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This work describes a concept for and a realization of a large-volume (200 L) certified source (calibration drum) for calibration of gamma-spectrometry systems used for Assay of Radioactive Waste Drums. The design of the calibration drum is based on the idea of modeling a homogenous source by a large number of point sources. We deal with the problem of finding an optimal number of point sources and their appropriate arrangement to obtain close modeling of a homogenous source. An original model for the arrangement of the sources is derived and presented. We demonstrate quantitatively that with the proposed arrangement of 448 point sources a very close modeling of a homogenous drum is achieved. A wooden matrix was constructed from 14 discs stuck together, in which 32 axial holes were drilled, following the abovementioned design. 470 standard sources - Certified Reference Materials - Modules (CRM-Modules) were produced by transferring equal masses of standardized 152Eu solution in each of them. The activity of each CRM-Module is 68,6 ± 1,4 kBq. The CRM-Modules are made from polycarbonate with cylindrical shape (Φ 25 × 56 mm). All modules were subjected to a large number of tests for validation of their individual metrological characteristics. The results from these tests allowed all CRM-Modules to be certified by the Bulgarian National Metrological Institute. 32 tubes, each containing 14 Certified Reference Materials - Modules (CRM-Modules) are placed in the holes. The calibration drum contains 448 CRM-Modules with overall activity 30,72 ± 0,65 MBq. After the final assembly, the calibration drum was subjected to a number of tests, which proved its metrological characteristics. As a result, the Large-volume source was also certified by the Bulgarian National Metrological Institute and it is in use for calibration of a system for assay of drummed radioactive waste.