NRX

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 experienced one of the world's first major reactor accidents 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.

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 the 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 indispensible tool in the blossoming field of condensed matter physics. 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. NRX operated for 45 years, being shut down permanently in 1992. It is currently undergoing decommissioning at the Chalk River Laboratories site.

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 8m and height of 3m. The core vessel held about 175 6cm-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 3.1m long, 3.1cm in diameter and weighing 55kg, 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 so many 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. An 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.

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 [1] (http://www.nrc.gov/reading-rm/basic-ref/glossary/crud.html). However, the word most likely entered our language in the 14th century, so this is a tall tale.

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.

External links

• The NRX incident, by Peter Jedicke (http://infotech.fanshawec.on.ca/faculty/jedicke/personal/nrx.htm)
• Nuclear Accidents (Georgia State University) (http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucacc.html)
• "The Canadian Nuclear FAQ" (http://www.nuclearfaq.ca) by Dr. Jeremy Whitlockde:NRX Reaktor