The release of radioactivity into the atmosphere was mainly caused by:

1. Unfiltered containment venting:
   In addition to the release of noble gases, which would also have occurred with filtered venting, this led to the release of mainly highly volatile fission products such as iodine and caesium.
2. Containment leakage:
   In the course of the accident, the design pressure and temperature of the containments were (in part significantly) exceeded in units 1-3. Leakage probably occurred during this process.

In addition to being released into the atmosphere, radioactive substances were also released into water – especially the water injected for emergency cooling. As there were no more closed cooling circuits, large quantities of contaminated water escaped through leaks in the containment vessels and accumulated in the buildings. In early April 2011, heavily contaminated water leaked into the sea. In addition, water - mainly groundwater - entered the buildings from the outside.

Various measures, including the sealing of leaks on buildings, were taken to successfully reduce the inflow of groundwater into the buildings. These include:

• Commissioning of groundwater drainage wells and drainage wells.
• Sealing leakages on buildings and building ducts
• Construction of a waterproof structural groundwater barrier directly in front of the quay wall
• Freezing the soil around the reactor buildings of units 1-4
• Sealing off a large part of the plant site and the harbour basin seabed

In addition, a purification plant for the contaminated water is in operation. Water that is not fed back into the reactors for cooling after treatment is temporarily stored in various tanks on the plant site. A constant expansion of the storage capacities has been necessary so far. Parts of the treated water are to be discharged into the sea from the end of 2023 on. This involves, in particular, groundwater that has been diverted around the power plant site. The concentration of radioactive substances still present in these waters is far below the legal limits.