Structural Design and Evaluation of a Steam Generator in PGSFR

A once-through steam generator in sodium-cooled fast reactor converts the sub-cooled feedwater to superheated steam by transferring heat from sodium in the intermediate heat transport system to water/steam and provides superheated steam under normal power conditions. It is a heat exchanger as well as a structural barrier between liquid sodium and water/steam and thus is regarded as one of the most critical components in the sodium-cooled fast reactor deciding the plant reliability and availability. The prototype Gen-IV sodium-cooled fast reactor(PGSFR) employs a vertical once-through shell-and-tube heat exchanger which has a sodium-to-water counterflow with single-walled straight heat transfer tubes. Its construction material is the 9Cr-1Mo-V steel for high heat transfer performance and high allowable strength at elevated temperature. It uses the straight tube so that it is possible to apply the single piece tube without any weld for intertube joint. Since straight tubes do not provide enough flexibility to accommodate the longitudinal expansion difference among tubes, flow distributors are applied for uniform flow at inlet shell. And the expansion bellows joint was joined on the main shell to provide the large flexibility to compensate for differential thermal expansion between shell and tube bundle. In this study, structural design for a PGSFR SG was described and structural integrities against representative operating duty cycle event were evaluated by ASME B&PV Section III Division 5. From the evaluation results, the structural design issues were drawn and feasibility of design improvement was discussed. (author)