Environmentalists are increasingly emerging from the closet to back nuclear power for a low carbon energy future that staves off the ravages of CO2-induced climate change.

Yet some green campaigners who accept the nuclear argument still oppose the technology because, they say, nuclear reactors require a king’s ransom to build.

Now, the Breakthrough Institute - a youthful group dedicated to “an ecologically vibrant planet” - has given those holdouts a good reason to drop their final objections and join the nuclear cause.

In their new report, How to Make Nuclear Cheap, Breakthrough notes that a number of alternative reactor types - the sort that regular SmartPlanet readers will recognize - augur affordable nuclear power.

Breakthrough explains that the reactors are inherently safer and operate more efficiently than the fleet of inferior conventional reactors that occupy the world’s nuclear power landscape today. Many of the alternatives can burn nuclear “waste” as fuel, thus answering the vexing question of what to do with waste, the study says. And they lend themselves to assembly-line construction.

Combine all those factors, and the cost of nuclear tumbles. The inherent safety is a big cost advantage because a lot of nuclear’s costs today relate to the extra engineering required to meet safety regulations, note the report’s authors who include Breakthrough’s Michael Shellenberger, one of the environmental stars of the new pro-nuclear feature film Pandora’s Promise.

The technologies have names like molten salt reactors, pebble bed reactors and fast reactors. Some of them run on thorium fuel rather than uranium. Each type ticks its own unique set of advantages. A molten salt reactor, for example, uses liquid fuel that cannot melt down (it’s already melted) and that drains harmlessly into a tank in the event of an emergency.

Many of them operate at high temperatures - a good thing for electricity-generating efficiency and for use as a source of industrial process heat, which is one of the great potentials for low-CO2 applications of future nuclear reactors.

I’ve been feeding SmartPlanet readers a steady dose of information on these reactors that are Betamaxes to the world’s established fleet of less capable VHS models. The general press is starting to pay more attention. TIME Magazine just penned a write-up of the Breakthrough report, a few months after The New York Times took note of thorium and other nuclear options.

One big stumbling block: The alternative nuclear movement will require a lot of investment before it hits the low cost sweet spot. To that end, the Breakthrough report points out that it is crucial for policymakers “to identify the technologies most amenable to commercialization and deployment,” and to “support a broad commitment to nuclear innovation aimed at expanding, rather than restricting, technological options.”

With environmental leadership like Breakthrough’s, more and more previously shamed nuclear supporters should start to boldly show their true colors.