Patent Application
20160155521
The present invention relates to neutron-absorbing glass and a neutron-absorbing material using the neutron-absorbing glass, and a management method of corium, an unloading method of corium, and a shutdown method of a nuclear reactor using the neutron-absorbing glass and the neutron-absorbing material.
In nuclear power plants such as boiling water nuclear power plants and pressurized water nuclear power plants, a plurality of fuel assemblies containing a nuclear fuel material (uranium pellet) is loaded in a reactor core of a nuclear reactor. When fuel assemblies are unloaded. In a normal operation cycle, the size of each fuel assembly is designed such that a state of criticality is not reached alone and thus, if the fuel assemblies are unloaded one by one, there is no danger of criticality and the fuel assemblies can be unloaded safely.
However, if an accident in which the nuclear fuel material (uranium pellet) contained in a fuel assembly loaded in a nuclear core of a nuclear reactor melts should occur like the nuclear power ant in the Three Mile Island nuclear power plant, a method of safe management by preventing a state of criticality of the molten nuclear fuel material (hereinafter, referred to as “corium”) from arising is needed. The corium remains inside a reactor pressure vessel or is leaked into a containment vessel thereof. Further, the corium is a result of melting of the uranium pellet in a fuel rod inside the nuclear reactor together with a surrounding structure. It is necessary to cut and unload the corium out of the nuclear reactor and also at this point, the unloading method to prevent a state of criticality from occurring is indispensable.
Patent Literature 1 proposes a glass composition containing SiO2, B2O3, Al2O3, La2O3, Gd2O3 and the like for transparent window glass having capabilities to block radiation such as X rays, γ rays and the like. An embodiment of Patent Literature 1 concretely discloses seven glass compositions in which SiO2 is in the range of 18 to 30 mol %, B2O3 is in the range of 18 to 38 mol %, Al2O3 is in the range of 2.8 to 19.8 mol %, La2O3 is in the range of 6 to 13 mol %, and Gd2O3 is in the range of 15 to 20 mol %.
Patent Literature 1: JP 2009-7194 A
B (boron) generates boric acid by reacting with water thus, if glass containing B is present in water, B may be dissolved from the glass to generate boric acid, which is dissolved in water. If boric acid is dissolved in water, the inside of a reactor becomes an acidic corrosive environment and structures inside the reactor and peripheral devices may be more likely to be corroded.
The glass composition described in Patent Literature 1 improves detergent resistance and acid resistance so that burning should not arise even after cleaning. The glass composition contains large quantities of Gd (gadolinium) and B absorb a large quantity of neutrons and so can absorb neutrons, but water resistance of the glass is low and the glass poses a problem that B is dissolved when used for a long time in a state soaked in water.
An of the present invention is to improve water resistance of neutron-absorbing glass.
To achieve the object the present invention is characterized in that neutron-absorbing glass that can be input into water includes gadolinium oxide, boron oxide and zinc oxide, wherein B2O3 is contained 42 to 65 mol % in terms of oxide above.
According to the present invention, water resistance of neutron-absorbing glass can be improved.
The present invention relates to neutron-absorbing glass and a neutron-absorbing material used in a nuclear reactor in which water is used as a moderator and suitable particularly when used in water inside the reactor. The present invention also relates to a management method of corium to which the neutron-absorbing glass or the neutron-absorbing, material is applied, an unloading method of the corium, and a shutdown method, of a nuclear reactor.
Hereinafter, an embodiment of the present invention will be described.
Neutron-absorbing glass according to the present embodiment contains gadolinium oxide, boron oxide, and zinc oxide and good water resistance and neutron absorption performance can be obtained by B2O3 being contained 42 to 65 mol % in terms of oxide above. With improved water resistance, B having absorbed neutron is less likely to be dissolved in water, which makes it easier to treat or dispose of water. When described in the specification that the oxide is “x to y mol %”, this means that the oxide is “x mol % or more and y mol % or less” (x mol %≦oxide≦y mol %). This also applies below.
Gd is expensive, but is an element having a neutron absorption cross-sect on about 60 times as large as that of B and can increase the quantity f absorbed neutrons by Gd being contained in glass. Gadolinium oxide and boron oxide are mainly in charge of neutron absorption. Table 1 shows elements whose neutron absorption is large and their neutron absorption cross-sections. Though dependent also on the state of irradiated neutrons, an element with an increasing neutron absorption cross section tends to have higher neutron absorption performance.
The content (42 to 65 mol %) of B2O3 is substantially increased to increase the quantity of absorbed neutrons, but at the same water resistance decreases and thus, zinc oxide having an effect of improving water resistance is contained. Glass productivity is also improved by containing zinc oxide. Further, having the total of Gd2O3, B2O3, and ZnO 70 mol % or more in terms of oxide above is effective to achieve all of glass productivity, water resistance, and neutron absorption performance.
The preferable composition range of neutron-absorbing glass is: Gd2O3 is 5 to 13 mol %, B2O3 is 42 to 65 mol %, ZnO is 5 to 45 mol %, and the total of at least one of Al2O3, ZrO2, and R2O (R: alkali metal) is 0 to 30 mol % in terms of oxide above. By selecting Gd2O3 and B2O3 in the above ranges, high neutron absorption performance is exhibited. Moreover, glass can be produced without being crystallized and thus, glass productivity is improved. By selecting ZnO in the above range, water resistance and glass productivity can be improved. In addition, by containing appropriate quantities of Al2O3, ZrO2, and R2O, water resistance can be improved without promoting crystallization, that is, lowering glass productivity. However, if the contents are too high, high-temperature viscosity increases for Al2O3 and crystallization occurs for ZrO2, which lowers glass productivity. In the case of R2O, problems of a dramatically increased volatilization amount of B2O3 or conversely reduced water resistance may arise.
Particularly preferably, the total of Gd2O3 and B2O3 is 52 to 70 mol % and the total of ZnO, Al2O3, ZrO2, and R2O is 30 to 48 mol % and that Gd2O3 is 5 to 10 mol %, B2O3 is 47 to 60 mol %, ZnO is 10 to 40 mol %, Al2O3 is 0 to 20 mol %, ZrO2 is 0 to 15 mol %, and R2O is 0 to 15 mol % in terms of oxide above is particularly effective for all of neutron absorption performance, water resistance, and glass productivity.
It is desirable that R2O be at least Li2O. As shown in Table 1, Li has a neutron absorption cross-section smaller than that or Gd or B, but is one of elements having a large quantity of absorbed neutrons and thus, neutron absorption performance can be improved by containing Li2O in neutron-absorbing glass. When two types or more of R2O (for example, Li2O and Na2O, Li2O and K2O) are contained, a mixed alkali effect specific to glass can be exhibited so that glass productivity and water resistance can be improved. However, if the quantity of R2O is too much, the volatilization amount of B may be extremely increased or conversely water resistance may be lowered during glass production and thus, care must be taken. Further, containing R2O can exhibit an abnormal boric acid phenomenon specific to glass so that elution of B into water can be limited or prevented.
By setting the density of neutron-absorbing glass to 3.2 to 4.7 g/cm3, glass can be allowed to sink in a stable manner when input into water. By selecting an appropriate shape and size, glass can be deposited on corium in a stable manner without dancing in water due to the circulation of water. As the shape of neutron-absorbing glass 1, as shown in
An appropriate average size is desirably less than 10 mm mesh and 1 mm mesh or more. If the size is too large, glass may be caught during input or it may be difficult for glass to come into contact with corium so that glass may not be distributed over the entire corium. On the other hand, if the size is too small, glass may dance in water due to a stream. A particularly preferable average size is less than 7 mm mesh and 2 mm mesh or more.
The neutron-absorbing material according to the present embodiment is, as shown in
Next,the management method of corium, the unloading method of the corium, and the shutdown method of a nuclear reactor will be described.
The shutdown method of a nuclear reactor according to the present embodiment is a method of shutting down a nuclear reactor in an emergency and a state of criticality can be prevented from being reached by inputting neutron-absorbing glass or neutron-absorbing materials according to the present embodiment into the nuclear reactor so that the neutron-absorbing glass or neutron-absorbing materials according to the present embodiment are piled up around fuel rods inside the nuclear reactor.
Hereinafter, glass productivity (ease of production), water resistance, density, characteristic temperature, and neutron absorption performance, which are evaluation items of the neutron-absorbing glass, will be described.
The productivity of neutron-absorbing glass is evaluated in a glass state produced at 1300 to 1400° C. Glass materials of 500 g in which predetermined amounts are formulated and mixed are put into a crucible and heated up to 1300 to 1400° C. at a rate of temperature rise of about 10° C./min in an electric furnace to melt the glass materials. The glass materials are stirred to make the glass uniform during the process and held for 2 to 3 hours. Then, the crucible is taken out of the electric furnace and the melt therein is poured into a stainless jig pre-heated to about 250° C. to produce glass.
If transparent glass is obtained under the above glass production conditions, the glass is evaluated as acceptable “◯” and if glass is crystallized (opacity), the glass is evaluated as unacceptable “×”. Even if the glass is in an uniform transparent glass state, if the quantity of volatility is large during glass production or if the glass has high-temperature viscosity and it is difficult to pour the glass, the glass is evaluated as “Δ”. If the glass productivity is good, good formability by heat is obtained and neutron-absorbing glass of various shapes as shown in
The water resistance of neutron-absorbing glass is determined based on a state of produced glass after putting the glass into an aqueous solution whose salinity concentration is 0.9% by weight and boiling the solution for three hours. If there is no change in appearance of glass and no corrosion is recognized, the glass is evaluated as acceptable “◯” and if dimming arises on the surface of glass or the structure is glass collapses, the glass is evaluated as unacceptable “×”. In addition, pH of water after the test is measured and if the water is acidic, the glass is evaluated as “Δ” even if there is no change in appearance.
The density of neutron-absorbing glass is measured by grinding the glass to powder and using a pycnometer method using a helium gas.
As characteristic temperatures of neutron-absorbing glass, the glass is ground to powder and a transition point Tg and a yield point Mg are measured based on differential thermal analysis (DTA).
For the neutron absorption performance of neutron-absorbing glass, the neutron absorption cross-section per unit volume is calculated using the quantities of the Gd element, B element, and Li element per unit volume determined from the composition and density of glass and the neutron absorption cross-sections of respective elements shown in Table 1 and if the density is equal to or larger than 2.52 g/cm3 of B4C, the glass is evaluated as acceptable “◯”, and if the density falls below 2.52 g/cm3, the glass is evaluated as unacceptable “×”.
Hereinafter, further details will be provided using examples. However, the present invention is not limited to examples described here.
In Example 1, the composition and characteristics of neutron-absorbing glass are examined. The example is shown in Table 2 and a comparative example is shown in Table 3. For the production of glass, reagents Gd2O3, B2O3, ZnO, Al2O3, ZrO2, Li2CO3, Na2CO3, K2CO3, SiO2, MgO, CaCO3, SrCO3, and BaCO3 manufactured by Kojundo Chemical Laboratory Co., Ltd. are used as materials.
As shown in Table 2, glass in Examples A-01 to 30 is all acceptable in neutron absorption performance, water resistance, and class productivity. Also, the density of glass is in the range of 3.2 to 4.7 g/cm3 and the glass can be allowed to sink in a stable manner after being input into water. The density of B4C is 2.52 g/cm3 and the density of glass is larger than this density. Further, the transition point Tg and the yield point Mg as characteristic temperatures are not high, which makes secondary treatment by heat easier. More specifically, (a) spherical shown in
In Comparative Examples B-01 to 25 in Table 3, in contrast to Examples A-01 to 30 shown in Table 2, some comparative examples are acceptable in glass productivity, but none is acceptable in both water resistance and neutron absorption performance. Comparative Examples B-01, B-02 are common borosilicate glass and zinc borate glass and are excellent in glass productivity and water resistance. However, the Gd element that absorbs a large quantity of neutrons is not contained and thus, compared with Examples A-01 to 30, the neutron absorption cross-section ratio is extremely small and Comparative Examples B-01, B-02 are inferior to B4C in neutron absorption performance. The density of Comparative Example B-02 is larger than that of B4C, but the density of Comparative Example B-01 is smaller than that of B4C. Comparative Examples B-03 to 05 are Gd2O—B2O3 based glass or Gd2O3—B2O3—SiO2 based glass and glass productivity thereof is good, but the content of B2O3 is very high and water resistance thereof is insufficient. Comparative Example B-06 is also Gd2O3—B2O3—SiO2 based glass, but the content or SiO2 is high and thus, high-temperature viscosity thereof is large, leading to poor glass productivity. Also, water resistance thereof is not sufficient.
Comparative Examples B-07 to 18 contain generally known ZnO, Al2O3, ZrO2, or alkaline-earth oxide to improve water resistance and glass productivity of Gd2O3—B2O3—SiO2 based glass. However, crystallization leads to opacity and sufficient water resistance cannot be obtained. It turns out that inclusion of ZnO, Al2O3, ZrO2, or alkaline-earth oxide in Gd2O3—B2O3—SiO2 based glass does not lead to improvements of glass productivity and water resistance. Further, in Comparative Examples B-19 to 25, inclusion of alkali metal oxide is examined and only Comparative Example B-19 that contains none of Al2O3, ZnO, ZrO2, and alkaline-earth oxide becomes uniform transparent glass and in others, like in Comparative Examples B-07 to 18, crystallization leads to opacity. However, Comparative Example B-19 contains a large quantity of alkali metal oxide and thus, the volatilization amount of B2O3 is large and also water resistance is insufficient. Water resistance of Comparative Examples B-20 to 25 in which opacity (crystallization) occurs is not good like in Comparative Examples B-07 to 18.
From examination results Of Examples A-01 to 30 and Comparative Examples B-01 to 25 described above, we found that as neutron-absorbing glass that can be input into water, all of glass productivity water resistance, and neutron absorption performance can be improved containing gadolinium oxide, boron oxide, and zinc oxide and B2O3 is 42 to 65 mol % in terms of oxide above.
In Example 2, the shape and size of neutron-absorbing glass is examined. Formability by heat of glass is good and thus, the production of neutron-absorbing glass of various shapes and sizes is attempted. First, the (a) spherical neutron-absorbing glass 1 shown in
In
The average size of the neutron-absorbing glass 1 in a spherical shape can roughly be controlled the outflow amount of the molten glass 13 from the glass melting furnace 11, the cutting rate of the cutters 15, 15′, and the groove size on the surface of the forming rolls 16, 16′ In the present embodiment, the diameter thereof is adjusted to be about 5 mm. Then, the sizes or less than 10 mm mesh and 1 mm mesh or more are obtained by using sieves of 10 mm mesh and 1 mm mesh. In this range of size, neutron-absorbing glass in a spherical shape can be obtained at high yield rates. If the size thereof is 10 mm or more, glass may be caught while being input into water or it may be difficult for glass to come into contact with corium so that glass may not be distributed over the entire corium. If the size thereof is less than 1 mm, on the other hand, glass may dance in water due to a stream. Preferably, the size is set to less than 7 mm mesh and 2 mm mesh or more by using sieves of 7 mm mesh and 2 mm mesh.
Next, (b) tablet-shaped neutron-absorbing glass shown in
(c) Granular neutron-absorbing glass shown in
(d) Bead-shaped neutron-absorbing glass shown in
In Example 3, compounding neutron-absorbing glass and B4C is examined. Powder neutron-absorbing glass and powder of B4C mixed, molded in a die, and heated in a hypoxia atmosphere to produce a sintered body f a neutron-absorbing material shown in
As a result of performing the same water resistance test as that in Example 1 using the obtained sintered body, good water resistance without being corroded can be obtained. The glass of Example A-14 and B4C both has a large neutron absorption cross-section per unit volume and thus, neutron absorption performance is good. On the other hand, B4C alone may generate boric acid by gradually reacting with water in the water to create an acidic corrosive environment. By combining with neutron-absorbing glass, the area where B4C is in contact with water can be reduced and also, with high water resistance of the neutron-absorbing glass, B is less likely to be dissolved into water even after a long period of exposure to water. In addition, a sintered body of B4C can be produced more easily. Further, the density can be increased when compared with a case in which B4C is used alone, the sintered body is less likely to be moved by a stream. The neutron-absorbing material is not limited to the use of being input into water and may also be developed for replacement of B4C powder loaded into a control rod or for replacement of a B4C sintered body used in a fast reactor.
Also in Example 4, like in Example 3, compounding neutron-absorbing glass and B4C is examined and the neutron-absorbing material 3′ shown in
The same water resistance test as that of Example 1 is performed using the obtained neutron-absorbing material. As a result, good water resistance without being corroded can be obtained. Regarding the neutron absorption performance, the glass of Example A-25 and B4C both have a large neutron absorption cross-section per unit volume and thus, it is needless to say that neutron absorption performance is good. When compared with Example 3, there is no need to grind neutron-absorbing glass to powder and mix with B4C powder uniformly for molding and sintering in Example 4 and therefore, neutron-absorbing materials can be produced from neutron-absorbing glass and B4C at low costs. Also, like Example 3, the neutron-absorbing material is not limited to the use of being input into water and may also be developed for replacement of B4C powder loaded into a control rod or for replacement of a B4C sintered body used in a fast reactor.
In Example 5, an example of the management method of corium to which the neutron-absorbing glass or neutron-absorbing material according to the present invention examined in Examples 1 to 4 is applied described above will be described.
To maintain subcriticality of corium and to promote safety, the neutron-absorbing glass or neutron-absorbing material is input into a nuclear reactor. In
The neutron-absorbing glass is uniform transparent glass, but has a property of being colored when irradiated with neutrons. With an increasing amount of irradiation of neutrons, the degree or coloring tends to increase and thus, by checking the degree of coloring of neutron-absorbing glass according to the present invention input into a nuclear reactor, it becomes possible to detect or predict the location of corium inside the reactor.
In Example 6, an example of the unloading method of corium to which the neutron-absorbing glass or neutron-absorbing material examined in Examples 1 to 4 described above is applied will be described.
As shown in
The above corium is described by taking a method of digging by excavation using a drill as an example, but the method of digging is not limited to the excavator and may also use a power shovel.
In Example 7, an example of controlling a nuclear fission reaction of a nuclear reactor by inputting the neutron-absorbing glass or neutron-absorbing material will be described.
As a method of emergency shutdown of a nuclear reactor other than the control rod, a method of injecting water of boric acid into the nuclear core of the nuclear reactor has been known. However, the injection of water of boric acid into the nuclear core may change the inside of the reactor to an acidic corrosive environment.
Thus, instead of injecting water of boric acid, the neutron-absorbing glass or neutron-absorbing material is input such that the neutron-absorbing glass or neutron-absorbing material is deposited around fuel rods inside the nuclear reactor. Accordingly, an emergency shutdown of the nuclear reactor can be performed by control ling the nuclear fission reaction of the nuclear reactor. When the neutron-absorbing glass or neutron-absorbing material is used, boric acid can be prevented from eluting into water inside the nuclear reactor or if boric acid elutes, pH can be prevented from decreasing. Thus, corrosion of structures inside the reactor can be prevented and also the reaction of nuclear fuel can be continued to be inhibited and therefore, the nuclear reactor can be shut down for a long period of time.
1, 1′: Neutron-absorbing glass
2, 2′: B4C particle
3, 3′: Neutron-absorbing material
4: In water
5, 5′: Corium
6: Drill
7: Siphon
8: Excavator
11: Glass melting furnace
12: Stirring blade
13: Molten glass
14: Plunger
15, 15′: Cutter
16, 16′: Forming roll
17: Container
18: Neutron absorber
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/069578 | 7/19/2013 | WO | 00 |
