A Study on the Straight Double-Walled Tube Steam Generator Design Against Sodium-Water Reaction in DECs

Abstract

The Sodium-cooled Fast Reactor (SFR) is one of the most promising concepts suggested for Generation-IV nuclear reactor systems. Some SFRs adopt Steam Generator (SG) as their heat exchange system between sodium and water. Sodium-water reaction occurs in the tube failure accident of a SG. The tube failure may propagate to adjacent tubes resulting in a large scale tube failure by this reaction. In an advanced loop-type SFR design promoted by Japan Atomic Energy Agency (JAEA), a straight double-walled tube SG is adopted to prevent this sodium-water reaction [1], [2]. The double-walled tube is expected to prevent water leakage by acting as double wall boundary and mitigate consequences of the sodium-water reaction. It is expected for the outer tubes to practically behave as waste resistant for the adjacent tubes to mitigate sodium water reaction consequences. Mitigation is expected in Design Extension Conditions (DECs) such as the loss of the mitigation function which might lead an initial water leakage to large scale tube failure. In addition to the prevention of the initial leakage, the initial water leakage rate is practically suppressed because of the narrow gap between the inner and the outer tube. In this paper, tube failure propagation has been calculated to assess property protection performance on outer tubes. The evaluation results showed that the total leakage rate is limited to one double-ended guillotine scale hence the double-walled tube SG has the property protection performance. By additional calculations assuming the loss of the mitigation function, a sever event in DECs is cleared. These calculations suggest that increase of the reliability of water blowdown system and enhancement of the pressure release system are effective for the boundary integrity between primary and secondary cooling systems. There is an issue to be addressed to adopt the concept described above, that is, the decrease of temperature difference between exchange tubes especially for structural integrity of the straight double-walled tube SG for its thermal contact resistance between double-walled tubes and its lack of bending part to release thermal stress. The dispersion of thermal contact resistance between tubes causes temperature difference there due to their heat transfer rate difference. To suppress this dispersion, the oxidized scale is reduced on the interface between the inner and the outer tubes by applying heat treatment using hydrogen furnace for the tube element production. Then, thermal contact resistance of the double-walled tube is successfully reduced at laboratory scale. Thus, these results suggest that the double-walled tube SG may suppress water leakage rate and sodium-water reaction consequences in DECs. Furthermore, temperature difference between exchange tubes due to oxidized scale on the interface between the inner and the outer tubes can be reduced at laboratory scale. Hereafter, the specifications of the double-walled tube SG will be determined including tolerance reinforcement of sodium boundary.