I think this idea will not work properly as nuclear explosion cause nuclear fall out. After a nuclear explosion tonns of dust and radioactive material mix with atmosphere and fall slowley for long time which cause blood cancer, hemophelia etc.

To create a nuclear winter effect we'll have to explode a lot of bombs in various heights and places to cover earth's atmosphere properly but in this case it may harm our ozone layer also. Another reason is that if we cover whole atmosphere with nuclear shelter then many plants and forests can disappear from earth due to not getting proper light and improper photosynthesis.

In all case it will be a bad idea for solving global warming problem.