Phenomenological and Experimental Study of the Tritium Distribution in the Effluents Resulting from the Sodium Hydrolysis

摘要

Within the framework of the dismantling of Sodium cooled fast neutron reactors (SFR) in France (PHENIX, SUPERPHENIX, RAPSODIE), several processes are under investigation regarding sodium disposal. One of them, called ELA (radioactive sodium waste treatment process), is based on the implementation of the sodium-water reaction, in a controlled and progressive way, to remove residual sodium mainly from the sodium purification systems called cold traps. This system contains, after draining, residual sodium and trapped impurities such as sodium oxide (Na_2O), sodium hydride (NaH) with traces of tritium (NaT). The hydrolysis of these various chemical species leads to the production of a liquid effluent, mainly composed of an aqueous solution of sodium hydroxide, and a gaseous effluent, mainly composed of an inert gas, hydrogen and steam. The tritium is present in both effluents. Within the gaseous effluent, it is expected to be released as HT form only. But, according to some experimental data gathered from tritiated sodium hydrolysis operations performed at the CEA in France, a non-negligible amount of tritium is released in the off-gas as HTO form (up to more than 90 % of the tritium found in the gaseous effluent). HTO being 10,000 times more radiotoxic than HT, a precise knowledge of the mechanisms governing the phase distribution of tritium is necessary to estimate the tritium distribution. Indeed, it will help designing the process needed to optimize the treatment of the off-gas before its release into the environment. In order to develop a numerical model able to predict the chemical composition of each effluent, a phenomenological description of the hydrolysis of residual sodium performed in the conditions of ELA has been proposed and is described in this paper. To validate some of the hypotheses made in this phenomenological description, and with the aim to validate the model that will be based on this scenario, a series of experiments has been performed in a laboratory-scale hydrolysis process designed at the CEA Cadarache, France. The experimental device consists of a 2-liter cylindrical glass reactor into which small quantities of sodium containing tritium were hydrolyzed by water injected drop by drop. A chemical characterization of the effluents generated by the process will allow us to study the influence of several parameters on the tritium distribution. An analysis of the experimental results obtained during this study is provided in this paper.