Numerical Investigation for Dynamic Transport and Thermohydraulic Properties of High-Temperature Vapor and Radioactive Substances in Severe Nuclear Accident

嚴重核事故中高溫蒸汽與放射性物質的輸運過程及熱工水力特性的數值研究

Student thesis: Doctoral Thesis

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Award date24 Aug 2021

Abstract

Nuclear energy is one of the most promising base-load power sources to alter the existing energy supply structure, facilitating the achievement of “Carbon Neutrality” and alleviating global warming. However, harnessing and controlling nuclear energy effectively and gaining sufficient public trust is still challenging. For any nuclear issues, nuclear safety is of fundamental importance that the severe nuclear accident imposes a significant threat on human being’s health and activities. Therefore, the robust and prompt mechanisms/systems for emergency response must be developed based on precise predictions and prognoses. The present thesis numerically investigates the thermohydraulic transport phenomena of gas and radionuclides released by a severe accident. This thesis employs multi-scales of mathematical models from micro-scale (µm) to regional-scale (km) aimed at different scenarios and physical pictures.

During normal operation of a nuclear reactor, a dynamic equilibrium is maintained that the heat generated by the reactor core is transferred smoothly to the cooling water. Once a severe accident happens, the shortage of coolants may induce high-temperature steam and hydrogen gas release into the containment. Then, the prediction of steam condensation and heat transfer through the containment surface is essential to protect the integrity of the containment. Generally, the efficiency of condensation is governed by the heat flux and droplet generation rate. Extensive efforts have been made to boost droplet growth and condensation efficiency via delicately designing micro/nanostructured surfaces. However, simultaneously achieving rapid droplet growth and removal is still challenging. This thesis investigates the condensation on hierarchical mesh-covered surfaces employing the mesoscopic kinetic-based lattice Boltzmann method (LBM). The mechanism of dynamic growth and transport of droplets inside and outside the micro-pores is unraveled by resolving the heat transfer process and tracking the solid-liquid-vapor interactions. The proposed meshed surface realizes a robust self-refresh capability to clear the pinned droplets timely. The deterioration of hydrophobicity is avoided, contributing to a sustaining and prolonged dropwise condensation. The optimal case cuts down the droplet residence time and departure radius of 18 % and 17 % through rational design of mesh structures, respectively. Besides, the number of large droplets throughout the condensation process can also be reduced to different levels. The results can provide viable references to design various desirable meshed surfaces, facilitating efficient condensation on the containment surface and also in diverse engineering scenarios and applications.

In extreme situations, the containment loses its integrity, and the radioactive substances will be released into the atmosphere. Under such circumstances, the leaked radionuclides dispersion subject to the atmospheric environment should be tracked. Attributed to its advantages of computational accuracy and cost, the combined Gaussian-Lagrangian model (puff model) is employed to resolve the long-distance (over 200km) transport process of radionuclides. The dispersion of I-131 and Cs-137 originated from Fukushima Daiichi nuclear disaster is first studied to validate the model. The airborne concentration and deposition rate of both radionuclides agree well with the measured data. Then, the potential consequences are evaluated by postulating severe accidents at Daya Bay nuclear power plant (DBNPP), which widely concern the public from the Greater Bay Area of China. Typical cases for each month are simulated, considering Hong Kong, Shenzhen, Guangzhou, and Macau. The results reveal that except for June to August, the northeasterly wind dominates the dispersion of radionuclides, resulting in large contaminated area in Shenzhen and Hong Kong. Continuous attention should be paid to Cs-137 accumulation at several monitoring sites. The area reaching 50mSv/week equivalent dose (thyroid) will not exceed 35km from DBNPP. Measures such as sheltering and evacuation should be taken at the area within 5km.

Following the dispersion across the suburban areas, the radionuclides will experience the local-scale (within 1km) transport and accumulation in the urban environment. The dispersion within the compact built environment can be disastrous, threatening human health instantaneously. This thesis uses the computational fluid dynamic (CFD) based models to study the airborne behaviors of different pollutants. The effects of atmospheric stability and building morphologies are analyzed by establishing quantitative indicators, including air exchange rate, heat removal rate, pollutant removal rate, heat transfer coefficient, and pollutant transfer coefficient. The correlations between indicators and thermal stabilities are developed, which provides explicit expressions to describe the influence of the changing bulk Richardson number (Rb). The stability threshold is demonstrated to be Rb≈0.7, where the flow speed near the ground approaches zero. The high temperature gradient is formed and acts as positive feedback to facilitate a more stratified condition. In addition, a complex building morphology composed of obstacle group, target group, one main street canyon, and several subsidiary street canyons is further proposed. The dimensional parameters along with the sources are set as variables. An orthogonal numerical test is conducted, indicating that the source conditions play a more critical role than the obstacles dimension in pollutants accumulation around targets. The flow and dispersion structures are finally categorized into five patterns, which are strongly relevant to the dimension parameters. Due to the upstream perturbation, the sidewalls of targets are contaminated more seriously than the obstacles, while both their windward and leeward walls have a lower concentration of pollutants.

    Research areas

  • Nuclear energy, severe accident, thermal hydraulics, dropwise condensation, droplet dynamic behavior, hierarchical structures, lattice Boltzmann method (LBM), atmospheric dispersion, dose assessment, Gaussian puff model, urban environment, thermal stratification, computational fluid dynamics (CFD)