In-service Inspection of Flow-Accelerated Corrosion in VVER plants

Authors : V.I. Baranenko¹, Yu. Yanchenko¹
                 M.O. Ampilogov¹, V.A. Yurmanov²
                      

Company : ¹VNIIAES, Moscow                  
                   ² ENES/NIKIET,Moscow

Country : Russia

Topic :

Wall thinning management
Non-Destructive
Examination methods

ABSTRACT :

The report looks at in-service inspections of flow-accelerated corrosion (FAC) of corrosion/erosion-prone piping. Analysis is provided of the documentation used by NPPs to identify wall thickness and record in-service inspection findings and of the results of .in-service inspection of wall thickness in VVER-440 and VVER-1000 plants. Some wall thickness measurements are below nominal design values. Other wall thickness measurements were found to be above nominal design preoperational wall thickness. Wall thickness results below nominal values are due to erosion-corrosion, while wall thickness measurements above nominal design values are caused by the fact that ultrasonic inspections give values for both pipe wall thickness of unaffected metal and thickness of corrosion product deposits. Quantitative testing results are given that prove the said explanation using actual case studies involving secondary system piping having significant deposits in VVER-440 and VVER-1000 plants. This fact is important for proper interpretation of ultrasonic wall thickness measurements.

Pipe wall thickness measurements taken in the same points during different periods of plant operation were compared to identify the pattern of FAC intensity and deposition growth and loss (washout) depending on the mode of plant operation. Measurements taken on NB465×16 feedwater piping bends in 1995-2002 were given as an example. These measurements were taken in 28-30 axial sections, including 12 circumferential measurements (12 hours) taken every an hour (30º) in each section.  Control data was processed to identify that reduction in wall thickness and increase in wall thickness were found to be axis-wise in each of the 28-30 sections and circumference-wise in every hour of the 12 hour cycle during the period between the measurement dates. Reductions in wall thickness were identified in an average of ~45% reference points of the total amount of follow-on measurements, while increase in wall thickness was noted in the majority locations. Over seven years of plant operation maximum reduction and increase in wall thickness were 1.3-4.3 mm and 1.6-4.7 mm, respectively.