Prediction of the Ru volatilization curve during boiling and drying accident in a reprocessing plant

Boiling and drying accident is thought to have the largest potential radiological consequence. If the cooling time of reprocessing fuel is short, volatile Ru (106Ru) gives a significant impact. Previous experiments on Ru volatilization were conducted by externally heating 100∼400 mL simulated waste. Under such conditions, a large temperature difference occurs inside the waste once it begins to dry. Therefore, it is difficult to correlate the obtained waste temperature vs. time curve with that during an accident. In the present experiment, the Ru volatilization curves were obtained when 4.6, 2.0, and 0.91 mL of simulated waste were heated at constant rates of 0.07, 0.2, and 0.8°C min−1, respectively. Under these conditions, the waste temperature was the same as the external heating temperature. In previous experiments, only one or two volatilization peaks were observed, but this time four peaks were observed. Until now, Ru volatilization was thought to occur through the oxidation of Ru nitrosyl nitrate in concentrated nitric acid and after acid evaporation its thermal decomposition. However, the appearance of four peaks cannot be explained by this mechanism and may be due to complex changes in the chemical form of Ru compound with increasing temperature. The experimental results can serve as basic data for predicting Ru volatilization during an accident. Using the Arrhenius-type equations for Ru volatilization derived from these basic data and the waste temperature change during the accident derived in our previous report, we attempted to predict the Ru volatilization curve over the accident progression time.