For the purpose of further increasing decontamination performance, the purification behavior of uranyl nitrate hexahydrate (UNH) crystals was studied in the static system. The approach to purification of UNH crystals recovered from the irradiated fast reactor core fuel using a dissolver solution was examined by an isothermal sweating process, which involves heating to the melting point of the crystal. Since transuranium (TRU) elements were present as liquid impurities in the UNH crystal, their decontamination factors (DFs) were improved using this process. The DFs of Cs and TRU elements for the UNH crystals had a tendency to increase with time and were kept virtually constant by 240 min. Not having been precipitated as Cs2Pu(NO3)(6) during the crystallization, Pu remained in the mother liquor and was removed by the sweating process. The purification rate coefficients of UNH crystals for Cs and TRU elements were calculated using these experimental data. These values for Pu, Cs-137, Am-241 and Cm-242 were evaluated to be 2.13 x 10(-4), 2.73 x 10(-4), 2.11 x 10(-4), and 3.13 x 10(-4) s(-1) at 57 degrees C, respectively.