Solidification of radioactive wastes with sulphur. Investigation of the leaching mechanism

The possibilities were studied of using sulfur as an additive and as the matrix material in the solidification of low and medium level radioactive waste. The leaching mechanism in the various resulting solid materials was investigated. Concentrated boric acid, aqueous waste and loaded ion-exchange resins were used as models of the radioactive waste. This selection allowed us to compare our results with literature data. For the characterization of the solidification and comparison with literature data we measured the leaching rate of inactive cesium and cobalt, the stability in water and the compressive strength. To investigate the leaching mechanism, we measured porosity and the sulfide concentration in the leaching liquid. The cesium and cobalt concentrations were determined by absorption spectrometry, the sulfide concentrations with an ion-selective silver/sulfide electrode. In the first phase, we tried to improve cement solidification by either adding sulfur to the cement-waste mixture or by impregnating the cement in molten sulfur. The addition of sulfur to the cement-mixture produced worse characteristics, impregnation with 5 % sulfur lowered the leaching rate and increased the compressive strength by a factor of three. Sulfur as the matrix poses other problems. In the presence of alkaline earth metals and at high pH-values we observed a substantial production of sulfide ion. The formation of soluble sulfide complexes increased the leaching rate. This problem was solved by adding Clinoptilolite in H+-form to buffer the pH-value at acceptable values. The study shows that the mechanism of leaching for Cs and Co out of the cement or the sulfur matrix is diffusion controlled. With sulfur matrices at high sulfide concentrations erosion is the dominating mechanism for bringing cesium and cobalt into the leaching fluid