See in the R&D sectors of government, the scientist collaborate with other scientist in the world until and unless anything they are working on is a top secret or knowing it can be a threat to national security. Thus if you have been alloted a project in DRDO you can't do that but if you wish to keep up your studies you can approach your seniors. Depending on your performance you will be sanctioned a study leave. This things are well often in R&D sectors.

Now you can also get chance to work in foreign laboratories. For example, in the particle accelerator (Large Hadron Collider) by CERN , many BARC scientist worked on the project about there detector physics and accelerator physics. So you can also get this kind of chance relevant to your field and obviously related to the defence perspective.

I hope this answer helps. All the very best for your future endeavors!