Navigation path

Qualification of new materials for replacement/repair of original materials in VVER components

 
Qualification of new materials for replacement/repair of original materials in VVER components

Description

 
The Project and had the purpose to qualify new welding material for VVER plants repairs. The developement of such qualification was necessary because the original welding material couldn't be procured any more by the Utilities or the manufacturers. In the frame of this PHARE Project three types of most probable repairs were identified and the associated welding materials were defined on the base of the requested design properties and on existing Nuclear Welding Materials already implemented on Western plants.
The Reactor Pressure Vessel (RPV) where suspected underclad cracks could need repair. The cracks could be located in the base material of the shell just below the stainless steel cladding of 8 mm thickness.

  • The Steam Generator (SG) collector made of stainless steel and welded on the vessel made of carbon steel

 
 
 
The TBT was retained for his simplified implementation which is always preferred for on site repairs.

Concerns about coarse grain zone under the fusion line of the first layer deposit were also investigated and several tests performed by Skoda showed that in the condition of repair implemented no coarse grain area appeared in the 15Kh2MFA ferrite base metal HAZ, under the first layer deposited, and the welding conditions of the second layer had no influence on the properties of the HAZ.
The last issue was to check to be sure that the choice of the material selected to be qualified was satisfactory was related to the specific mixing of the Nickel base alloy (Inconel 52 type) with the cladding of the RPV. The cladding used on VVER is a Niobium stabilized Stainless Steel never used in France combined with Nickel base Alloys. It was veryfied that no sensitivity to hot cracking in the mixing area occurred as no defect or singularity was recorded in this specific area.
As a conclusion, it was confirmed that the selected materials, had the required properties for their respective repairs. These properties were:

  • best behavior with the medium in contact with the new material (corrosion, fatigue corrosion)

  • adapted mechanical properties in relation with the design requirements.

Two precautions were suggested consequently to the implementation of the qualification program.

  • <span style="" font-size:="" "="">The use of Inconel 52 type material for RPV underclad repair is a very good compromise. In the same way, the mechanical properties of the Inconel are little lower than those of the 15Kh2MFA base material but higher than those of the original cladding. For this reason, before any implementation of this repair it was suggested to perform previously a specific design review of the considered repair.

Beneficiary Organization Details
Dukovany NPP (Czech Republic)
http://www.cez.cz
Project Aims
Components of VVER reactors were designed and manufactured in accordance with old Soviet rules.
These rules specified lists of materials and welding consumable which only can be used for manufacturing or repairing these components. Any other material was defined as a "new material" and had to be qualified, before its use, according to Soviet requirements.
At the time of the project, due to the situation in Eastern countries, some of these materials were no longer produced and their replacement by similar materials according to PWR rules was the sole solution.
Nevertheless, direct application of materials used for PWR, qualified in accordance with mechanical design codes was not possible because Soviet rules did not consider them and because no practical procedure existed to define the requirements to be fulfilled to obtain the qualification.
The objective of the PH2.03/97 project was to establish a "Simplified Qualification Programme" and to implement it for some attractive materials.
Project Results
The project was completed according to the specified Terms of Reference. A simplified qualification programme for replaced welding materials was developed for the following VVER-440 activities:

  • repair of reactor pressure vessel;
  • repair of weld in austenitic stainless steel piping;
  • repair of dissimilar weld in steam generator.

A proposal for Qualification Test Programme was developed and submitted to the Czech Nuclear Safety Requlatory Authority (SUJB).
Further Project Results
Further information on the project results could be sought from the beneficiary organizations.


General Information

Title: 
Qualification of new materials for replacement/repair of original materials in VVER components
Programme: 
PHARE
Budget year: 
1997
Countries: 
Czech Republic
Types of activities: 
Design safety
NPP: 
Dukovany
Installation types: 
VVER 440-213
Duration (months): 
180
Contractors: 
NRI Rez
Status: 
Completed
Old reference: 
PH2.03/97
Effective contract date: 
01/07/2000
Closure date: 
31/12/2001