Tritium removal in fluoride-salt-cooled high-temperature reactors
Creators: name: Fernandez Puertas, Eneritz
Publisher: name: Imperial College London
Year: 2016

description: The Fluoride-Salt-Cooled High-Temperature Reactor (FHR) is a new nuclear reactor concept characterized by the use of a very high-temperature fuel, a very high-temperature coolant (~ 700 ºC) and operation at near atmospheric pressure. These operating conditions make this reactor type safer and more efficient than the current water cooled reactors. However, this reactor concept has some unresolved issues; one of them being the formation of tritium in the coolant salt. If tritium is not removed from the reactor, it will permeate the structural materials and find its way out to the environment. Graphite has been proved to have the ability to adsorb tritium at high temperatures, and thus, it can be considered as a tritium removal technology. The aim of the present work was to study the feasibility of using graphite or other carbon materials as an effective tritium removal technology in FHRs. To this end, an experimental system to measure the adsorption of hydrogen isotopes on different carbon materials immersed in molten salts was designed and commissioned. The experimental system consisted of a stainless steel vessel heated by a tubular furnace to operating temperatures from 500 to 700 ºC. The amount of hydrogen adsorbed was calculated by monitoring the pressure of the vessel. Experiments were performed to measure hydrogen adsorption on a selected activated carbon and graphite immersed in FLiNaK for different temperatures and pressures. Hydrogen solubility in FLiNaK was also measured for the same operating conditions. Tritium and hydrogen have very similar properties, and therefore hydrogen was used instead of the radioactive tritium. Both hydrogen solubility in FLiNaK and adsorption on carbon samples were found to decrease with increasing temperature. However, no correlation was found between the ratio of adsorption to solubility and the temperature. In addition, for a selected temperature and pressure the hydrogen adsorption on carbon materials with different properties was studied. This was carried out in order to understand the effect that the properties of the carbon materials have on their hydrogen adsorption capacity. Experimental results showed that the properties of the sample did not have a big impact on their hydrogen adsorption capacity. However, characterization performed on the spent carbon samples showed that FLiNaK dopes the carbon materials with potassium, which enhances their hydrogen adsorption capacity. Additional experiments were performed using deuterium gas, in order to extrapolate the results to tritium and give an estimation of its adsorption properties on these carbon materials. 5 Activated carbon was found to adsorb roughly 20 % more deuterium than hydrogen. However, deuterium adsorption on graphite was found to be lower than the one of hydrogen. Furthermore, models were developed to describe the dynamics of hydrogen and deuterium dissolution in FLiNaK and adsorption on carbon materials. This information was used to develop a model describing tritium transport in FHRs, including tritium removal from the secondary cooling salt using activated carbon. The model gave an estimation of the amount of activated carbon that would be required in order to remove at least 90 % of the estimated tritium production in FHRs.