Apsara, the One-Megawatt Pool Reactor Built at Trombay, Went Critical on 4 August 1956 as Asia's First Outside the Soviet Union, and Returned as the 2 MW Apsara-U on 10 September 2018

Apsara first reached criticality, the point at which a nuclear chain reaction keeps itself going, on 4 August 1956 at the Atomic Energy Establishment, Trombay, on the edge of Bombay (now Mumbai). The Bhabha Atomic Research Centre (BARC), the successor to that establishment, lists it as a one-megawatt swimming-pool reactor and calls it the first research reactor in Asia. The historian Jayita Sarkar words the claim more narrowly: the first reactor to become operational in Asia outside the Soviet Union. A 1959 report by N. B. Prasad and A. S. Rao of the Trombay establishment describes it as the first atomic reactor to go into operation in Asia.

Its rebuilt successor, Apsara-U, was born at Trombay at 18:41 on 10 September 2018, according to the Department of Atomic Energy’s release of the next day. The release puts the gap between the two reactors at nearly 62 years.

A pool eight metres deep, with the core on a trolley

A swimming-pool reactor keeps its fuel in a deep tank of ordinary water. BARC’s newsletter of January and February 2021 gives Apsara’s pool as stainless-steel lined, 8.4 metres long, 2.9 metres wide and 8 metres deep, filled with demineralised light water. The fuel was plates of enriched uranium alloyed with aluminium, clad in aluminium. BARC’s reactor table puts the total fuel at 4.5 kilograms, with cadmium shut-off rods and light water as both moderator and coolant. Moderator means the material that slows neutrons so they are more likely to split uranium atoms. A BARC conference paper adds that the water also served as reflector and as radiation shielding.

The core hung from a movable trolley and could be parked in three positions, so that it sat next to whichever experimental facility a team needed. The 1959 report lists a thermal column (a block of material that delivers slowed neutrons), eight experimental holes, a through channel beside the core for oscillation experiments and a special shielding corner. BARC’s table gives the maximum neutron flux as 10^13 neutrons per square centimetre per second, or 10 trillion. The newsletter says the average was about 10^12.

The 1 MW figure is the design maximum. A BARC paper by S. M. Sundaram, presented at a 1985 International Atomic Energy Agency seminar and published in 1987, lists Apsara at 400 kilowatts.

British drawings, Indian builders

The sources agree that Apsara drew on foreign help and disagree on which country. Sarkar writes that in late 1954 the United Kingdom Atomic Energy Authority gave the Department of Atomic Energy engineering drawings and technical data for a pool-type reactor, along with six kilograms of enriched uranium fuel rods. Department engineers then built the reactor using British know-how and imported fuel rods, valves and other components, and she adds that France’s Commissariat a l’Energie Atomique also supplied enriched uranium. World Nuclear News, in 2018, calls Apsara a French-supplied reactor and dates the start of construction at Trombay to May 1955.

The Indian share is documented in several places. Virendra Singh of the Tata Institute of Fundamental Research (TIFR) writes that Apsara’s control systems were built at TIFR. Sarkar notes that Indian personnel gained technical experience by constructing the reactor themselves. At the opening in January 1957, as Sarkar quotes the speech, Prime Minister Jawaharlal Nehru said the pool reactor was “the work, almost entirely, of our young Indian scientists and builders.”

Homi Bhabha had set up the Atomic Energy Establishment, Trombay in January 1954, BARC says, as a base for the multidisciplinary work his nuclear programme needed. His own career is covered in the profile of Homi Bhabha and the atomic programme.

Fifty years of experiments, isotopes and training

The BARC newsletter lists what the reactor was used for over about five decades: neutron physics, fission physics, radiochemistry, biology, irradiation techniques and research and development on reactor technology. It says the neutron activation analysis technique developed with Apsara found wide use in chemistry, archaeology and forensic science. In that method a sample is placed in the neutron beam, and the radiation it gives off afterwards identifies the elements inside it, even in tiny amounts.

Apsara’s shielding corner was also a test bench. The newsletter says shielding experiments there checked the design of the shields used in the Dhruva reactor, in India’s pressurised heavy water power reactors and in the 500 MWe Prototype Fast Breeder Reactor, whose first criticality in April 2026 is covered in a separate article.

Two BARC papers date the isotope programme. Mazumdar and Singh write that India’s radioisotope production began with Apsara’s commissioning in August 1956, that the 40 MW CIRUS reactor, commissioned in 1960, moved it to industrial scale, and that Dhruva, operating since 1985, improved both quality and quantity. The 1987 Sundaram paper says the Trombay research reactors were used effectively as a training tool for the manpower of the nuclear power programme. The newsletter makes the same point about Apsara and CIRUS: people who gained early experience operating them went on to lead later nuclear projects. It adds that the simple design, with an open pool and a movable core, made Apsara popular with researchers.

The opening of January 1957 and the name

Singh writes that Nehru formally inaugurated the Trombay laboratories on 20 January 1957. K. S. Parthasarathy, who worked in the Department of Atomic Energy, gave an account in The Tribune in 2003 of what came before and after. In his telling, M. Sundaram, a draftsman who was fiddling with a small telephone exchange, heard someone behind him ask to be connected urgently to Nehru, and Bhabha then informed Nehru that the pool reactor had been made critical on 4 August 1956. Parthasarathy writes that Nehru named the reactor Apsara at the opening, describing it as a celestial damsel or a water nymph. The telephone story rests on that one recollection.

The Tata Institute’s archive carries Nehru’s words from the same day. He said that whatever the circumstances, “we shall never use this atomic energy for evil purposes,” and added that attaching conditions to such an assurance would take it nowhere.

Shut down in 2009, or 2010

Official and BARC sources give two dates for the end of the first Apsara. The Press Information Bureau release of 2018 says the reactor was shut down in 2009, and the 2021 newsletter says it was shut down for good in 2009. A BARC conference paper says it was shut in May 2009 for partial decommissioning and upgrading to 2 MW. BARC’s reactor table says it was permanently shut down in 2010, and a 2022 BARC paper groups it with CIRUS as permanently shut down in 2010. The sources do not explain the difference, and this article uses 2009 for the shutdown for upgrading.

Apsara-U: a 2 MW core with fuel made in India

The Department of Atomic Energy’s release says Apsara-U was made indigenously and uses plate-type dispersion fuel elements of low-enriched uranium. It says the higher neutron flux will raise India’s own production of radioisotopes for medical use by about fifty percent, and the reactor will also serve research in nuclear physics, materials science and radiation shielding.

BARC’s table gives the particulars. Maximum power is 2 MW thermal, the fuel is uranium silicide plates in aluminium alloy cladding, the core sits on a grid of 64 lattice positions and the maximum neutron flux is 61 trillion (6.1 x 10^13) per square centimetre per second, about six times the old reactor’s design figure. The reflector is beryllium oxide and the control rods are hafnium. The newsletter says the core still hangs from a movable trolley parked in three positions, as in the old Apsara, and describes two safety-minded changes: the cooling water flows downward through the core and passes through a delay tank so short-lived radioactivity decays before it circulates, and a layer of hot water on top of the pool cuts the radiation at the pool top. BARC calls the hot-layer concept the first of its kind in India. The BARC conference paper adds a new reactor building meeting seismic qualification criteria and two independent shutdown devices. The newsletter says low-enriched uranium was chosen to meet the international requirement.

Apsara-U is in routine service. In its Founder’s Day address of 2025, BARC reported that Dhruva and Apsara-U had operated safely over the year, that the reactors were used to irradiate about 500 samples for medical and industrial radioisotopes among other work, and that the indigenously developed uranium silicide fuel had reached a burn-up of 55,000 megawatt-days per tonne in Apsara-U.

Where Apsara sits among the Trombay reactors

BARC’s reactor table and newsletter date the other reactors at Trombay: CIRUS, built with Canada, critical on 10 July 1960; ZERLINA, a 100 watt heavy-water reactor for lattice studies, on 14 January 1961; and the 100 MW Dhruva on 8 August 1985, with a maximum flux of 1.8 x 10^14. One neutral point from the foreign record belongs here. Sarkar quotes a September 1961 US National Security Council study that judged Apsara and ZERLINA capable of producing only small, research quantities of plutonium, and CIRUS capable of producing a significant amount if operated for that purpose.

A 2022 BARC paper says a 40 MW High Flux Research Reactor is being designed for the centre’s Visakhapatnam site to keep up the radioisotope programme.