[en] In this research the precipitation method refers to the ASTM E 320-79 standard, where the precipitate is in the form of CsClO_4 salt. Several aspects related to the precipitation process are studied in order to determine its effectiveness and to obtain relatively nuclear grade of "1"3"7Cs. The precipitation was done by using HClO_4 at 0 °C. The aspects under examination includes the influence of CsNO_3 addition as a carrier and HClO_4 as well as the polarity of the washing agents used. The experiment used "1"3"7Cs standard (NIST) for the determination of detector efficiency, measurement precision and accuracy, and optimum condition during the precipitation process. Measurement for detector efficiency, precision and accuracy was repeated at 5 to 7 times at counting time of 1500 seconds, by which values of (5.31±0.066) x 10"-"2%, 1.119% and 0.542% for detector efficiency, precision bias and accuracy bias respectively was obtained. The optimum addition of the carrier was determined by varying carrier composition from 100 to 225 mg of CsNO_3 at 0 °C for 1 hour. The resulting CsClO_4 was subsequently measured for its "1"3"7Cs content at 661.8 keV. The result showed that the maximum recovery precipitation obtained was 83.87% at carrier addition of 225 mg. The optimum amount of two washing agents used during the process was also studied, where acetone and a mixture of ase tone and ethanol were used. The polarity of the washing agents was indicated by their dielectric constant (from 20 to 22.4). Analysis results indicate that the counts of "1"3"7Cs in the CsClO_4 after washing and the counts of the supernatant do not show significant difference. It is therefore concluded that polarity difference within those range have no significant effect on the solubility of CsClO_4. The precipitation of the irradiated U_3Si_2/Al at optimum condition results in "1"3"7Cs concentration of 0.011 μg/g sample or (4.415±0.035) x 10"2 μg/g for a 3 x 3 x 1.37 mm"3 irradiated U_3Si_2/Al sample. (author)