Preparation and characterization of carbons for the retention of halogens in the condenser vacuum system of a thermonuclear plant

Activated carbons were prepared by air and carbon dioxide activation, from almond tree pruning, with the aim of obtaining carbons that reproduce the textural and mechanical properties of the carbons currently used in the filtering system of the condenser vacuum installation of a Thermonuclear Plant (CNA; Central Nuclear de Almaraz in Caceres, Spain), produced from coconut shell. The variables studied in non-catalytic gasification series with air were the temperature (215-270 °C) and the time (1-16 h) and the influence of the addition of one catalyst (Co) and the time (1-2 h) in catalytic gasification. In the case of activation with CO₂, the influence of the temperature (700-950 °C) and the time (1-8 h) was studied. The resulting carbons were characterized in terms of their BET surface, porosity, and pore size distribution. The N₂ adsorption isotherms at 77 K for both series showed a type I behaviour, typical of microporous materials. The isotherms showed that with both gasificant agents the temperature rise produced an increase in the carbon porosity. With regards to the activation time, a positive effect on the N₂ adsorbed volume on the carbons was observed. The best carbons of each series, as well as the CNA (carbon currently used in the CNA), were characterized by mercury porosimetry and iodine solution adsorption isotherms. The results obtained allowed to state that several of the carbons produced had characteristics similar to the carbon that is target of reproduction (which has S_BET of 741 m² g⁻¹, V_mi of 0.39 cm³ g⁻¹ and a iodine retention capacity of 429.3 mg g⁻¹): carbon C (gasification with CO₂ at 850 °C during 1 h), with S_BET of 523 m² g⁻¹, V_mi of 0.33 cm³ g⁻¹ and a iodine retention capacity of 402.5 mg g⁻¹, and carbon D (gasification with CO₂ at 900 °C during 1 h), whose S_BET is 672 m² g⁻¹, V_mi is 0.28 cm³ g⁻¹ and has a iodine retention capacity of 345.2 mg g⁻¹.