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Nuclear Regulation Authority Outline of the New Safety Standards for Light Water Reactors for Electric Power Generation
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GA 1 - 2 - 3 - 4 - AP 1 - 2 - 3 - 4 - 5 - NT 1 - 2 - 3 - 4 - 5 - 6Vol.1No.2
Vol.1, No.1, NT3 EJAM(1-1-NT3) - Seal-Welding over Cracks for Preventing SCC Propagation

Seal-Welding over Cracks for Preventing SCC Propagation

 
Takashi Ito, Hitachi-GE Nuclear Energy, Ltd.
Tooru Ootsubo, Toshiba Corporation
 

KEYWORDS:
seal-welding, SCC, overlay

 
1. Technical summary
 
Classification
(I: Inspection, II: Repair,III: Replacement, IV: Preventive Maintenance, V: Others)
 
This repair method overlays and seals an existing crack to prevent its contact with the reactor water. It aims to prevent propagation of stress corrosion cracking (SCC). SCC resistant materials are used for overlaying. Laser welding and TIG welding have been developed as overlaying method.
Seal welding method does not need machining / grinding for removing cracks, preparing grooves, nor buttering by weld.
This simple method can be applied to narrow location such as lower plenum of BWR, and it takes short time to complete repair work.

 

Figure1

Fig. 1 Concept of Seal Welding

Figure2

Fig. 2 Image of repairing work for CRD stub-tube

 
2. Scope

(1) Components
BWR and PWR core internals, including pressure boundary components. Core shroud, shroud support, control rod drive (CRD) housing and stub-tube , for example

(2) Materials
Both base metal and weld of austenitic stainless steel and high nickel alloy

(3) Condition
This method can only be applied to location where the necessary structural strength is ensured by the remaining portion.


Figure3

Fig. 3 Lower Plenum of BWR

 
3. Features

(1)Prevent propagation of exiting SCC crack without removing it
(2)Applicable to narrow location and complicated configuration
(3)Need short time to complete repair
(4)Repair work can be done whichever in the air or in the water

Figure4

Fig. 4 Image of repairing work for CRD stub-tube

Fig5_1 Fig5_3
Fig5_5
Fig5_2

Fig. 5 Examples of CRD stub-tube seal welding mock up


4. Examples of Application

The method has been applied to a penetration of the control rod drive mechanism of reactor vessel head of a PWR in Japan.

 
5. Reference
Guidelines on repairing methods (seal welding) (JANTI-VIP-01), Japan Nuclear Technology Institute (in Japanese )
 
6. Contact
Japan Society of Maintenology (ejam@jsm.or.jp)