Life Evaluation of a Nozzle Charged by Low Primary Loads and Severe Cyclic Thermal Shocks

The paper presents the results of both elastic and inelastic analysis of a nozzle provided by an inner thermal baffle subjected to low primary loads and to severe cyclic thermal downshocks. The structural component is in fact charged by cold thermal shocks that are present in PEC reactor mechanisms for possible scrams and by loads deriving by the operative service of the sodium plant placed in C.R.E. Casaccia to which is connected. The nozzle has been instrumented on the outer surface with thermocouples and resistive strain gages in axial and circumferencial directions; the operative service conditions were verified in a series of ten experimental cycles. The study of the stress strain fields in the component needed both a flexibility piping analysis for the definition of the charges deriving from the plant service and a thermo-mechanics analysis of the stresses caused by the thermal transient. A refined non-linear thermal effects analysis was performed giving great care to the study of the thermal effects on the component caused by the little sodium by-pass flow between the nozzle itself and the thermal baffle.