NRX (National Research Experimental) was a heavy-water-moderated, light-water-cooled, nuclear research reactor at the Canadian Chalk River Laboratories, which came into operation in 1947 at a design power rating of 10 MW (thermal), increasing to 42 MW by 1954. It was Canada's most expensive science facility and the world's most powerful nuclear research reactor at its construction. NRX was remarkable for its heat output and the number of free neutrons it generated. In the late 1940s, the NRX reactor had the highest neutron flux in the world: 10–20 times that of a graphite reactor of comparable power, due to its small physical size made possible by the use of a heavy water moderator. NRX experienced the world's first major reactor accident outside of Russia on 12 December 1952. The reactor began operation on 22 July 1947 under the National Research Council of Canada and was taken over by Atomic Energy of Canada Limited (AECL) shortly before the 1952 accident. The accident was cleaned up, and the reactor was restarted within two years. NRX operated for 45 years, then shut down permanently on 30 March 1993. Decommissioning is underway at the Chalk River Laboratories site. NRX was the successor to Canada's first reactor, ZEEP. Because the operating life of a research reactor was not expected to be very long, in 1948, planning started for the construction of a successor facility, the National Research Universal reactor, which started self-sustained operation (or "went critical") in 1957.
Design Two main processes govern a heavy water-moderated reactor. First, the water slows down (moderates) the neutrons which are produced by nuclear fission, increasing the chances of the high energy neutrons causing further fission reactions. Second, control rods absorb neutrons and adjust the power level or shut down the reactor in the course of regular operation. Inserting the control rods or removing the heavy water moderator can stop the reaction. The NRX reactor incorporated a calandria, a sealed vertical aluminium cylindrical vessel with a diameter of 8.75 feet (2.667 m) and height of 10.5 feet (3.20 m). The calandria vessel held 198 calandria tubes with inside diameter 2-1/4" (57.15 mm) connected to the top and bottom tube sheets in a hexagonal lattice. The calandria contained approximately 3,300 US gallons (12,491 litres) heavy water, and the uranium fuel load was 10.5 short tons (9,525 kg). A helium cover gas was used to vent the heavy water system and to carry gaseous activation products to a recombiner system. Air could not be used as a cover gas, as its irradiation would result in production of corrosive nitric acid. The heavy water level in the reactor could be adjusted to help set the power level. Fuel elements or experimental items were sitting in the vertical tubes and surrounded by air. This design was a forerunner of the CANDU reactors. The fuel elements contained fuel rods 120.5 inches (3.060m) long, with the fuel segment 1.360 in (34.54 mm) diameter, with an outer aluminum fuel sheath diameter between 1.66-1.74 inches (42.16-44.20 mm) depending on the fuel rod type. Surrounding the fuel elements were aluminum coolant tubes, collectively carrying up to 3,500 imperial gallons (15,900 litres) of cooling water from the Ottawa River flowing through them. An air flowrate of 70,000 lb/hour (32,000 kg/hour) was used to cool the graphite reflector shields, by flowing through the space between the inner and outer reflectors (known as the J-rod annulus). Twelve of the vertical tubes contained control rods made of boron carbide powder inside steel tubes. These could be raised and lowered to control the reaction, with any seven inserted being enough to absorb sufficient neutrons that no chain reaction could happen. The rods were held up by electromagnets so that a power failure would cause them to fall into the tubes and terminate the reaction. A pneumatic system could use air pressure from above to quickly force them into the reactor core or from below to slowly raise them from it. Four were called the safeguard bank while the other eight were controlled in an automatic sequence.
History NRX was for a time the world's most powerful research reactor, vaulting Canada into the forefront of physics research. Emerging from a World War II cooperative effort between Britain, the United States, and Canada, NRX was a multipurpose research reactor used to develop new isotopes, test materials and fuels, and produce neutron radiation beams, that became an indispensable tool in the blossoming field of condensed matter physics. The nuclear physics design of NRX emerged from the Montreal Laboratory of Canada's National Research Council, which was established at the University of Montreal during WWII to engage a team of Canadian, British, and other European scientists in top-secret heavy-water reactor research. When the decision was made to build the NRX at what is now known as Chalk River Laboratories, the detailed engineering design was contracted to Canada's Defence Industries Limited (DIL), who subcontracted construction to Fraser Brace Ltd. In the early days of cancer radiation therapy, the NRX reactor was the world's only source of the isotope cobalt-60, first used to bombard tumours in 1951. In 1994, Dr. Bertram Brockhouse shared the Nobel Prize in Physics for his work in the 1950s at NRX, which advanced the detection and analysis techniques used in the field of neutron scattering for condensed matter research. Based on this design, the CIRUS reactor was built in India. It was ultimately used to produce plutonium for India's Operation Smiling Buddha nuclear test.
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