JP2013024826A - Control member for light water reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control member for a light water reactor in which recriticality caused by melted fuel rods can be effectively prevented by putting the control member as a neutron-absorbing control material into a reactor pressure vessel when a refrigeration system in the light water reactor has troubles causing meltdown.SOLUTION: Boron atoms 5 are used as a granular material which spreads in a hollow outer shell part 4 and floats in water.

Description

  The present invention relates to a light water reactor control member that is put into a light water reactor as a member that absorbs neutrons in an emergency and is effective in preventing recriticality when a fuel rod is melted.

In conventional light water reactors, control rods are automatically inserted between the fuel rods arranged in the reactor pressure vessel in response to an earthquake, and the nuclear fission reaction of the nuclear fuel in the fuel rods stops. Thereafter, the decay heat of the nuclear fuel is cooled. When the external power supply is lost due to the earthquake, the emergency diesel generator is started, and the operation of the cooling system facility is maintained by the generated power. However, if the emergency diesel generator and cooling system facilities are damaged by a tsunami after the earthquake, the reactor pressure vessel cannot be cooled, and the temperature of the primary cooling water in the reactor pressure vessel rises, generating steam. After a few hours, the upper part of the fuel rod is exposed on the water surface. When the fuel rod is exposed on the water surface and becomes 1200 ° C. or higher, the water / zirconium reaction in which hydrogen is generated by a chemical reaction between the zirconium alloy constituting the fuel cladding tube and water vapor becomes active. This is an exothermic reaction, and the temperature of the fuel cladding tube increases at an accelerated rate and exceeds the melting point (about 1800 ° C.) of the zirconium alloy in a short time. At about 2800 ° C., the nuclear fuel begins to melt.

  Debris-like debris may occur when the core melts as the fuel cladding tube or nuclear fuel melts and collapses. Further, there is a possibility that the outside of the fuel assembly structure or the like is covered with oxide (crust) that has been melted and solidified. When the shape of the nuclear fuel collapses into debris or oxide (crust), the boron control rod that absorbs neutrons melts at its melting point (about 2300 ° C) and mixes with debris and oxide (crust). However, depending on the progress of core melting, boron may be temporarily unevenly distributed in the nuclear fuel, resulting in insufficient neutron absorption. Criticality can occur. On the other hand, there is a possibility that part of the nuclear fuel that has fallen down is not a lump but is in the form of sand and accumulates in the lower part of the reactor pressure vessel. Also in this case, if a large debris region where water exists in the voids is formed, the abundance ratio of water and nuclear fuel becomes a condition suitable for criticality, and recriticality may occur.

  When recriticality occurs, the heat generation is much larger than the decay heat, so cooling becomes even more difficult and the control of the reactor becomes impossible. According to the past example, all the fuel rods are exposed on the water surface about 5 hours after the problem of the cooling function of the core occurs, and about 10 hours later, the entire core is melted. It should be noted that care must be taken not to cause recriticality when the molten fuel is taken out of the reactor whose temperature has been stopped after melting the core.

In order to prevent the recriticality as described above, there is a method of injecting boric acid water that absorbs neutrons into a reactor pressure vessel (see, for example, Patent Document 1). Boric acid water is an aqueous solution of boron compound. Among boron isotopes, 10B has a very large neutron absorption cross section, and is therefore used as a control rod for neutron absorption in a nuclear reactor. However, since boric acid water can only exist outside the above-mentioned debris, the effect of preventing recriticality due to boric acid water absorbing neutrons is small. In addition, when an oxide (crust) exists outside the debris, leakage of neutrons from the debris to the outside becomes more difficult due to the presence of the oxide (crust), and recriticality is likely to occur. Moreover, since the boric acid water density | concentration becomes thin with subsequent water injection, it is necessary to continue supplementing.

JP 2007-101332 A

  Therefore, the present invention provides a recriticality when a fuel rod is melted by introducing it into a reactor pressure vessel as a control material that absorbs neutrons when there is a risk of melting of the cooling system of a light water reactor and core melting. An object of the present invention is to provide a light water reactor control member that can be effectively prevented.

  In place of boric acid water, boron powder that absorbs neutrons can be injected into the reactor pressure vessel and mixed into molten fuel or crust. Boron has a specific gravity of about 2.3 and is heavier than water. . Although the cooling water remaining in the reactor pressure vessel is convected by the heat generated by the fuel rods, its stirring action can be expected, but even with fine powder with a particle size of several microns, the bottom of the reactor pressure vessel will gradually increase in the cooling water over time. There is a possibility of sedimentation. If boron powder settles at the bottom of the reactor pressure vessel prior to core melting, the recriticality prevention effect is diminished.

  Further, boron powder may block the gap between the fuel rods before the core melts. The fuel rods are usually arranged regularly with a gap of 2 to 4 mm in the fuel assembly, and cooling water exists in the gap. However, if a large amount of boron powder is injected to close the gap, Even in the presence of fuel, there is a risk of hindering the heat removal of the fuel. Therefore, a method of injecting boron powder is not practical.

  The present invention has been made based on the above examination results, and is characterized in that a boron and / or boron compound is dispersed in a holding body to form a granular body floating in water.

  According to the light water reactor control member of the present invention (hereinafter abbreviated as “control member”), the emergency diesel generator and the cooling system equipment are damaged in addition to the loss of the external power source, resulting in a problem in the cooling function of the core. In this case, the control member is put into the reactor pressure vessel by an appropriate means. Then, the control member floats on the free liquid level of the cooling water in the reactor pressure vessel, and when the water level drops and the upper part of the fuel rod is exposed on the water surface, the control member moves downward together with the water level and the control member melts. By mixing with the cladding tube and fuel, neutrons are efficiently absorbed and effective in preventing recriticality. In this case, since the control member floats on the free liquid level of the cooling water in the reactor pressure vessel, there is no risk of closing the gap between the fuel rods.

  Here, in the past, injecting seawater from the outside of the light water reactor to cool the reactor core was a measure after making a decision on decommissioning. In this regard, in the present invention, after injecting the control member into the reactor pressure vessel, water injection into the core can be resumed by restoring the external power supply or repairing the cooling system equipment, so that melting of the core can be avoided. The control member floating on the free liquid surface of the cooling water in the reactor pressure vessel can be easily removed after the light water reactor is cooled down. Therefore, the control member can be supplied into the reactor pressure vessel at an early stage from a preventive viewpoint without making the decision of decommissioning.

  In the present invention, even when a carrier in which boron and / or a compound of boron (hereinafter referred to as “boron etc.”) is dispersed by melting of the fuel cladding tube or the heat generated from the nuclear fuel is melted, Since it does not settle in the cooling water, neutrons are absorbed efficiently by mixing with the fuel cladding tube and nuclear fuel in which boron is melted, and it is effective in preventing recriticality.

  In addition, even when some of the damaged nuclear fuel becomes sand particles and settles in the lower part of the reactor pressure vessel, boron and other substances are mixed in the sand particles to efficiently absorb neutrons and recriticality. It is effective for prevention. Furthermore, even when molten nuclear fuel passes through the reactor pressure vessel and falls into or out of the reactor containment vessel, it is effective in preventing recriticality by mixing boron etc. into the molten fuel. .

  Here, it is desirable that the holding body has a lump shape with a hollow outer shell portion inside. The lump shape is a concept including not only a spherical shape and a rectangular shape but also an arbitrary shape such as a scale shape or a foil shape.

  More specifically, a mode in which boron or the like is dispersed inside the outer shell portion is included. Or the aspect in which boron etc. have adhered to the inner wall part of an outer shell part, and the aspect in which boron etc. were accommodated in the hollow part inside an outer shell part are also contained.

  The holding body is not limited to the hollow shape as described above, and a porous body such as expanded graphite can also be used. In the present invention, the control member can be configured by filling the void formed in the expanded graphite with boron. Alternatively, the control member can be configured by filling the pores of a porous metal (for example, stainless steel) with boron and sealing the pores exposed on the surface by plastic working or the like.

  In addition, the granular material may be a mode in which one is present alone or a mode in which a plurality is combined. For example, the granular material may have a form in which boron or the like is dispersed inside one outer shell. Moreover, the aspect with which such a granular material couple | bonded may be sufficient.

  As the boron compound, for example, one or more of boron carbide (B4C), boron oxide (B2O3), and boron nitride (BN) can be used.

  The control member of the present invention is a granular material in which boron or the like is dispersed in a holding body and floats on water. Therefore, the specific gravity of the entire control member is less than 1. Since the gaps between the fuel rods are regularly arranged in the fuel assembly, usually with a gap of 2 to 4 mm, the particle diameter of the control member (passing outermost diameter) is 1 mm or less in order not to close the gap. Is desirable. As described above, the shape of the granular material is not necessarily spherical.

  When the holding body for dispersing boron or the like reaches its melting point, the control member is broken and boron or the like is separated. Therefore, it is desirable that the melting point of the material of the holding body be as high as possible in order to float the control member in water for as long a time as possible until the core is melted. However, even if boron or the like is separated into the water, the cooling water remaining in the reactor pressure vessel is convected due to the heat generated by the fuel rods, so that the stirring action immediately sinks to the bottom of the reactor pressure vessel. Absent. Therefore, there are no particular restrictions on the melting point of the material of the holder for dispersing boron or the like. Further, when the melting point of the material of the holder for dispersing boron or the like is higher than the melting point (about 1800 ° C.) of the zirconium alloy constituting the fuel cladding tube, the control member melts into the molten fuel cladding tube while maintaining the original shape. . However, the effect of boron and the like absorbing neutrons is independent of whether or not the control member maintains its original shape.

  According to the present invention, when a cooling system facility of a light water reactor fails and there is a risk of melting the core, it is injected into a reactor pressure vessel as a control material that absorbs neutrons, and recriticality when the fuel rod is melted is reduced. It can be effectively prevented. In addition, if the core melt can be avoided, the light water reactor can be removed easily by scooping up the control member floating on the free liquid level of the cooling water in the reactor pressure vessel after the cold shutdown. The control member can be supplied into the reactor pressure vessel at an early stage from a preventive viewpoint without making a decision.

It is sectional drawing which shows the state in the middle of preparation of the control member of 1st Embodiment of this invention. It is sectional drawing which shows the control member of 1st Embodiment of this invention. It is sectional drawing which shows the state in the middle of preparation of the control member of 2nd Embodiment of this invention. It is sectional drawing which shows the control member of 2nd Embodiment of this invention. It is sectional drawing which shows the state in the middle of preparation of the control member in the modification of 2nd Embodiment of this invention. It is sectional drawing which shows the control member in the modification of 2nd Embodiment of this invention. It is sectional drawing which shows the state in the middle of preparation of the control member in the modification of 3rd Embodiment of this invention. It is sectional drawing which shows the control member in the modification of 3rd Embodiment of this invention.

1. First Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, the periphery of paraffin wax particles 2 is wrapped with a paste 1 in which fine powder of alumina is kneaded together with a binder, and further, micron-order boron powder 3 is adhered to the outside thereof. Such composite particles are prepared by mixing paraffin wax, binder resin, and fine alumina powder with a known mixing device to produce particles having a particle size of about 1 mm by coating the paraffin wax particles 2 with the paste 1. It can be produced by mixing powder 3. When this is sintered at about 1500 ° C., the paraffin wax melts and volatilizes, while the boron atoms in the boron powder 3 diffuse into the alumina. As a result, as shown in FIG. 2, a hollow ball (control member) 6 in which boron atoms 5 were dispersed in an outer shell portion (holding body) 4 of alumina was produced. If the outer diameter of the completed ball 6 is 1.0 mm, the inner diameter is 0.9 mm, and the volume ratio of alumina and boron is 1: 2, it can be determined whether or not this floats in water by the following calculation. The specific gravity of alumina is 4.0, and the specific gravity of boron is 2.3.

(Equation 1)
Ball volume: V ₁ = 4πR ³ = 4 × π × (0.5) ³ = 1.57 mm ³
Void volume inside the ball: V ₂ = 4πR ³ = 4 × π × (0.45) ³ = 1.14 mm ³
Ball weight: W = γ (V ₁ −V ₂ ) = 2.87 × 10 ⁻³ (g)
X (1.57-1.14) = 1.23 * 10 < ^-3 > (g)
Weight of water excluded by ball: W ₀ = γ ₀ × V ₁ = 1.0 × 10 ⁻³ (g) × 1.57
= 1.57 × 10 ⁻³ (g)
π: Circumference ratio = 3.14
R: radius γ: specific gravity of the sintered body of alumina and boron constituting the ball = 2.87 × 10 ⁻³ (g)
γ ₀ : Specific gravity of water = 1.0 × 10 ⁻³ (g)

Since W <W _{0 according} to the above calculation, the ball 6 floats on water. Since the melting point of alumina is 2054 ° C., the ball melts at this temperature, but is rapidly mixed into the already melted fuel cladding tube (melting point: about 1800 ° C.). Furthermore, boron is mixed in the nuclear fuel (melting point: about 2800 ° C.) that is melted thereafter, so that neutrons are efficiently absorbed and the effect of preventing recriticality is exhibited. In the present embodiment, other ceramics such as magnesium oxide, yttrium oxide, and zirconium oxide can be used instead of alumina.

2. Second Embodiment Boron powder 10 having an average particle size of 10 μm and paraffin wax 11 are heat-kneaded so as to have a volume ratio of 1: 1, and then a spherical kneaded product having a particle size of about 1 mm is obtained by a known granulation method. Produced. Next, the spherical kneaded product and alumina fine powder 12 having an average particle diameter of 0.5 μm were mixed with a rotary mixer to produce composite particles having the alumina fine powder 12 adhered to the surface (FIG. 3). reference). Next, the composite particles are sintered in the atmosphere at 1500 ° C. for 120 minutes, whereby a hollow ball (control member) in which the boron powder 10 is fixed to the hollow inner peripheral surface of the outer shell portion (holding body) 13 of alumina. 14 was produced (see FIG. 4). In addition, part or all of the boron powder 10 can be diffused into the outer shell portion 13 by adjusting the sintering temperature and the sintering time.

  5 and 6 show a modification of the second embodiment. In this modified example, boron powder 10 having an average particle size of 10 μm and paraffin wax 11 are heat-kneaded so as to have a volume ratio of 1: 1, and then a spherical kneaded material is produced, and alumina fine powder is formed on the surface of the kneaded material. Spherical composite particles having a particle size of about 0.5 mm with 12 attached thereto were produced. A bonded particle (see FIG. 5) in which a plurality of these composite particles are bonded is sintered, and a hollow ball (control member) 14 having a particle diameter of 1 mm or less in which boron is diffused inside the outer shell portion (holding body) 13 of alumina. Was prepared (see FIG. 6).

3. Third Embodiment In the first embodiment, a fine powder of stainless steel was used instead of the fine alumina powder. Composite particles equivalent to those shown in FIG. 1 were prepared using SUS430 powder having an average particle diameter of 2 to 5 μm. This is sintered at 750 ° C. and 1000 ° C. in a non-oxidizing atmosphere, and a hollow ball (control member) in which boron is alloyed to the outer shell of stainless steel and boron is fixed to the outer peripheral surface of the outer shell. Produced.

  7 and 8 show a modification of the third embodiment. In this modified example, a paste 1 in which fine powder of stainless steel is kneaded together with a binder wraps around the paraffin wax particles 2, and further a micron-order boron powder 3 is adhered to the outer side of the spherical particle having a particle size of about 0.5 mm. The composite particles were prepared. A bonded particle (see FIG. 7) in which a plurality of the composite particles are bonded together was sintered, and a hollow ball (control member) 6 having a particle diameter of 1 mm or less formed by alloying boron into the outer shell portion 4 of stainless steel was produced.

4). Fourth Embodiment In Embodiment 2, stainless steel fine powder was used instead of alumina fine powder. Composite particles equivalent to those shown in FIG. 3 were prepared using SUS430 powder having an average particle diameter of 2 to 5 μm. This is a hollow ball (control member) which is sintered at 750 ° C. and 1000 ° C. in a non-oxidizing atmosphere, and boron is alloyed to the outer shell of stainless steel and boron is fixed to the inner peripheral surface of the outer shell. Was made.

  In the third and fourth embodiments, when the fuel rod is exposed on the water surface and becomes 1200 ° C. or higher, the water-zirconium reaction in which hydrogen is generated by the chemical reaction between the zirconium alloy in the fuel cladding tube and water vapor becomes active. This is an exothermic reaction, and the temperature rises at an accelerated rate and exceeds the melting point (about 1800 ° C.) of the zirconium alloy in a short time. On the other hand, although the melting point of stainless steel is about 1400 ° C., boron generates a eutectic liquid phase at 1149 ° C. with iron, so it melts around 1100 ° C. Therefore, the outer shell made of stainless steel melts at 1100 ° C. or higher, and boron quickly mixes with the molten fuel cladding. Further, the temperature rapidly rises and the nuclear fuel melts at about 2800 ° C. As described above, boron is mixed in the molten fuel cladding tube, the nuclear fuel, the fuel cladding tube and the fuel mixture, or the like. Boron absorbs neutrons, and is effective in preventing recriticality.

5. Fifth Embodiment In the first embodiment, iron-boron alloy powder is used instead of fine alumina powder. Using an iron-boron alloy powder having an average particle diameter of 1 μm, composite particles equivalent to those shown in FIG. 1 were produced. This is sintered at 750 ° C. and 1000 ° C. in a non-oxidizing atmosphere, and boron is alloyed in the outer shell of the iron-boron alloy and at the same time a hollow ball (control member) with boron fixed to the outer peripheral surface of the outer shell. ) Was produced.

6). Sixth Embodiment In Embodiment 2, iron-boron alloy powder was used instead of fine alumina powder. Composite particles equivalent to those shown in FIG. 3 were prepared using an iron-boron alloy powder having an average particle diameter of 1 μm. This is sintered at 750 ° C. and 1000 ° C. in a non-oxidizing atmosphere, and boron is alloyed in the outer shell of the iron-boron alloy and a hollow ball with boron fixed to the inner peripheral surface of the outer shell (control) Member).

  Also in the fifth and sixth embodiments, the outer shell portion made of an iron-boron alloy is melted at 1100 ° C. or higher and mixed in the melted fuel cladding tube, the nuclear fuel, the fuel cladding tube and the fuel mixture, or the like. Fit. Therefore, the same operation and effect as the third and fourth embodiments can be obtained.

7). Seventh Embodiment A control member is produced by filling expanded graphite powder (particle size of 1 mm or less) with boron. Expanded graphite is manufactured by inserting a predetermined substance between sheets of graphite using a chemical reaction, and burning and gasifying the substance inserted between the layers by rapid heating. The release of gas generated at that time explosively spreads between the layers, so that the graphite expands in the stacking direction of the layers. The control member is manufactured by vacuum impregnating boric acid water into a large amount of voids formed by the expansion of graphite and evaporating the water by heating it. Since this control member includes a large amount of voids, it can be floated on water. Since graphite and water have poor wettability, the cooling water in the reactor pressure vessel does not enter the gap.

  A method for injecting the control member as described above into the reactor pressure vessel is not particularly limited. For example, there is a method of using an emergency core cooling device. That is, in a pressurized water type light water reactor, three systems can be used: an accumulator injection system in which water stored at a high pressure is charged, a high pressure injection system in which a pump is used to send water at a high pressure, and a low pressure injection system that is operated after the pressure is lowered. In the boiling water type light water reactor, a high pressure core spray, a low pressure core spray, and a low pressure injection system in which water is supplied at a high pressure from the upper part of the reactor pressure vessel can be used. In addition, the boiling water reactor has a suppression pool (chamber) that supplements the capacity of the reactor containment vessel. The control member can be injected together with fresh water or seawater through a system that can be employed.

  The present invention can be used as a control material that absorbs neutrons when a failure occurs in a cooling system facility of a light water reactor.

1 Paste 2 Paraffin wax particles 3 Boron powder 4 Outer shell 5 Boron atom 6 Ball (control member)
10 Boron powder 11 Paraffin wax 12 Alumina fine powder 13 Outer shell 14 Ball (control member)

Claims (9)

  A control member for a light water reactor, characterized in that boron and / or a compound of boron is dispersed in a holding body to form a granular body that floats on water.   2. The light water reactor control member according to claim 1, wherein the holding body has a lump shape having a hollow outer shell portion inside. 3.   3. The light water reactor control member according to claim 1, wherein one of the granular bodies is a single body or a plurality of the granular bodies are combined.   4. The light water reactor control member according to claim 2, wherein boron and / or a boron compound is dispersed in the outer shell portion. 5.   4. The light water reactor control member according to claim 2, wherein boron and / or a compound of boron adheres to an inner wall portion of the outer shell portion.   The light water reactor control member according to any one of claims 1 to 5, wherein the holding body is made of expanded graphite, and boron and / or a compound of boron adheres to the expanded graphite.   The light water reactor control member according to claim 1, wherein the boron compound is one or more selected from boron carbide, boron oxide, and boron nitride.   The control member for a light water reactor according to any one of claims 1 to 7, wherein the holding body is one or more selected from aluminum oxide, magnesium oxide, yttrium oxide, and zirconium oxide. The light water reactor control member according to claim 1, wherein the holding body is made of stainless steel or iron-boron alloy.

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