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Core Shroud Replacement Work

Main Essential Technology

Toshiba has developed the technology and machines for the Core Shroud Replacement Method consisting of Chemical Decontamination, Shroud Support Machining, Shroud Welding, etc.

Principle

Principle

  • Metal oxides, including radio-nuclides, on the inside surface of reactor components and piping are dissolved by chemical reaction and removed by ion exchange.
  • Oxalic Acid is used to dissolve mainly iron oxides (which decompose to carbon dioxide gas and water by UV light after decontamination).
  • Hydrogen Permanganate is used to dissolve mainly chromium oxides (which decompose to manganese ions by adding oxalic acid after oxidation).
  • Some remarkable features of this process are:
  1. The ability to effectively decontaminate a large area in a short period of time.
  2. A high Decontamination Factor (DF)
  3. Minimal radwaste generation and easy treatment of liquid waste.
  4. No detrimental effects on structural materials.
  5. Minimal radiation exposure during the decontamination process.
  6. Compact decontamination equipment.
Dose rate Reduction

Dose rate Reduction

DF: Decontamination Factor=(Dose rate before Decon.)/(Dose rate after Decon.)

  • The dose rate in the RPV was continuously monitored by scintillation detectors installed at the top of three CRD (Control Rod Drive) housings. The average under water dose rate at the RPV bottom head region decreased to approximately 1/40th of its original reading.
Shroud Support Machining Tool

Shroud Support Machining Tool 1

 

The new core shroud must be installed and welded to the existing shroud support. Therefore, it is necessary to precision machine the upper face of the shroud support to an exact flatness. Toshiba has developed the Shroud Support Machining Tool to perform this operation.

Installation into the RPV

Installation into the RPV

After installation using the overhead crane, the milling head of the shroud support machining tool turns circumferentially to machine the upper face of the shroud support. The machining tool is operated under full designed to remote control and machine both in air and under water.

Shroud Support Machining Tool

Shroud Support Machining Tool 2

Shroud Support Machining Tool 3

  • Full penetration welding is performed between the lower face of the core shroud and the upper face of the existing core shroud support.
    To accomplish this operation, Toshiba has developed Remote Automatic Welding Machines for OD (outside diameter) and ID (inside diameter) welding.
  • The OD welding machine was designed to be very thin in order to operate in the narrow gap between the core shroud and 20 jet pump assemblies.
  • After fabrication and assembly, a full size mockup test was done to verify the performance, operability and handling of this machine.
Narrow Groove Welding

Narrow Groove Welding 1

Narrow Groove Welding 2

  • The new core shroud is welded to the existing shroud support cylinder with a new narrow groove welding technique.
  • The welded joint of the new narrow groove weld has less of a bevel angle than the original weld design. This makes it possible to reduce the heat input and shorten the welding time.

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