case with the conventional power flattening method ( This is the case (when three fuel rods of Gd-1.0% are used) due to local contamination of high-concentration burnable poison. From the figure, it can be clearly seen that the power increase is controlled in the early stage of combustion of the loaded fuel assembly, and then the power is flattened.

[Effects of the Invention] As described above, the present invention disperses burnable poisons at a low concentration in most of the fuel pellets in a fuel assembly, so that new fuel assemblies loaded during reactor power operation and the fuel around them It is possible to suppress the increase in the output of the assembly and control the rate of increase in output at the initial stage of combustion of new fuel, ensuring the integrity of the fuel, and making it possible to change the fuel during output operation, which improves the fuel utilization rate and This has the excellent effect of significantly improving the operating availability.

[Brief explanation of drawings]

Figure 1 is a perspective view of fuel pellets containing burnable poison;
Fig. 2 is a partially broken explanatory diagram of a fuel rod using the same, Fig. 3 is a perspective view of a nuclear reactor fuel assembly incorporating the fuel rod,
FIG. 4 is a cross-sectional view thereof, FIG. 5 is a graph showing the relationship between fuel burn-up and infinite multiplication factor according to one embodiment of the present invention, and FIG. 6 is a graph showing the relationship between fuel burn-up and infinite multiplication factor according to another embodiment of the present invention. It is a graph showing the relationship between magnifications. lO...Fuel pellets containing burnable poison, 12...
Cladding tube, 16... Fuel rod, 20... Fuel assembly, 2
2... Upper tie plate, 24... Lower tie plate, 26... Spacer. Patent applicant Representative of Power Reactor and Nuclear Fuel Development Corporation
Figure 2 Figure 3

Claims (1)

[Claims] 1. The burnable poison is dispersed in most of the fuel pellets in the fuel assembly at such a low concentration that the combustion rate changes over time in the early stages of combustion, making it possible to load fuel during power operation. A nuclear reactor fuel assembly characterized by: