B.Tech

Both CSE and ECE are good options but it is better to opt CSE only if you are really interested in Software field because after completing B.Tech in ECE or in any other branch you can easily come to software field but you cannot enter any other core departments if you are a CSE graduate. Since you want to do M.Tech in Nuclear Engineering my suggestion is to opt ECE because it provides practical learning and lets you be prepared for your Masters. I don't say CSE doesn't provide practical learning but I say it doesn't need because maximum subjects are theoretical and doesn't require any practical knowledge to clear those subjects and that can take you away from learning things practically.