Continuous monitoring of lithium in dynamic Pb–17Li systems

During operation of the liquid metal blanket, lithium depletion may occur from the Pb–17Li alloy. It is thus necessary to have a reliable and accurate method for the monitoring of the Li content and for its adjustment. Continuous methods are the most attractive. In the frame of this work, three methods available for the on-line determination of the Li content of Pb–Li alloys (electrochemical sensor, electrical resistivity meter and plugging indicator) have been assessed in a liquid metal loop system. Although the tested electrochemical sensors were susceptible to thermal shock, the results obtained under static conditions show that the concept is satisfying: the sensor is specific to lithium, simple to use and it can detect a composition change of ±0.15 at% Li. The solid electrolyte (β-alumina) is the key element but no difficulty is expected to get material with improved mechanical properties taking into account recent developments in this field. The resistivity meter is robust and very sensitive to composition change, the minimum detectable limit being ±0.15 at% Li. It is easy to handle but its replacement in a loop could be slightly more complicated than the electrochemical sensor. The plugging indicator does not need calibration and it is very robust. However, changes in the composition of the liquid alloy must be higher than 1 at% Li to be detected. The plugging temperature is well related to the liquidus temperature of the alloy but a good knowledge of the Li–Pb phase diagram is necessary to deduce the Li concentration. Therefore, it is thought that the monitors with greatest potential for continuous monitoring of the Li content in the liquid metal blanket are the electrochemical sensor and the resistivity meter.