RELAP5/Mod3.3 thermal-hydraulics characterization of the steam generator mock-up during operational transients in STEAM facility in support of the design of the DEMO WCLL BoP

•STEAM will host the mock-up of the EU-DEMO Steam Generators.•STEAM primary loop has been investigated with RELAP5/Mod3.3 system code.•DEMO normal operations have been studied including pulse-dwell transitions.•Different primary loop management strategies have been numerically simulated.•Setting the primary average temperature minimizes the loop operative fluctuations.