Last week, Exelon announced that it will be shuttering two major nuclear plants in Illinois. The Clinton Power Station is scheduled to close on June 1, 2017, and the Quad Cities Generating Station will close on June 1, 2018. Exelon’s announcement came after an Illinois legislative proposal that would have provided support for the nuclear plants failed.

Exelon’s announcement is part of a recent trend in the nuclear sector, as low natural gas prices and the rapid growth of wind and solar generation fueled in part by state and federal subsidies have put the industry on shaky economic ground in some merchant markets. A recent report noted that U.S. utilities have closed or announced premature closure of seven nuclear-powered energy generation facilities in the past three years, while at least eight more are at risk of early closure in the next two years. The report—published by leading environmental and clean energy groups—noted that the United States’ share of carbon-free electricity is declining, despite the rapid rise of renewable energy sources like wind and solar. This decline is largely due to the closure of nuclear and hydro-powered electricity generation facilities.

According to the Nuclear Energy Institute, nuclear plants currently account for more than 60% of carbon-free generation in the United States, while hydro represents nearly 20% of carbon free generation. If most of the capacity of shuttered nuclear facilities is replaced with gas-fired generation, a significant percentage of the United States’ carbon-free energy will be lost.

The threatened erosion of the nation’s carbon-free nuclear energy supply has prompted a recent chorus of calls for policy support for nuclear energy. While the growth of renewable technologies like wind and solar have been greatly aided by state renewable portfolio standards (RPSs) and federal tax incentives, nuclear generators have largely been left out. A leading proposal calls for expansion of state RPS programs—which typically require utilities to procure a certain percentage of their electricity from renewable sources—to include other low-carbon technologies like nuclear. So-called “low-carbon portfolio standards” would stimulate demand and help existing nuclear facilities continue operating through their useful life. Many existing nuclear facilities are designed to continue operating for decades, and could greatly help the United States meet its climate goals.