Democratic presidential candidate Julián Castro proposed phasing out nuclear energy during an NPR interview Friday.

The former Obama administration official acknowledged that nuclear energy is preferable to carbon-based fuels, such as oil and coal, but said his environmental policies would center almost entirely on wind and solar energy. NPR host Lulu Garcia-Navarro voiced concerns that green energy "cannot in the short term make up for the traditional sources of energy."

"We need to move away from coal, oil, and gas and phase out nuclear," Castro said. "Nuclear is definitely preferable when it comes to carbon emissions versus those other three and the way that I think about it is, sort of the worst-first approach to working on immediately getting the worst of the types of energy that produces carbon emissions out first."

Castro's plan would put the United States on a path to 100 percent clean renewable energy in the electricity sector within 15 years.

"I have set a goal in my plan of getting us to an electricity sector that is clean renewable and zero-emission by 2035," Castro said. "And, you know, nuclear has a role in that but I don’t think that it has the primary role. I think ultimately renewables will have the primary role."

Castro's climate plan would cost $10 trillion over the next 10 years.

An analysis challenging the LOADMATCH model, a grid designed to evaluate the viability of electricity systems, found renewable energy is not a sustainable option.

"In a system where variable renewable resources make up over 95 percent of the U.S. energy supply, renewable energy forecast errors would be a significant source of uncertainty in the daily operation of power systems," according to the National Academy of Sciences.

Nuclear energy has the highest capacity factor of any energy source. Nuclear power plants are providing their maximum power output about 92 percent of the time, making them especially reliable, according to the Department of Energy. Nuclear power plants produce nearly no carbon emissions.

"Nuclear power plants are typically used more often because they require less maintenance and are designed to operate for longer stretches before refueling (typically every 1.5 or 2 years)," the Department of Energy says. "Natural gas and coal capacity factors are generally lower due to routine maintenance and/or refueling at these facilities."