Nuclear power’s tenure as the leading source of zero-carbon electricity may be coming to an end.

After decades of stalled nuclear plant development and the recent surge of increasingly cheap wind and solar deployments, the newcomers are pulling ahead. In the first five months of 2018, renewables produced 20.17 percent of U.S. electricity and nuclear produced 20.14 percent, according to Energy Information Administration data compiled by Ken Bossong of the Sun Day Campaign.

A similar record was hit in the first three months of 2017.

In the two most recent months included in the data set, April and May, renewables outproduced nuclear by more than 10 percent. 

To be fair, the renewables category here encompasses wind, hydropower, large- and small-scale solar, geothermal and biomass. Nuclear still more than doubles the generation of the second-largest single clean energy source, hydropower.

That said, the data offers a snapshot of how the mix of clean energy is shifting. Renewables out-generate nuclear in more than half of states, according to an analysis of EIA data by the Sun Day Campaign.

Sun Day has a very clear stake in this race: Its mission is to promote sustainable energy and help “phase out the use of nuclear power.”

The nonprofit is not alone among advocates who want clean energy, but want to end the top source of zero-carbon power. Environmental groups have long taken aim at the radioactive waste and perceived risks of nuclear power plants, and they often play a key role in working out the deals that arrange for plant retirements.

The baseload nature of nuclear plants — providing massive amounts of electricity around the clock — contrasts with the intermittent production of wind and solar. Nuclear is also considered more readily dispatchable than are wind and solar. Hence the intense interest in energy storage to balance out renewable generation throughout the days and seasons.

The key question for decarbonization is which other power sources are eating up nuclear’s market share.

Sun Day’s messaging lumps nuclear plants in with coal plants as outdated and “environmentally polluting” power sources. If coal retires, it will be replaced by less polluting sources. Unlike coal, if nuclear retirements get replaced by natural gas, greenhouse gas emissions go up. 

Natural gas remains the go-to flexible capacity source for the grid; big batteries have begun to play that role in only a few locations so far.

The renewable sector can cheer its growing prominence, but ending nuclear will just make it harder to decarbonize with wind and solar.