For undersea cables radiation doses between 2.5 and 25 rad are to be expected during a working life of 25 years. The majority of previous investigations of the radiation sensitivity of optical fibers, however, apply dose rates > 1 rad/s and total dose values > 3 x 10E3 rad. The present paper describes Co60 irradiations of a Ge-doped single mode fiber with dose rates between 10E-4 and 20 rad/s. Expected loss increase of an undersea optical fiber cable at 1550 nm wavelength with a temperature of 2øC is derived by three different methods. The most comfortable and reliable one, the 'dose rate transformation method', yields, for example, a loss of only 0.0244 dB/100 km after 25 years of irradiation with a dose rate of 0.4 rad/a. This method could be the basis of a standard test procedure for the effect of natural radioactivity on optical fibers for very long repeaterless terrestrial and undersea cables

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