JP2015206672A - Control rod for nuclear reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an accident resistance control rod not being fused earlier than fuel at the time of a severe accident of a nuclear power plant and having neutron absorbers remaining at a reactor core for a long period of time even when being fused and dropping down.SOLUTION: A control rod for a nuclear reactor includes: neutron absorption sections 31 having neutron absorbers; a plurality of housing members 21 housing the neutron absorption sections 31; a coupling member integrating the plurality of housing members 21; and eutectic prevention members 33. Each of the eutectic prevention members 33 is provided between the neutron absorption section 31 and the housing member 21, contains eutectic preventing oxide having neutron absorption effect and high temperature stability and prevents the neutron absorption section 31 and the housing member 21 from being brought into direct contact with each other.

Description

  Embodiments described herein relate generally to a reactor control rod.

  The control rod of the boiling water reactor has a cross-shaped cross section, a central coupling member extending in the axial direction (vertical direction), and four plate-like wings around the central coupling member in four directions. It has spread. The wing extends in the axial direction of the tie rod. Further, a handle is joined to the upper end portion of the central coupling member, and a lower support member having a drop speed limiter is attached to the lower end portion of the central coupling member.

Each wing includes, for example, a neutron absorber such as B ₄ C, and the neutron absorber is covered with a sheath having a substantially U-shaped cross section made of austenitic stainless steel.

JP 2011-174828 A JP 2013-246102 A

In a severe accident at a nuclear power plant, it is assumed that the core will be melted due to insufficient cooling of the core with coolant. In such an event, the control rods melt and fall in addition to the core fuel. Boron carbide (B ₄ C) used for control rods is likely to eutectic with the austenitic stainless steel used for the structural material parts (wings) of the control rods, resulting in a lower melting point and melting at an early stage. The

  After the control rod melts, a “recritical” event can be considered where the reactor becomes critical again due to water injection into the reactor. Recriticality can cause damage to the reactor pressure vessel, the containment vessel, or the reactor building, and can lead to the release of radioactive material to the environment.

FIG. 7 is a diagram showing a melting point of a structural material for a control rod. As shown in FIG. 7, the melting point of austenitic stainless steel is 1723K. Therefore, the melting point of the control rod made of austenitic stainless steel is lower than UO ₂ having a melting point of 3123K. Further, B ₄ C itself has a melting point of 2723 K, and further, the melting point decreases to 1500 K due to the eutectic with austenitic stainless steel. For this reason, there is a possibility that the control rod melts first depending on the temperature in the furnace. Thereafter, fuel melting remains in a partial range, and a core of a certain height is formed by the melted fuel, and then water injection can cause recriticality at a certain height.

  Also, the melting core has a high temperature due to the generation of decay heat. Further, when the fuel shape is collapsed, it is difficult to secure a sufficient coolant for cooling to the inside. For this reason, water is likely to exist as water vapor. In this case, the possibility of reaching criticality is low because the amount of hydrogen, which is a moderator for decelerating fast neutrons generated by fission into thermal neutrons that are prone to fission, is insufficient. On the other hand, the decay heat decreases with the passage of time, and therefore, after several months have passed since the melting of the core, the coolant in the furnace becomes liquid water instead of water vapor. This can lead to criticality. In addition, boron in the above-described control rod that was present at the beginning may flow out with the coolant as time passes, and may become critical due to a reduction in the reactivity suppression effect.

  Accordingly, an embodiment of the present invention aims to provide an accident resistance control rod that does not melt earlier than the fuel at the time of a severe accident at a nuclear power plant, and that the neutron absorber remains in the core for a long time even when melted and dropped. To do.

  In order to achieve the above-described object, a reactor control rod according to the present embodiment includes a neutron absorber having a neutron absorber, a plurality of storage members containing the neutron absorber, and the plurality of storage members integrated. And an eutectic-preventing oxide having a neutron absorption effect and high-temperature stability, and is provided between the neutron absorption part and the storage member and directly between the neutron absorption part and the storage member And a eutectic prevention member that prevents unwanted contact.

  According to the embodiment of the present invention, it is possible to provide an accident resistance control rod in which a neutron absorbing material stays in the core for a long period of time even when it melts and falls at the time of a severe accident at a nuclear power plant and even when melted and dropped.

It is a horizontal sectional view showing the state where the control rod for reactors by a 1st embodiment was inserted in the core. It is a conceptual perspective view which shows the structure of the wing | wing of the nuclear reactor control rod by 1st Embodiment. It is a conceptual perspective view which shows the structure of the neutron absorption part inserted in the wing of the control rod for reactors by 1st Embodiment. FIG. 4 is a conceptual longitudinal sectional view taken along arrows IV-IV in FIG. 2 showing a configuration of a wing of a nuclear reactor control rod according to the first embodiment. It is a notional longitudinal cross-sectional view which shows the structure of the wing | wing of the control rod for reactors by 2nd Embodiment. It is a notional longitudinal cross-sectional view which shows the structure of the wing | wing of the control rod for reactors by 3rd Embodiment. It is a figure which shows melting | fusing point of a control-rod structure material.

  Hereinafter, a control rod for a nuclear reactor according to an embodiment of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a horizontal sectional view showing a state in which a nuclear reactor control rod according to a first embodiment is inserted into a core. The nuclear reactor control rod 10 is positioned so that it can be inserted into the gap between the rectangular columnar fuel assemblies 1 arranged in a lattice pattern in a core (not shown) from below.

  The nuclear reactor control rod 10 includes four plate-like wings 20 extending in the horizontal direction and the vertical direction, and a rod-like central coupling member 11 that is coupled and integrated to extend in the vertical direction. The four wings 20 extend in a cross shape in the horizontal direction around the central coupling member 11. Further, in the nuclear reactor control rod 10, a central coupling member 11 is positioned at the substantially horizontal center of four square fuel assemblies 1, and each wing 20 is sandwiched between adjacent fuel assemblies 1. Are arranged as follows.

  FIG. 2 is a conceptual perspective view showing the configuration of the wing of the nuclear reactor control rod. FIG. 3 is a conceptual perspective view showing a configuration of a neutron absorbing portion inserted into the wing of the nuclear reactor control rod. FIG. 4 is a conceptual vertical cross-sectional view taken along arrow IV-IV in FIG.

  The wing 20 includes a storage member 21 and a neutron absorption part 31 stored in the storage member 21. The storage member 21 has a U-shaped outer shape in a horizontal section, and extends in the axial direction, that is, in the vertical direction in the installed state. The storage member 21 is formed by cutting out from a plate material, for example, by making the tip portion of the wing 20 in a horizontally extending direction a smooth curved surface.

  The storage member 21 is formed with a plurality of cylindrical storage holes 22 that are horizontally spaced apart from each other in the vertical direction of the wing 20. The storage hole 22 is open on the side coupled to the central coupling member 11 (FIG. 1), and is closed without penetrating on the opposite side, that is, the side where the wing 20 extends in the horizontal direction. The storage hole 22 is formed in the storage member 21 by, for example, drilling from the side that is connected to the central coupling member 11. The material of the storage member 21 is, for example, austenitic stainless steel.

A neutron absorbing portion 31 is stored in each storage hole 22. The neutron absorber 31 is solid and is formed in a cylindrical shape. The material of the neutron absorber 31 is, for example, sintered B ₄ C.

  A eutectic prevention member 33 is provided on the outer surface of the neutron absorber 31. The eutectic prevention member 33 is arranged so as to prevent the neutron absorbing portion 31 and the storage member 21 from coming into direct contact. Therefore, the eutectic prevention member 33 does not necessarily need to completely cover the outer surface of the neutron absorption part 31, but may completely cover the possibility of partial melting of the storage member 21. preferable.

  From the viewpoint of minimizing the insertion property of the neutron absorber 31 provided with the eutectic prevention member 33 in the storage hole 22 and the contact area between the eutectic prevention member 33 and the storage member 21 in the storage hole 22. A gap 35 is formed between the eutectic prevention member 33 and the storage member 21. In order to secure the gap 35 in the circumferential direction, the eutectic prevention member 33 may be fixed with a spacer (not shown). Alternatively, the eutectic prevention member 33 may be placed on the storage hole 22 without using a spacer or the like, that is, a line contact may be used instead of a surface contact.

  The material of the eutectic prevention member 33 is selected as a requirement that it has a neutron absorption effect, is a material that is stable at high temperatures, and is also available in terms of cost.

The eutectic preventing member 33 may be formed by forming a lanthanoid metal or lanthanoid oxide film for the eutectic preventing member 33 described later on the outer surface of the neutron absorbing portion 31 by thermal spraying or the like. Alternatively, the neutron absorption part 31 may be housed as a lanthanoid oxide container for the eutectic prevention member 33. In this case, the neutron absorber 31 is not limited to a solid shape, and may be, for example, powdered B ₄ C.

The inventors conducted a thermodynamic equilibrium analysis (reaction analysis) of the candidate material as a material of the eutectic prevention member 33 at 1150 ° C. with 1 mol of the oxide of the candidate material and 10 mol of water vapor. The ratio of the number of moles of the product amount after the steam reaction to the number of moles of the original candidate material (1 mole) was determined. Here, 1150 ° C. is a temperature at which B ₄ C reacts with water vapor above this temperature. That is, the reaction of B ₄ C with water vapor starts at 1150 ° C.

As described above, B ₄ C reacted with water vapor to produce a reaction product having a molar number of 10 times or more. However, a part of the rare earth element and a part of the transition metal remained at 1 mole, It turns out that it does not react. Specifically, the material that does not react with water vapor is, in the rare earth element, gadolinium (Gd), lutetium (Lu), thulium (Tm), erbium (Er), which are candidate lanthanoid elements (Ln), These oxides were dysprosium (Dy), gallium (Ga), europium (Eu), and samarium (Sm), and the results of these reaction analyzes were all 1 mol and did not react with water vapor. . As transition metals, mercury (Hg), hafnium (Hf), cadmium (Cd), and silver (Ag) oxides did not react with water vapor as well.

  Among these, Gd, Eu, Dy, Sm, Er, Hf, and Cd are nuclides having a large neutron absorption cross section. Further, these oxides are in a stable form, and are therefore materials that do not cause eutectic with austenitic stainless steel. These materials are therefore available stable materials. Hereinafter, among these, the oxides of lanthanoid elements Gd, Eu, Dy, Sm, and Er will be referred to as oxides for preventing eutectic.

The highest oxidation number that the element can take is almost 3 for lanthanoid elements of rare earth elements and 4 for Hf. From the results of the reaction analysis, it is known that if it is combined with oxygen up to this maximum oxidation number, no further oxidation reaction occurs, so that it does not react with water vapor to generate hydrogen. The above-described lanthanoid element oxide Ln ₂ O ₃ and Hf oxide HfO ₂ have a melting point of 2000 ° C. or higher and high thermal stability. Therefore, it is the most effective form as a candidate material capable of suppressing hydrogen generation.

  Note that the oxides of Sm, Gd, and Eu can take two forms: a high-temperature monoclinic system called B-type and a low-temperature cubic system called C-type. For example, Eu undergoes a phase transition at 1000 ° C. and changes the crystal system of the stable phase. That is, phase transition occurs in the process of temperature rise due to an accident, and if the density is changed, structural destruction may occur, which is not preferable. Also, since the atomic density of Eu is higher in the high temperature phase, the high temperature phase is preferable as the control rod material. Moreover, it is said that the phase transition in 1000-1300 degreeC is irreversible. For this reason, it can be used in the state of the stable high temperature phase from the beginning by using the oxide made into the high temperature type crystal system beforehand.

  As described above, the eutectic prevention member 33 in the present embodiment has an eutectic prevention oxide as a material.

  In the nuclear reactor control rod 10 according to the present embodiment, a eutectic prevention member 33 having an eutectic prevention oxide that has a high melting point and is stable at a high temperature is disposed on the outer periphery of the neutron absorber 31. For this reason, even when the ambient temperature of the nuclear reactor control rod 10 rises, the state in which the eutectic prevention member 33 is interposed between the storage member 21 and the neutron absorbing portion 31 stored in the storage member 21 continues. To do. As a result, the storage member 21 made of austenitic stainless steel and the neutron absorbing portion 31 stored in the storage member 21 are not in direct contact, and no eutectic is generated. Therefore, the configuration of the nuclear reactor control rod 10 can be maintained up to a higher temperature than before. Furthermore, the use of a substance that does not generate hydrogen due to reaction with high-temperature steam at the time of an accident makes the nuclear reactor control rod 10 safer.

  Further, the eutectic prevention oxide used for the eutectic prevention member 33 has a neutron absorption effect, and can further increase the reactivity value of the nuclear reactor control rod 10 or further extend the life.

[Second Embodiment]
FIG. 5 is a conceptual longitudinal sectional view showing a configuration of a wing of a nuclear reactor control rod according to the second embodiment. This embodiment is a modification of the first embodiment.

  In the nuclear reactor control rod 10 according to the second embodiment, the eutectic prevention member 33 is formed concentrically along the inner surface of each storage hole 22 formed in the storage member 21. The diameter of the outer surface in the radial direction of the eutectic prevention member 33 is formed to be approximately the same as the diameter of the inner surface of the storage hole 22.

  In this case, the eutectic prevention member 33 may be provided, for example, by inserting what is formed into a cylindrical container shape along the inner surface of the storage hole 22. Alternatively, a cylindrical container-shaped eutectic prevention member 33 may be arranged in a line at intervals in a mold having an outer shape of the wing 20, and the austenitic stainless steel storage member 21 may be formed by casting. .

  A gap 36 is formed between the inner surface of the eutectic prevention member 33 and the outer surface of the neutron absorber 31. Note that the gap width need not be uniform in the circumferential angle direction. That is, the eutectic prevention member 33 and the neutron absorber 31 may be in contact with each other.

  When the thermal expansion coefficient of the neutron absorbing portion 31 is larger than that of the eutectic preventing member 33, the eutectic preventing member 33 restrains the thermal expansion of the neutron absorbing portion 31 when the temperature rises. In the present embodiment, by securing a gap 36 between the outer surface of the neutron absorbing portion 31 and the inner surface of the eutectic preventing member 33, the neutron absorbing portion 31 can be damaged or collapsed by tapping with the eutectic preventing member 33. Sex can be excluded.

[Third Embodiment]
FIG. 6 is a conceptual longitudinal sectional view showing a configuration of a wing of a nuclear reactor control rod according to the third embodiment. This embodiment is a modification of the first or second embodiment.

  In the nuclear reactor control rod 10 according to the third embodiment, the eutectic prevention member 33 has a cylindrical shape extending in the longitudinal direction. A gap 35 is formed between the outer surface of the eutectic prevention member 33 and the inner surface of the storage hole 22. In addition, a gap 36 is formed between the inner surface of the eutectic prevention member 33 and the outer surface of the neutron absorber 31. The gap 35 and the gap 36 do not need to be uniform in the circumferential direction. That is, the eutectic prevention member 33 and the storage hole 22, and the eutectic prevention member 33 and the neutron absorber 31 may be in contact with each other.

  By providing a gap between the neutron absorber 31 and the storage hole 22, mechanical interaction due to temperature change between the neutron absorber 31 and the storage member 21 is reduced, and the neutron absorber 31, the eutectic prevention member 33 and the storage are reduced. The structural soundness of each member 21 can be improved.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. For example, in the embodiment, the storage member 21 of the wing 20 is formed from, for example, a plate material, and the storage hole 22 is formed by drilling. However, the present invention is not limited to this. For example, the present invention can also be applied to a wing having a configuration in which the solid neutron absorbing portions 31 are stored side by side in a container-shaped storage member formed by bending a plate into a U shape.

Further, in the embodiment, the material of the neutron absorbing portion 31 shows the case of B _{4 C,} but is not limited thereto. For example, metal hafnium (Hf) may be used.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope 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.

  DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 10 ... Reactor control rod, 11 ... Central coupling member (coupling member), 20 ... Wing, 21 ... Sheath part (housing member), 22 ... Storage hole, 31 ... Neutron absorption part, 33 ... Eutectic prevention member, 35, 36 ... gap

Claims (6)

A neutron absorber having a neutron absorber;
A plurality of storage members enclosing the neutron absorber;
A coupling member that integrates the plurality of storage members;
A co-crystal preventing oxide having a neutron absorption effect and high-temperature stability is provided between the neutron absorption part and the storage member to prevent direct contact between the neutron absorption part and the storage member. An anti-crystallizing member;
A control rod for a nuclear reactor. Each of the plurality of storage members has a flat plate shape that is radially arranged around an axis extending in the vertical direction and spaced in the circumferential direction and extends in the direction of the axis and the radial direction,
The coupling member has a rod shape extending in the axial direction for coupling the plurality of storage members at the center.
The control rod for a nuclear reactor according to claim 1, wherein:   The oxide for preventing eutectic includes at least one of gadolinium oxide, europium oxide, dysprosium oxide, samarium oxide, and erbium oxide. Control rod for nuclear reactors.   The said neutron absorption part is a solid shape, The said eutectic prevention member is formed so that it may closely_contact | adhere to the circumference | surroundings of the said neutron absorption part, The Claim 1 thru | or 3 characterized by the above-mentioned. The reactor control rod as described.   The said eutectic prevention member is formed in a container shape, The said neutron absorption part is accommodated in the said eutectic prevention member of the said container shape, The Claim 1 thru | or 3 characterized by the above-mentioned. The reactor control rod as described.   Each of the plurality of storage members is formed with a plurality of storage holes for storing the neutron absorbing portion, and the eutectic prevention member is provided along the inner surface of the storage hole. The control rod for a nuclear reactor according to any one of claims 1 to 3, wherein the control rod is a reactor.

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