This paper describes the dimensioning of the EVITA semi-open loop, which has been built at BR2 to irradiate full size JHR fuel elements and qualify them for the foreseen exploitation parameters. One explains here the followed methodology to quantify the required additional head for the booster pump and to determine the pressure profile along the circuit and the safety margin on the fuel. This methodology relies only on a priori calculations without any measurement on full size installation subpart as usual before the assembly in controlled zone. The article also explains how the original JHR thermal hydraulic safety calculation scheme was adapted to the BR2 environment. One also compares the measurement results on the fully built installation with our previsions. Our models compare well with the experimental results, excepted for the pressure loss on the fuel plates.