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When California鈥檚 largest electric utility announced last week that it would close the state鈥檚 last operational nuclear power plant, supporters were quick to call the moment a potential game changer for America鈥檚 energy future.

The basic message, after all, is that officials in America鈥檚 most populous state, while eager to battle against climate change, have decided that nuclear reactors aren鈥檛 needed in the fight.

And the move, in which state regulators nudged Pacific Gas & Electric Co. toward a plan to close its Diablo Canyon reactor, comes as other states have also been closing nuclear plants or planning to do so. Solar and wind power are surging, and PG&E said the Diablo Canyon power will be replaced by renewables.

Could the end of the line be coming into view for a power source that used to be hailed by some as the future of clean power?

Actually, it looks far too early to draw that conclusion. The reality is that a battle still rages and may go on for some time. Some new reactors are being built. Some governors even in other politically liberal states are trying to save old reactors rather than scrap them. And though cheap natural gas may have called the economics of nuclear plants into question, environmentalists are divided over whether a nuclear phaseout would be wise.

鈥淚 think the stakes are becoming higher as these closures are happening,鈥澛爏ays Jesse Jenkins, a researcher at the聽Massachusetts Institute of Technology's Energy Initiative. 鈥淲e have a pretty big decision to make as a country about whether or not we鈥檙e going to give up this source of power or to build on that foundation. That could be one of the biggest decisions we鈥檒l have when it comes to our targets and climate goals.鈥�

Opponents of nuclear power point to the plants鈥� safety risks, the problem of their radioactive waste, and the fast-falling costs of cleaner alternatives.

The other side says there鈥檚 still a key hurdle in deploying more solar arrays and wind turbines: They only provide power when the sun is shining or the wind blowing 鈥� unless it gets much cheaper to store that power to be used on demand.

For his part, Mr. Jenkins recently published a paper, with co-authors from the Argonne National Laboratory, noting this challenge of bringing storage costs down.

鈥淭he least-cost generation mix includes a diverse mix of resources,鈥� they write, envisioning a future in which 鈥渨ind, solar, and flexible nuclear technologies co-exist.鈥�

Another argument made by supporters of nuclear power: Today鈥檚 reactors aren鈥檛 the same as yesterday鈥檚. Newer ones can be smaller, less costly, and leave behind less radioactive waste.

This battle, in short, is far from settled by the move to close Diablo Canyon. The news has simply brought the debate into sharper relief.

A nuclear-free future?

Proponents of the shutdown plan say it鈥檚 a step toward ending both nuclear hazards and carbon emissions 鈥� and a model of long-term planning.

, supported by a coalition of environmental groups and labor unions, calls for replacing the plant鈥檚 2,200-megawatt capacity with a blend of renewable sources, efficiency upgrades, and energy storage. It includes a commitment from PG&E to source 55 percent of its overall energy sales from renewables. It also provides $350 million for existing workers鈥� severance, retention, and retraining, as well as $49.5 million to compensate the county for lost taxes and jobs.

Though still subject to approval by the state land commission, the agreement represents 鈥渁 big step in the transition in California鈥檚 clean energy future, in which the grid is dominated by [power] generation from renewables,鈥� says Peter Miller, senior scientist with the Natural Resources Defense Council, one of the groups that has lobbied for the Diablo Canyon shutdown.

鈥淚t shows it鈥檚 possible to develop [renewable] sources at a scale and speed that allow states to replace retiring resources,鈥� he says.

The deal鈥檚 success, Mr. Miller and others contend, would make a strong case for the existing shift away from nuclear power 鈥� a movement that has gained momentum over the past few years.

Between 2013 and 2014, energy companies across the country on four nuclear power plants. This month, Exelon Corp.聽 to shutter two nuclear plants in Illinois by 2017 and 2018, respectively.

In general, companies questioned the economics: The costs of maintaining aging reactors could not hold up against the declining price of natural gas and growing subsidies for renewables. The two Illinois plants ranked among Exelon鈥檚 top performers, for instance, yet they lost a combined $800 million over the past seven years, the company reported.

Safety has also been an issue. Less than two years after the Fukushima accident in Japan in 2011, a generator leak at the San Onofre nuclear plant in California drew domestic attention to the public health and environmental risks of such facilities.

The persistence of such concerns, along with unease around the use of a power source that could double as a weapon of mass destruction, suggests that the time is ripe to put an end to nuclear energy, some environmentalists say. And with renewables becoming cheaper than ever, a carbon-free and nuclear-free energy future may be within the nation鈥檚 grasp, they say.

The nuclear industry 鈥渋s in reverse, it鈥檚 going down the drain,鈥� says Daniel Hirsch, director of the environmental and nuclear policy program at the University of California, Santa Cruz. 鈥淒iablo Canyon shows that we can get rid of the risks of nuclear [power] without adding to the risk of global warming. It tells the world what we all need to do.鈥�

'A viable option'

But that worldview on energy, if gaining in California, isn鈥檛 necessarily sinking in nationwide. In another politically liberal state, New York, Democratic Gov. Andrew Cuomo (D) has been trying to save some of the state鈥檚 economically squeezed reactors 聽

Indeed, plans to launch reactors that use safer, more efficient technology are already under way in and 鈥� though remain.

Innovators around the country have also been working on even more advanced nuclear technology that would address problems associated with traditional reactors, such as size, inflexibility, and cost. The engineers behind Silicon Valley-based Oklo, for instance, envision a nuclear reactor small enough to manufacture in a factory 鈥� like cars or prefab houses.

鈥淲e want to get the cost down, get them out the door really quickly,鈥� says nuclear engineer Jacob DeWitte, the company鈥檚 co-founder and chief executive. 鈥淲e want to show that this is a viable option.鈥�

鈥淣ationally, it鈥檚 a mixed bag,鈥� notes Ali Mosleh, a professor of engineering at the University of California, Los Angeles and a former appointee of the US Nuclear Waste Technical Review Board. 鈥淧lants are shutting down, but there鈥檚 also construction here and there. And there鈥檚 momentum for small reactors and next-generation reactors.鈥�

And maybe, some say, that鈥檚 the way it needs to be.

Nuclear energy provides , while renewables 鈥� wind, solar, geothermal, biomass, and hydroelectricity 鈥� produce 13 percent combined. Cutting nuclear from the equation could leave the US with a bigger challenge in terms of achieving its climate goals, says Kerry Emanuel, a climate scientist and meteorologist at the Massachusetts Institute of Technology

鈥淭he numbers just don鈥檛 add up,鈥� he says. 鈥淚n the last two years we have shut down prematurely more nuclear energy than we have added solar and wind.鈥�

At the same time, the need for carbon-free energy is growing. To prevent dangerous levels of global warming, the world鈥檚 nations must achieve net-zero greenhouse gas emissions , the United Nations reports. As the world鈥檚 second-largest energy consumer, the US plays a key role in implementing practices toward that goal.

And so some advocate the need to consider the use of multiple sources of clean energy 鈥� including the nation鈥檚 existing fleet of nuclear power 鈥� at least until the necessary climate targets are achieved.

鈥淭he way I view it, the best way of success is by thinking about deploying all those things,鈥� says Armond Cohen, co-founder and executive director of the Clean Air Task Force, a Boston-based research and public advocacy nonprofit. 鈥淭hat鈥檚 the future we want to create. We want to have those options.鈥�