NRX
Encyclopedia
NRX was a heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

 moderated
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

, light water cooled, nuclear research reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 at the Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 Chalk River Laboratories
Chalk River Laboratories
The Chalk River Laboratories is a Canadian nuclear research facility located near Chalk River, about north-west of Ottawa in the province of Ontario.CRL is a site of major research and development to support and advance nuclear technology, in particular CANDU reactor...

, 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.
Although nuclear reactors would not generate electricity until 1951, 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
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

 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
Radiation therapy
Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

 for treating cancer, neutron scattering
Neutron scattering
Neutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...

 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
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

: 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
National Research Council of Canada
The National Research Council is an agency of the Government of Canada which conducts scientific research and development.- History :...

, and was taken over by Atomic Energy of Canada Limited
Atomic Energy of Canada Limited
Atomic Energy of Canada Limited or AECL is a Canadian federal Crown corporation and Canada's largest nuclear science and technology laboratory...

 (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 on 8 April 1993. It is currently undergoing decommissioning at the Chalk River Laboratories site.

Design

A heavy water moderated reactor is governed by two main processes. First, the water slows down (moderates
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

) the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s which are produced by nuclear fission, increasing the chances of the high energy neutrons causing further fission reactions. Second, control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s absorb neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s 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 calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....

, a sealed vertical aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 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
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 gas to displace air and prevent corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

. 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. This design was a forerunner of the CANDU reactors.

The fuel elements contained fuel rods 3.1 m long, 31 mm in diameter and weighing 55 kg, containing uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 tube with up to 250 litres per second of cooling water from the Ottawa River
Ottawa River
The Ottawa River is a river in the Canadian provinces of Ontario and Quebec. For most of its length, it now defines the border between these two provinces.-Geography:...

 flowing through it. Between the coolant sheath and the calandria an air flow of 8 kg/second was maintained.

Twelve of the vertical tubes contained control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s made of boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...

 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 electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...

s, 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 pushbutton
Pushbutton
Pushbutton is a UK-based digital agency specialising in designing, developing, and delivering interactive television.As one of the industry forerunners creating rich interactive television experiences, Pushbutton have graduated from traditional 'red button' services for Sky, ITV, Disney, National...

s 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.

History

NRX was for a time the world's most powerful research reactor
Research reactor
Research reactors are nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used for electricity production, heat generation, or maritime propulsion.-Purpose:...

, vaulting Canada into the forefront of physics research. Emerging from a World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 cooperative effort between Britain, the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, NRX was a multipurpose research reactor used to develop new isotopes, test materials and fuels, and produce neutron radiation
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...

 beams, that became an indispensable tool in the blossoming field of condensed matter physics
Condensed matter physics
Condensed matter physics deals with the physical properties of condensed phases of matter. These properties appear when a number of atoms at the supramolecular and macromolecular scale interact strongly and adhere to each other or are otherwise highly concentrated in a system. The most familiar...

.

The nuclear physics design of NRX emerged from the "Montreal Laboratory" of Canada's National Research Council
National Research Council of Canada
The National Research Council is an agency of the Government of Canada which conducts scientific research and development.- History :...

, 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
Chalk River Laboratories
The Chalk River Laboratories is a Canadian nuclear research facility located near Chalk River, about north-west of Ottawa in the province of Ontario.CRL is a site of major research and development to support and advance nuclear technology, in particular CANDU reactor...

, 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
Bertram Brockhouse
Bertram Neville Brockhouse, was a Canadian physicist. He was awarded the Nobel Prize in Physics "for pioneering contributions to the development of neutron scattering techniques for studies of condensed matter", in particular "for the development of neutron spectroscopy".-Life:Brockhouse was...

 shared the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

 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
CIRUS reactor
CIRUS is a research reactor at the Bhabha Atomic Research Center in Trombay near Mumbai, India. CIRUS was supplied by Canada in 1954, but uses heavy water supplied by the United States. It is the second oldest reactor in India. It is modeled on the Canadian Chalk River National Research...

, based on this design, was built in India. It was ultimately used to produce plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 for India's Operation Smiling Buddha nuclear test.

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. 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.

Accident

On December 12, 1952, the NRX reactor suffered a partial meltdown
Nuclear meltdown
Nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission...

 due to operator error and mechanical problems in the shut-off systems. For test purposes, some of the tubes were disconnected from high pressure water cooling and connected by hoses to a temporary cooling system and one was cooled only by airflow.

During tests on low power, with low coolant flux through the core, the supervisor noticed several control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s being pulled from the core, and found an operator in the basement opening pneumatic valves. Wrongly opened valves were immediately closed, but some of the control rods did not reenter the core and stuck in almost withdrawn positions, but still low enough for their status lights to indicate them as lowered. Due to a miscommunication between the supervisor and the control room operator, wrong buttons were pressed when the supervisor asked for lowering the control rods into the core. Instead of sealing the withdrawn control rods to the pneumatic system, the safeguard bank of four control rods was withdrawn from the core. Operator however noticed that the power level exponentially increases, doubling each 2 seconds, and tripped the reactor. Three of the safeguard control rods however were not inserted into the core and the fourth took abnormally long time, about 90 seconds, to slide back, while the power kept rising. After just 10 seconds 17 MW were reached. The cooling water boiled in the tubes connected to the temporary cooling system, and some of them ruptured; the positive void coefficient
Void coefficient
In nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant...

 of the reactor led to yet higher power increase rate. About 14 seconds later valves were opened to drain the heavy water from the calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....

, which however took some time; power increased for 5 more seconds, peaked at 80 MW, then went down as the moderator level decreased and was at zero 25 seconds later. Meanwhile some fuel elements melted
Fuel element failure
A fuel element failure is a rupture in a nuclear reactor's fuel cladding that allows the nuclear fuel or fission products in the form of dissolved radioisotopes or hot particles to enter the reactor coolant or storage water....

 and the calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....

 was pierced at several places; helium leaked and air was aspired inside. Hydrogen and other gases evolved by high-temperature reaction of metals with cooling water, and 3–4 minutes later oxyhydrogen
Oxyhydrogen
Oxyhydrogen is a mixture of hydrogen and oxygen gases, typically in a 2:1 molar ratio, the same proportion as water. This gaseous mixture is used for torches for the processing of refractory materials and was the first gaseous mixture used for welding...

 exploded in the calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....

. During the incident, some gaseous fission products were vented to the atmosphere and heavy water in calandria was contaminated with the cooling water and the fission products.

To remove decay heat
Decay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...

, the water cooling system was kept operating, leaking contaminated coolant to the floor. About 10 kCi of radioactive materials, contained in about a million gallons (about 4000 m3) of water, were dumped to the basement of the reactor building during next few days.

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, including future US president Jimmy Carter
Jimmy Carter
James Earl "Jimmy" Carter, Jr. is an American politician who served as the 39th President of the United States and was the recipient of the 2002 Nobel Peace Prize, the only U.S. President to have received the Prize after leaving office...

. The NRX reactor core and calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....

, damaged beyond repair, were removed and buried, and an improved replacement was installed; the refurbished reactor was operating again within two years.

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.

External links


See also

  • Nuclear reactor
    Nuclear reactor
    A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

  • Nuclear power
    Nuclear power
    Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

  • Nuclear fission
    Nuclear fission
    In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

  • Nuclear power plant
    Nuclear power plant
    A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

  • Nuclear meltdown
    Nuclear meltdown
    Nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission...

  • Nuclear waste
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