This probably isn’t the kind of meltdown you are thinking of. This is a good kind of melting.
The Kankakee River in Illinois has been iced up this winter. The ice can jam the flow of the river, causing flooding. So operators at the nearby Dresden nuclear power plant, in coordination with the local emergency management agency, started releasing water from the Dresden cooling lake into the river. The lake is evidently at a warmer temperature, so releasing lake water will hopefully melt the river ice.
The lake is warmer than the surrounding environment because it accepts the waste heat from the nuclear plant. Water is taken from either the river or the lake, and then pumped through the nuclear plants’ main condensers. When the water passes through the condensers, it is warmed up, and then returned to either the river, the cooling lake, or passed through cooling towers. There are evidently three different modes of operation for the turbine cycle cooling, as described in the Wikipedia article below.
The heat in the lake comes from nuclear reactors. Nuclear fission created the heat, although it is rather low-grade heat by the time it passes through the turbine and removed by the circ water flowing through the condenser tubes.