SFA inspection and repair stands for power-grade reactors
Together with Gidropress EDB, OKBM and SSC RIAR, R&D Company "Sosny" is involved in development of stands for inspection and repair of irradiated fuel assemblies of power reactors. Sosny specialists carried out a feasibility study and gave justification to a concept of VVER-1000 SFA inspection and repair for Ukranian and Bulgarian NPPs. Under the NPP-2006 Project they also developed a preliminary design of a control and measurement system for the repair and inspection stand. In 2009, systems for ultrasonic leak test and eddy-current defectoscopy of the TVSA fuel claddings at the Kalinin NPP and an ultrasonic system for fuel rod leak tests at the Temelin NPP, the Czech Republic, will be commissioned.
The developed equipment and methods allow for an express nondestructive assay of spent fuel in the NPP cooling pools, in particular:
- visual examinations of SFAs and fuel elements;
- dimensional measurements;
- detection of leaky fuel elements in SFAs;
- eddy-current defectoscopy of fuel elements.
The concept of inspection and repair stands adds to the number of SFA and fuel rod inspection methods available. The new methods and equipment significantly increase operational and cost effectiveness of the VVER fuel inspection process as compared to traditional ones.
Sosny specialists developed a system for eddy-current defectoscopy of fuel claddings to detect and identify uniformity defects (cracks, corrosion, hydrogenates) on the external and internal surface of the claddings. The fuel elements undergo eddy-current defectoscopy after they are removed from a SFA. The system response characteristics comply with applicable international standards (ASTM E 426-98, JIS G 0583-2004, etc).
The patented method for detection of leaky fuel rods in SFAs (Patent No.2262757 РФ G21 c17/06, 17/07) was tested on nine leaky SFAs of the VVER-440 and VVER-1000 reactors. The test results showed high performance and efficiency of the method and allowed for applying it as a standard technique for the VVER fuel examinations at SSC RIAR.
The method is based on under-cladding water detection. A tiny ultrasonic sensor on the upper plug of the fuel element excites ultrasonic waves in the cladding and registers them after they travel downward and back. The water available under the cladding of a leaky fuel element causes power dissipation in the fuel element and an abrupt decrease of the signal amplitude.
The ultrasonic fuel element leak test system for the TVSA-T inspection and repair stand for the Temelin NPP developed by R&D Company "Sosny" includes a leak test manipulator that has 52 ultrasonic sensors. Inspection of all 312 fuel rods of a fuel assembly requires a consecutive installation of the manipulator in six different orientations, each time with a 60° rotation about the fuel assembly (FA). Special software controls the consecutive connection of the ultrasonic sensors to the measurement system and determins the signal amplitude. The software can print out and save the results as a chart of the FA cross-section indicating tight and leaky fuel rods.
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