Erosion corrosion of secondary circuit piping is a serious problem in PWRs. There has been some incidents and accidents in NPPs were people have died due to breaks of piping in the secondary circuit (Surry, Fukujima etc.). In Loviisa NPP in Finland there were 2 guillotine breaks in the feed water lines in the 1990s. The conditions in the secondary circuit of the VVER 1000 units are practically similar to the VVER 440 units. Accordingly there is a potential risk of flaw induced wall thinning of secondary piping of the SUNPP.
The erosion corrosion problem can be managed by improving the water chemistry, changing material of piping and by monitoring wall thinning by measuring the wall thickness of selected potentially dangerous parts. In VVER 1000 the water chemistry is neutral according to design specification. By increasing the pH to above 9, the wall thinning in the secondary piping will decrease remarkably. With a basic water chemistry the corrosion problem will be shifted to the Cu/Ni condenser tubes. Furthermore dissolved Cu can adhere to steam generator tubes (stainless steel) causing pitting etc. corrosion on those tubes. If the plant has condenser tubes of other material than Cu/Ni, (e.g. Ti or SMO; Mo alloyed stainless steel) one can play with the water chemistry and use more basic water. This can be managed by adding hydrazine or morfoline to the secondary water. As a result the formation of magnetite on the pipe surfaces will increase and protect the piping from erosion. In most of the VVER plants, however, the condenser tubes are made of Cu/Ni, so the higher pH cannot be utilized. For these NPPs a pipe wall thickness monitoring management is extremely important. That is the main reason to the supply of ultrasonic equipment for monitoring pipe wall thickness in SUNPP in this project.
The main objective of this project was to provide ultrasonic transducers and associated equipment for measuring of wall thickness of the secondary piping at SUNPPs. The aim is at elaborating an In-service Inspection procedure for monitoring erosion corrosion of selected pipes of the secondary circuit and reduce the risk of pipe breaks in future operation.
The delivery included a complete set of ultrasonic transducers including connecting cables. The following probes were delivered:
- 2 units of bi-crystal, straight beam probes with refracted longitudinal, 5 MHz frequency and contact area around 12,5 mm;
- 2 units of bi-crystal, straight beam probes with refracted longitudinal 10 MHz frequency and contact area of 7,5 mm;
- 1 unit of bi-crystal straight beam probe with refracted longitudinal, 5 MHz frequency contact area of around 14 mm, temperature limit 600 degrees C;
- 1 unit of mono-crystal, normal beam probe with refracted longitudinal 4 MHz frequency and crystal diameter around 24 mm;
- 1 unit of mono-crystal transverse beam probe refracted by 45, 60, 70 and 80 degrees 4MHz frequency and active element size around 8*9 mm (total 8 probes).
A more detailed description of the scope of supply of equipment is given in the Technical Specification (TS). The delivery of the specified transducers, cables, hard- and software, etc. was completed according to the scope of the TS.
Quality of the results, lesson learned, recommendations for follow-up
The project included only supply of equipment. The delivery of the specified equipment was successful.
Further information on the project results could be sought from the JRC/IE archive or from beneficiary organizations.