[en] Models being developed for numerical simulation of ductile fracture require a number of input parameters, which are known as micro-mechanical parameters. The micro-mechanical parameters of the primary heat transport (PHT) piping material of pressurised heavy water reactor (PHWR) are not known. A study has been carried out to understand the micro-void development and to determine the three important micro-mechanical parameters of this material namely, initial void volume fraction (f_o), critical void volume fraction (f_c) and void volume fraction at final failure (f_F). Initial void volume fraction (f_o) was determined by measuring inclusion volume fraction by quantitative image analysis of photomicrographs taken on the as polished surface of the samples cut of PHT piping piece. Micro-void development in the material was studied using scanning electron microscope (SEM) on specimens sectioned from the tensile tested specimens. For this purpose, nine smooth round bar tensile testing specimens were subjected to uniaxial tension test. Six samples underwent interrupted test while three were loaded till fracture occurred. The critical void volume fraction (f_c) and the void volume fraction at fracture (f_F) were estimated from the fracture surfaces of the tensile specimens. The study of sections from the interrupted tests showed that voids were present in the material around inclusions when the diameter reduction during tension test was 10.5% or more. However, no micro-voids were seen in the specimens in which the diameter reduction was 4.5%. The mean values of the three micro-mechanical parameters f_o, f_c and f_F from the study were found to be 0.0011, 0.118 and 0.368, respectively. The average void volume fraction at a location where the diameter reduction was 44% was found to be 0.00463 which is more than four times the initial void volume fraction. (author)