Recovery of radioactivity as solids from nonflammable organic low-level radioactive wastes using supercritical water mixed with RuO 2

Abstract

We investigated the distribution behavior of iron, cobalt, cesium, iodine and strontium attached to nonflammable organic materials, in solid, liquid and gas phases during the decomposition of these materials in supercritical water mixed with RuO 2. The distributions of these elements under various conditions (initial amounts, with/without precipitation reagent) were determined by using their radioisotopes as simulated low-level radioactive wastes in order to ease the detection of trace amounts of elements even in solid and gas phases. Iron and cobalt were found only in the solid phase when iron hydroxide was added as a precipitation reagent before the supercritical water reaction. Cesium, iodine and strontium were found in the liquid phase after the reaction. Therefore, by adding precipitation reagents such as sodium tetraphenylborate, and Na 2CO 3 (or NaHCO 3) and AgNO 3 aqueous solutions to each resultant liquid phase containing cesium, strontium and iodine, respectively, these elements can be successfully recovered only in the solid phase. The gases produced during the decomposition of the organic material contain no radioactivity under all conditions in this study. These results indicate that all of the elements investigated in this study (iron, cobalt, cesium, iodine and strontium) can be recovered successfully by this supercritical water process using RuO 2. Consequently, this process is suggested as a predominant candidate for the treatment of nonflammable organic materials in low-level radioactive wastes (LLW).