Combustion characteristics and migration behavior of nuclides during incineration of low-level radioactive waste

Abstract

Incineration is a main method of disposal low-level radioactive waste (LLRW). However, the combustion characteristics and migration of nuclides during incineration is still unclear. This study first investigated the effect of package mass, feeding time interval, air distribution, and water vapor supply on the combustion characteristics of LLRW and NOx emission. Optimal parameters were obtained based on combustion efficiency and NOx concentration. Then, the migration of nuclides was studied in conditions with the optimal parameters. The results showed that the combustion efficiency and NOx concentration were affected by furnace temperature, redox atmosphere and their distribution inside the furnace. Package mass, feeding time interval, and air distribution were optimized to balance the thermal decomposition rate of the waste and the oxidation rate of the combustible gas generated from thermal decomposition of the waste. Water vapor supply can decrease the furnace temperature and the thermal decomposition rate of the waste. The optimal parameters of package mass, feeding time interval, air distribution, and the feeding rate of water vapor is 100 g/bag, 40 seconds, 0.5/0.23/0.23/0.23 (excess air coefficient) for the first layer to fourth layer of air inlet, and 25 kg per hours, respectively. During incineration, more than 99% nuclides were fixed in bottom ash. The order of the ratio of nuclides in flue gas, fly ash, and bottom ash is bottom ash > flue gas > fly ash. Based on the ratio of nuclides in fly ash and flue gas, the order of volatility of nuclides during incineration is Cr > C s> Mn > Co > Nb > Zr.