Development of Surface Modification Technology for CEDM Nozzle and Fatigue Enhancement of Ni-based Alloy

Abstract

Control element drive mechanism (CEDM) nozzle is manufactured as welded on the reactor vessel and currently uses Inconel 690 alloy. The top of the reactor is equipped with about 100 CEDM nozzles with an internal diameter of about 70 mm. Relatively large inlet/outlet nozzles are equipped with two outlet nozzles and four inlet nozzles on the reactor wall. The inner diameter of the nozzle is vulnerable to stress corrosion cracking (SCC) and fatigue. An ultrasonic nanocrystal surface modification (UNSM) treatment is applied to the inner diameter of the nozzle. The ultimate goal is to improve the service life of parts by introducing compressive residual stress and suppressing primary water stress corrosion cracking (PWSCC).
The main purpose is to design and fabricate a UNSM treatment device for the internal diameter processing of CEDM nozzles. In order to develop the UNSM system, the basic technology such as the development of UNSM tooling is developed and the mechanical properties and fatigue performance of Inconel 690 alloy before and after UNSM treatment are evaluated and compared. The inner diameter of the nozzle was treated by a newly developed UNSM treatment device under the optimized treatment parameters. It was found that the mechanical properties and fatigue performance of nozzle were improved in comparison with the untreated nozzle, which may be attributed to the increase in hardness and induced compressive residual stress.