NRX

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For the NRX series of nuclear thermal rocket designs, see NERVA.

NRX 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. At the time of its construction it was Canada's most expensive science facility and the world's most powerful nuclear research reactor ^[1]. Although nuclear reactors would not generate electricity until 1951^[2], NRX was remarkable both in terms of its heat output and the number of neutrons it generated. When a nuclear reactor is operating its nuclear chain reaction generates many billions of free neutrons, and in the late 1940s NRX was the most intense neutron source in the world. As a result it opened up whole new areas of science that had not been possible before: Radiation therapy for treating cancer, neutron scattering for researching advances in materials from steel to semiconductors, production of radioisotopes for medical purposes, and R&D essential for the birth of nuclear power: generating electricity from a nuclear reaction.

NRX experienced one of the world's first major reactor accidents 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 restarted within two years. NRX operated for 45 years, being shut down permanently in 1992. It is currently undergoing decommissioning at the Chalk River Laboratories site.

1 Design
2 History
3 References
4 External links
5 See also

[edit] Design

A heavy water moderated reactor is governed by two main processes. 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 normal operation. Either inserting the control rods or removing the heavy water moderator can stop the reaction.

The NRX reactor incorporated a sealed vertical aluminium cylindrical vessel with a diameter of 8 m and height of 3 m. The core vessel held about 175 six-centimetre-diameter vertical tubes in a hexagonal lattice, 14,000 litres of heavy water and helium gas. The level of water in the reactor could be adjusted to help set the power level. Sitting in the vertical tubes and surrounded by air were fuel elements or experimental items.

The fuel elements contained fuel rods 3.1 m long, 31 mm in diameter and weighing 55 kg, containing uranium fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant tube with up to 250 litres per second of cooling water from the Ottawa River flowing through it.

Twelve of the vertical tubes contained control rods made of boron powder inside steel tubes. These could be raised and lowered to control the reaction, with 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 of these were called the safeguard bank while the other eight were controlled in an automatic sequence. Two pushbuttons on the main panel in the control room activated magnets to seal the rods to the pneumatic system, and the pushbutton to cause the pneumatic blowdown of the rods into the core was located a few feet away.

[edit] 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 the beams of neutral particles, called neutrons, 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 Defense Industries Ltd. (DIL), who subcontracted construction to Fraser Brace Ltd.

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.

The CIRUS reactor, based on this design, was built in India. It was ultimately used to produce plutonium for India's Operation Smiling Buddha nuclear test.^[3]

On December 12, 1952, the NRX reactor suffered a partial meltdown due to operator error and mechanical problems in the shut-off systems. Some fuel cladding burst, and as a result there was a release of radioactive material to the environment, although most of the release was contained within the NRX building. Clean-up of the site required several months of work, partially carried out by 150 US Navy personnel who had been training in the area, as well as future US president Jimmy Carter.^[4] The NRX core was removed and buried, and a new core put in its place; the refurbished reactor was operating again within two years.^[5] The lessons learned in the 1952 accident advanced the field of reactor safety significantly, and the concepts it highlighted (diversity and independence of safety systems, guaranteed shutdown capability, efficiency of man-machine interface) became fundamentals of reactor design.

It is claimed that the term "crud" originally stood for "Chalk River Unidentified Deposit", used to describe the radioactive scaling that builds up on internal reactor components, first observed in the NRX facility.^[6] However, crud can also stand for "Corrosion Related Unidentified Deposit" and similar expressions and is commonly used with no relation to the Chalk River plant.