Gaseous effluents come from process equipment and, to a lesser extent, from the workshops’ ventilation systems. Gaseous fission products are mostly collected from the shearing process and after dissolution of the used fuel assemblies and then undergo successive treatments. For example, almost all the tritium is trapped as tritiated water with the remainder released as tritiated steam. Iodine-129 is absorbed on element-specific filters made of zeolite and silver nitrate. Particulates are stopped by high-efficiency filters (99.9% efficiency). Minor residual releases are mainly ruthenium and krypton-85, which have an extremely small radiological impact.

The main part of the gaseous effluents is released through stacks that are high enough (about 100 m) to provide sufficient atmospheric dispersion to reduce the impact to acceptable levels. Flow rate and activity are continuously monitored through automatic systems and some other controls are performed through sampling and laboratory measurements. Independent checks on these measurements are performed by the safety authorities.

As an example, gaseous releases at the La Hague in France measured in 2002 are shown in Table 16.2.