Coal does not magically materialize either, it needs ot be mined, transported, processes and then transported some more. You'd have to account for that in order to make a fair comparison.

You may also want to take into account how localized and preventable the emissions are. In this particular case, burning fossil fules to heat up homes, already implies no expensive filtration systems, because installing them would be a private investement and one that likely makes no sense given they could equally well replace coal furnace with gas one for less the price.

What's more important is Poland has one of the highest electricity prices in Europe. Even accounting the downsides, it totally makes sense to replace the base of the energy mix with nuclear power and leave coal/gas for when there's a shortage of power. At that point moving to electical heating should make the actual, both financial and envioronmental, cost of inevitable emissions more 'efficient' and manageable. So two ghouls with one rod?

Nuclear requires least mining and materials over lifecycle vs any alternative so in this regard it's clean.

It has low land footprint as a bonus point

It outputs comparable or even smaller waste volumes vs alternatives per kwh over lifecycle. Both nuclear and renewables do create toxic waste over lifecycle. For eg ren do use much more copper both internally and for the grid. Copper mining is associated with arsenic and other dangerous chemicals that must be isolated forever, otherwise you get nasty spills like recently in Africa.

To sum it up, yes, nuclear is as clean as any good ren alternative