Background: A connection between a low alloy steel pressure vessel nozzle and a stainless steel main pipe safe-end in a pressurized water reactor（PWR） nuclear power plant is a dissimilar metal weld

which is at key location of pressure boundary and serves in high temperature primary water environments.Since 1990

cracking failure incidents happened on the welds in the primary water coolant of several PWRs in the countries such as USA

Sweden and Japan.Purpose: This paper is to briefly review the typical cases involving this problem in engineering and relevant researches.Methods: Methods such as nondestructive testing

crack and microstructure observation

finite element analysis were used in the failure analysis.Results: The cause of the failure cases was primary water stress corrosion cracking（PWSCC）.Conclusions: It is concluded that the welds of 600 Alloy type such as 82/182 are susceptible to PWSCC but those of 690 Alloy Type such as 52/152 have excellent resistance to the cracking

and more researches are necessary.