Failure Analysis Of The Latch Arm Of CRDM In Nuclear Reactor And Study On Its Typical Materials’ Wear Mechanism

Nuclear reactor control rod drive mechanism(CRDM)is an important device for nuclear reactor reactivity control.The latch arm component is the motion executive component of CRDM.As the core component of CRDM,the latch arm needs to bear the dynamic load of the drive rod and the control rod during the operation of the CRDM,which causes the latch arm to wear.In this study,the failure analysis of the lifting latch arm samples after the 6.5 million and 10 million steps thermal life tests were carried out to study the wear evolution trend of the latch arm during service.The wear mechanism and evolution trend of three kinds of cobalt-based alloys(oxy-acetylene welding cobalt-based alloy:Stellite6-OAW,casting cobalt-based alloy:Stellite6,forging cobalt-based alloy:Stellite6B)under different impact kinetic energy are studied.The following conclusions are obtained:(1)The failure analysis of the latch arm samples after 6.5 million steps and 10million steps thermal life tests shows that the wear amount of the upper tooth surface is greater than that of the lower tooth surface,and the wear amount of the upper tooth is the largest.In the process from 6.5 million steps to 10 million steps,the part with the largest increase in wear is the lower teeth.The results proved that the double-toothed latch arm has a mechanism of mutual wear of two surfaces(the upper tooth surface of the upper and lower teeth),and alternately withstands material removal,which proves the superiority of the double-toothed latch arm structure.(2)The main wear mechanism of the tooth surface of the lifting latch arm is plastic deformation caused by impact wear,and repeated plastic deformation results in a smooth wear scar surface.The high strain on the carbide surface caused by mechanical impact wear will cause the carbide to fracture,and corrosion will make the carbide more likely to peel off.(3)The impact wear test results show that with the increase of impact kinetic energy E_i increases,the energy absorption rates of the three cobalt-based alloys all show a trend of first decreasing and then increasing.The energy absorption rate of the Stellite6-OAW alloy is the highest,but due to the hardness of the material itself influencing factors such as the size and distribution of carbides in the alloy,the wear of Stellite6-OAW alloy is the lightest,followed by Stellite6B alloy,and the wear of Stellite6 alloy is the most serious.The wear of the three materials all show the phenomenon of first intensifying and then reducing,mainly because as the impact kinetic energy E_i increases,the energy absorbed in a single impact cycle also increases,resulting in a corresponding increase in the energy used for material removal,so the wear gradually increased.When the impact kinetic energy continues to increase,the degree of plastic deformation of the grinding pair material is correspondingly increased,resulting in a smaller average contact stress and a decrease in the degree of wear.(4)The wear mechanism of Stellite6-OAW alloy is mainly to produce plastic deformation and stacking of materials,accompanied by a slight oxidation reaction.The wear mechanism of Stellite6 alloy is mainly to produce plastic deformation and the occurrence of carbide fragmentation,and the formation of delamination.The main wear mechanism of Stellite6B alloy is plastic deformation and material stacking.Among them,based on all the results,the impact and wear resistance of Stellite6-OAW alloy is significantly better than that of Stellite6 alloy and Stellite6B alloy.