A crystallization process has been under development for the recovery of U as uranyl nitrate hexahydrate (UNH) crystals. Two experiments, solubility of Cs2Pu(NO3)(6) and U crystallization, were performed to evaluate the influence of HNO3 concentration in solution on the precipitation behavior of Cs2Pu(NO3)(6). The solubility of Cs2Pu(NO3)(6) in a uranyl nitrate solution has been found to decrease with increasing HNO3 concentration in the solution. These experimental results imply that Cs2Pu(NO3)(6) is readily formed in high HNO3 concentration solutions. Based on the solubility of Cs2Pu(NO3)(6) in uranyl nitrate solutions, U crystallization experiments were performed with a range of HNO3 concentrations in the feed solution. At an HNO3 concentration of 4.5 mol/dm(3) in the feed solution, the HNO3 concentration of the mother liquor was 6.5 mol/dm(3). The decontamination factor (DF) of Cs for the UNH crystals was 8.61 after crystal washing. In this case, Cs2Pu(NO3)(6) precipitated upon cooling of the feed solution and was not separated from the UNH crystals after crystal washing. On the other hand, the DF of Cs was 174 after the UNH crystals were washed when the HNO3 concentration in the feed solution was 3.2 mol/dm(3). With a lower HNO3 concentration in the feed solution, nearly all Cs remained in the mother liquor during the U crystallization process. Therefore, to suppress Cs2Pu(NO3)(6) precipitation, a lower HNO3 concentration must be maintained in the feed solution.