Many of the works presented in literature for the analysis of flow stability with supercritical pressure fluids do not make any allowance for the presence of heating structures, whose role is considered somehow negligible considering similar experience achieved for the boiling channel cases. The present work, actually, demonstrates the contrary, providing a warning to analysts and designers of experimental apparatuses. The work is the follow-up of previous analyses with an in-house code and CFD codes for studying natural circulation, in which the presence of heating structures was identified as an important contributor in the prediction of stability, as it could have been recognised since the very beginning. After using CFD codes in previous works, the RELAP5/MOD3.3 is here used as a tool to analyse the effects of the above mentioned parameters. Numerical effects are also considered making use of the two adopted numerical discretisation schemes available in the code, being the semi-implicit scheme and the nearly implicit one. After examining these effects, in order to better discuss their impact, counterpart operating conditions are addressed for boiling channel instabilities, pointing out the different effects of the heating structures in the two cases.
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