Nuclear-waste Storage Primer

Basically, four options exist for storing high-level used nuclear waste--pools, select-alloy casks, concrete silos, and vaults.

The most common contrivance is to isolate the waste by using a pool of water to entomb metal containers that carry the spent nuclear waste. Such pools are built to be quake proof, based along the designs used for nuclear reactors.

Select-alloy-casks destined for storage are coated internally with neutron-absorbing (to prevent the possibility of the waste's achieving a self-sustaining chain reaction). After emplacement of the spent waste, the cask is filled with helium (to forestall corrosion). Then, to dissipate its heat, it is submerged in a water pool for three to four years. Following this preconditioning, the cask is placed in a ferro-concrete building. (Select-alloy-casks also are employed in transporting radioactive materials.)

Silo storage comprises the placing of slender stainless-steel canisters, which encase the nuclear waste, into large concrete facilities. The canisters stay cool by natural air convection.

With vault storage, the canisters containing the high-level nuclear wastes are placed in concrete enclosures that are buried deep underground. As for silo storage, cooling of the canisters is by convection.

Pool or alloy-cask storage of 5000 metric tons of nuclear waste typically requires 50,000 to 100,000 square meters (one-half to one-million square feet) of space, with the alloy casks needing more space than the pools. Such storage is considered safer than vault or silo storage in terms of resistance to earthquakes.

The previous article is based on an April 1, 1998, report prepared for the Nikkei English News service.