The easiest test of whether one is dealing with a serious environmentalist is quite simple: anyone claiming to be a friend to the environment who simultaneously makes it their priority to shut down the most abundant carbon-free energy source in present day is at best no serious friend to the environment, showing a ludicrous disregard for the most basic concept of triage.
In this case, the calculus of triage is quite simple, and quite brutal - taking Fukushima as an example of some of the very worst consequences of a potential nuclear accident in terms of modern nuclear reactors (insomuch as a 40-year old reactor design can be called "modern"), such consequences amount to a catastrophic loss of property and perhaps even livelihoods to a localized region - but it pales in comparison to the global and devastating consequences of unchecked climate change, and pales even in comparison to the premature deaths brought about from ordinary pollution from more polluting sources like coal.
Few environmentalists are willing to take such a self-marginalizing position (although clearly that number is far from zero, if the ongoing campaigns against Vermont Yankee and Indian Point are any indication). Those that do inevitably fall into two categories - those that (disingenuously) assert that the gap can be filled nearly immediately with renewable sources (despite the mathematical difficulties of such a claim), falling back onto the idea of natural gas as a "bridge fuel" (again failing the basic arithmetic rule that half the carbon dioxide emission of coal, easily the dirtiest source available, is still far greater than zero, or close enough when all emissions are factored in over the entire lifecycle), or when pressed, falling back upon the idea of "energy austerity" - asserting (again, under highly questionable premises) that the energy deficit can simply be closed by using less - either by efficiency or simply by imposed austerity.
A natural experiment for this position is to look to Germany's Energiewende, which purports to do just that - trading carbon-free baseload from nuclear today for a promise of carbon-free intermittent electricity from renewables tomorrow. As to its efficacy, the evidence speaks for itself - Germany's carbon emissions increased last year by 1.2% - namely because the chief replacement for nuclear energy has come not from renewables but perversely from burning more brown coal and natural gas. Claiming that substituting definite and indisputable risks (if only from the environmental costs of coal burning alone) for an uncertain, possible (and by all accounts, remote) risk represents a positive environmental trade-off is laughably absurd. Worse, it represents the very opposite of intelligent triage - again, taking for granted the idea that the risk of a nuclear catastrophe is non-negligible (debatable, but assumed for the sake of argument), basic logic dictates eliminating the worst and most certain environmental harms first - the very opposite of what is being done.
Inevitably then we come to the default position it seems of most "mainstream" environmentalists today, perhaps realizing the absurd parody of triage implied by prioritizing the closure of existing, operational plants which emit no carbon in their operations over the most significant environmental offenders, instead re-focus their message on opposing the development of new nuclear units (essentially hoping to simply run out the clock on the matter). The most curious arguments invoked inevitably come down to very selective applications of both arithmetic and economics - those being that nuclear takes too long to build compared to renewables (which causes one to question whether they've done the math at the time it would take to build out an equivalent capacity) or that it simply costs too much while in the same breath insisting that the federal government must provide financial support to their energy sources of choice. (As for the latter, there is yet another quick test of the seriousness of the convenient economic principle invoked - ask the proponent whether their argument applies equally when it is their own ox being gored. If the answer is "no," the argument about economics can be clearly dismissed as specious special pleading.)
Give the boosters of natural gas as an eponymous "bridge fuel" credit for one thing - at least their position doesn't rely upon logical gymnastics (although it does depend on where you call home in the wintertime).
60% of the U.S. carbon-free electricity portfolio. In what constitutes an existential fight for the simultaneous survival of the human race combined with an unparalleled drive to lift out billions from crushing poverty, what sane leader then treats the vanguard as disposable? Contrary to popular belief, we do not currently suffer from an embarrassment of riches when it comes to options for stabilizing carbon output, especially if economic considerations are factored in (as they should be).
The fact is, prioritizing carbon-emitting sources like natural gas over nuclear - be it for the present (if temporary) economic realities (again, where warm weather and plenty of pipeline capacity persists) or for more ideological reasons (i.e., avoiding nuclear energy at all costs) - poses a real and significant handicap in our ability to combat climate change.
Sane triage allows for the idea of swapping out the worst sources (like coal) for "better" sources (like natural gas) - but what serious advocate for action on climate change should advocate turning upon their own vanguard - especially when arguably the nuclear solution has the potential to cut across ideological boundaries, particularly to those who might otherwise be otherwise ill-disposed to work as allies (i.e., ideological conservatives)? One needn't even believe in the reality of climate change for solutions which mitigate carbon to have real consequences - something which itself ultimately demonstrates nuclear's cross-cutting value proposition as a key tool in climate change mitigation.