Don Mosier | Rimini Road
The thin-walled stainless-steel canisters used to store spent nuclear fuel at San Onofre have been a concern since they were chosen instead of the much thicker canisters used in Europe. Previous articles have highlighted the difficulty in transporting these canisters from the cooling pools to the vertical storage system which resulted in the near-drop disaster last August. Another problem has emerged that the Nuclear Regulatory Commission (NRC) is investigating. The vertical receptacle (“overpack”: concrete and neutron-absorbing components) into which the loaded canister is lowered has guide rings or channels that orient the canister. The accident in August involved the canister getting stuck on the guide rings. Even if the canister is properly lowered, contact with the guide rings or channels results in gouging of the canister outer walls. Here are the details from the NRC safety analysis:
“A single, base Holtec HI-STORM overpack design is provided which is capable of storing each type of multi-purpose canister. The overpack inner cavity is sized to accommodate the canisters. The inner diameter of the overpack inner shell is 73-1/2 inches and the height of the cavity is 191-1/2 inches. The overpack inner shell is provided with channels distributed around the inner cavity to present an inside diameter of 69-1/2 inches. The carbon steel channels are intended to offer a flexible medium to absorb some of the impact during a non-mechanistic tip-over, while still allowing the cooling air flow through the ventilated overpack.” (source: NRC HI-STORM 100 Final Safety Analysis Report (FSAR) – ML16138A100).
The outer canister diameter is 68-3/8 inches, leaving only 9/16-inch clearance around the canister. This narrow clearance results in gouging of the 1/2-inch thick stainless-steel canisters as they are lowered into the overpack receptacle.
The NRC approved use of these thin-walled canisters despite knowing that they were subject to damage during loading, although it states it may have acted differently if it had known the extent of the gouging problem. Although we hoped that the current investigation would result in a moratorium on any new nuclear fuel transfer until thicker canisters and foolproof loading procedures can be adopted, we were once again disappointed. Unfortunately, the NRC ignored these concerns and decided on May 21st to allow canister storage to resume. The announcement came with no details about why the procedure was determined to be “safe.” NRC oversight, such as it is, tends to be reactive rather than proactive. Reaction after a major accident will be too late for 8 million residents of Southern California.