Feasibility study for cryogenic pellets production with pure gaseous helium cooling for ITER disruption mitigation system
Résumé
CEA/DSBT designed and operated a pellet injection test bench cooled by liquid helium (LHe) to study the production and the acceleration of large cryogenic pellets using protium, deuterium, neon and mixtures for the ITER Disruption Mitigation System (DMS). The ø28.5 mm protium pellets were successfully formed in less than 30 minutes and accelerated above 500 m/s. However, LHe will not be available in the ITER tokamak and shall be replaced by a supercritical helium (SHe) circulation.
To evaluate the feasibility of SHe cooling, the CEA/DSBT test bench was adapted and operated with gaseous helium (GHe) at 1.25 bar. The main difference between GHe and LHe cooling is the temperature variation induced by the thermal heat removal. In LHe cooling, the heat exchange occurs at a constant temperature due to the presence of the latent heat (liquid to gas). In the opposite, in GHe cooling, only the specific heat is available for heat removal resulting in a temperature increase. This temperature rise is a drawback during the pellet formation, where the cell temperature should be kept as low as possible.
This paper presents a comparison study with LHe and GHe cooling using the current cold cell. The key parameters reviewed were the cell temperature, desublimation pressure, shell thickness and mixture percentage. Protium pellets were the most extensively compared, as they are the fastest to produce and the most characterised with LHe. All other types of DMS pellets were briefly studied. The results from the GHe experiments showed similar formation duration, pellet aspects and speeds as those with LHe. This proof of principle for gaseous cooling is a significant step in the design study of the ITER-DMS cold cell.
This work is being carried out within the framework contract “Cold Head Development and Cryogenic Supply Assessment” between the ITER Organisation and CEA.