This paper presents a summary of current earthquake design criteria used in former Soviet Regulations for equipment and piping systems of nuclear power plants in light of those used in United States and Japan. The detailed comparative seismic analysis of PWR (WWER) Primary Coolant Loop System (PCLS) according to Former Soviet (Russian) PNAE Code and ASME BPV Code with some comments regarding to Japan Code JEAG - 4601 was undertaken for better understanding of the differences and coincidences of seismic design criteria and requirements. The selection of these three guides for the study has very simple explanation: according to ASME BVPC, JEAG and PNAE the huge majority of existing NPPs has been designed.