The Sacramento Municipal Utility District is turning the grounds of a decommissioned nuclear power plant into one of the largest battery storage sites in the region. SMUD plans to install a 640-megawatt-hour battery system at the Rancho Seco Nuclear Generating Station site near Herald, California, a facility that first produced electricity in 1975 and was permanently shut down in 1989 after Sacramento County voters chose to close it. The project would convert already-disturbed industrial land into flexible grid storage, reusing transmission infrastructure that once carried nuclear baseload power.
Why Rancho Seco’s second life as battery storage matters right now
California’s grid faces growing pressure to store surplus solar energy generated during the day and dispatch it during evening demand peaks. Gas-fired peaker plants still fill that gap across much of the state. A 640-megawatt-hour battery at Rancho Seco could absorb hours of excess renewable generation and release it when the grid needs it most, directly reducing SMUD’s reliance on fossil-fueled backup capacity.
The site offers a practical advantage that new greenfield projects do not: it already sits on land zoned and wired for large-scale power generation. Rancho Seco began producing electricity in 1975 and was closed on June 7, 1989, the day after Sacramento County voted to shut it down. The plant is fully decommissioned, but its substation, switchgear, and high-voltage transmission lines remain. Connecting a battery to that existing grid infrastructure avoids the years-long interconnection queue that delays storage projects across California.
SMUD has already demonstrated its intent to redevelop the property for clean energy. The utility approved the Rancho Seco Solar II project on October 18, 2018, a plan that included photovoltaic solar arrays and associated grid facilities on the same land. Environmental review documents filed under CEQA docket SCH No. 2017092042 described battery storage alongside those solar installations, establishing a regulatory foundation for the current effort.
Federal licenses, spent fuel, and the site’s layered regulatory record
One complication sets Rancho Seco apart from a typical brownfield redevelopment: nuclear waste is still on the property. The U.S. Nuclear Regulatory Commission issued a license to SMUD to store spent fuel from the reactor in an Independent Spent Fuel Storage Installation at the plant site near Herald. That installation remains active because no permanent federal repository for commercial spent fuel exists. Any large construction project on the grounds must account for the buffer zones and security requirements surrounding the dry cask storage area.
State emergency planning records confirm Rancho Seco is fully decommissioned yet still listed as a site with an active spent fuel installation. The coexistence of decommissioned reactor land and licensed fuel storage creates a regulatory overlay that battery developers at other retired nuclear sites will likely face as well. How SMUD manages that overlap could set a precedent for similar conversions at shuttered plants across the country.
Earlier environmental review for the Rancho Seco Solar II project flagged habitat concerns on the property. California Department of Fish and Wildlife findings identified impacts to the California Tiger Salamander, a listed species, and required mitigation measures before construction could proceed. Whether the battery component triggers additional review or falls within the scope of the existing environmental clearance has not been publicly resolved in available filings.
Open questions about SMUD’s 640-megawatt-hour battery timeline
Several details remain absent from the public record. No SMUD board resolution or detailed project application specifying the exact battery technology, construction timeline, or interconnection study results has surfaced in available filings. The 640-megawatt-hour capacity figure circulates in reporting about the project, but the formal procurement documents, vendor selection, and engineering specifications have not appeared in accessible state or utility records as of early June 2026.
The relationship between the battery project and the spent fuel installation also lacks public clarity. Neither the NRC nor the California Governor’s Office of Emergency Services has released statements confirming whether the storage addition requires modifications to the existing emergency planning zones or ISFSI buffer areas. For a project of this scale on land that still holds radioactive material under federal license, that regulatory question is not academic. It could determine construction timelines and site layout.
SMUD ratepayers and neighboring communities in the Herald area have a direct stake in how quickly this project advances. If the utility can bring the battery online within a few years, it gains a tool to cut peak-hour natural gas generation and lower system costs. If permitting complications related to the spent fuel installation or species mitigation slow the schedule, the grid benefits get pushed further out. The next concrete marker to watch is whether SMUD files an updated CEQA addendum or a new environmental document that addresses battery-specific construction impacts on the Rancho Seco property. That filing would signal whether the project is moving from concept to construction or still working through the regulatory layers that come with building on a former nuclear site.
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*This article was researched with the help of AI, with human editors creating the final content.
