Guest Commentary written by

William Simpson

William Simpson is an author, researcher, naturalist and activist. He is executive director of the Wild Horse Fire Brigade.

San Luis Obispo County is studying the construction of a major seawater desalination plant along its 90-mile coastline, to provide a drought-proof water supply for 16 partner water agencies. One of the leading options involves pairing or expanding the plant at or near Diablo Canyon, California’s last operating nuclear power station.

Desalination is extremely energy-intensive. Reverse-osmosis systems — which force seawater through membranes to filter out salt — typically require 4,000 to 5,000 or more kilowatt-hours of electricity per acre-foot of water produced. (An acre-foot is the amount of water needed to cover an acre of land under a foot of water.)

With California’s grid already under strain, securing reliable power is a big challenge. Proponents argue using Diablo Canyon offers clear advantages. The plant already operates a small on-site desalination facility producing up to 1.5 million gallons per day. 

Nuclear reactors provide steady 24/7 baseload power at very low marginal cost — sometimes described as near “free” for on-site use. The electricity is zero-carbon, and utilizing existing infrastructure would significantly reduce costs and new demand on the electrical grid.

Last month the U.S. Nuclear Regulatory Commission approved 20-year license renewals for Diablo Canyon’s two units, potentially extending operations to 2044 and 2045. Although state law currently limits operations to 2030, the federal approval keeps longer-term options open.

This path carries substantial risks. Critics contend economic pressures and electric grid reliability concerns shouldn’t be prioritized over public health, safety and the environment.

Diablo Canyon’s reactors are 1980s-era, Westinghouse pressurized water reactors, located in a highly seismic coastal zone. The Diablo Canyon plant lies near multiple active faults, including the Hosgri Fault, which is 3 miles offshore, and the Shoreline Fault, which is less than a mile away.

Independent experts and advocacy groups like Mothers for Peace warn that seismic analyses may underestimate the combined hazard of simultaneous fault ruptures, which could produce stronger ground motion than the plant was designed to withstand. A severe earthquake during extended operation could potentially trigger a Fukushima-scale accident affecting the Central Coast and parts of Southern California. 

In 2011 in Fukushima, Japan, a 9.0 magnitude earthquake and subsequent tsunami damaged a nuclear plant, releasing radioactive material into the environment and causing tens of thousands to evacuate. 

A major concern at Diablo Canyon is reactor vessel embrittlement, especially in Unit 1, which was built with faulty material. Decades of neutron radiation can make the steel pressure vessel brittle, increasing the risk of cracking or failure during a seismic event or rapid cooldown. Critics, including UC Berkeley-affiliated experts, argue that the Nuclear Regulatory Commission granted exemptions to Diablo Canyon without sufficient long-term data.

The plant’s cooling system draws in 2 billion to 2.5 billion gallons of seawater daily and discharges it approximately 20 degrees warmer. This process kills billions of fish larvae, plankton and marine organisms annually. 

It also degrades roughly 14 square miles of nearshore habitat — an area the size of San Luis Obispo. Environmental groups, including the California Coastkeeper Alliance, call it one of the most destructive coastal operations in the state.

Extended operation also would generate and store more high-level radioactive waste onsite in a seismically active zone, with no permanent repository. Critics worry about corrosion of the plant’s dry casks in salty coastal air and potential seismic damage to its spent fuel pools.

The plant extension also is economically controversial. Some analyses estimate total costs through 2045 at $20 billion to $45 billion, including possible disaster or retrofit expenses. Critics allege PG&E inflated costs to secure state subsidies, while safer renewable energy and storage alternatives receive less consideration.

The San Luis Obispo County DESAL feasibility study is evaluating multiple sites, technologies and energy sources, including possibly pairing desalination with a planned Morro Bay offshore wind operation. However, wind’s intermittency makes it less suitable for a facility that needs to run at high capacity around the clock.

Public hearings were last month but no final decisions were made. Whether a nuclear-powered desalination plant moves forward will depend on technical, economic, and environmental findings, as well as public input and politics. 

Supporters say it’s a pragmatic solution for drought-proof water using reliable, low-carbon power. Critics see extending an aging nuclear plant in earthquake country as an unacceptable gamble.

The lessons of Fukushima serve as a reminder of what can happen when risks are underestimated.