While the safety, cost, and long-term environmental impacts of nuclear energy have been contested for the past half century, there is no debate that nuclear power plants are a significant source of carbon-free electricity. So is this the best time to be shutting them down? It is in California. One of the state’s biggest utilities—Pacific Gas and Electric Company—environmental groups, and labor organizations have proposed shuttering Diablo Canyon, California’s last nuclear power plant, when its…