JPWO2015008369A1 - Neutron-absorbing glass and neutron-absorbing material using the same, and a method for managing molten fuel, a method for taking out molten fuel, and a method for shutting down a reactor using the same - Google Patents

Abstract

Neutron-absorbing glass that can be put into water, which contains gadolinium oxide, boron oxide, silicon oxide, zirconium oxide and alkali metal oxides. 5 to 20 mol%.

Description

  The present invention relates to a neutron absorbing glass, a neutron absorbing material using the neutron absorbing glass, a molten fuel management method, a molten fuel take-out method, and a reactor shutdown method to which these are applied.

  In a nuclear power plant such as a boiling water nuclear power plant and a pressurized water nuclear power plant, a plurality of fuel assemblies including nuclear fuel material (uranium pellets) are loaded in a reactor core. When unloading a fuel assembly in a normal operation cycle, the fuel assembly is designed to be of a size that is not critical as a whole, so there is a risk that it will become critical if the fuel assembly is unloaded one by one. And can be safely carried out.

  However, in the unlikely event that an accident occurs in which nuclear fuel material (uranium pellets) contained in the fuel assemblies loaded in the reactor core melts, such as the nuclear plant at the Three Mile Nuclear Power Station, There is a need for a method for preventing criticality of the molten nuclear fuel material (hereinafter referred to as “molten fuel”) and managing it safely. This molten fuel is accumulated in the reactor pressure vessel or leaked into the containment vessel. Further, the molten fuel is obtained by melting uranium pellets in the fuel rods inside the nuclear reactor together with surrounding structures. In addition, it is necessary to cut the molten fuel and carry it out of the nuclear reactor. In such a case, a removal method for preventing the occurrence of criticality is indispensable.

Patent Document 1 discloses SiO ₂ , B ₂ O ₃ , Al ₂ O ₃ , La ₂ O _3, Gd ₂ O _{3, and the} like for transparent window glass having the ability to shield radiation such as X-rays and γ-rays. Including glass compositions have been proposed. SiO ₂ is 18 to 30 mol% than the embodiment of Patent Document 1, B ₂ O ₃ is 18 to 38 mol%, Al ₂ O ₃ is 2.8 to 19.8 mol%, La ₂ O ₃ is from 6 to 13 mol% and Seven types of glass compositions having Gd ₂ O _{3 in} the range of 15 to 20 mol% are specifically disclosed.

JP 2009-7194 A

  Since B (boron) reacts with water to produce boric acid, if glass containing B is present in water, B may be dissolved from the glass to form boric acid and dissolve in water. When boric acid dissolves in water, the inside of the furnace becomes an acidic corrosive environment, which may corrode the internal structure and peripheral equipment.

  The glass composition described in Patent Document 1 has improved detergent resistance and acid resistance so that no scorching will occur even if washing or the like is performed. This glass composition contains a large amount of neutron absorption Gd (gadolinium) and B, so it can absorb neutrons, but the water resistance of the glass is low, and the problem that B dissolves when used for a long time in the state of being immersed in water. is there.

  An object of this invention is to improve the water resistance of neutron absorption glass.

To achieve the above object, the present invention provides a neutron absorbing glass as possible put into water, gadolinium oxide, boron oxide, silicon oxide, include zirconium oxide and alkali metal oxide, B ₂ O in the following in terms of oxide ₃ 40-60 mol% and R ₂ O: characterized in that it contains 5 to 20 mol% of (R alkali metal).

  According to the present invention, the water resistance of the neutron absorbing glass can be improved.

It is a schematic external view which shows the typical shape of neutron absorption glass. The B ₄ C powder, which is an example of a schematic external view of a neutron absorbing material sintered at neutron absorbing glass. The B ₄ C particulate which is an example of a schematic cross-sectional view of a neutron absorbing material coated with a neutron absorbing glass. It is an example of the schematic sectional drawing of the state which the neutron absorber (neutron absorption glass or neutron absorption material) contacted the surface of the molten fuel. It is an example of the schematic sectional drawing of the method of taking out safely the molten fuel inside a nuclear reactor to the exterior of a nuclear reactor. It is a differential thermal analysis (DTA) curve of a typical glass. It is an example of the schematic sectional drawing of the installation which produces neutron absorption glass. It is an example of the schematic sectional drawing of the installation which produces a neutron absorption material.

  The present invention relates to a neutron absorbing glass and a neutron absorbing material used in a nuclear reactor that uses water as a moderator, and is particularly suitable for use when introduced into water in a nuclear reactor. The present invention also relates to a molten fuel management method, a molten fuel take-out method, and a reactor shutdown method to which the neutron absorbing glass or the neutron absorbing material is applied.

  Hereinafter, embodiments of the present invention will be described.

The neutron absorption glass of the present embodiment includes gadolinium oxide, boron oxide, silicon oxide, zirconium oxide, and alkali metal oxide, and B ₂ O ₃ is 40 to 60 mol% and R ₂ O (R : Alkali metal) is 5 to 20 mol%, and good water resistance and neutron absorption performance can be obtained. The improved water resistance makes it easier for B to absorb neutrons to dissolve in water, thus facilitating water treatment and disposal. In addition, when the oxide is described as “x to y mol%” in the text, it indicates that “x mol% to y mol%” (x mol% ≦ oxide ≦ y mol%). The same applies to the following.

  Although Gd is expensive, it is an element having a neutron absorption cross section about 60 times that of B. By containing Gd in glass, the amount of neutron absorption can be increased. Gadolinium oxide and boron oxide are mainly responsible for neutron absorption. Table 1 shows elements with large neutron absorption and their neutron absorption cross sections. Although depending on the state of radiated neutrons, elements with a larger neutron absorption cross section tend to have higher neutron absorption performance.

Also, in order to increase the neutron absorption, the B ₂ O ₃ content (40-60 mol%) is increased, but on the other hand, the water resistance decreases, so silicon oxide and zirconium oxide are effective in improving water resistance. And an alkali metal oxide. Among them, by containing an appropriate amount of R ₂ O (5 to 20 mol%), a boric acid abnormal phenomenon peculiar to glass is expressed, and a decrease in water resistance is suppressed or prevented.

A preferred composition range of neutron absorbing glass, Gd ₂ O ₃ is 5 to 12 mol% in the following in terms of oxide, B ₂ O ₃ is 40 to 60 mol%, SiO ₂ is 8 to 35 mol%, ZrO ₂ is 3 12 mol% and R ₂ O is 5 to 20 mol%. More preferably, Gd ₂ O ₃ is 6 to 9 mol%, B ₂ O ₃ is 45 to 55 mol%, SiO ₂ is 15 to 30 mol%, ZrO ₂ is 5 to 10 mol% in terms of the following oxides: R ₂ O is 5 to 15 mol%.

R ₂ O desirably contains at least Li ₂ O. Li is smaller than the neutron absorption cross sections of Gd and B as shown in Table 1, but is one of the elements with a large neutron absorption. Therefore, the neutron absorption glass contains Li ₂ O, so the neutron absorption performance is improved. Can be improved. In addition, when two or more types of R ₂ O (for example, Li ₂ O and Na ₂ O, Li ₂ O and K ₂ O, etc.) are contained, a mixed alkali effect peculiar to glass can be expressed, and glass manufacturability and water resistance are improved. it can. However, since excessive addition of R ₂ O significantly increases the volatilization amount of B during glass production, it is necessary to be careful.

In addition, by setting the density of the neutron absorbing glass to 3.0 to 3.7 g / cm ³ , the neutron absorbing glass can be stably settled even if it is put into water. Moreover, by suitable shape and size, it does not fly underwater by the circulation of water, but can be deposited on molten fuel stably. Examples of the shape include (a) spherical, (b) tablet, (c) granular, or (d) bead-like neutron absorbing glass 1 as shown in FIG. Unlike ceramics, glass is a material with good thermoformability, and thus the shapes (a) to (d) can be manufactured at low cost.

  A suitable average size is desirably less than 10 mm mesh and 1 mm mesh or more. If the size is too large, there is a possibility that the molten fuel will not catch on the molten fuel due to being caught in the middle or difficult to contact the molten fuel. On the other hand, if the size is too small, there is a possibility that the water will fly underwater. A more preferred average size is less than 7 mm mesh and 3 mm mesh or more.

The neutron absorbing material of the present embodiment is a neutron absorbing material 3 obtained by sintering B ₄ C (boron carbide) particles 2 containing a large number of B having high neutron absorption performance with neutron absorbing glass 1 ′ as shown in FIG. . Further, as shown in FIG. 3, a neutron absorbing material 3 ′ in which the surface of granular B ₄ C particles 2 ′ is covered with neutron absorbing glass 1 ′. B ₄ C is one of the commonly known neutron absorbing materials and is widely used in nuclear reactors as a neutron shielding material and a nuclear reaction control material. For example, in boiling water reactors, control rods packed with B ₄ C are used to control nuclear fission reactions during normal operation and emergencies. However, B ₄ C is difficult to sinter by itself, and B may elute into water due to surface oxidation or the like, which may create an acidic corrosive environment. By combining such B ₄ C and the neutron absorption glass of the present embodiment, it becomes easy to obtain a desired shape and size, and good water resistance and neutron absorption performance can be obtained.

  Next, a molten fuel management method, a molten fuel take-out method, and a reactor shutdown method according to this embodiment will be described. FIG. 4 shows a state in which a neutron absorber 18 (neutron absorbing glass 1 or neutron absorbing material 3 or 3 ′) is in contact with the surface of the molten fuel 5 accumulated in the reactor pressure vessel or leaking into the containment vessel. Indicates. The neutron absorber 18 is introduced into the underwater 4 from above the molten fuel 5 managed in the underwater 4. When the neutron absorber 18 is in contact with the molten fuel 5 or is present in the vicinity of the molten fuel 5, the neutron from the molten fuel 5 can be absorbed and the subcritical state of the molten fuel 5 can be maintained.

  FIG. 5 shows a schematic cross-sectional view of a method for safely taking out molten fuel in which the uranium pellets and the like in the fuel rod are dissolved together with surrounding structures to the outside of the reactor. The molten fuel 5 is excavated from the state shown in FIG. 4 (the state where the neutron absorber 18 is in contact with the surface of the molten fuel 5). Molten fuel 5 'flies underwater due to excavation, but the neutron absorber 18 also flies along with the molten fuel 5' to prevent recriticality and safely take out the molten fuel 5 'outside the reactor. Can do. In addition, if the excavator 8 having the suction pipe 7 around the drill 6 is used, the excavated molten fuel 5 ′ can be sucked while being cut, so the amount scattered around is reduced, and the outside of the reactor can be more safely removed. It is possible to take it out.

  Further, the reactor shutdown method of the present embodiment is a method of shutting down the reactor in an emergency, and the neutron absorbing glass or neutron absorbing material of the present embodiment is introduced into the reactor, When the neutron absorbing glass or neutron absorbing material of the present embodiment is deposited around the fuel rod, it is possible to prevent reaching the criticality.

  Hereinafter, glass manufacturability (ease of production), water resistance, density, characteristic temperature, and neutron absorption performance, which are evaluation items of neutron absorption glass, will be described.

  The manufacturability of neutron absorbing glass was evaluated in the state of glass produced at 1300-1400 ° C. The glass was mixed and mixed with a predetermined amount of glass raw material 500 g in a crucible, heated to 1300-1400 ° C. at a heating rate of about 10 ° C./min in an electric furnace, and melted. At that time, the mixture was held for 2 to 3 hours with stirring to make the glass uniform. Thereafter, the crucible was taken out from the electric furnace, and the melt therein was poured into a stainless steel jig that had been heated to about 250 ° C. in advance to produce glass.

  In such a glass production condition, it was evaluated as a pass “◯” when it was in a uniform transparent glass state, and a “fail” when it was crystallized (white turbidity). Moreover, even if it was in the state of uniform transparent glass, it evaluated as "(triangle | delta)" when there was much volatilization at the time of glass preparation, or when high temperature viscosity was high and it was difficult to pour. If the glass productivity is good, good thermoformability is obtained, and neutron absorbing glasses having various shapes and sizes are easily obtained as shown in FIG.

  The water resistance of the neutron absorbing glass was judged by the state of the glass after the glass prepared in an aqueous solution having a salt concentration of 0.9% by weight was boiled for 3 hours. When the introduced glass did not change in appearance and no corrosion was observed, it was evaluated as “good”, and when the surface of the glass was white burned or the glass was structurally broken, it was evaluated as “failed”. Further, the pH of water after the test was also measured, and even if no change in appearance was observed, it was set as “Δ” if it was acidic.

  The density of the neutron absorbing glass was measured by a pycnometer method using glass as powder and helium gas.

Characteristic temperature of the neutron absorbing glass, the glass powder was measured transition point T _g and yield point M _g by differential thermal analysis (DTA). FIG. 6 shows a DTA curve of a typical glass. Starting temperature transition point T _g of the first endothermic peak, the peak temperature is yield point M _g. These characteristic temperature is defined by the viscosity, T _g 10 ^13.3 poise, M _g is equivalent to a temperature at which the viscosity of 10 ¹¹ poise.

The neutron absorption performance of the neutron absorption glass is determined by using the quantities of Gd element, B element and Li element per unit volume obtained from the composition and density of the glass, and the neutron absorption cross sections of each element shown in Table 1. The neutron absorption cross section per unit volume was calculated, and when the density was equal to or greater than B ₄ C of 2.52 g / cm ³ , it was evaluated as “good”, and when it was lower, it was evaluated as “failed”.

  Hereinafter, it demonstrates in detail using an Example. However, the present invention is not limited to the description of the embodiments taken up here.

In Example 1, the composition and characteristics of neutron absorbing glass were examined. Examples are shown in Table 2, and Comparative Examples are shown in Table 3. For the production of glass, the reagents Gd ₂ O ₃ , B ₂ O ₃ , SiO ₂ , ZrO ₂ , Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , Al ₂ O ₃ manufactured by High Purity Chemical Laboratory are used as raw materials. ZnO, MgO, CaCO ₃ , SrCO ₃ , and BaCO ₃ were used.

As shown in Table 2, the glasses of Examples A-01 to 22 passed all of the neutron absorption performance, water resistance and glass manufacturability. Also, the density is in the range of 3.0 to 3.7 g / cm ³ , and it is possible to settle stably even if it is put into water. B ₄ C has a density of 2.52 g / cm ³ , which is larger than that. Furthermore, since no higher transition point T _g and sag M _g is a characteristic temperature, secondary processing is likely due to heat. Specifically, (a) the spherical shape shown in FIG. 1 can be heated and pressed (b) into a tablet shape, or the cullet can be heated (c) into a granular shape.

In contrast to Examples A-01 to 22 shown in Table 2, in Comparative Examples B-01 to 26 in Table 3, some of the glass fabrication properties were passed, but the water resistance and neutron absorption performance were No one was able to pass both. Comparative Example B-01 was a general borosilicate glass, and had good glass manufacturability and water resistance. However, since it does not contain a Gd element having a large neutron absorption, the neutron absorption cross-sectional area ratio is extremely small as compared with Examples A-01 to 22, and the neutron absorption performance of B ₄ C is inferior. Moreover, the density of Comparative Example B-01 was smaller than B ₄ C. Comparative Example B-from 02 to 04 is a Gd ₂ O ₃ -B ₂ O ₃ based glass or a _{Gd 2 O 3 -B 2 O 3} -SiO 2 based glass, glass manufacture was good, B ₂ The O ₃ content was very high and the water resistance was insufficient. Comparative Example B-05 was also a Gd ₂ O ₃ —B ₂ O ₃ —SiO ₂ glass, but had a high high temperature viscosity and poor glass manufacturability. Further, the water resistance was not sufficient.

Comparative Examples B-06 to 12 are generally well known Al ₂ O ₃ , ZnO or Gd ₂ O ₃ —B ₂ O ₃ —SiO ₂ based glass in order to improve the water resistance and glass manufacturability. Contains alkaline earth oxides. However, it became cloudy due to crystallization, and sufficient water resistance could not be obtained. Comparative Examples B-13 to 19 examined the inclusion of alkali metal oxides, but only Comparative Example B-13 containing no Al ₂ O ₃ , ZnO and alkaline earth oxides was a uniform transparent glass. . However, since it contains a large amount of alkali metal oxides, the amount of volatilization of B ₂ O ₃ is large and the water resistance is insufficient. Other Comparative Examples B-14 to 19 cause white turbidity (crystallization) due to the inclusion of Al ₂ O ₃ , ZnO or alkaline earth oxides as well as Comparative Examples B-06 to 12, and water resistance Was not good. In Comparative Examples B-20 to 26, a system further containing ZrO ₂ was examined. However, all of them were clouded (crystallized), and the glass production was not good. Moreover, it cannot be said that these water resistance is also favorable, and white turbidity (crystallization) may have a bad influence on water resistance.

From the above examination results of Examples A-01 to 22 and Comparative Examples B-01 to 26, as neutron absorbing glass that can be put into water, gadolinium oxide, boron oxide, silicon oxide, zirconium oxide, and alkali metal oxides are used. wherein, B ₂ O ₃ is 40 to 60 mol% in the following in terms of oxide and R ₂ O: by (R alkali metal) is 5 to 20 mol%, the neutron absorbing performance, water resistance and glass manufacturability I found that I could improve everything. In order to improve water resistance, the boric acid abnormality phenomenon peculiar to glass by containing both B ₂ O ₃ and R ₂ O was utilized.

  In Example 2, the shape and size of the neutron absorption glass were examined. Since glass has good moldability by heat, we made neutron-absorbing glasses of various shapes and sizes. First, (a) spherical neutron absorbing glass 1 shown in FIG. 1 was produced. As this neutron absorbing glass 1, the glass of Example A-11 in Table 2 was used. The equipment used is shown in FIG. This equipment is basically the same as equipment for producing marbles.

In FIG. 7, the neutron absorption glass of Example A-11 was melted at 1300 to 1400 ° C. in a glass melting furnace 11, and the stirring blade 12 was rotated to make the molten glass 13 uniform. By raising the plunger 14 from the lower part of the glass melting furnace 11, a predetermined amount of the molten glass 13 is poured out, sequentially cut by the cutters 15 and 15 ', and dropped between the rotating forming rolls 16 and 16'. It was. In order to make the molten glass 13 into a spherical shape, semi-circular grooves are continuously formed on the surfaces of the forming rolls 16 and 16 ', and the grooves face each other. The molten glass 13 that passed between the forming rolls 16 and 16 ′ was cooled and turned into a spherical neutron absorbing glass 1. Then, to remove thermal strain of the neutron-absorbing glass 1 of the resulting spherical, it was performed stress relief at a temperature slightly higher than the transition point T _g. Transition point T _g of the Example A-11 is because it is 532 ° C., and stress relief heat treatment at about 550 ° C.. By removing the thermal strain, the mechanical strength and water resistance of the neutron absorbing glass can be improved.

  The average size of the spherical neutron-absorbing glass 1 is roughly determined by the flow rate of the molten glass 13 from the glass melting furnace 11, the cutting speed of the cutters 15 and 15 ', and the groove size of the surfaces of the forming rolls 16 and 16'. Can be controlled. In this example, the diameter was adjusted to about 5 mm. Then, using a 10 mm mesh and a 1 mm mesh sieve, a size of less than 10 mm mesh and 1 mm mesh or more was obtained. With such a size, a spherical neutron absorbing glass can be obtained with a high yield. When the size is 10 mm or more, there is a possibility that the molten fuel may not be spread over the molten fuel due to being caught in the middle when it is poured into the water or difficult to contact the molten fuel. On the other hand, if the size is less than 1 mm, there is a risk that the water will drift underwater. Preferably, a 7 mm mesh and a 3 mm mesh sieve are used and the mesh size is less than 7 mm and 3 mm mesh or more.

  Next, (b) a tablet-like neutron absorbing glass shown in FIG. 1 was produced in the same manner as described above using the glass of Example A-11. The tablet-shaped neutron absorbing glass was produced by hot pressing the spherically produced neutron absorbing glass. Thereafter, in the same manner as described above, the strain was removed and sieved to obtain a desired size. Since the tablet shape is less likely to roll than the spherical shape, handling is easy. Further, since the surface area per weight is larger than that of a sphere, an improvement in neutron absorption performance can be expected.

  The (c) granular neutron absorbing glass shown in FIG. 1 was also produced using the glass of Example A-11 in the same manner as described above. First, the glass of Example A-11 was melted and produced, and pulverized to an appropriate size cullet by a crusher. The cullet was heated to about 800 ° C. in a tunnel furnace, and the edges were rounded to form particles. At that time, the same tunnel furnace was also used for strain relief. Thereafter, a sieve was applied in the same manner as described above to obtain a desired size.

  The (d) bead-like neutron absorbing glass shown in FIG. 1 was also produced using the glass of Example A-11 in the same manner as described above. First, a glass tube having a diameter of about 5 mm was produced from the glass of Example A-11. The glass tube was scratched at intervals of about 5 mm and cut by thermal shock. This was heated to about 800 ° C. in a tunnel furnace in the same manner as described above, and the edges were rounded to form beads. At that time, the same tunnel furnace was also used for strain relief. Thereafter, a sieve was applied in the same manner as described above to obtain a desired size. In the bead shape, the surface area can be further increased, which is considered to contribute to the improvement of neutron absorption performance.

In Example 3, the combination of neutron absorption glass and B ₄ C was studied. Neutron-absorbing glass and B ₄ C powder were mixed, molded in a mold, and heated in a low-oxygen atmosphere to produce a sintered body of neutron-absorbing material 3 shown in FIG. The reason for heating in a low oxygen atmosphere is to suppress or prevent the oxidation of B ₄ C as much as possible. As the neutron absorbing glass, the glass of Example A-07 shown in Table 2 was used and pulverized to 30 μm or less by a stamp mill and a jet mill. B ₄ C used a commercially available powder of 150 μm or less. 30% by volume of the neutron-absorbing glass powder of Example A-07 and 70% by volume of B ₄ C powder were mixed and mixed, using a mold with a diameter of 5 mm and a thickness of 5 mm under the conditions of 1 ton / cm ² . A large number of cylindrical shaped bodies were produced. These compacts were passed through a tunnel furnace in a low-oxygen atmosphere, and the glass powder of Example A-07 was softened and fluidized at about 800 ° C. to produce a sintered body of a neutron absorbing material. The obtained sintered body had a volume shrinkage of about 10 to 20%.

As a result of carrying out the same water resistance test as in Example 1 using the obtained sintered body, good water resistance was obtained without being corroded. Further, since both the glass of Example A-07 and B ₄ C have a large neutron absorption cross section per unit volume, the neutron absorption performance is good. On the other hand, B ₄ C alone may react with water gradually in water to produce boric acid, resulting in an acidic corrosive environment. By combining with neutron absorbing glass, the area where B ₄ C comes into contact with water can be reduced, and the water resistance of neutron absorbing glass is high, so that B does not easily dissolve even when exposed to water for a long time. Moreover, it becomes easy to make a sintered body of B ₄ C. Furthermore, the density can be increased compared with the case of using B ₄ C alone, and it has the feature that it is difficult to move due to the water flow. Moreover, this neutron absorbing material is not limited to the application to be put into water, but can be developed for substitution of B ₄ C powder loaded in a control rod, substitution of a B ₄ C sintered body used in a fast reactor, and the like.

In Example 4 as well, in the same manner as in Example 3, the combination of neutron absorbing glass and B ₄ C was studied, and the neutron absorbing material 3 ′ shown in FIG. As the neutron absorbing glass 1 ′, the glass of Example A-16 shown in Table 2 was used. In addition, commercially available granular particles of 1 to 3 mm were used as the B ₄ C particles 2 ′. FIG. 8 shows the equipment used for producing the neutron absorbing material 3 ′ shown in FIG. In FIG. 8, the equipment shown in FIG. 7 is devised so that granular B ₄ C particles 2 ′ can be put into the molten glass 13 at 1300 to 1400 ° C. from the plunger 14. The B ₄ C particles 2 ′ were introduced into the upper container 17 of the glass melting furnace 13 and heated with the residual heat of the glass melting furnace 13. Further, in order to prevent oxidation of B ₄ C, the inside of the container 17 was made an inert atmosphere. Granular B ₄ C particles 2 ′ were sequentially dropped from the container 17, and were poured from the lower part of the glass melting furnace 11 together with the molten glass 13. It was cut with cutters 15 and 15 ′ and dropped between forming rolls 16 and 16 ′ in the same manner as in Example 2 to produce a spherical neutron absorbing material 3 ′ as shown in FIG.

In FIG. 3, neutron absorption glass is applied to the surface portion of one B ₄ C particle. However, in Example 4, there were many cases where a plurality of B ₄ C particles entered. This is not a problem unless the size of the neutron absorbing material 3 ′ becomes too large. Thereafter, the resulting neutron absorbing material 3 'was subjected to heat treatment slightly temperatures greater about 550 ° C. above the transition point T _g of the Example A-16, the neutron absorbing glass 2' to remove heat distortion.

The same water resistance test as that of Example 1 was performed using the obtained neutron absorbing material. As a result, good water resistance was obtained without being corroded. Further, regarding the neutron absorption performance, it goes without saying that both the glass of Example A-16 and B ₄ C have a large neutron absorption cross-sectional area per unit volume, and thus are good. The fourth embodiment, as compared to Example 3, and the neutron absorbing glass powder, B ₄ C powder and uniformly mixed, molded, does not require firing, the neutron absorbing glass and B ₄ C The neutron absorbing material can be manufactured at low cost. Further, as in Example 3, this neutron absorbing material is not limited to the application to be put into water, but it is an alternative to the B ₄ C powder loaded in the control rod or the B ₄ C sintered body used in the fast reactor. There is a possibility that it can also be deployed.

  In the fifth embodiment, an example of a molten fuel management method to which the neutron absorbing glass or the neutron absorbing material studied in the first to fourth embodiments will be described.

  In order to maintain the subcriticality of the molten fuel and improve safety, neutron absorbing glass or neutron absorbing material is introduced into the reactor. In FIG. 4, a lump of molten fuel 5 is submerged in water 4, and a neutron absorber 18 (neutron absorbing glass, neutron absorbing material) is introduced into water 4 so as to directly cover the upper surface of the lump of molten fuel 5. Touching. Since the density of the neutron absorber 18 is sufficiently larger than that of water, it is likely to deposit on the surface of the molten fuel 5. Further, when there are cracks in the molten fuel 5 mass or when there is a gap between the molten fuel 5 masses, the neutron absorber 18 enters these cracks or gaps. Thus, even if a positive reactivity is applied to the molten fuel 5 for some reason, the neutrons generated from the molten fuel 5 are shielded and the chain reaction is suppressed so that the criticality is not reached. can do. It is effective to make the size of the neutron absorber 18 smaller than the mass of the molten fuel 5.

  Moreover, although neutron absorption glass is uniform transparent glass, it has the characteristic to color by irradiation of a neutron. Since the degree of coloring tends to increase as the amount of neutron irradiation increases, the position of the molten fuel in the nuclear reactor is detected by examining the degree of coloring of the neutron absorbing glass of the present invention introduced into the nuclear reactor. It is also possible to make predictions.

  In the sixth embodiment, an example of a method for taking out molten fuel using the neutron absorbing glass or the neutron absorbing material studied in the first to fourth embodiments will be described.

  As shown in FIG. 5, the neutron absorber 18 is introduced into the reactor so as not to cause recriticality when the molten fuel 5 is taken out. The molten fuel 5 is crushed by the drill 6 of the excavator 8 and the particulate molten fuel 5 ′ is sucked through the suction pipe 7 of the excavator 8. At this time, a part of the excavated particulate molten fuel 5 ′ may not be sucked into the suction pipe 7 of the excavator 8 and may be scattered in the surrounding water 4. In this state, there is a possibility that the volume ratio of the particulate molten fuel 5 ′ in the water 4 changes and becomes recritical. Therefore, the neutron absorber 18 is also scattered together with the particulate molten fuel 5 ′ scattered in the water 4 so that neutrons in the water 4 can be absorbed and blocked. Thereby, a chain reaction can be suppressed and it can prevent reaching a recriticality also during excavation work. Further, even if the neutron absorber 18 is damaged by cutting with the drill 6 of the excavator 8 during excavation work, the neutron absorption performance is not impaired.

  The molten fuel has been described by taking an example of a method of excavating with a drill, but the excavating method may be a power shovel and is not limited to an excavator.

  In the seventh embodiment, an example of controlling the nuclear fission reaction by introducing neutron absorbing glass or neutron absorbing material will be described.

  Conventionally, there is a method of injecting boric acid water into the core of a nuclear reactor as one of the methods for emergency shutdown of a nuclear reactor other than a control rod. However, when boric acid water is introduced into the core, there is a possibility that the inside of the furnace may become an acidic corrosive environment.

  Therefore, instead of injecting boric acid water, the neutron absorbing glass or neutron absorbing material described above is introduced so that the neutron absorbing glass or neutron absorbing material is deposited around the fuel rod inside the reactor. Thereby, the nuclear fission reaction of the nuclear reactor can be controlled and the nuclear reactor can be stopped urgently. Further, when neutron absorbing glass or neutron absorbing material is used, it is possible to prevent boric acid from eluting into the water inside the reactor, or to prevent pH from being lowered even if boric acid is eluted. For this reason, corrosion of the reactor internal structure can be prevented and the reaction of the nuclear fuel can be continuously suppressed, so that the nuclear reactor can be stopped for a long period of time.

1, 1 ': Neutron absorption glass
2, 2 ': B ₄ C particles
3, 3 ': Neutron absorbing material
4: Underwater
5, 5 ': Molten fuel
6: Drill
7: Suction tube
8: Cutting machine
11: Glass melting furnace
12: Stir feather
13: Molten glass
14: Plunger
15, 15 ': Cutter
16, 16 ': Forming roll
17: Container
18: Neutron absorber

Claims (14)

In neutron absorbing glass that can be put into water, it contains gadolinium oxide, boron oxide, silicon oxide, zirconium oxide and alkali metal oxides, and B ₂ O ₃ is converted to 40 to 60 mol% and R ₂ O (in terms of the following oxides) A neutron-absorbing glass characterized by containing 5 to 20 mol% of R: alkali metal). In the claims 1, neutron absorbing glass, wherein Gd ₂ O ₃ in the following in terms of oxide 5-12 mole%, the SiO ₂ 8 to 35 mol% and in that it comprises ZrO ₂ 3 to 12 mol%. 3. The method according to claim 1, wherein Gd ₂ O ₃ is 6 to 9 mol%, B ₂ O ₃ is 45 to 55 mol%, SiO ₂ is 15 to 30 mol%, and ZrO ₂ is 5 to 10 in terms of the following oxides. mol% and neutron absorbing glass which comprises the R ₂ O 5 to 15 mol%. In any one of claims 1 to 3, a neutron absorbing glass wherein R ₂ O is characterized in that it comprises at least Li ₂ O. 5. The neutron absorbing glass according to claim 1, wherein the neutron absorbing glass has a density of 3.0 to 3.7 g / cm ³ .   6. The neutron absorbing glass according to claim 1, wherein the neutron absorbing glass has a granular shape, a spherical shape, a tablet shape, or a bead shape.   7. The neutron absorbing glass according to claim 1, wherein an average size of the neutron absorbing glass is less than 10 mm mesh and 1 mm mesh or more.   The neutron absorption glass according to any one of claims 1 to 7, wherein an average size of the neutron absorption glass is less than 7 mm mesh and 3 mm mesh or more. A neutron absorbing material comprising B ₄ C powder and the neutron absorbing glass according to claim 1. A neutron-absorbing material characterized in that the surface of granular B ₄ C is coated with the neutron-absorbing glass according to claim 1.   In the method for managing the molten fuel in the reactor pressure vessel or the containment vessel, the neutron absorbing glass according to any one of claims 1 to 8 or the neutron absorbing material according to claim 9 or 10 is applied to the molten fuel disposed in water. The molten fuel management method, wherein the molten neutron absorbing glass or the neutron absorbing material is brought into contact with the surface of the molten fuel.   In the method of taking out the molten fuel inside the nuclear reactor to the outside of the nuclear reactor, the molten fuel in which the neutron absorbing glass according to any one of claims 1 to 8 or the neutron absorbing material according to claim 9 or 10 is disposed in water. The molten fuel is taken out from the reactor, the neutron absorbing glass or the neutron absorbing material is brought into contact with the surface of the molten fuel, the molten fuel is crushed and taken out of the reactor.   13. The method for taking out molten fuel according to claim 12, wherein the molten fuel is taken out of the reactor by being sucked while being crushed.   In the method of shutting down a nuclear reactor, the neutron absorbing glass according to any one of claims 1 to 8 or the neutron absorbing material according to claim 9 or 10 is introduced into the reactor, and the neutron absorbing glass or the neutron absorbing material is provided. A method of shutting down a nuclear reactor, comprising depositing around a fuel rod inside the nuclear reactor.

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