Pulse generated in the Transient Reactor Test facility at Idaho National Laboratory can reach a minimum full width at half maximum of 100 ms. for a maximum power of 5500 MW with the current configuration. To reproduce conditions of a Reactivity Insertion Accident postulated for Light Water Reactor, the width of the pulse shall decrease down below 50 ms. to increase even more the maximum power. To reach this goal, it is intended to initiate higher power transient that could quickly be clipped to maintain the total energy deposition within the 2500 MJ limit of the reactor operation license. One of the pulse clipping options under consideration is the implementation of a fast-negative reactivity insertion system by injecting helium 3 in the core. Initial calculation shows that to ensure fast clipping of the pulse, the new system, in a form of a cartridge pre-inserted in the core, shall undergo a quick pressurization from near vacuum to minimum density of 2.2 kg/m3 in less than 5 ms. The HENRI (Helium 3 Negative Reactivity Insertion) facility has been designed and built at the Oregon State University to test the feasibility of such system, design a fast response pressurization system and test the repeatability of the process. To support this task, preliminary CFD calculations are performed. In this first stage of the project, only the gas dynamics part of the process is considered. Initial results show that the ultimate atomic density could physically be reached within the 5 ms. limit assuming an ultra-fast opening device.