Thermal-hydraulic assessment of Once-Through Steam Generators for EU-DEMO WCLL Breeding Blanket primary cooling system application

Once-Through Steam Generators (OTSGs) were recently selected to be installed in the Primary Heat Transfer Systems (PHTS) related to EU-DEMO WCLL Breeding Blanket (BB). Referring to the Balance of Plant Direct Coupling Design (BoP-DCD) option, these components are used to deliver the thermal power removed from the two principal blanket subsystems, i.e. First Wall (FW) and Breeder Zone (BZ), to the Power Conversion System (PCS). OTSG design foresees a vertical component with primary water moving downward inside tube bundle and secondary fluid flowing throughout shell in counter current. The latter moves firstly downwards through the annular downcomer and then goes up in the central riser, where it boils up to super-heated steam conditions. Then, it is deflected by the upper tubesheet, flows again downwards along the steam downcomer and exits through the laterally connected steam nozzles. In the last years, as part of the research activities associated to the Work Package Balance of Plant (WPBoP), a detailed analysis of the steam generator, aimed at deeply understanding the main thermal-hydraulic aspects characterizing its performances during the pulsed regime of DEMO normal operations, was performed. To fulfil this scope, a complete model of the component was prepared by using the best-estimate system code RELAP5/ MOD3.3, selected to be also the reference design verification tool. During operations, OTSG experiences full-load and low-load alternative phases with rapid transitions from one another. The steam generator capability to follow such load variations avoiding the occurrence of instabilities in the component was studied. The computational activity carried out was a preliminary thermal-hydraulic char-acterization of the component conceptual design during both pulse and dwell phases. The feedbacks of this analysis were also fundamental for the conceptualization of the steam generator mock-up to be installed in the planned STEAM facility that will be built at the ENEA Research center of Brasimone.