Abstract <p>Ensuring sustainable nuclear power development requires addressing the issues of dealing with iodine-129 (¹²⁹I), which is a long-lived radionuclide accumulated in spent nuclear fuel (SNF) and transferred to process gas and liquid streams during SNF reprocessing at radiochemical plants. The paper considers archival materials (for the period rom 1973 to 2000) related to specific features of ¹²⁹I distribution in the gas supply system of the Mayak radiochemical plants and patterns of its migration in the environment of the Mayak observation zone. The following sources of ¹²⁹I release into the environment were considered: routine emissions from the radiochemical plant stacks, accidental emissions in 1957 and 1967, discharges of liquid radioactive waste (LRW) into surface storage reservoirs (Karachay), and wind transfer of water aerosols generated from the Karachay water area. It was shown that maximum population doses from ¹²⁹I did not exceed 2−5 μSv/year and were registered in the mid-1980s during simultaneous operation of the DB and RT-1 radiochemical plants, when no standard system for ¹²⁹I capture was installed at the RT-1 plant yet and the total release from the plant stacks to the atmosphere reached 0.25 TBq/year. The relative distribution of ¹²⁹I transferred to the B and DB radiochemical plants was estimated as follows: ~30−40% was emitted from the stacks to the atmosphere, ~40% accumulated in the sorbent of the gas treatment systems, and ~20% remained in LRW. The SNF dissolution at these radiochemical plants was performed in the presence of significant amounts of mercury capable of keeping iodine within a solution. At the RT-1 plant, where SNF dissolution was carried out without mercury, reprocessing of about 100 t of VVER SNF per year resulted in the generation of about 0.13 TBq ¹²⁹I/year, out of which ~0.051 TBq/year (~40% of ¹²⁹I) was emitted to the atmosphere. In 1979−2024, the total amount of ¹²⁹I delivered to the RT-1 plant (as a component of VVER SNF) was estimated at 6.5 TBq (reprocessing of other types of SNF was not taken into account), out of which ~2 TBq was emitted to the atmosphere and ~3−4 TBq accumulated in sorbents. The atmospheric emission (<i>D</i>_I) and ¹²⁹I/⁹⁰Sr activity ratio (<i>R</i>_I/Sr) for additional sources of ¹²⁹I emission into the environment were estimated as <i>D</i>_I ≈ 7.4 Ci (0.3 TBq), <i>R</i>_I/Sr ~ 2.7 × 10^–5 (the 1957 accident); <i>D</i>_I ≈ 0.032 Ci (1.2 GBq), <i>R</i>_I/Sr ~3.1 × 10^–4 (the 1967 accident); <i>D</i>_I ≈ 1.2 Ci (0.045 TBq), <i>R</i>_I/Sr ~3.6 × 10^–3 (wind transfer of water aerosols from the Karachay water area). The results obtained can be used for developing a conceptual framework of dealing with ¹²⁹I when designing a new high-capacity radiochemical plant.</p>