A Study on the straight double-walled tube steam generator design against sodium-water reaction in DECs

DECを想定したナトリウム-水反応に対する直管型2重管SGの設計に関する研究

神田 大徳; 江沼 康弘  ; 二神 敏  ; 河村 雅也*; 宇敷 洋*; 小雲 信哉*; 市原 隆司*; 中島 崇*

Kanda, Hironori; Enuma, Yasuhiro; Futagami, Satoshi; Kawamura, Masaya*; Ushiki, Hiroshi*; Ogumo, Shinya*; Ichihara, Takashi*; Nakashima, Takashi*

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. In the tube failure accident within a SG, sodium-water reaction occurs. By this reaction, the tube failure may propagate to adjacent tubes resulting in a large scale tube failure. 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. 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 sodium. 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 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 decreased on the interface between the inner and the outer tubes by applying heat treatment using hydrogen furnace for the tube element production.