JP2013036859A - Neutron shielding material, manufacturing method therefor, storage rack for spent nuclear fuel, and transport cask - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a neutron shielding material which suppresses gas generation and the occurence of swelling and which is excellent in neutron absorption capacity even while being used under water.SOLUTION: A neutron shielding material 10 is constituted by a base member 11 having the composition of a B-Al alloy, a cladding material 12 which has the composition of an Al alloy except for the B-Al alloy or an Al single body and which clads the base member 11, and an oxide film 13 provided on a surface of the cladding material.

Description

  The present invention relates to a neutron shielding material using an aluminum alloy to which boron (B) is added, a method for producing the same, a storage rack for spent nuclear fuel, and a transport cask.

  Spent nuclear fuel discharged from the core after a certain period of operation of the nuclear reactor is housed in a storage rack in the storage pool installed in the power plant until reprocessing, and the decay heat generated is stored in the pool water. Is removed.

In recent years, the capacity of the storage pool has become tight, and the storage capacity has been increased by effectively utilizing the space in the storage pool. In addition, a large intermediate storage facility is installed outside the power plant, where spent nuclear fuel is stored dry.
In these spent nuclear fuel storage racks, transport casks, and storage canisters, stainless steel (B-SUS) added with boron (B) having a neutron shielding ability is generally used in Japan as a neutron shielding material. Yes.

On the other hand, the aluminum alloy (B-Al) to which boron is added has a thermal conductivity of about 240 W / m · K, and the cooling performance is significantly higher than that of the B-SUS material (about 17 W / m · K). Excellent. Furthermore, since B-Al is lightweight with a specific gravity of about 1/3 compared to B-SUS, the transportation cost can be significantly reduced.
For this reason, B-Al is used as a neutron shielding material in Europe and the United States, and its application is also examined in Japan (for example, Patent Documents 1 to 3).

Japanese Patent Laid-Open No. 2005-10000 JP 2007-155664 A JP 2007-33242 A

  However, when B-Al is used in water, the phenomenon of gas generation and surface bulging has been confirmed by a report from the US Nuclear Regulatory Commission (NRC) (NRC INFORMATION NOTICE 2009-26). ing.

In addition, when B-Al is used in water as a neutron shielding material, not only the total amount of generated gas is increased, but also the amount of hydrogen generated based on the hydration reaction of the following formula is increased as compared with the case where Al is used alone ( (See Examples below).
2Al + 2OH ⁻ + 6H ₂ O → 2Al (OH) ⁴ + + 3H ₂

In order to suppress the generation of hydrogen, which is an explosive gas, a possible countermeasure is to form a strong oxide film on the surface and suppress the Al hydration reaction represented by the above formula. As a general method for forming an oxide film on the surface of Al, alumite treatment or boehmite treatment may be mentioned.
However, when alumite treatment or boehmite treatment is performed on B-Al, there is a problem that the boron compound on the surface falls off and an oxide film is not formed. Furthermore, there is a problem that the neutron absorption ability is lowered due to dropping of boron having the neutron absorption ability.

  The present invention has been made in consideration of such circumstances, and a neutron shielding material that is excellent in neutron absorption ability, in which generation of gas and surface swelling is suppressed even when used in water, and a method for producing the same, and spent nuclear fuel It is an object to provide a storage rack and a transport cask.

  In a neutron shielding material, a base member having a B-Al alloy composition, a coating material having a composition of an Al alloy other than B-Al alloy or Al alone, and covering the base member, and provided on the surface of the coating material And an oxide film.

  ADVANTAGE OF THE INVENTION According to this invention, even if it uses it in water, generation | occurrence | production of gas and surface swelling is suppressed, and the neutron shielding material which is excellent in neutron absorption capability, its manufacturing method, the storage rack of used nuclear fuel, and a transport cask are provided. .

(A) The figure which shows embodiment of the neutron shielding material which concerns on this invention, (B) The sectional drawing. The flowchart which shows embodiment of the manufacturing method of the neutron shielding material which concerns on this invention. Process drawing which shows the manufacturing method of the neutron shielding material by the cold rolling method. (A) The figure which shows the neutron shielding material which concerns on other embodiment, (B) The sectional drawing. (A) The figure which shows the neutron shielding material which concerns on other embodiment, (B) The sectional drawing. The perspective view which shows embodiment of the storage rack which concerns on this invention. The perspective view which shows embodiment of the transport cask which concerns on this invention. The schematic diagram of the gas generation amount measuring device of the sample immersed in water. (A) and (B) are the graph and table | surface which show the relationship between the time of immersion of a sample in water, and gas generation amount.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1, the neutron shielding material 10 includes a base member 11 having a B—Al alloy composition and a covering material 12 having a composition of an Al alloy other than the B—Al alloy or Al alone and covering the base member 11. And an oxide film 13 provided on the surface of the covering material 12.

The base member 11 is mainly made of aluminum, which is lightweight and has high thermal conductivity, and is doped with boron isotope ( ¹⁰ B) having a large neutron cross section in a range of 0.4 to 2 wt%. Here, if the boron addition rate is less than 0.4 wt%, the neutron shielding effect of the base member 11 is reduced, and if it exceeds 2 wt%, the heat dissipation effect of the base member 11 is reduced.
Boron added to aluminum may contain boron carbide (B ₄ C) powder as well as boron metal fine particles dispersed in an aluminum matrix.

In addition to the case of using Al alone, which is a raw material of the B-Al alloy constituting the base member 11, or JIS standard 1000 series pure aluminum, the covering material 12 is made of Al other than B-Al alloy such as 7000 series duralumin. Alloys can be used.
That is, the covering material 12 can be suitably used as long as it is an Al-based material on which a stable and dense oxide film 13 is formed on the surface by an alumite treatment or a boehmite treatment described later.

The thickness of the covering material 12 is desirably included in the range of 0.1 to 2 mm.
When the thickness of the covering material 12 is less than 0.1 mm, peeling due to scratches or impacts is likely to occur, and damage is likely to occur during processing. And when the thickness of the coating | covering material 12 exceeds 2 mm, the neutron shielding capability of the neutron shielding material 10 will fall.

  As shown in FIG. 2, the neutron shielding material manufacturing method first undergoes a step (S11) of forming the base member 11 into a predetermined shape. Next, the outer surface of the base member 11 is subjected to a step (S12) of coating the coating material 12 by any one of a cold rolling method, a friction stir welding method, a thermal spraying method, a powder sintering method, and a powder extrusion method. . Then, corrosion resistance is imparted by forming an oxide film on the surface of the covering material 12 by an alumite method or a boehmite method (S13).

FIG. 3 is a process diagram showing an example of coating processing by a cold rolling method.
The B-Al base material and the same Al material were covered with a thickness of 3 mm on both sides of 11 mm thick B-Al, and the first cold rolling was performed at a rolling rate of 30%. As a result, out of the total plate thickness of 12 mm, B-Al had a plate thickness of 8 mm, and Al on both outer sides thereof had a plate thickness of 2 mm.
In the second pass, cold rolling was performed at a rolling rate of 25%. Of the total plate thickness of 9 mm, B-Al had a plate thickness of 7 mm, and Al on both outer sides thereof had a plate thickness of 1 mm.
In the third pass, cold rolling was performed at a rolling rate of 15%, and out of the total plate thickness of 7.6 mm, B-Al had a plate thickness of 6 mm, and Al on both outer sides thereof had a plate thickness of 0.8 mm.
In the final fourth pass, cold rolling was performed at a rolling rate of 34%, and B-Al was 4 mm in thickness with a total thickness of 5 mm, and Al on both outer sides was 0.5 mm in thickness.

Here, a rolling rate shall be shown by following Formula.
Rolling ratio (%) = (original thickness−sheet thickness after rolling) / original thickness × 100

Next, the formation process (S13; FIG. 2) of the oxide film 13 (FIG. 1) will be described.
The base member 11 covered with the covering material 12 is immersed in high-temperature pure water for about 2 hours by boehmite treatment to form an oxide film 13 on the surface thereof.
The boehmite method may be treated in saturated steam in addition to the case of being immersed in high-temperature pure water as described above.

As another method for forming an oxide film, anodizing may be mentioned.
Anodizing is a method for forming a film by anodizing aluminum. Anodization is to perform electrolysis of water mainly using a target material (base member 11 covered with the covering material 12) as an anode and mainly using a strong acid as an electrolyte.
As an electrolytic solution for alumite treatment, sulfuric acid is mainly used, but organic acids such as oxalic acid, chromic acid, phosphoric acid and the like are sometimes used.
The thickness of the oxide film 13 can be controlled by changing the energization time and the current density during the anodizing treatment.

Next, the effect of the oxide film 13 will be outlined.
If the B-Al base member 11 is used in water with only the Al coating material 12 coated, a natural oxide film is formed on the Al surface.
In this natural oxide film, when water is present, a porous hydrated oxide layer grows relatively thick due to a hydration reaction.

However, since this hydrated oxide layer lacks denseness and stability, corrosion (pitting corrosion) of aluminum proceeds from the gap.
This pitting corrosion of aluminum causes Cl ^{− to} be adsorbed on intermetallic compounds and other defects existing in the vicinity thereof, solubilizing the hydrated oxide layer, and further advances the surface corrosion of aluminum. At this time, it is known that hydrogen gas is also generated in addition to oxygen gas and nitrogen gas.

Then, the case of using only B-Al base member 11 in the water, the amount of H ₂ and other gases as will be described later will be further increased. Further, even if the oxide film 13 is directly formed on the B—Al base member 11, the boron compound on the surface falls off and the oxide film cannot be formed.

  As shown in FIGS. 4 and 5, the surface of the base member 11 may have a curved surface or a complicated shape. In this case, the coating material 12 cannot adopt the cold rolling method, but is provided on the surface of the base member 11 by any one of the friction stir welding method, the thermal spraying method, the powder sintering method, and the powder extrusion method. Can do.

With Friction Stir Welding (FSW), the tool is rotated with a strong force against the boundary part to be welded, and the boundary part is softened and plastically flowed by frictional heat. This is a joining method.
In the present embodiment, the material of the covering material 12 is a foil-like body having a thickness of 0.1 to 2 mm, the B-Al base member 11 is wrapped with the foil-like body, and the edges of the foil-like body are bonded together. The base member 11 is covered by bonding.

  Thermal spraying is a method of forming the coating material 12 by heating and melting the material of the coating material 12 and directly spraying the material on the base member 11. A combustion flame, plasma (arc discharge), or the like is used as a heat source, and the material of the covering material 12 is made into droplets and sprayed onto the surface of the base member 11 by a high-speed gas flow or the like. By doing so, the covering material 12 is formed. The coating material 12 may be formed by spraying solid phase particles at a high speed. The manufacturing method of the neutron shielding material by the thermal spraying method has an advantage that it is easy and low in cost.

The powder sintering method is a method of forming the covering material 12 by placing a temporary sintered body of the raw material powder of the covering material 12 on the surface of the base member 11 and performing main sintering in a continuous furnace at 400 to 700 ° C. .
Alternatively, there is a method in which the base member 11 and the covering material 12 are integrally sintered. In this method, the B-Al billet of the base member 11 is manufactured from a mixture of Al powder and B ₄ C powder by an isotropic pressure pressing method (HIP) or a cold isostatic pressing method (CIP).
The powder of the covering material 12 is applied to the outside of the B-Al billet, and further subjected to CIP or HIP treatment to produce a composite billet. The composite billet is heated and sintered at 300 ° C. to produce a temporary sintered body, and then subjected to main sintering at 400 to 700 ° C. to produce a neutron shielding material.

  In the powder extrusion method, a plastic fluid obtained by adding a binder to raw material powder is injected by applying pressure to a mold having a target shape. The obtained molded body is put into a heating furnace, the binder is evaporated, preliminarily sintered to leave only the metal, and further transferred to a sintering furnace to perform main sintering of the metal.

FIG. 6 is a spent nuclear fuel storage rack 30 made using a neutron shielding material.
In the storage rack 30, rectangular tubes 31 formed by molding the plate-like neutron shielding material 10a (FIG. 1) are arranged in a staggered pattern. A plurality of spent fuel assemblies (not shown) taken out from the reactor core by a crane (not shown) are inserted into each of the square tubes 31.
The storage rack 30 is arranged in a storage pool (not shown) provided in the power plant, and the spent fuel assembly accommodated in the square tube 31 is stored for a certain period of time, and the spent nuclear fuel collapses. Heat is removed.

FIG. 7 shows a partial cut model of a spent nuclear fuel transport cask 40 made using a neutron shielding material.
The fuel assembly in which decay heat is attenuated in the storage rack 30 (FIG. 6) is refilled into a transport cask 40 for transport to a reprocessing plant or the like. The transport cask 40 is provided with a fuel basket 41 formed by molding a plate-like neutron shielding material 10a, and a fuel assembly 42 is accommodated therein.

The experimental data by the Example and comparative example of this invention are shown, and the effect of this invention is confirmed.
As a sample 26, an example including a B-Al base member 11, an Al coating material 12, and an oxide film 13 as in the neutron shielding material 10 (FIG. 1), and Comparative Example 1 (Al) in which the oxide film 13 is omitted. And the comparative example 2 (B-Al) which abbreviate | omitted Al coating | covering material 12 is prepared.
In addition, the comparative example in which the oxide film is directly formed on the surface of B-Al is not verified since the boron compound on the surface falls off and a stable oxide film is not formed as described above.

FIG. 8 shows the gas generation amount measuring device 20 of the sample 26 immersed in water.
This gas generation amount measuring device 20 has a container 22 having a vent hole 25 so that the internal pressure becomes atmospheric pressure, and a transparent end having a sealing end 21a outside the container 22 and a tapered opening end 21b inside. The pipe 21 and the temperature control part 24 which hold | maintains temperature so that the thermometer 23 of the pure water accommodated in the inside of the container 22 may be 30 degreeC are comprised.

FIG. 9A starts from the time when the sample 26 is put in a state in which pure water is distributed to the sealing end 21a of the transparent tube 21, and the sample 26 (Example, Comparative Example 1, Comparative Example) is observed over about 250 hours. It is the result of having measured each gas generation amount of Example 2).
According to this, in Comparative Example 1 (Al) and Comparative Example 2 (B-Al), no gas was generated immediately after immersion, but gas was generated after about 25 hours, and then rapidly generated up to 80 hours, then 200 hours. Gas generation continued until.
On the other hand, in the example (B-Al + oxide film), no gas generation was observed even when the immersion time was extended to 1000 hours.

As shown in FIG. 9 (B), when the amount of gas generated per unit surface area of the sample 26 was derived, 9.55 × 10 ⁻⁴ ml / cm ² in B-Al (Comparative Example 2), Al (Comparative Example) In 1), 5.82 × 10 ⁻⁴ ml / cm ² and B-Al + oxide film (Example) were unable to be measured.
When this generated gas was analyzed, hydrogen gas was contained in addition to oxygen and nitrogen, and the hydrogen gas concentration was 1.1% for B-Al (Comparative Example 2) and 0.5% for Al (Comparative Example 1). The result was obtained.

From the above experimental results, it has been found that B-Al generates more gas than Al due to corrosion due to use in water, and in particular, the amount of hydrogen generated increases. This is considered to be due to the influence of the boron compound precipitated on the surface of B-Al. Furthermore, in B-Al and Al accompanied by gas generation when used underwater, white spot-like corrosion marks resulting from the hydration reaction were confirmed on the surface.
On the other hand, as in the example, the B-Al base material coated with Al and further formed with an oxide film on the surface did not show corrosion marks and gas generation due to underwater use.

  According to the neutron shielding material of at least one embodiment described above, since the gas generation is suppressed even when used in water and the neutron absorption capability is excellent, the constituent material suitable for the storage rack and the transport cask for spent nuclear fuel. Is provided.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  10 (10a, 10b, 10c) ... neutron shielding material, 11 ... base member, 12 ... coating material, 13 ... oxide film, 20 ... gas generation amount measuring device, 21 ... transparent tube, 21a ... sealed end, 21b ... opening End, 22 ... Container, 23 ... Thermometer, 24 ... Temperature control part, 25 ... Vent, 26 ... Sample, 30 ... Storage rack, 31 ... Square tube, 40 ... Transport cask, 41 ... Fuel basket, 42 ... Fuel assembly body.

Claims (6)

A base member having a B-Al alloy composition;
A coating material that has a composition of an Al alloy other than B-Al alloy or Al alone and covers the base member;
A neutron shielding material comprising: an oxide film provided on a surface of the coating material.   The neutron shielding material according to claim 1, wherein a thickness of the covering material is included in a range of 0.1 to 2 mm.   The neutron shielding material according to claim 1 or 2 is produced by any one of a cold rolling method, a friction stir welding method, a thermal spraying method, a powder sintering method, and a powder extrusion method. Manufacturing method.   4. The method for producing a neutron shielding material according to claim 3, wherein an oxide film is formed on the surface of the coating material by an alumite method or a boehmite method.   A storage rack for spent nuclear fuel using the neutron shielding material according to claim 1.   A transport cask for spent nuclear fuel using the neutron shielding material according to claim 1.

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