The Trojan Independent Spent Fuel Storage Installation was designed to safely store the spent fuel from the decommissioned Trojan nuclear power plant for decades, until it can be moved to a permanent federal repository. The storage system is specifically designed to withstand foreseeable hazards, including a 9.0 subduction zone earthquake off the coast of the Pacific Northwest.
The Trojan ISFSI is one of many spent nuclear fuel storage facilities in the United States that have been issued licenses by the Nuclear Regulatory Commission. Until a federal waste repository is built and the spent fuel can be moved to a permanent storage location, the Trojan storage facility will be operated by Portland General Electric and monitored by the NRC and the Oregon Dept. of Energy.
The ISFSI is an above-ground, air-cooled dry cask storage system. Cooling is accomplished using a passive convection system containing no moving parts, and does not require a power source. The ISFSI consists of:
Stainless steel inner canisters
The inner canisters are sealed, transportable, stainless steel cylinders containing fuel assemblies, fuel containers and fuel debris cans. Each canister is stored in a concrete cask. The 34 canisters at Trojan contain 790 fuel assemblies as well as fuel debris.
The concrete casks containing the steel inner canisters at Trojan sit on the ISFSI Storage Pad, protecting the canisters and their contents from hazards and providing shielding from the irradiated fuel. The temperature of the air exiting the cask is continuously monitored. These casks have a thick inner liner made of carbon steel surrounded by thick concrete walls. Each cask weighs about 150 tons. The spent fuel is cooled via passive convection of air passing through the gap between the inside of the cask and the outside of the steel canister. No power is needed to aid in the cooling process.
The ISFSI storage pad
The 34 concrete casks sit on a heavy concrete pad, located adjacent to the former Trojan Nuclear Plant site. The thick, rebar-reinforced pad has an engineered underlayment of rock and gravel.
The ISFSI storage pad is surrounded by a secured area, monitored and protected round the clock.
The Trojan ISFSI was designed and licensed to withstand a “seismic margin earthquake.” This could include a subduction zone earthquake off the Oregon coast of a magnitude similar to the one that shook Japan on March 10, 2011 as well as more localized seismic events in the vicinity of the storage area.
No significant physical damage would be expected from earthquakes within planning parameters, and because the casks use a passive cooling system, a power disruption at the facility would not affect them.
No significant movement of the casks is postulated to occur as a result of the seismic margin earthquake event. However, even in the unlikely event that a cask or casks were to overturn, the integrity of the inner canisters would not be compromised. (Note that these canisters were designed to be used as shipping containers for eventual transport to a federal waste repository, so they are engineered to withstand risks associated with being moved, loaded and unloaded.)
Tsunami and flooding readiness
The Trojan ISFSI is well-protected by its location, 45 feet above mean sea level and approximately 72 miles from the mouth of the Columbia River. As was demonstrated during the tsunami generated by the Alaskan earthquake of 1964, tsunami effects at the mouth are dissipated inside the river due to the characteristics of the estuary. Therefore, the risk of tsunami inundation at the Trojan site was considered low enough that no tsunami plan was required when the facility was licensed.
The ISFSI is also not considered vulnerable to other postulated flooding events on the nearby Columbia River, including a failure of the Grand Coulee Dam. Even if the site were to flood, water would also serve to provide passive cooling to the casks.