Study on Hydrodynamic Instability-based Critical Heat Flux Enhancement Mechanism of Nanofluids and Liquid Metal Fin Concept

Abstract

In the light water reactors, a number of safety systems have been installed to prevent the progression of the accidents and return to the safety condition. The CFD (core damage frequency) is low enough like 1.0E-4/RY and 1.0E-5/RY for OPR-1000, APR-1400 respectively due to the safety systems. However, although the safety systems have been established in the plants, some severe accidents happened. If the prevention of severe accident is absolutely impossible, it is required to terminate the progression of severe accidents through the power cooling system. While the molten fuel was adequately cooled and certainly retained in reactor vessel in the TMI nuclear accident, the molten fuel was escaped from the reactor vessel in the Fukushima nuclear accident. The different results between TMI and Fukushima accidents give one of the most important lessons that the integrity of reactor vessel should be protected to minimize the spreading of radioactive material. It is called as in-vessel retention (IVR) or in-vessel corium confinement and cooling. One of the effective features for the IVR is an external reactor vessel cooling (ERVC) strategy. In general, ERVC strategy gives sufficient thermal margin for small and medium-sized reactors like AP600 and AP1000. However, it is not certain whether the IVR-ERVC strategy provides enough thermal margin to prevent CHF phenomenon even for large-sized nuclear power plants. In this study, the nanofluid and liquid metal was presented as flooding material to ensure the safety margin to CHF. Nanofluid is a colloidal with dispersed nanoparticles. It is known that the nanoparticle coated layer induced by boiling was formed on heat surface. One of the most interesting characteristics of nanofluids is their capability to enhance CHF significantly at relatively dilute concentrations due to this layer. In this work, specific pool/flow boiling test facilities were designed for IVR-ERVC test. In pool boiling test, a variety of factors related to CHF were invested. The graphene oxide (3500~5300 W/mK), Al2O3 (30 W/mK), SiO2 (1.3 W/mK) nanofliuds were selected and prepared basis on the thermal conductivity. CHF tests of the pool boiling were conducted to select the prospective candidate for improved IVR-ERVC. The graphene-oxide nanofluid shows the highest enhancement (195 %) in term of CHF value. The effects of the heater inclination on CHF were investigated. The CHF enhancement ratio was decreased as the heater inclination was increased. The coolant contains the boric acid (H3BO3, 5000ppm), lithium hydroxide (LiOH, 3ppm) and tri-sodium phosphate (TSP, Na3PO412H2O, saturation). These chemicals were added to working fluid to make the real plant situation. Although highly-concentrated chemicals have some influences on CHF enhancement, the graphene oxide nanofluid is dominant factor to enhance the CHF. After boiling tests, a nanoparticles coated layer was observed on the heater surface. The enhancement of CHF is related to the buildup of a deposition layer of nanoparticles on the heater surface. The main reason of CHF enhancement is explained as increased wettability in many experiments. Reverse results were observed when the graphene oxide was coated on the heater surface. Some models related to improved wettability for nanofluids were limited to explain this unique case. A hydrodynamic instability theory is a traditional model to predict the CHF for pool boiling. Kevin-Helmholtz and Rayleigh-Taylor instabilities were included to establish the theoretical ground in this model. The test heater were designed and prepared to allow direct observation of the Rayleigh-Taylor wavelengths. Higher CHF results have shorter Rayleigh-Taylor wavelengths in all cases. The experimental correlation was presented based on experimental data from observing the Rayleigh-Taylor wavelengths. After analyzing the experimental results and conclusions about pool boiling, the flow boiling CHF tests were designed and conducted. The test facility was designed to be scaled-down to 1/25 of APR-1400 dimension. Under the ERVC situation, the natural circulation occurs between the reactor vessel outer wall and the surrounding insulation. To simulate this condition, a pump was used to control the mass flux passing over the heater surface. Working fluids are he graphene oxide (3500~5300 W/mK), Al2O3 (30 W/mK), SiO2 (1.3 W/mK) nanofliuds. Graphene oxide nanofluid show the highest CHF enhancement at 50 kW/m2 mass flux condition. The ratio of the CHF enhancement was about ~80% compared with the case of water under the same conditions. As the mass flux is increased, the ratio of CHF enhancement is also increased. The parameters were the RT wavelength and the advancing contact angle. The effects of these parameters were confirmed with a 3-dimensional hemispherical geometry facility. Flooding the liquid metal into reactor vessel cavity was proposed and studied to ensure the safe margin for IVR-ERVC strategy. The heat transfer mode is a single-phase heat transfer due to the high boiling point of liquid metal. It allows to remove the decay heat on IVR-ERVC strategy without the concern about CHF. The SWOT (strengths, weaknesses, opportunities, and threats) analysis was conducted to evaluate the IVR-ERVC. Some research approaches were conducted to overcome the weaknesses of liquid metal flooded IVR-ERVC. Improved heat transfer or reduced heat flux was confirmed by experimental results for a small-scaled facility to simulate the boiling phenomena under IVR-ERVC condition. The heat transfer area could be enlarged over 2 times on the basis of the original area for the reactor vessel. This phenomenon was named as “liquid metal fin”. The capability of heat removal was determined by some factors. The maximum heat flux was reduced about 3 times compared with that of the case without liquid metal. A number of power plant has been operated to satisfy energy demand. The nuclear power plants makes the large amount of energy constantly and easily. It is impossible to stop the operation of the nuclear power plants based on this situation. Consequently, it is required to construct and operate more reliable nuclear plants in order to protect the human, public and the environment from radiological hazards. IVR-ERVC strategy is the effective way to terminate the progression of severe accidents. Flooding the graphene oxide nanofluid and the gallium liquid metal could improve the IVR-ERVC strategy.