JPH0583158B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is applicable to fast breeder reactors, thermal neutron reactors, and medium-speed neutron reactors of the type in which nuclear fuel pins, each consisting of nuclear fuel pellets stacked and sealed in a cladding tube, are loaded into the reactor core. The present invention relates to a non-homogeneous nuclear reactor such as the above, and more specifically to a nuclear reactor that can automatically suppress an abnormal increase in reactor output to prevent a nuclear reactor runaway.

<Prior Art> A fast breeder reactor will be described below as an example of the above-mentioned non-homogeneous nuclear reactor. Figure 2 is a schematic explanatory diagram of a normal fast breeder reactor, in which 1 represents the reactor vessel, 2 represents the shielding plug, 3 represents the upper core mechanism, and 4 represents the reactor core, which is further enriched in plutonium. It has a core fuel region 5 and a radial blanket fuel region 6. Core fuel area 5
The blanket fuel area 6 is constructed by arranging a large number of fuel assemblies with a hexagonal cross section so that their longitudinal directions are vertical, and each assembly is made by arranging a large number of fuel pins in a vertical direction. ing. core 4
The heat generated by nuclear fission is received by the sodium coolant flowing through each fuel assembly through the coolant inlet pipe 7 at the bottom of the reactor vessel 1 and the fuel assembly lower inlet pipe 8. It is taken out from the outlet pipe 9.

As shown in an enlarged view in FIG. 3, the fuel pins constituting the core fuel region 5 are formed by stacking core fuel pellets 11 with a high plutonium enrichment in the axial center of a cylindrical cladding tube 10, and Blanket fuel pellets 12 made of depleted uranium containing a large amount of uranium-238 are stacked at the bottom, and both ends of the cladding tube are sealed with upper end plugs 13 and lower end plugs 14. A spring 15 is usually inserted between the uppermost pellet of the pellet stack and the upper end plug 13 to press the pellet stack, and the space where the spring 15 is installed is used for nuclear fission of the core fuel. This serves as a gas plenum 16 for storing the fission product gas generated.

On the other hand, the fuel pin constituting the blanket fuel region 6 has substantially the same structure as the fuel pin shown in FIG. 3, except that only blanket fuel pellets are stacked and sealed in the cladding tube. A core 4 is formed by arranging blanket fuel assemblies made up of fuel pins around the outer periphery of a region 5 made up of core fuel assemblies. and core fuel area 5
Neutrons generated by nuclear fission of core fuel pellets in the blanket fuel region 6 can be absorbed by uranium-238 in the blanket fuel pellets in the blanket fuel region 6 and converted into plutonium.

By the way, in general, in large-scale fast breeder reactors for power generation, blanket fuel is also arranged inside the core fuel region 5 so that nuclear fission does not concentrate in the center of the reactor core. There are two methods for arranging this blanket fuel. One method is to arrange assemblies of the same type as the blanket fuel assemblies in a distributed or circumferential manner at appropriate radial positions inside the core fuel region, and the core thus arranged is radially non-contact. It is called a homogeneous core. The other type is called an axially non-homogeneous core, in which plutonium-rich core fuel pellets 11 in the fuel pins constituting the core fuel assembly are also deposited in the center of the stack, as shown in Figure 3. , internal blanket fuel pellets 12a containing uranium-238 as the main component (see Figure 4)
Use a fuel pin loaded with In addition, in the fuel pin of FIG. 4, the same reference numerals are given to the same components as in FIG. 3, and the explanation thereof will be omitted.

<Problems to be solved by the invention> In either case of a radially non-homogeneous core or an axially non-homogeneous core as described above, in order to increase the thermal output of a fast breeder reactor, it is necessary to increase the size of the core. However, as the core becomes larger, the power coefficient of the reactor tends to become positive. If the power coefficient is positive, if the reactor power increases for some reason, nuclear fission will increase even if the control rods are not withdrawn, and the thermal output will gradually increase, which will affect the safety of the reactor. I don't like it. On the other hand, even if the output coefficient is negative, it is possible that the control rod may be pulled out by mistake, and there is a risk that such an erroneous operation will cause an abnormal increase in output. Therefore, nuclear reactors must be equipped with means to automatically suppress abnormal reactor output.

Generally, as a means for suppressing output in the event of an abnormality, it has been conventional practice to install multiple systems of emergency reactor shutdown devices, and the devices are designed to automatically operate to shut down the reactor in an emergency. Furthermore, it is desirable to install multiple safety devices in nuclear facilities, and in particular, emergency shutdown in the event of an abnormality is required to work reliably even in the event of an earthquake or loss of power. Therefore, multiple emergency reactor shutdown means that operate on different principles are to be installed, and in addition to reactor shutdown using mechanical devices, reactor runaway suppression means that reliably operate based on physical phenomena must be prepared. It is required to keep it.

Therefore, this invention makes it possible to automatically suppress abnormal power increases through simple physical phenomena by improving the fuel pins that constitute the axially non-homogeneous core described above. This was done with the aim of providing a nuclear reactor that can ensure safety.

<Means for Solving the Problems> According to the present invention, the above object is achieved by improving the fuel pins constituting the axially non-homogeneous core as shown in FIG.

That is, the heterogeneous nuclear reactor according to the present invention uses fuel pellets 12 for internal blankets of fuel pins for a conventional axially heterogeneous core as shown in FIG.
a The inner diameter of the fuel cladding tube 10 corresponding to the lowermost end of the stacked portion is reduced to support the fuel pellets for the internal blanket so that they do not move downward within the cladding tube, and to stack the fuel pellets for the internal blanket below. The core fuel pellets 11 and the blanket fuel pellets 12 are all or partially hollow pellets having an axial hole in the center thereof.

<Effect> When the reactor power increases abnormally for some reason, the temperature in the center of the reactor core increases. Due to this temperature rise, the core fuel pellets (hollow pellets) below the internal blanket fuel pellets melt around the central hole and move downward by gravity through the hole. Due to the downward movement of the nuclear fuel material by melting, the nuclear fission chain reaction established in the entire reactor core is stopped. This is because the contribution of nuclear fuel material to the chain reaction is large when it is in the center of the reactor core, and becomes smaller as it moves to the periphery of the reactor core.
Therefore, if the latter situation occurs, the chain reaction will be automatically stopped.

<Embodiments> The present invention will be described in detail below with reference to embodiments shown in the drawings. FIG. 1 shows a partially enlarged view of a fuel pin used in the nuclear reactor of the present invention. The same reference numbers as in the figures are provided.

The difference between the fuel pin of the present invention and the conventional one shown in FIG. 4 is that the inner diameter of the portion 10a of the fuel cladding tube 10 corresponding to the lowest end of the stacked portion of the fuel pellets 12a for the internal blanket is reduced. The fuel pellets 12a are supported so as not to move downward within the cladding tube 10, and all or part of the core fuel pellets 11 and blanket fuel pellets 12 stacked below the internal blanket fuel pellets 12a. is a hollow pellet.

The inside diameter of the reduced portion 10a of the cladding tube is usually at least about 0.1 mm smaller than the inside diameter of the other portions. The cladding tube is generally manufactured by drawing the entire stainless steel pipe, and by slightly tightening the drawing at a predetermined location, the reduced inner diameter portion 10a can be easily formed. Depending on the degree of reduction in the inner diameter, the corresponding portions of the internal blanket fuel pellets 12a may be slightly shaved off in the circumferential direction as in the illustrated embodiment.

In addition, fuel pellets 12a for the internal blanket
A hole 21 perpendicular to the axial direction is formed in the center of the core fuel pellet 11 sealed lower down. As in the illustrated embodiment, the holes 22 may be drilled through the blanket fuel pellets 12 further below, but if the holes are drilled up to the lowest blanket fuel pellet 12a, Since neutrons leak through the holes in the bottom pellet 12b during normal operation of the furnace, the fuel pellet 12 for the bottom blanket is
It is desirable to fill b with a material without holes.

A core fuel region of a large fast breeder reactor is formed by collecting dozens to hundreds of such fuel pins to form a core fuel assembly and further arranging a large number of these core fuel assemblies.

Assuming that the output of the reactor of the present invention having the fuel pin as described above increases abnormally for some reason, the temperature at the center of the reactor core will first rise due to the abnormal increase in output. When the temperature reaches approximately 2700°C or higher, the center of the core fuel pellet 11 located below the internal blanket fuel pellet 12a in the cladding tube, that is, the area around the void 21, begins to melt, and the molten nuclear fuel material is absorbed by gravity. The pellet then passes through the holes 21 and 22 and gradually moves downward. Thus, as the nuclear fuel material moves from the center of the core to the periphery, its contribution to the fission chain reaction becomes smaller, and the chain reaction automatically tends to stop. At this time, the fuel pellets 12a for the internal blanket do not melt because they generate little heat due to nuclear fission, and are supported by the reduced inner diameter portion 10a of the cladding tube, so the nuclear fuel material in the upper part of the core falls into the center of the core. There is no danger of accelerating nuclear fission, and the chain reaction will be stopped solely by the melting and falling of the nuclear fuel material at the bottom of the core.

The above-described temperature rise in the center of the reactor core due to the abnormal increase in reactor power, and the movement of molten nuclear fuel material produced by this temperature rise to the lower part of the reactor core due to gravity, are both obvious physical phenomena. Therefore, even if the core of the nuclear reactor according to the present invention causes an unexpected power increase accident and other reactor shutdown devices are inoperative, the nuclear fission chain reaction can be automatically and reliably stopped only by the above physical phenomenon. It is possible to do so.

Incidentally, since the core fuel pellet 11 below the internal blanket fuel pellet 12a of the fuel pin used in this invention is a hollow pellet, there arises a disadvantage that the amount of heat generated by nuclear fission during normal operation is correspondingly reduced. In order to avoid this, it is sufficient to increase the plutonium enrichment in the nuclear fuel material by the volume ratio of the pores in the hollow pellet.

<Effects of the Invention> Since the present invention is a nuclear reactor having a fuel pin configured as described above, it is possible to automatically and reliably initiate a nuclear fission chain reaction of nuclear fuel material in the reactor core with only a simple physical phenomenon when the reactor output abnormally increases. It is possible to stop and suppress the nuclear reactor, thereby significantly improving the safety of the entire reactor core.

Although this invention has been explained above using a fast breeder reactor as an example, similar effects can be obtained when a similar configuration is applied to other non-homogeneous nuclear reactors, such as thermal neutron reactors or medium speed neutron reactors. will be easily understood by those skilled in the art.

[Brief explanation of drawings]

Figure 1 is a partially enlarged sectional view of a fuel pin used in the core fuel region of a nuclear reactor according to the present invention, Figure 2 is a schematic illustration of a fast breeder reactor, which is a typical non-homogeneous nuclear reactor, and Figure 3 is a high speed FIG. 4 is a cross-sectional view of a typical fuel pin used in the core fuel region of a breeder reactor, and FIG. 4 is a cross-sectional view of a conventional fuel pin for an axially heterogeneous core. DESCRIPTION OF SYMBOLS 10... Fuel cladding tube, 10a... Inner diameter reduced part of cladding tube, 11... Core fuel pellet, 12...
Blanket fuel pellets, 12a...Inner blanket fuel pellets, 21, 22...Vacancies.

Claims (1)

[Claims] 1 Core fuel pellets with high plutonium enrichment are stacked in the axial center of the fuel cladding tube, fuel pellets for blankets are stacked on top and bottom of the core fuel pellets, and fuel pellets for internal blanket are stacked in the center of the stacked part of the core fuel pellets. In a heterogeneous nuclear reactor having a fuel pin in which fuel pellets are charged, the inner diameter of the fuel cladding tube portion corresponding to the lowermost end of the stacked portion of fuel pellets for the internal blanket is reduced so that the fuel pellets for the internal blanket can flow inside the cladding tube. In addition to supporting the fuel pellets so that they do not move downward, an axial hole is drilled through the center of all or part of the core fuel pellets and the blanket fuel pellets stacked below the internal blanket fuel pellets. A non-homogeneous nuclear reactor characterized by hollow pellets.