Abstract: Once-through steam generator（OTSG） is one of the main equipments of ACPR50 S small modular reactor.High flux of neutrons in OTSG will cause steam activation,increase the dose rate at main steam outlet,thus polluting entire secondary loop of the reactor.We carried out a preliminary analysis of ¹⁶N generation and transport based on physical calculation of current Shielding design of OTSG.The results show that the neutron flux magnitude in the OTSG has been reduced to a low level.Activity of ¹⁶N in the OTSG outlet is approximately tens of Bq/cm³.The results indicate that the material and thickness of current shielding structure of OTSG is satisfactory to meet the preliminary design goal under normal operating conditions.