[en] In this article we will describe the radionuclide standardization system TDCR, Triple-to-Double Coincidence Ratio, of the National Metrology Laboratory of Ionizing Radiation and its stage of development to provide a method to standardize radionuclides difficult by other techniques. The TDCR is an absolute measurement method, therefore, it does not depend on a reference standard to determine the radioactive activity of the samples, initially developed to standardize radionuclides that decay by pure beta emission (electrons and positrons), such as ³H, ¹⁴C, ⁴⁵Ca and ⁶³Ni and electronic capture, such as the ⁵⁵Fe. However, it is possible to standardize alpha emitters, such as ²⁴¹Am, and other radionuclides with a complex decay scheme. The method uses three photomultiplier tubes aligned at an angle of 120° to each other, and double and triple coincidences between the three photomultiplier tubes are collected. The method is currently used in 26 radionuclide metrology laboratories around the world. In this article we also present the radionuclides that will be standardized by the method. (author)