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discovered 02_2012

discovered 02.12 COLLABORATIONS WWW.Hzdr.DE Thermal shock with consequences One safety issue concerning the emergency core cooling is the potentially large impact on pipes and structural components through a sudden cool-down. This applies especially to the walls of reactor pressure vessels as, following many years of constant operation, they have been bombarded with neutrons. A change in material properties (e.g. increasing embrittlement) could be the consequence. Furthermore, directly after an emergency shutdown, the pressure vessel still has temperatures extending well beyond 200 degrees Celsius. If the subcooled emergency water, which according to the plant can have temperatures between 10 and 60 degrees, were allowed to flow into the reactor vessel without being mixed, its wall would be exposed to considerable thermo-mechanical stress. The sudden cool- down of the wall (also known as PTS – pressurized thermal shock) could under certain circumstances lead to a cracking fissuration or even to the failure of the pressure vessel. The risk of such damage depends, on the one hand, on the wall’s current mechanical properties and on the other hand on the temperature of the water entering the reactor pressure vessel. This temperature is a result of the flow mixture from the steam as well as the cold and hot water inside the main coolant pipe and is therefore governed by the thermal fluid dynamics of the fluids inside the pipe. Researchers at the HZDR have been investigating this kind of scenario for years. The Structural Materials Division has been studying the fracture mechanical properties of the vessels’ material, while researchers in the Fluid Dynamics Institute have been looking into how well the cold emergency water mixes with the hot water in the main pipe. The Experimental Thermal Fluid Dynamics Division as well as the Computational Fluid Dynamics (CFD) Division at Dresden’s Helmholtz Center are both active in this field. The analysis of failure sequences as well as the evaluation and optimization of emergency measures is the specialty of the Reactor Safety Division. Investigations covering plant dynamics and the insights gained from experiments on thermal shock as well as CFD simulations and the emergency cooling supply help to increase the safety of nuclear plants today and in the future. Mixing phenomena and plant safety The mixing of cold and hot water in the main coolant pipe is one of the phenomena of fluid dynamics, on which the complex flow dynamics in the emergency core cooling and thus the safety of the reactor components ultimately depend. The water inside the pipeline evaporates at a certain temperature depending on the prevalent pressure. The cold emergency coolant that streams in comes into contact with PRESSURE TANK: Functional principle of the test facility for analysis of thermal shock scenarios inside a pressurized water reactor.