San Onofre uses dry-cask storage similar to Diablo

Storage of high level radioactive waste at California’s other nuclear power plant, the San Onofre Nuclear Generating Station near San Clemente, is handled similarly to Diablo Canyon nuclear power plant.

Both plants use a combination of storage in pools and dry casks, with the bulk of the plants’ used fuel remaining in the pools. According to San Onofre’s operator, Southern California Edison, the plant’s two spent fuel pools contain 2,452 assemblies, roughly divided equally between the pools.

The plant has 42 dry-cask canisters with 24 assemblies per cask, for a total of 1,008 assemblies in dry storage. This means 29 percent of the plant’s spent fuel is in dry casks.Like Diablo Canyon, San Onofre was built with the assumption that the federal government would take the fuel for storage or reprocessing into new fuel. Its original spent fuel pool racks held 800 assemblies.

In 1991, the plant installed higher-density racks when it became evident that the federal government was not going to take the fuel. Southern California Edison and the Nuclear Regulatory Commission believe the pools are safe, even at the higher storage density.

The pools are 38 feet deep, and water is circulated through the pool to prevent any from boiling away. The 14-foot-tall assemblies are covered by 23 feet of water.

As the pools continued to fill up, the utility received permission from the NRC to establish a dry-cask storage facility. The first casks were filled in 2003.

They are located at an area where a decommissioned reactor once stood. In 1992, an older reactor was shut down after operating for 24 years.