Assessment of Hill-Side Effect on RV CEDM Nozzles

CEDM (Control Element Drive Mechanism) controls the power of nuclear power plant by absorbing neutrons produced by nuclear fission. CEDM nozzles are installed in the reactor vessel head by shrinkage fitting and each CEDM nozzle has different length and passes through the spherical head at a different angle which may cause different stress distribution. Formerly, CEDM nozzle at the center line had been considered as a nozzle which is in the most severe condition and also it was selected to get the maximum interaction effects between the discontinuity regions. However, recent reports on nozzles which are installed in a dome infer that there are hill-side effects which may cause higher stresses than stresses on center lined nozzle and those effects should be considered. In this paper, stress distributions at three different locations of the closure head with the CEDM nozzles were evaluated. As a result, the stress intensities at a particular location of the nozzles on the hill-side are higher than those of a nozzle on the center line.