Padmanabha Krishnagopala Iyengar (29 June 1931 – 21 December 2011; best known as P. K. Iyenger), was an Indian nuclear physicist who is widely known for his central role in the development of the nuclear program of India. Iyengar previously served as the director of BARC and former chairman of the Atomic Energy Commission of India, he raised his voice and opposition against the nuclear agreement between India and the United States and expressed that the deal favoured the United States.
Iyengar joined the Tata Institute for Fundamental Research, Department of Atomic Energy in 1952 as a junior research scientist, undertaking a wide variety of research in neutron scattering. He later got shifted to Atomic Energy Establishment (later renamed as Bhabha Atomic Research Centre) when it was formed in 1954. In 1956, Iyengar was trained in Canada working under Nobel laureate in Physics Bertram Neville Brockhouse, contributing to path-breaking research on lattice dynamics in germanium. At the DAE, he built up and headed the team of physicists and chemists that gained international recognition for their original research contributions in this field. In 1960s, he indigenously designed the PURNIMA reactor and headed the team that successfully commissioned the reactor on 18 May 1972 at BARC.
When Ramanna took over as director of Bhabha Atomic Research Centre in 1972, the mantle of directorship of the Physics Group (PG) was handed over to Iyengar. He was one of the key scientist in the development of India's first nuclear device. The team, under Raja Ramanna tested the device under the code name Smiling Buddha on 18 May 1974. Iyengar played a leading role in the peaceful nuclear explosion at Pokharan-I, for which he was conferred the Padma Bhushan in 1975.
Iyengar took over as Director of the Bhabha Atomic Research Centre in 1984. As director, one of his first tasks was to take charge of the construction of the Dhruva reactor, the completion of which was then in question, and bring it to a successful conclusion under his leadership. Recognizing the importance of transferring newly developed technology from research institutes to industry, he introduced a Technology Transfer Cell at the BARC to assist and speed the process. He motivated basic research in fields ranging from molecular biology, to chemistry and material science. He nucleated new technologies like lasers and accelerators, which led to the establishment of a new Centre for Advanced Technology, at Indore.
Iyengar was appointed chairman of the Atomic Energy Commission of India and secretary to the Department of Atomic Energy in 1990. He was also appointed as chairman of the Nuclear Power Corporation of India. Under his leadership the Department of Atomic Energy vigorously pursued the nuclear power programme with the commissioning of two new power reactors at Narora and Kakrapar, and continued with the development of new reactor systems, such as liquid-sodium based fast reactors. Equal emphasis was laid on enhanced production of heavy water, nuclear fuel and special nuclear materials. He also initiated proposals for the export of heavy-water, research reactors, hardware for nuclear applications to earn precious foreign exchange.
Regarding Iyengar's involvement in Indian cold fusion research, the Indian newspaper Daily News and Analysis wrote: "Iyengar also pioneered cold fusion experiments in the 1980s to prove the hypothesis that nuclear fusion can occur at ordinary temperatures under certain scenarios. The experiments were discontinued after Iyengar's exit from the nuclear establishment by some conservative scientists." 
Iyengar has been the recipient of many high civilian awards and honours. After retirement Iyengar served in various positions such as member of the Atomic Energy Commission, scientific advisor to the Government of Kerala, on the board of the Global Technology Development Centre, president of the Indian Nuclear Society, and a member of the Inter-governmental Indo-French Forum, besides serving on various national committees. Iyengar’s later interests focused on advances in nuclear technology for nuclear applications, issues of nuclear policy and national security, science education and the application of science in nation-building. He participated in various international meetings on non-proliferation issues. Most recently, as a founder trustee of the Agastya International Foundation, he focused on rural education and instilling creativity and scientific temperament in rural children.