How India Developed Nuclear Weapons

In the Himalayas, a chilling chapter was written in the history After the Chinese detonated their first nuclear test in 1964, America and India, which had just fought a border war with china, were both justifiably concerned about this. The CIA knew it needed more information on China’s growing nuclear capability but had few ways to detect because of the extreme remoteness of Chinese testing grounds, conventional surveillance in that pre-satellite era.
The solution to this intelligence dilemma was a joint American-Indian effort to plant a nuclear-powered sensing device given by USA on a high Himalayan peak in order to listen into China and monitor its missile launches.India used that device to learn the operation of nuclear technology.Later India told USA that the device was lost in heavy snowfall.
CIRUS, a research reactor at BARC supplied by Canada in 1954 was used to produce plutonium stockpile which is used in India's initial weapons,uses heavy water supplied by the U.S.
India perform its first nuclear test in pokhran in 1974 with the help of CIRUS and nuclear-powered sensing device.In this way India developed its nuclear weapons.

References:
Spies in the Himalayas Secret Missions and Perilous Climbs
M. S. Kohli and Kenneth Conboy