Lithium Hideout and Return in the CANDU Heat Transport System during Shutdown and Start-up

Lithium hydroxide is used to control the pH_a (pH apparent) of the Heat Transport System (HTS) coolant in CANDU^R reactors. The recommended range of the lithium concentration in the coolant is between 0.38 ppm (5.5x10^-5 m) and 0.60 ppm (8.7x10^-5 m) to minimize carbon steel corrosion in the HTS and magnetite deposition in the core during normal operation; this corresponds to pH_a values between 10.2 and 10.4. Similar pH_a and lithium concentrations should be maintained during shutdown and start-up. However, maintaining the pH_a of the HTS coolant within specification during shutdown and start-up has been difficult for some CANDU stations, especially when the HTS is taken to a Low Level Drain State (LLDS), because of lithium hideout and return. This paper presents the results from lithium adsorption and desorption studies on iron oxides under relevant shutdown and start-up chemistry conditions performed to elucidate the mechanisms of the observed lithium hideout and return. The results show that lithium hideout and return are driven largely by changes in the solubility of magnetite as the HTS coolant chemistry changes during shutdown; changes in lithium concentration were inversely correlated with the solubility of magnetite. When the HTS system is de-pressurized and drained to a low coolant level, the ingress of air rapidly oxidizes the dissolved Fe (II) in the coolant, 2Fe⁺² + ¹/₂O² + 3 H₂ = 2FEOOH + 4 H⁺, resulting in the formation of lepidocrocite or maghemite, which have much lower solubilities but larger surface areas than does magnetite. The large surface area of the Fe (III) oxides can adsorb significant quantities of lithium from the coolant, leading to lithium hideout and a pH_a decrease. During start-up, the chemistry of the coolant changes from oxidizing to reducing, and lepidocrocite and other Fe (III) oxides are reduced to Fe (II), gradually dissolving as their solubility increases with increasing temperature. The adsorbed lithium is released and the lithium concentration and pH_a of the HTS coolant increase. At same time, lithium adsorbed on magnetite surfaces is also released as the coolant temperature and the solubility of magnetite increase. The paper will illustrate the lithium hideout phenomenon using CANDU station data and present a summary of the key results from the lithium adsorption experiments. It will conclude with some comments on possible mitigating strategies. (authors)