Storage buildings for the temporary storage of spent fuel elements for the period between removal from the nuclear power plant and reprocessing or direct ultimate waste disposal. The elements are stored in specially designed cast-iron containers, in particular in so-called Castor® containers which meet all safety functions such as radiation shield, retention of radioactive substances, mechanical integrity even in the event of earthquakes and aircraft crashes. These containers are stored in conventional warehouses. The containers are cooled during interim storage by the natural convection of air.

Interim storage is planned for periods of several decades. In a country such as the United States, which does not currently reprocess its spent fuel, waste has remained in storage in reactor pools or dry silos for more than 30 years in some cases.

Sweden, which does not reprocess its fuel, has built the CLAB storage facility in Oskarshamn, featuring large interim storage pools 30 m underground. This facility will be used until a repository is built.

In France, intermediate-level waste must also be stored, only 36% of which has been conditioned, with the remainder awaiting recovery and processing. Some legacy waste must also be dealt with, for example waste from the earliest power stations using the graphite gas process, which is stored in Marcoule. The ongoing process of dismantling retired power plants also produces low-level long-lived waste, which is being held in interim storage pending definition of a final management strategy.