1. No, nuclear energy is not clean energy, especially nuclear fission based energy. Nuclear fission based reactors like the ones in power plants and nuclear submarines involve radioactive and extremely toxic substances as end products. And the radiation levels from these are dangerous. Disposal of these end products is a huge environmental concern in all the nations using nuclear energy.
2. Locos will have to be very heavy and will need thick lead walls to prevent radiation leakage. This means, very complex and very costly construction.
3. Even the tiniest...
more... mistake, something like a minor derailment can mean disaster for 100's of passengers.
4. Maintenance will become very costly and very risky for the loco shed staff. They'll have to practically risk their lives to just maintain the loco.
Better to use Nuclear energy for Large scale power generation and use electric locomotives. For self-powered locos like diesel locos, Fuel cells can be used to generate energy instead of diesel engines. This has been researched in some countries, though probably not yet in IR.