Decontamination of gravels contaminated with uranium

• Rock types of high radioactivity were quartz, lamprophyre, and schist.
• The larger the gravel size, the more the concentration of the gravel is reduced.
• Average removal efficiency of gravel after the third washing was about 37%.
• Removal efficiency of the contaminated gravel was not related to its size.
• The higher the gravel weight, the more the concentration of gravel is reduced.

Gravel washing equipment and electrokinetic–electrodialytic decontamination equipment were manufactured to decontaminate gravel contaminated with uranium. The removal efficiency according to the gravel size and weight and the removal efficiency according to the lapsed time using the manufactured equipment were investigated through several experiments. The volume of gravel in the high uranium concentration group was about 10%, the rock types of which were quartz, lamprophyre, and schist. The larger the gravel size, the higher the contaminated concentration of gravel. The average uranium (238U) concentration of gravel after the first washing was about 1.45 Bq/g, and the average removal efficiency of gravel after the third washing was about 37%. In addition, the removal efficiency of the contaminated gravel was not related to its size. The contaminated concentration of the gravel decreased with an increasing gravel weight. In addition, the removal efficiency of contaminated gravel was not related to its weight. When the electrokinetic–electrodialytic decontamination period of 5 days, 10 days, 15 days, and 20 days elapsed, the 238U in the gravel was removed by about 40%, 65%, 72%, and 81%. The more the electrokinetic–electrodialytic decontamination time elapsed, the more the removal efficiency ratio of 238U decreased. Finally, the gravel with a size of less than 10 cm was treated by soil washing and electrokinetic decontamination methods with soil, and gravel with size of more than 10 cm but less than 20 cm was treated by gravel washing and electrokinetic–electrodialytic decontamination methods. Gravel with a size of more than 20 cm is treated by a gravel washing method, and gravel contaminated with a high concentration of uranium was treated by crushing and ball mill washing methods.