Recently this was in news. There was a rumor that the plutonium device planted in Nanda Devi exploded and caused floods in India.
https://www.bbc.com/news/world-asia-india-56102459
The rumour supposed that perhaps the plutonium capsules (which continue to generate heat by radioactive decay, since that's their purpose) had melted through ice and the meltwater deluged downstream. It's not improbable that this may happen someday, or that their presence perhaps already contributes meltwater into the Rishi Ganga, but the location of their demise is not the tributary valley where the February 2021 flood appears to have originated.
Part of the confusion is that the origin point of the flood was not immediately known, only that it came from somewhere within the Nanda Devi Sanctuary, a large nature reserve area that shares the name with the famous mountain contained within.