A new reprocessing system for spent nuclear fuels based on a precipitation method is proposed to recover uranium and transuranium elements from spent nuclear fuels in high ratios and to achieve extreme safety without any potential dangers. Experiments were carried out for a simulated fuel solution containing uranium and 17 major elements. The main reprocessing processes are as follows: (a) dissolution of U02 fuel under mild conditions; (b) neutralization of the dissolved fuel solution with Na2C03-NaHC03 mixed solutions, followed by the separation of precipitated fission products by centrifugation; (c) separation of cesium by a precipitation method using a tetraphenylborate ion; and (d) recovery of uranium (U) as a precipitate of the hydrolyzed compound from an alkaline solution. As a result, 99.95% of the U was recovered with the least amount of fission products, i.e., 10-5 g or even less in the recovered 1 g of U with the only exceptions being zirconium and molybdenum.