JP6448224B2 - Reactor pressure vessel reactor bottom protection structure - Google Patents

Description

  Embodiments described herein relate generally to a reactor bottom vessel protection structure for a reactor pressure vessel that protects a welded portion to which an in-core device that penetrates the reactor bottom of a reactor pressure vessel is fixed.

  In a boiling water reactor, the level of reactor water in the reactor is usually above the core region, and the fuel rods in the core region are located in the reactor water.

  If water injection stops into the furnace due to some trouble such as a piping failure, and the water level drops and the fuel rod is exposed, water drop is detected and water injection into the reactor is performed using the emergency core water injection system. This prevents the fuel rod from continuing to be exposed.

  However, if a severe accident such as an earthquake or tsunami occurs and the exposure of the fuel rods cannot be prevented by in-core water injection by the emergency core water injection system, the temperature of the fuel rods may rise and eventually the core melts There is.

  In the unlikely event of core melting, the hot core melt may fall and damage the bottom of the reactor. Furthermore, when the core melt flows out of the pressure vessel due to damage to the bottom of the reactor core, the core melt falls into the reactor containment vessel, and a large amount of non-condensable gases such as carbon dioxide and hydrogen are reacted with the concrete. It is considered that the reactor containment vessel may be damaged.

  Therefore, even if the core melts, if the core melt can be held in the reactor pressure vessel, it is possible to avoid reaction between the core melt and concrete and damage to the reactor containment vessel It becomes. Further, since the diffusion region of the core melt can be kept in the reactor pressure vessel, the time and cost required for fuel removal and decommissioning measures are greatly reduced.

  When the core melt falls and accumulates on the bottom of the reactor pressure vessel, the most dangerous part is the in-core equipment (for example, control rod drive mechanism ( It is known that the CRD) housing and the like) are welded parts fixed by welding.

  When the welded portion is damaged, the core melt flows out from the damaged portion. Therefore, it is considered important to protect the welded part reliably in order to prevent the core melt from flowing out of the reactor pressure vessel.

  Conventionally, various techniques for holding a core melt inside a reactor pressure vessel when core melting occurs have been studied. In Patent Document 1, a plurality of water pipes are provided at the bottom of the reactor, and the core melt is efficiently cooled by pouring water into a gap formed between the core melt and the reactor pressure vessel. A technique for preventing the damage of the apparatus is disclosed.

JP 2000-162358 A

  However, normally, the in-reactor equipment is welded and fixed to the reactor pressure vessel via a stub tube provided at the bottom of the reactor pressure vessel, and the weld is formed at a position protruding into the reactor. The For this reason, since a gap is not formed between the core melt and the welded portion, the technique disclosed in Patent Document 1 cannot sufficiently cool the core melt contacting the welded portion, and prevents damage to the welded portion. It was not enough to do.

  Also, it has been difficult to realize a protective structure that can protect the welded portion without breaking when a very hot core melt falls to the bottom of the furnace.

  The present invention was made in consideration of such circumstances, and the reactor bottom portion of the reactor pressure vessel that protects the welded portion to which the in-furnace equipment passing through the reactor bottom portion of the reactor pressure vessel is fixed with high soundness. The purpose is to provide a protective structure.

In the reactor bottom vessel protection structure for a reactor pressure vessel according to an embodiment of the present invention, an in-reactor device that penetrates a through hole provided in the reactor bottom portion of the reactor pressure vessel is arranged around a welded portion that is fixed by welding. A space formed by the heat insulating part, the metal cover part that is arranged to cover at least a part of the outer surface of the heat insulating part, and holds the heat insulating part, and the heat insulating part and the metal cover part A low elastic modulus part having a lower elastic modulus than the heat insulating part, a certain gap is provided between the in-furnace equipment and the furnace bottom part and the metal cover part, and the upper part of the metal cover part is disposed in the furnace And a fixing portion for fixing to the internal device.

  According to the embodiment of the present invention, there is provided a reactor bottom vessel protection structure for a reactor pressure vessel that protects a welded portion to which an in-core device passing through the reactor bottom portion of the reactor pressure vessel is fixed with high soundness.

Schematic of a reactor pressure vessel to which this embodiment is applied. (A) The expanded sectional view which shows the structure of the furnace bottom part protection structure concerning 1st Embodiment, (B) The cross-sectional view of II. (A) The expanded sectional view which shows the 1st modification of the furnace bottom part protection structure concerning 1st Embodiment, (B) The cross-sectional view of II-II. (A) The expanded sectional view block diagram which shows the 2nd modification of the furnace bottom part protection structure concerning 1st Embodiment, (B) The cross-sectional view of III-III. The expanded sectional view which shows the structure of the furnace bottom part protection structure concerning 2nd Embodiment, (B) The cross-sectional view of IV-IV.

Hereinafter, this embodiment is described based on an accompanying drawing.
FIG. 1 shows a schematic diagram of a reactor pressure vessel 50 in a boiling water reactor to which the present embodiment is applied. Hereinafter, the CRD housing 12 will be described as an example of the in-furnace equipment provided through the reactor bottom 13 of the reactor pressure vessel 50.

  The CRD housing 12 passes through the reactor bottom 13 and is inserted into the reactor pressure vessel 50 and fixed to the reactor pressure vessel 50. A control rod drive mechanism (not shown) is inserted and accommodated in the CRD housing 12 from below. Then, the control rod is inserted into the core region 11 using this control rod drive mechanism.

(First embodiment)
FIG. 2A is an enlarged cross-sectional view showing the configuration of the furnace bottom portion protection structure 10 according to the first embodiment (region A shown in FIG. 1). FIG. 2B shows a cross-sectional view taken along line II.

  As shown in FIG. 2A, the reactor bottom protection structure 10 (hereinafter abbreviated as protection structure 10) of the reactor pressure vessel 50 according to the embodiment is provided in the reactor bottom 13 of the reactor pressure vessel 50. The CRD housing 12 penetrating the formed through-hole 20 is disposed around the welded portion 15 where the CRD housing 12 is fixed by welding, and is disposed so as to cover at least a part of the outer surface of the heat-insulated portion 16. The metal cover part 18 which hold | maintains the part 16, and the low elastic modulus part 17 which is provided in the space formed by the heat insulation part 16 and the metal cover part 18 and whose elastic modulus is lower than the heat insulation part 16 is provided.

  The CRD housing 12 is inserted into the furnace through a through hole 20 provided in the furnace bottom 13. On the other hand, the stub tube 14 is formed on the inner surface of the furnace bottom 13, and is formed in the vicinity of the through hole 20 so as to protrude into the furnace in parallel with the inserted CRD housing 12.

  As for the welding part 15, the front-end | tip of the stub tube 14 protruded inside the furnace and the side surface of the CRD housing 12 are fixed by welding. Thereby, the CRD housing 12 is fixed to and supported by the furnace bottom portion 13.

  The protective structure 10 is formed in a cylindrical shape so as to cover the periphery of the welded portion 15, and its inner surface side is formed along the outer periphery of the CRD housing 12 and the stub tube 14 so as to surround the welded portion 15. ing.

  The heat insulating portion 16 is formed in a cylindrical shape and is disposed around the welded portion 15. The material constituting the heat insulating portion 16 is not melted even when it comes into contact with the core melt generated when core melting occurs, and heat conduction to the welded portion 15 can be prevented, that is, it has a high melting point and low heat. A heat insulating material with conductivity is selected.

  Specifically, a heat insulating material having a melting point of 2500 ° C. or higher and a thermal conductivity of 2 W / (mK) or lower is desirable. Examples of the heat insulating material constituting the heat insulating portion 16 include silicon carbide and zirconium. A porous heat insulating material is preferable because it can realize low thermal conductivity.

  The low elastic modulus portion 17 is provided in a space formed by the heat insulating portion 16 and the metal cover portion 18 on the inner peripheral side of the heat insulating portion 16. As the material constituting the low elastic modulus portion 17, a low elastic modulus material having a lower elastic modulus than the heat insulating portion 16 and having deformability is selected.

  A material that has a high melting point (approximately 2500 ° C. or higher) and low thermal conductivity (approximately 1 W / (mK) or less) and does not impair the heat insulating performance of the heat insulating portion 16 is desirable. Examples of the material constituting the low elastic modulus portion 17 include fibrous ceramics such as fibrous zirconia, fibrous alumina, and fibrous silica.

  The metal cover portion 18 is formed so as to cover the heat insulating portion 16 and the low elastic modulus portion 17, and holds the heat insulating portion 16 having a low mechanical strength with a metal material. Examples of the metal material include stainless steel and nickel-base alloy.

  The fixing portion 19 is disposed on the upper portion of the protective structure 10 and is a member for fixing the protective structure 10 to the CRD housing 12. The fixing is performed by welding. Note that the fixing of the protective structure 10 to the CRD housing 12 is not limited to the configuration performed via the fixing portion 19, and the protective structure 10 and the CRD housing 12 may be fixed directly by welding.

Here, the effect of providing the low elastic modulus portion 17 will be described.
When core melting occurs and the molten core is deposited on the bottom 13, it is assumed that the temperature of the protective structure 10 rises to about 1200 ° C. due to decay heat generated from the core melt.

For example, when porous zirconia is selected as the constituent material of the heat insulating portion 16 and stainless steel is selected as the constituent material of the metal cover portion 18, the thermal expansion coefficient of zirconia is about 7 × 10 ⁻⁶ / K, The thermal expansion coefficient of stainless steel is about 16.2 × 10 ⁻⁶ / K. For this reason, the difference in thermal expansion coefficient is large, and stress due to the thermal expansion difference is generated.

  In particular, since the inner peripheral surface side of the heat insulating portion 16 is close to the CRD housing 12 via the metal cover portion 18, the generated stress cannot be released and a large stress is applied in the radial direction.

  Although the porous material can improve the heat insulating performance, the mechanical strength is lowered, and therefore, the heat insulating portion 16 may be damaged due to the stress due to the difference in thermal expansion.

  As in the present embodiment, by providing the low elastic modulus portion 17 having deformability on the inner peripheral surface side of the heat insulating portion 16, the radial stress caused by the thermal expansion difference between the heat insulating portion 16 and the metal cover portion 18 is reduced. It can be reduced. Thereby, even if it is a case where a core melts, it becomes possible to prevent the damage of the heat insulation part 16 by the thermal expansion difference of the heat insulation part 16 and the metal cover part 18. FIG.

  For this reason, even if it becomes a high temperature environment, the protection structure 10 can protect the welding part 15 from a core melt, maintaining a protection function healthy, without being damaged.

  FIG. 3A is an enlarged cross-sectional view showing a second modification of the protective structure 10 according to the first embodiment. FIG. 3B shows a cross-sectional view of II-II.

  The difference from the protective structure 10 according to the first embodiment shown in FIG. 2 (A) is that the heat insulating portion 16 is divided and provided in the circumferential direction, and the low elastic modulus portion 17 is divided between the heat insulating portions 16. It is in the point provided in.

  In this modified example, the cylindrical heat insulating portion 16 has a plurality of divided structures, and the low elastic modulus portions 17 are formed between the heat insulating portions 16 of the divided structures. By doing in this way, the heat insulation part 16 is released from restraint by a cylindrical shape, and the some movable range can be provided in the heat insulation part 16 of a divided structure by the deformability which the low elastic modulus part 17 has.

  Thereby, the stress of the circumferential direction which acts on the heat insulation part 16 from the metal cover part 18 by a thermal expansion difference can be reduced.

  FIG. 4A is an enlarged cross-sectional view illustrating a second modification of the protective structure 10 according to the first embodiment. FIG. 4B shows a cross-sectional view of III-III.

  The difference from the protective structure 10 according to the first embodiment shown in FIG. 2 (A) is that a low elastic modulus portion 17 is provided at the peripheral edge portion of the heat insulating portion 16.

  In this modification, the low elastic modulus part 17 is provided in the peripheral part (corner part) of the heat insulation part 16 where the stress from the metal cover part 18 due to the thermal expansion difference is concentrated. Thereby, the stress concerning the peripheral part of the heat insulation part 16 can be reduced, and the failure | damage of the heat insulation part 16 can be prevented reliably.

  In addition, the formation method of the low elastic modulus part 17 is not limited to the method shown in FIGS. 2 to 4, and the heat insulating part 16 can be formed by combining the methods shown in FIGS. 2 to 4. It is possible to reduce the acting stress more effectively.

(Second Embodiment)
FIG. 5A is an enlarged cross-sectional view showing the configuration of the protective structure 10 according to the second embodiment. And FIG. 5 (B) has shown the cross-sectional view of IV-IV. In addition, about the structure same as 1st Embodiment, the same code | symbol is attached | subjected and the overlapping description is abbreviate | omitted.

  The difference from the first embodiment is that the fixing portion 19 fixes the CRD housing 12 and the metal cover portion 18 by providing a certain gap 21. Specifically, a gap 21 of 2 mm or more is provided and fixed between the CRD housing 12 and the metal cover portion 18.

  By providing the gap 21, contact between the CRD housing 12 and the protective structure 10 can be avoided when thermal expansion of the CRD housing 12 occurs during melting of the core. Thereby, since the stress to the heat insulation part 16 which arises by contact with the CRD housing 12 can be avoided, damage to the heat insulation part 16 can be prevented.

  According to the reactor bottom vessel protection structure of the reactor pressure vessel of each embodiment described above, by providing a low elastic modulus portion in a part of the protection structure that covers and protects the welded portion using a heat insulating member, Since the stress acting on the member can be reduced, the welded portion can be protected with high soundness.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel 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 modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  In each of the above-described embodiments, the case where the furnace device is applied to the CRD housing 12 has been described. However, the present invention is not limited to the CRD housing 12 and is not limited to the CRD housing 12. It can also be applied to various instrumentation piping such as housings and core differential pressure gauge nozzles.

  In each of the above embodiments, the case where the present invention is applied to a reactor pressure vessel in a boiling water reactor has been described. However, the present invention is not limited to a boiling water reactor. For example, a reactor such as a pressurized water reactor It is also applicable to.

10 Reactor bottom protection structure 11 Core region 12 CRD housing (in-furnace equipment)
13 Furnace bottom part 14 Stub tube 15 Welding part 16 Heat insulation part 17 Low elastic modulus part 18 Metal cover part 19 Fixing part 20 Through-hole 21 Void 50 Reactor pressure vessel

Claims (6)

A heat insulating portion disposed around a welded portion in which an in-reactor device penetrating a through hole provided in the bottom of the reactor pressure vessel is fixed by welding;
A metal cover part that is arranged to cover at least a part of the outer surface of the heat insulating part and holds the heat insulating part;
A low elastic modulus portion provided in a space formed by the heat insulating portion and the metal cover portion, and having a lower elastic modulus than the heat insulating portion;
A nuclear reactor, comprising: a fixed part that provides a certain gap between the in-furnace equipment and the bottom of the furnace and the metal cover part, and fixes an upper part of the metal cover part to the in-furnace equipment. Pressure vessel furnace bottom protection structure.   2. The reactor bottom vessel protection structure according to claim 1, wherein the low elastic modulus portion is provided on an inner peripheral surface side of the heat insulating portion. The heat insulating portion is provided by being divided in the circumferential direction,
The reactor bottom vessel protection structure for a reactor pressure vessel according to claim 1 or 2, wherein the low elastic modulus portion is provided between the divided heat insulating portions.   The reactor bottom vessel protection structure for a reactor pressure vessel according to any one of claims 1 to 3, wherein the low elastic modulus portion is provided at a peripheral portion of the heat insulating portion.   5. The reactor bottom protection structure for a reactor pressure vessel according to claim 1, wherein the heat insulating portion is porous zirconia. 6.   The reactor bottom vessel protection structure for a reactor pressure vessel according to any one of claims 1 to 5, wherein the low elastic modulus portion is a fibrous ceramic.

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