Diablo Canyon is Shutting Down. Is California Ready?

February 23, 2021
Nuclear Regulatory Commission/flickr
Mark Specht
Senior Energy Analyst

En español

In this moment, California’s electrical grid faces no shortage of challenges. There’s the year-round risk that utility power lines will spark wildfires, and there’s the real possibility that an extreme summer heatwave will trigger more rotating blackouts. But there’s another issue looming on the horizon that California hasn’t even begun to address: replacing the Diablo Canyon nuclear power plant with clean energy when it shuts down mid-decade.

In 2018, California regulators approved the decision to shut down Diablo Canyon, California’s last nuclear power plant. Shortly afterwards, California state legislators passed a law to ensure that Diablo Canyon’s retirement does not lead to an increase in global warming emissions when the power plant’s two nuclear reactors go offline in 2024 and 2025.

But has the state been doing enough to replace Diablo Canyon without increasing global warming emissions?

We crunched the numbers, and the answer is no. Today, the Union of Concerned Scientists (UCS) released a new analysis, Countdown to Shutdown, with all the details. But for those of you looking for the tl;dr version, you’re in the right place.

How did UCS conduct this analysis?

When we set out to do this analysis, the first question we had to answer was, “how will we figure out if Diablo Canyon’s retirement will increase global warming emissions?”

It might sound like a simple question, but it’s really not. There are so many other factors driving changes in California’s electricity sector (e.g. renewable portfolio standard and grid reliability requirements), so we needed a way to pinpoint the effect of Diablo Canyon’s retirement on global warming emissions.

To pinpoint this effect, we used grid modeling tools to project global warming emissions in two different worlds. First, we modeled the real world that we all live in, where Diablo Canyon will shut down mid-decade. Then, we modeled an alternative world where Diablo Canyon remains online. Finally, by comparing the emissions trajectories in these two different worlds, we were able to pinpoint the effect of Diablo Canyon’s retirement on global warming emissions.

In essence, the emissions in the alternative world (where Diablo Canyon remains online) serve as a baseline – as long as emissions in the real world stay below those levels, then we’re golden. In other words, the only way to ensure that we replace Diablo Canyon without increasing global warming emissions is to keep emissions below the baseline levels in that alternative world.

Will Diablo Canyon’s retirement increase global warming emissions?

Unfortunately, the answer is yes.

Assuming that California continues on its current decarbonization pathway most recently endorsed by regulators at the California Public Utilities Commission, the UCS analysis shows that, between now and 2030, California’s electricity sector will emit an extra 15.5 million metric tons (MMT) of global warming emissions due to the retirement of Diablo Canyon. And because those emissions would come from existing natural gas power plants that operate more frequently, air pollution emissions from gas plants would increase in tandem. We estimated that the increase in emissions of one type of air pollutant, nitrogen oxides (NOx), would be roughly equivalent to the NOx emissions from 1,890 diesel school buses operating over the next decade.

Furthermore, the UCS analysis indicates that the current decarbonization pathway for the state’s electricity sector doesn’t reduce 2030 emissions enough to fully replace Diablo Canyon in addition to meeting California’s clean energy requirements (i.e. the renewable portfolio standard). Thus, the analysis demonstrates that California’s electricity sector will not decarbonize enough by 2030 to comply with state law; further decarbonization is necessary.

Without further action, existing natural gas power plants in California will operate more frequently due to the retirement of Diablo Canyon, increasing global warming and air pollution emissions.

Does this mean Diablo Canyon should stay online?

Absolutely not. That is definitely NOT the take-away here.

Since the very beginning, the Diablo Canyon nuclear power plant has been plagued by seismic safety concerns. And as with any nuclear power plant, there are the inevitable challenges of nuclear waste disposal. But those concerns weren’t even the final straw.

Pacific Gas and Electric (PG&E) decided to shut down Diablo Canyon mainly for economic reasons. The nuclear plant uses ocean water to cool its reactors, and the cooling technology is so destructive to ocean ecosystem that the technology is being completely phased out of power plants in California. PG&E was faced with a choice: spend a massive amount of money to upgrade the cooling technology at Diablo Canyon or shut down the plant entirely. In the end, PG&E determined the upgrades wouldn’t be worth it, and they decided to shutter the plant.

Between the seismic safety concerns and the costly upgrades, closing Diablo Canyon is still the right choice. And California needs to do more to replace Diablo Canyon with clean resources in order to prevent an increase in emissions and carry on in the transition to clean electricity.

What kinds of resources do we need to replace Diablo Canyon?

Just as the UCS analysis was able to pinpoint the effects of Diablo Canyon’s retirement on global warming emissions, the analysis was also able to pinpoint exactly what resources need to be built by 2030 to replace the power plant. In general, a diverse mix of renewables and energy storage are the most economic approach, and the exact mix of resources is heavily dependent on how aggressively California decarbonizes its electricity sector.

Under the current, relatively unambitious plans for decarbonizing California’s electricity sector, we need many gigawatts of wind and energy storage resources to replace Diablo Canyon. However, solar and geothermal resources also play an important role when planning for more aggressive decarbonization targets (which we absolutely should be doing!). California is already planning to build many of these resources eventually in order to reach its 100 percent clean electricity goals. From this analysis, it’s clear that accelerating the deployment of a diverse mix of renewable and energy storage technologies is the key to replacing Diablo Canyon.

Replacing Diablo Canyon will require a diverse mix of renewables and energy storage, including clean resources like wind power.

What next?

First and foremost, regulators at the California Public Utilities Commission need to act now to ensure that Diablo Canyon’s retirement does not lead to an increase in global warming emissions. As it stands, the current plans for California’s electricity sector will result in a significant uptick in cumulative emissions due to Diablo Canyon’s retirement.

There are a couple different ways that California regulators could address this issue. For example, they could order additional procurement of new resources over and above what’s already being planned. Alternatively, they could choose a more ambitious decarbonization pathway for California’s electricity sector, which is UCS’s preferred approach since it would both ensure full replacement of Diablo Canyon while advancing California towards its state-wide decarbonization goals.

As the electrical grid continues to evolve, nuclear plants across the country are at risk of shutting down. California showed its leadership in managing the transition to clean electricity when the state committed to replacing its last nuclear power plant without increasing emissions. Now California must follow through on that promise.