Forgetting Fukushima

[Anomalocaris]

How tectonically active is the Indian sub-continent??

Depends on where.

Much of India outside of the Himalayas is cratonic, that is to say built on stable, monolithic, very old, Precambrian metamorphic basement shield rocks like the middle of Canada. Major earthquakes are as unlikely here as anywhere else on land.

Himalaya belt is very tectonically active, and experience major quakes regularly.

Fukushima shows nuclear powerplants are in fact capable of resisting the very largest earthquakes, provided they are not actually built right over the displaced fault, as some in Japan are.