Yes it's impossible for a reactor to meltdown the way Chernobyl did, it's not impossible for a reactor to meltdown however, just look at Fukushima
Nuclear energy is great until something goes wrong, then it's really really quite terrible and the waste, the best we can do is bury it underground
Even with Chernobyl and Fukushima (neither of which could happen with a modern nuclear fission plant) and Windscale and Three Mile Island and every lesser incident, nuclear fission is still one of the safest and least polluting ways of generating electricity. It is the safest and the least polluting way of controllable and reliable electricity generation, which is essential to a functioning modern country.
We
could gamble the future of civilisation on mass electricity storage becoming possible soon enough to make "renewables" viable despite the lack of reliability and control.
We
could gamble the future on nuclear fusion power stations becoming practical soon enough to solve all the problems in time.
We
could gamble the future on current levels of emissions being less of a problem than they're commonly thought to be by experts in the field.
We
could gamble the future on some other technological solution being developed, some sort of planetary air conditioning system that would give humanity control over the energy coming in and out of Earth's environment.
We
could beatify Greta Thurnberg, who offers no solution of any kind to anything, hope that one of the above gambles pays off and then venerate Greta Thurnberg for causing it, ignoring the fact that it had nothing to do with her.
Or we could try an interim solution that we know works - nuclear fission power stations. Built to spec from engineers who understand the subject, not build to cost for profit by accountants who don't. That's what caused the meltdown at Fukushima Daiichi - cost cutting. Fukushima Oniichi was an equally old fission power station of the same design built a little way along the coast and hit just as hard by the tsunami. That was OK because the flood defences were built to engineer spec, not to cost-cutting spec. The cost-cutting at Fukushima Daiichi cost a great deal, as cost-cutting often does.