JPS6122800B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a spent fuel transport container (hereinafter referred to as a cask) that maintains the inside of the container at a subcritical state both in normal times and during abnormal accidents.

A cask is used to safely transport several fuel assemblies used in a nuclear reactor. Therefore, no matter what kind of fuel assembly is placed inside the cask or how it changes, the inside of the cask must always remain subcritical.

First, in order to clarify the purpose of the present invention, a conventional cask will be explained. In FIG. 1, the cask is surrounded from the outside by a shock absorber 8, fins 7, a container body 6, an attached container leg 9, a cooling water 5, and a hollow cylinder 4. The interior of the hollow cylinder 4 is partitioned by a fuel holding frame 3 and a grid-like basket 1, and a hollow prismatic bottle 2 is provided inside the basket 1, into which the fuel assembly is placed. ing.

Generally, the material of the basket 1 is steel, and the material of the bottle 2 is stainless steel. Water is placed inside the hollow cylinder 4 other than the basket 1, bottle 2, and fuel assembly to cool the fuel assembly, and on the other hand, this water shields radiation emitted from the fuel assembly.

In a normal cask in which spent fuel is placed in a predetermined position, the effective multiplication factor (keff) of neutrons is sufficiently smaller than 1.0 over all fuel burnup levels, as shown by curve 61A in Figure 5. It is subcritical.

However, in an abnormal case where the basket 1 bottle 2 is crushed due to some kind of accident, and only fuel and water are mixed at a certain ratio and collected in a part of the cask and become homogeneous (hereinafter this case is referred to as an abnormal accident). ) may exceed the critical value depending on the burnup of the fuel, as shown by the curve 60A in FIG.

Since the actual cask basket 1 is generally made of steel and has a thickness of about 8 mm, it is unlikely that it will break. Even if it were to break, in order for the abnormal accident mentioned above to occur, it is thought that exceeding criticality would not be achieved unless the conditions such as the mixing ratio of fuel and water and the extent to which the fuel spreads within the cask were met. ,
This assumes an abnormal accident in the unlikely event of an accident.

The present invention was made in view of the above circumstances, and
In order to further increase the safety of the cask, we assume possible phenomena and provide a cask that can sufficiently cope with any of these phenomena. That is, the first objective is to obtain a cask with increased strength of the basket so that abnormal accidents such as the basket breaking do not occur. The second objective is to obtain a cask with consideration given to nuclear properties so that even if the basket is broken and the above-mentioned abnormal accident occurs, the inside of the cask can be maintained at a sufficiently subcritical state.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2, explanations for the same numbers as in FIG. 1 will be omitted. The interior of the hollow cylinder 4 of the cask includes a fuel holding frame 3 and a partially hollow interior 21,
The hollow part is partitioned by a lattice-shaped basket 20 containing a water-soluble substance with a large neutron absorption cross section (hereinafter referred to as a neutron absorber), and a hollow prismatic bottle 2 is placed inside the basket 20. A fuel assembly is housed in the bottle 2.
In FIG. 3, a lattice-shaped basket 20 is partially hollow 21, into which a neutron absorber is placed. A bottle 2 is disposed within a lattice-shaped basket 20, and a fuel assembly 10 is housed therein. The other part of the basket 20 is filled with water 3A to shield radiation emitted from the fuel assembly 10,
Meanwhile, the fuel assembly 10 is cooled. The neutron absorber filled in the hollow 21 is in the form of powder or granules. This is to ensure that in the event of an abnormal accident, the neutron absorber will quickly diffuse into the water and even dissolve, keeping the cask in a subcritical state. As the neutron absorber, for example, boric acid such as sodium pentaborate is used.

FIG. 4 shows a cask of another embodiment. In FIG. 4, the entire inside of the basket 30 is made into a hollow 31.
A neutron absorber is filled inside the neutron absorber. A bottle 2 is provided inside the basket 30, into which the fuel assembly 10 is placed, and its surroundings are filled with water 3A.

Next, the cask action will be explained. By making the inside of the basket 20 hollow, the cask has better overall steel properties (tensile, compressive,
(torsion, etc.) and can withstand even stronger forces.

Even if the basket is broken, the neutron absorber stored in the hollow 21 of the basket 20 quickly diffuses into the water and further melts to absorb the neutrons generated from the fuel assembly 10. Therefore, as shown in FIG. 5, even in the event of an abnormal accident, the effective multiplication factor inside the cask does not exceed 1.0 and the inside of the cask can be kept subcritical.

Therefore, even during normal times, the inside of the cask can be more completely kept subcritical, resulting in extremely high safety.

The example shown in FIG. 4 has the same effects as the example shown in FIG. 3, has sufficient strength, and is easier to manufacture.

In this way, the cask of the present invention can increase the safety of the cask during normal times, prevent abnormal accidents from occurring in advance, and even if an abnormal accident occurs, the inside of the cask can be kept subcritical. It is extremely safe and can be maintained at high temperatures.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of a conventional cask, Fig. 2 is a sectional view of an embodiment of the cask of the present invention, Fig. 3 is a sectional view showing a part of the cask in Fig. 2 in detail, and Fig. 4. 5 is a cross-sectional view showing in detail a part of a cask according to another embodiment of the present invention, and FIG. 5 is a comparison diagram of the effective multiplication factor (keff) of the cask during normal times and during an abnormal accident. 20, 30, 40...Basket, 21...Hollow part into which the neutron absorber is placed, 31...Hollow part into which the neutron absorber is placed.

Claims (1)

[Scope of Claims] 1. A spent fuel transport container whose outermost periphery is surrounded by a shock absorber, and in which a fin, a container main body, a cooling water section, and a hollow cylinder are housed in this order,
A lattice-shaped basket containing a water-soluble neutron absorbent having a large neutron absorption cross section is housed inside the hollow cylinder, a bottle for storing and fixing the spent fuel assembly is housed inside this basket, and the A spent fuel transport container comprising a hollow cylinder filled with water. 2. According to claim 1, the basket is characterized in that the inside of its structural material is partially hollow, and this hollow part is filled with a neutron absorbent that is water-soluble and has a large neutron absorption cross section. spent fuel transport container. 3. The basket according to claim 1, wherein the inside of the structural material is entirely hollow, and the hollow part is filled with a neutron absorbent that is water-soluble and has a large neutron absorption cross section. Spent fuel transport container. 4. The spent fuel transport container according to claim 1, wherein the water-soluble neutron absorber having a large neutron absorption cross section is formed in the form of powder or granules.