At the same time, state energy-efficiency mandates would force it to reduce generation, while new measures liberalizing the electricity market meant PG&E would lose much of its customer base to other providers. With PG&E having to shoehorn more renewable power into a shrinking sales base, existing capacity would have to close to make room.

The obvious choice was Diablo, which would not be as adept at backing up new wind and solar power as nimble gas turbines are. California already has to curtail solar power surges on some days, and PG&Es projections showed that rising solar capacity would make surges bigger and more frequent.

Increasingly, not only PG&E's gas plants but Diablo itself would have to ramp down to accommodate surges that would otherwise crash the grid. That’s a difficult operation for a nuclear plant, as is ramping back up when the sun sets.

Of course, the problem of solar oversupply could be solved by simply curtailing excess solar power during surges—it doesn’t help to replace clean nuclear power with clean solar power—but PG&E has a renewables quota to meet. The company foresaw that it would have to curtail Diablo more and more often and that swelling intermittent overcapacity would more generally depress electricity markets, pushing the marginally profitable plant into the red.

So to comply with the state’s decarbonization rules PG&E agreed to shut down a zero-emissions plant and replace it with a mix of gas and renewables with much higher carbon emissions. That seemingly irrational decision was perfectly rational from the utility’s viewpoint, because California does not recognize nuclear power as low-carbon energy nor discourage a utility for replacing it with fossil fuels.

All of this was spelled out in the Joint Proposal and in press conferences by PG&E and anti-nuclear groups. The only obfuscations were the breezy assurances from all parties that Diablo would be replaced pristinely by renewables and demand reduction. Almost none of the former was specified, and much of the latter is an illusion of the anticipated offloading of PG&Es customers to other companies.

In fact, PG&Es own scenarios show that closing Diablo to make way for solar is a serious blunder from a decarbonization standpoint.

The company’s 2030 forecast shows that with its program of closing Diablo, increasing renewables and reducing demand through efficiency and customer defections it would still generate 24,000 gigawatt-hours of electricity from natural gas.

But if it kept Diablo open and did not build any new renewables, it could reduce natural-gas generation by 25 percent to 18,000 gigawatt-hours—with a reduction of 2.4 million tons of carbon dioxide annually.

A Diablo closure would be a boon for the market share of the renewables sector, but a set-back for decarbonization.

Can Nuclear Survive?

Gas prices look to remain low for a long time, but even if they rise there is no way out of the dynamic of over-capacity and plunging electricity prices as long as subsidizing wind and solar is the government’s main energy initiative.