JPS58637B2 - Nuclear reactor core using fuel assembly with molten fuel retainer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is capable of retaining part or most of the molten fuel that spouts out within a substantially fixed area when some kind of accident occurs in a nuclear reactor and fuel is damaged. This relates to a nuclear reactor core using a fuel assembly that allows for

The types of accidents in nuclear reactors can be roughly divided into initial events: those that begin with initial events in the entire reactor core, such as reactivity insertion accidents and coolant accidents, and those that are localized, such as coolant flow blockage accidents in fuel assemblies. It can be classified into

In any case, the output will increase, and when the fuel temperature increases and the fuel material melts, the temperature will be 2700-2800℃
With cladding materials made of alloys such as stainless steel, vanadium, and nickel, fuel failure occurs and molten fuel is released into the coolant.

A typical case of fuel failure is considered to be when internal pressure increases due to thermal expansion of the fuel, volumetric expansion due to fuel melting, and release of fission product gas from the molten fuel apply stress to the cladding tube, causing it to break.

In other words, as the fuel heats up and melts due to an increase in output during a reactivity accident, the fission product gas trapped within the solid fuel is released while increasing its volume, causing a rapid increase in internal pressure and straining the cladding. When the amount of strain exceeds the failure limit of the cladding tube, fuel failure occurs and a mixture of fission product gas and molten fuel is ejected.

Therefore, when a fuel failure occurs and molten fuel is ejected, most of the ejected material falls and is widely deposited on the core structure and the bottom of the reactor vessel, making it extremely difficult to remove.

Furthermore, if too much molten fuel is accumulated and retained in one place, there is a risk that the fission chain reaction will be maintained due to the critical shape, that is, the size and shape necessary for the system containing fissile material to reach criticality. .

The purpose of the present invention is to solve this problem, and is designed to retain part or most of the molten fuel in the event that an accident occurs in a nuclear reactor and the fuel cladding tube is damaged. It has a molten fuel retention function that allows the molten fuel to be in a critical shape, and
Moreover, it is an object of the present invention to provide a nuclear reactor core that can also perform a neutron shielding function.

Hereinafter, the present invention will be explained in detail based on the drawings.

FIG. 1 is an illustration of an example of a nuclear fuel assembly used in the present invention.

This fuel assembly 1 is for a fast reactor, and includes a hexagonal cylindrical wrapper tube 3 in which a large number of fuel elements 2 are arranged in a bundle, and the wrapper tube 3 for handling during fuel exchange.
It includes a handling head 4 formed at the upper end and a cylindrical entrance nozzle 5 provided at the lower end of the wrapper tube 3.

A large number of orifice holes 6 are provided on the side wall of the entrance nozzle 5.
is bored and communicates with the inside of the wrapper tube 3 and the opening of the handling head 4.

Above the entrance nozzle 5 and below the bundle of fuel elements 2, a molten fuel holder 9 made of a high melting point material and having a large number of vertical holes 8 is provided. An upper shielding body 10 is arranged.

The material of the molten fuel holder 9 should be one that has good coexistence with the coolant (usually liquid sodium in the case of a fast neutron reactor) and molten fuel so as not to cause a significant reaction when it comes into contact with these substances. For example, tantalum, tungstentrium compounds, uranium compounds, graphite, etc. can be used.

Furthermore, these materials have excellent neutron shielding ability and are effective as shields for neutron irradiation of nuclear reactor structures or core support structures located below the reactor core.

This fuel assembly 1 is supported by having its entrance nozzle part 5 inserted into a support structure attached to the lower part of the reactor vessel, and the coolant flows in from the orifice hole 6, and the entrance nozzle part 5 and the melting holder 9 through the longitudinal bore 8 of the fuel element 2 and exits the handling head 4.

In the unlikely event that some kind of accident occurs, the output increases and the fuel elements 2 that are bundled and stored in the fuel assembly 1
If the cladding tube is damaged and molten fuel spouts out, most of the molten fuel falls and reaches the upper surface of the molten fuel holder 9.

Since the falling molten fuel does not have much fluidity, most of it remains on the upper surface of the molten fuel holder 9 and is held there.

As mentioned above, since the holder 9 is made of a high melting point material, it can be held reliably without the risk of being attacked by high temperature molten fuel.

FIG. 2 is an explanatory diagram of another example of the fuel assembly used in the present invention.

The difference from FIG. 1 is that a lower shield 11 is provided above the molten fuel holder 9.

As mentioned above, the molten fuel holders 9 made of tantalum, tungsten triumium compounds, etc. themselves have excellent shielding ability against neutrons and can be used as a shield for the lower part of the reactor core, but in cases where these alone are insufficient. ,
For example, by providing a lower shield 11 made of stainless steel, shielding against neutrons can be made more reliable.

Although either of the molten fuel holder 9 and the lower shield 11 may be positioned upward, it is easier to attach the molten fuel holder 9 if it is positioned downward as shown in FIG.

The adhesion of the molten fuel holding body 9 to the trumpet tube 3 can be achieved, for example, by the third
As shown in the figure, this may be done by fitting, or as shown in FIG. This may be done by welding to the pipe wall of No. 3.

Next, a nuclear reactor of the present invention constructed from a fuel assembly having such a structure will be explained.

FIG. 5 shows an example of this.

A support structure 15 for fuel assemblies is provided in the lower part of the reactor vessel (not shown), and supports a large number of fuel assemblies 1a.
, 1b, 1c, . . . have their respective entrance nozzles 5 inserted and fixed.

The structure of each fuel assembly 1a, 1b, 1c, . . . is similar to that shown in FIGS. Columnar molten fuel holding bodies 9a, 9b, 9c made of high melting point material
,... are arranged respectively.

Here, the molten fuel holding bodies of the fuel assembly in the center of the core are arranged such that the height of each column is high and becomes gradually lower toward the periphery of the core.

In other words, since the upper surface of the molten fuel holder 9 is high at the center of the core and becomes lower toward the periphery of the core, the fuel cladding tube is damaged and the molten fuel that falls and accumulates on the upper surface of the molten fuel holder 9 becomes critical. shape can be avoided.

Furthermore, in the core of a fast reactor, neutron flux is high in the center and low in the periphery.

As mentioned above, the molten fuel holder has the ability to shield neutrons, and its column height is also changed in accordance with the height of the neutron flux, so irradiation damage due to neutron irradiation in the lower part of the core can be avoided.

Since the present invention is a nuclear reactor core configured as described above,
In the event that some kind of accident occurs in a nuclear reactor and the fuel cladding tube is damaged, a part or most of the molten fuel can be reliably retained and the molten fuel can be prevented from becoming in a critical shape. Furthermore, it can perform a neutron shielding function, and has many excellent effects, such as being able to avoid irradiation damage due to neutron irradiation in the lower part of the reactor core by changing the height of the molten fuel column according to the level of neutron flux. It is something.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an example of a nuclear fuel assembly used in the present invention,
Figure 2 is an explanatory diagram of other examples of nuclear fuel assemblies, Figures 3 and 4.
Each figure is an explanatory view showing how to attach a molten fuel holding body to a trumpet tube, and FIG. 5 is an explanatory view of an embodiment of a nuclear reactor core according to the present invention. 1...Fuel assembly, 2...Fuel element,
3...Trumpet pipe, 4...Handling head, 5...Entrance nozzle, 8...
... Vertical hole, 9 ... Molten fuel holding body, 15.
...Fuel assembly support structure.

Claims (1)

[Claims] 1 A nuclear fuel assembly consisting of a columnar molten fuel holder made of a high melting point material having a large number of longitudinal holes provided above the entrance nozzle and below the fuel element bundle, and the column height of the molten fuel holder. In the reactor core, areas with high neutron flux in the reactor are set high, and areas with low neutron flux are set low.