Specifications for benchmark analyses of transient thermal-hydraulics in reactor vessel and primary heat transport system during decay heat removal operation

Kobayashi, Jun ; Tanaka, Masaaki  ; Hamase, Erina ; Ezure, Toshiki 

In a sodium-cooled fast reactor, a diversified auxiliary cooling system to remove decay heat from the core is required to enhance its safety. The decay heat removal systems (DHRSs) include a direct reactor auxiliary cooling system (DRACS) with a heat exchanger in the upper plenum (UP) of the reactor vessel (RV), a primary reactor auxiliary cooling system (PRACS) with a heat exchanger in the primary heat transport system (PHTS), an intermediate reactor auxiliary cooling system (IRACS) with a heat exchanger in the secondary heat transport system (SHTS), a heat removal system which employs a steam generator, and a reactor vessel auxiliary cooling system (RVACS) that effects cooling from outside the RV. In the operation of the DRACS with a dipped-type direct heat exchanger (D-DHX) installed in the UP of the RV (UP-RV), the thermal interaction, called core-plenum interaction (CPI), regarding the thermal-hydraulic phenomena in the UP-RV and the core is observed. The CPI includes the penetration flow of the sodium at a low temperature from the D-DHX into the core assemblies, the flow in the gap between assemblies, and the radial heat transfer through sodium in the gap. On the other hand, in the operation of the PRACS or IRACS, where the flowrate in the PHTS is maintained, the core coolability is affected by plant operating conditions. Two transient tests conducted at the PLANDTL-DHX sodium test facility in Japan Atomic Energy Agency were provided to develop an appropriate numerical analysis model for prediction of transient thermal-hydraulics in the DHRSs, the core, and the PHTS. In this document, the geometry information of the RV and the PHTS, including the heat exchangers for the DHRS, and the measured flowrate and temperature transients at each inlet of the intermediate heat exchanger (IHX) on the SHTS side and DHRS were specified as the boundary conditions for the benchmark analyses.