Dissolve 10.2 g of uranium trioxide in 20 mL of conc. hydrochloric acid, add about 50 mL of boiling water to it and finally add 2 g of copper sulfate pentahydrate and 100 mL of 3M hydrofluoric acid. Dissolve 5 g of ascorbic acid in 50 mL boiling water. Heat the uranyl solution to 100°C in a microwave oven and add 5 ml of the ascorbic acid solution heat again and add 5 mL of the ascorbic acid solution, repeat this until all of the ascorbic acid solution is consumed. Let the solution settle for 1h, filter with washing with hot water and dry the precipitated uranium tetrafluoride.
Reduction: (UO2)2+(aq) + C6H8O6(aq) + 2H+(aq) → U4+(aq) + C6H6O6(aq) + 2H2O(l)
Precipitation: U4+(aq) + 4HF(aq) + 2.5H2O(l) → UF4·2.5H2O(s) + 4H+(aq)