M. Sc. degree
Indian Institute of Technology Madras, India, 2022

Abhishek Saraswat

Supervisor: Dr. Sateesh Gedupudi

Development of a two-phase detection probe for high temperature lead-lithium liquid metal applications

Keywords:

• Counting gases and liquids
• Materials and associated technologies

Abstract:

Liquid-gas two-phase flow is a common occurrence in various industrial applications such as thermal power plants, steelmaking and refining processes etc. However, for nuclear fusion applications, where a lithium based liquid metal breeder/coolant is utilized, the existence of a two-phase flow may lead to certain critical issues including fuel inefficiency resulting from the reduced tritium breeding ratio, probability of mechanical failures due to generation of hot spots and safety considerations related to improper nuclear shielding. Out of the candidate liquid breeders, lead-lithium eutectic (Pb-16Li; hereafter referred to as PbLi) has gained immense focus for its various advantages and is utilized in several tritium breeding blanket concepts. Therefore, the development of a two-phase detection tool for PbLi liquid metal environment is imperative. Numerous two-phase measurement studies have been performed on room temperature/low melting liquid metals like GaInSn, Hg, Na etc. However, corrosive nature of PbLi coupled with constraints owing to high temperature operations severely restricts the application of commercially available diagnostic tools. This has indeed resulted in a lack of two-phase detection experiments in PbLi environment.

This work primarily aims to bridge the existing gap with the development and preliminary validation of a compact sensor probe as a measurement tool to experimentally study two-phase regimes in PbLi liquid metal environment. In this study, an electrical conductivity and temperature measurement based multivariable two-phase detection probe is developed using electrical insulation coating of high purity alumina (Al2O3). The probe is calibrated in a specifically designed liquid metal-gas two-phase test facility. Functional validation of the fabricated probe is achieved in high temperature PbLi-Ar (liquid-gas) two-phase vertical column with bulk PbLi temperature upto 400°C and time-averaged void fractions upto 0.95, covering two-phase flow regimes from well dispersed bubbly flow upto localized annular flows with void fractions similar to an in-box Loss of Coolant Accident characterized by a very large high pressure gas ingress inside bulk PbLi. Estimations of time-averaged void fraction, bubble frequency and average bubble residence time are performed. The experimental data corroborates high reliability and excellent temporal resolution towards individual bubble detection using electrical conductivity based principle, while the observed bulk temperature trends coherent with estimated void fractions provide qualitative insights into the presence of two phase flows. A decrease in the experimentally measured bubble frequency with an increase in the time-averaged void fractions above 0.6 indicates coalescence of bubbles in the PbLi environment.