Laurence, though in the media we often hear zero-emission credits referred to as subsidies, or

"a sum of money granted by the government or a public body to assist an industry or business so that the price of a commodity or service may remain low or competitive..."

they aren't subsidies at all. The credit isn't paid by a public body, but by electricity customers as part of their electricity bill. It's credited to utility-scale generators which emit no CO2 as a waste product of generating electricity. Those generators include nuclear plants, solar farms, wind farms, geothermal plants, and hydroelectric dams.

The amount paid to each generator corresponds to the Social Cost of Carbon (SCC), a metric invented by the U.S. Environmental Protection Agency as a way to quantify the social costs imposed on society by each ton of CO2 emitted. They include financial costs, health costs, property losses, and other costs incurred as a result of climate change. Obviously, coming up with that number is a big job - one that's extremely subjective, and controversial. But EPA's best estimate (as of 2019) was that each ton of CO2 emitted was responsible for $42 in damages to society.

Because zero-emission sources don't impose that cost on society, the intent of the credit is to reward them a corresponding amount for "not generating carbon" - kind of like a backwards carbon tax. In that sense it levels the playing field - each source receives what its zero-carbon contribution is worth.

So when you hear anti-nuclear or renewables advocates refer to a zero-emission credit as a "nuclear subsidy", call them out on it. Tell them solar and wind farms get exactly the same credit, and it's based on the amount of clean energy they generate. If renewables can't generate as much clean energy, that's not your problem - tell them they should be investing in a nuke plant.