Assessment, uncertainty, and sensitivity analysis of an Intermediate Break LOCA in the LSTF facility with the code CATHARE Thermal-hydraulic simulations are required to demonstrate the safety of a Nuclear Power Plant in both normal and accidental conditions. These simulations are performed for the French nuclear fleet with the system code CATHARE, which is developed at CEA in the Thermalhydraulic and Fluid Mechanics Section (STMF). The CATHARE code is based on six conservative equations (mass, momentum, and energy for liquid and vapor phase) with semi-empirical closure laws that require validation and uncertainty quantification. This 6-months work is realized at STMF within an international OECD/NEA project, called ATRIUM (Application Tests for Realization of Inverse Uncertainty quantification and validation Methodologies in thermal-hydraulics).