The 73rd and final multi-purpose canister (MPC) of used nuclear fuel from Southern California Edison’s (SCE’s) twin unit San Onofre Nuclear Generating Station (SONGS) in the USA has been placed in its designated below-grade storage vault., US-based Holtec International said on 7 August. 

Holtec International is contractor for the decommissioning of San Onofre, which was closed by SCE in 2013 in face of regulatory delays and after serious safety issues were discovered with its steam generators. The fuel transfer is seen as an interim step towards eventual relocation at a federal off-site facility once it is constructed.

“Our commitment remains ensuring spent nuclear fuel is safely stored and that it can be transported to an off-site facility in the future,” said Doug Bauder, SCE vice president and chief nuclear officer. “We are developing a strategic plan to help us explore opportunities for advancing various alternatives to get the spent fuel off-site, as well as make sure that our fuel is ready for pickup when the opportunity presents itself.”

SCE expects to release the strategic plan early next year, which will consider alternatives for relocating the used fuel to an off-site facility. The options will include permanent disposal or temporary storage at a site.

All of the used fuel is now contained in 123 canisters in dry storage. which allows SCE to focus on dismantling SONGS. SCE issued a deconstruction notice in January and began work to dismantle the plant in February. The first projects completed include asbestos removal from inside the containment domes and shipping the unit 1 reactor pressure vessel to a disposal facility in Utah.

“For the next stages at San Onofre, we’ve developed a streamlined organisation that is focused on providing oversight of our decommissioning contractor and safely managing the spent fuel,” Bauder said. “Much of the work coming up will be inside the containment domes and preparing for the removal of lots of steel and concrete.”

The HI-STORM UMAX subterranean MPC storage system was selected by the SCE because of its ability to withstand any conceivable threat, “including earthquake intensity levels exceeding those ever recorded”, Holtec noted. The system is intended for an extended service life by design and manufacturing measures such as the use of low-carbon stainless steel material, peened weld seams and Holtec’s proprietary low heat input welding process. Each MPC was dried using a forced helium dehydration system, which is known to be uniquely effective in drying the used fuel especially the water-logged and damaged used fuel, Holtec said. 

To support the transfer of used nuclear fuel from wet storage to dry storage, significant enhancements were developed and implemented for the SONGS project and are being utilised across the Holtec fleet. These enhancements include the development of a training programme using a Systematic Approach to Training, detailed loading procedures with process improvements, and the development and utilisation of camera systems and load monitoring equipment to make the loading of canisters into the storage modules safer.

A robust ageing management plan is being established for the SONGS storage system to ensure that any degradation would be identified long before it could become an issue, Holtec added. The MPCs are licensed for transport and are ready to be shipped if and when a consolidated interim storage facility (CISF) becomes available. Removing the storage canisters from a corrosive salt-air environment such as SONGS to a more conducive environment is possible. Holtec is currently working to establish the HI-STORE CISF in the dry and arid land of southeastern New Mexico which will use the same subterranean storage system as SONGS.