Restraining effect of nitrous acid on volatile ruthenium release from simulated high-level liquid waste following boil down

The release of volatile ruthenium (Ru) compounds into the environment is a cause of concern in evaporation to dryness accident. It results from the loss of the cooling function of high-level liquid waste (HLLW). Although coexisting nitrous acid (HNO2) is considered to restrain Ru volatilization in nonthickened HLLW, such an effect has not been confirmed in the thickened HLLW expected under accident conditions. An experimental solution containing 2–5 mol/L HNO3 was prepared using simulated HLLW (s-HLLW) and heated while continuously controlling its HNO2 concentration up to 0.06 mol/L during measuring the release of volatile Ru. Results confirm a substantial effect of HNO2 on reducing volatile Ru release in the solutions containing 2–5 mol/L HNO3. This corresponds to the condition where the s-HLLW is boiled down to 1/3 of its initial volume. It is also confirmed that volatile Ru release increases with s-HLLW thickening. More volatile Ru is released as the heating power increases. These results highlight the importance of the parameters relevant to a Ru release model applicable to the source-term analysis of accident situations.