Caesium behaviour in liquid sodium—The effect of carbon

Abstract On the basis of previous thermodynamic calculations and experimental work, caesium has been identified as the most volatile major fission product with respect to release from liquid sodium into the gas phase. Sorption of the caesium by graphite has been shown to be very effective in reducing the volatility of the caesium, and the formation of stable caesium-graphite compounds reduces the escaping tendency of caesium to less than that of iodine in liquid sodium. The caesium forms two compounds one of which is relatively unstable in liquid sodium and is probably a surface adsorption compound. The more stable fraction of the adsorbed caesium is probably a lamellar caesium-graphite compound and is almost fully stable in vacuum at 500°C in the absence of sodium. The rate of reaction for the breakdown of the more stable caesium-graphite compound in sodium has been measured over a range of temperatures and the corresponding activation energy has been calculated. Graphite has been found to be equally effective for caesium sorption when it is used in the gas phase above the liquid sodium. Charcoal has been shown to be even more efficient than graphite in trapping caesium, and the caesium-charcoal compound which is formed is almost fully stable in sodium at 500°C and in air at the same temperature. Pre-treatment of the charcoal with sodium at 500°C for 3 days has little effect on its performance with respect to caesium sorption.