The Fukushima accident reveals the need for additional safety systems for nuclear power plants. One promising option is the supercritical carbon-dioxide (sCO2) heat removal system, which consists of a simple Brayton cycle. This study provides an overview of the extensions and validation of the thermal-hydraulic system code ATHLET for the simulation of sCO2 power cycles, especially with regard to the sCO2 heat removal system. The properties of CO2, heat transfer and pressure drop correlations, as well as compact heat exchanger and turbomachinery modelling are considered.
The supercritical carbon dioxide (sCO2) heat removal system, which is based on a closed Brayton cycle with sCO2 as a working fluid, is an innovative, self-propelling and modular heat removal system for existing and future nuclear power plants. By changing the number of CO2 cycles, the heat removal capacity can be adapted. In this paper, up to four sCO2 cycles are analyzed in interaction with a pressurized water reactor, using the thermal-hydraulic system code ATHLET and considering a long-term station blackout and loss of ultimate heat sink scenario with conservatively high and low decay heat curves. The presented start-up procedure for the heat removal system might require further optimization due to the non-linear thermal gradients. Independent from the start-up, a heat removal system with three or four CO2 cycles keeps the primary loop temperatures sufficiently low. However, with only three cycles, the core is almost uncovered, and the danger of recriticality may occur due to cold leg deboration. Controlling the turbine inlet temperature via the turbomachinery speed and subsequent shutdown of single cycles successfully adapts the operation of the heat removal system to the declining decay heat. This enables reliable decay heat removal for more than 72 h.
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