A method for dismantling a steam generator or heat exchanger, such as found in nuclear power plants, which steam generator or heat exchanger includes a plurality of primary circuit tubes with a contaminated inner surface and wherein one or more tubes are sealed with a plug at both end is provided, the method comprising a) opening one or both ends of each sealed tube by creating an opening in or removing, the plug (13); b) introducing a viscous polymer to cure inside the tube wherein the polymer fills the tube across the full tube cross-section at least at the tube ends, immobilizing contaminations in the filled portion inside the tube (11); c) curing the polymer, then detaching the tubes with cured polymer the detached tubes being sealed by the polymer d) sorting out the detached tubes with polymer.