Full scope scaling analysis and recommendations for PWR SGTR scenarios

Scaling studies were conducted in the field of nuclear thermal-hydraulics to analyze the scaling distortions in a Steam Generator Tube Rupture (SGTR) scenario. Test 4 of the NEA/OECD/ROSA-2 Large-Scale Test Facility (LSTF) was chosen as the basis for the analysis of scaling distortions. Firstly, a post test calculation of the experiment was performed with the RELAP5 system code along with a phenomenological assessment. This model was then scaled over a wide range of sizes to investigate the possible scaling distortions by means of the Power-to-Volume Scaling Tool (PVST) which is an in-house tool developed and validated by the Universitat Politecnica de Catalunya (UPC) for scaling nodalizations. The paper demonstrates that the main scaling distortion observed in Test 4 is attributed to the lack of preservation of the L/D ratio in the SGTR assembly nozzle. This rationale is quite important in the design of break units in experimental facilities to preserve the friction and the mass flows of the reference (prototype) plant and scenario. Subsequently, the paper presents a series of guidelines for minimizing scaling distortions and reports the results obtained by applying them, the proportional scaling distortion was reduced by more than tenfold. Additionally, the paper analyzes the RELAP5 models involved in the conservation of momentum equation for the SGTR assembly nozzle, and their potential scaling distortions.