The CIRUS reactor (Canada India Reactor Utility Services) was a 40 MW-thermal research reactor located at the Bhabha Atomic Research Center (BARC) in Trombay, near Mumbai, India. It was the second nuclear reactor built in India and was a vertical tank-type thermal reactor modeled on the Canadian NRX reactor.
The reactor's origin dates back to 1954, when it was supplied by Canada, with the United States providing the heavy water (deuterium oxide). It achieved its first criticality on July 10, 1960, and became fully operational in 1963. The reactor was pivotal in India's nuclear program, solving the problem of providing a powerful tool for research, training engineers, and developing the indigenous Pressurized Heavy Water Reactor (PHWR) systems.
The mechanism of the reactor involved using natural uranium metal as fuel, heavy water as a moderator, and light water as a coolant. The reactor's operation was crucial as it produced the plutonium used for India's first nuclear test, Pokhran-I (Codename Smiling Buddha), in 1974.
The CIRUS reactor connects directly to the history of India's nuclear weapons program and the concept of peaceful nuclear explosions (PNEs), as its use of foreign-supplied materials for the 1974 test led to international concerns. It was not under IAEA safeguards when sold, though both Canada and the U.S. stipulated its use only for peaceful purposes.
The reactor was permanently shut down on December 31, 2010, as a commitment under the Indo-US civil nuclear cooperation agreement. This decision was made despite the reactor's refurbishment between 1997 and 2003 and its potential to operate for a few more years. The facility is now undergoing a deferred dismantling (decommissioning) strategy.