Draining of primary cooling circuits in actively cooled reactor components: modelling the Electrostatic Residual Ion Dump of the ITER Neutral Beam Test Facility

Draining is an important process for in vessel actively cooled reactor components exposed to effects of nuc lear reactions. Activated corrosion products formed by the water interaction with metals in cooling circuits shall be controlled through coolant evacuation and component confinement in order to avoid spreading of contamination. The process shall be applied to the primary cooling circuits and to the in vessel actively cooled components of the nuclear fusion experiment ITER, i ncluding the components of the Neutral Beam I njectors whose prototype MITICA is under construction at the ITER Neutral Beam T est F acil ity in Padova. This work is focused on the Electrostatic Residual Ion Dump of MITICA whose cooling circuits must be drained before carrying out maintenance and inspection operations. Injection of pressurized nitrogen as inert gas is foreseen for blow ing ou t of water from cooling circuits . The process is simulated by means of transient two phase analyses with identification of flow regimes Parametric simulations as function of the inlet pressure are carried out to characterize the blowing out process in te rms of evacuation time and draining efficiency. The gas and water flow rates are evaluated recognising Rayleigh Taylor instabilities which occur when the light fluid (nitrogen) is pushing the heavy fluid ( The large diameter pipe (up to DN200) limit s the maximum allowable pressure for blowing out to 5 bar(g) in order to design and manufacture the draining system in accordance with the Sound Engineering Practice (low hazard level without CE marking applying the Pressure Equipment Directive Among all the applied draining pressures in the range 1.5 5 bar, 3 bar is selected to perform the blowing out because the margin with respect to the PED limit is consistent with simulated fluctuations W ith this boundary condition and a nitrogen flow rate of about 3 kg/s the overall draining time is estimated in 37 s with a draining efficiency parameter of 99%.