With two reactors operational in early 1960s, Bhabha was involved in learning and development of know-how to manufacture nuclear weapons. The atomic energy act was amended in 1962 to give far more control to the
central government. Bhabha was also aggressively lobbying for nuclear weapons and made several public speeches on the matter. He also estimated that a nuclear device with a 10
kt yield would cost US$350,000. The reactors were not producing fuel at the expected rate and with Nehru's death in 1964, the programme slowed down. The incoming prime minister
Lal Bahadur Shastri gave approval for the Subterranean Nuclear Explosion Project in 1964 under Bhabha's insistence. However, Shastri did not want to commit to a weapons test yet, and later appointed physicist
Vikram Sarabhai as the head of the nuclear programme. Because of Sarabhai's non-violent
Gandhian beliefs, he directed the programme towards peaceful purposes rather than military development. Meanwhile, the design work on the bomb proceeded under physicist
Raja Ramanna, who continued the nuclear weapons technology research after Bhabha's death in 1966. The weapons programme was directed towards the production of plutonium rather than uranium and then in 1969, enough plutonium had been accumulated for the production of a single nuclear bomb. Simultaneous work on the fabrication of the bomb core and
implosion design was conducted by teams led by physicist
V. S. Ramamurthy. The detonation system development began in April 1970 with
Pranab R. Dastidar collaborating with
W. D. Patwardhan at the
High Energy Materials Research Laboratory (ERDL) of the
Defence Research and Development Organisation (DRDO). In July, physicist
B. D. Nagchaudhuri was appointed as the scientific adviser to the
Defense Minister and as Director of the DRDO. Nagchaudhuri and Ramanna worked together to recruit the team and set up the requirements necessary for a nuclear weapon test.
Terminal Ballistics Research Laboratory (TBRL) of the DRDO developed the explosive lenses for the implosion system. Srinivasan and K. Subba Rao were tasked with developing fission models and prediction of the test's efficiency. In April 1971, Nagchaudhuri appointed N. S. Venkatesan as the new Director of TBRL to help develop the implosion system.
V.K. Iya was in charge of developing the
neutron initiator system. In the same year, Sethna succeeded Sarabhai as the chairman of the Atomic Energy Commission. In December 1971, during the
Indo-Pakistani War, the U.S. government sent a
carrier battle group led by the into the
Bay of Bengal in an attempt to intimidate India. The Soviet Union responded by sending its own naval force to deter the U.S. from involving militarily. This event is indicated as a reason for India's pursuance of the nuclear programme. After India gained military and political initiative over Pakistan in the
Indo-Pakistani war of 1971, the work on building a nuclear device continued. The hardware began to be built in early 1972 and the Prime Minister authorised the development of a nuclear test device in September 1972. == Nuclear test ==