Nuclear facility sites such as an enrichment plant and fabrication plant are allowed to be released from the regulatory control of nuclear safety after the plants are decommissioned. It is necessary to confirm that the site has been decontaminated successfully, prior to be released. A determination method for U-238 concentration of background level in environment and for probate of vast site areas was proposed, in which the gamma-ray emission from the progeny radionuclides of U-238, such as Th-234, Pa-234m, Ra-226, are measured by in-situ gamma-ray spectrometry with a portable germanium semiconductor detector (portable Ge detector). Validity of the determination method of U-238 concentration from the progeny radionuclides was examined by the comparison between the U-238 concentration determined by the in-situ measurement with the portable Ge detector and that directly measured by ICP-MS. The U-238 concentration by the in-situ measurement was determined from peak counting rate at 186 keV of the gamma-ray emission corresponding to Ra-226. The determined U-238 concentration was in the order of 0.01 Bq/g in radioactive concentration, and was in comparable level with the concentrations decided by the ICP-MS. The proposed method utilizing gamma-ray estimation from the progeny radionuclides may be available for the U-238 concentration determination in vast land areas.

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