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. At the time of its construction, it was Canada's most expensive science facility and the world's most powerful nuclear research reactor.[1] NRX was remarkable both in terms of its heat output and the number of free neutrons it generated. When a nuclear reactor such as NRX is operating, its nuclear chain reaction generates many free neutrons. In the late 1940s, NRX was the most intense neutron source in the world.
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, then shut down permanently on 30 March 1993.[2] 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 construction of a successor facility, the National Research Universal reactor, which started self-sustained operation (or "went critical") in 1957.
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 calandria, a sealed vertical aluminium cylindrical vessel with a diameter of 8 metres (26ft) and height of 3 metres (9.8ft). The core vessel held about 175 six-centimetre-diameter (2.4in) vertical tubes in a hexagonal lattice, 14,000 litres (3,100impgal; 3,700USgal) of heavy water and helium gas to displace air and prevent corrosion. 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 metres (10ft) long, 31 millimetres (1.2in) in diameter and weighing 55 kilograms (121lb), containing uranium fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant tube with up to 250 litres per second (3,300imp gal/min) of cooling water from the Ottawa River flowing through it. Between the coolant sheath and the calandria an air flow of 8 kilograms per second (1,100lb/min) was maintained.
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 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 insertion of the rods into the core was located a few feet away.
History
NRX and Zeep buildings 1945
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.[3]
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.
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.[5][failed verification] Crud has since become common parlance for "Corrosion Related Unidentified Deposit" and similar expressions and is commonly used with no relation to the Chalk River plant.[citation needed]
Accident
On December 12, 1952, the NRX reactor suffered a partial meltdown due to operator error and mechanical problems in the shut-off systems. For test purposes, some of the fuel channels had been disconnected from high pressure water cooling and were connected by hoses to a temporary cooling system; one low-power channel was cooled only by airflow.[6]
During tests at low power, with low coolant flux through the core, the supervisor noticed several control rods being pulled from the core; an operator in the basement had incorrectly opened pneumatic valves. The 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 to lower 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 accidentally withdrawn from the core. The operator noticed that the power level was exponentially increasing, 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 an abnormally long time, about 90 seconds, to insert, while the power kept rising. After just 10 seconds the core reached 17 MW(thermal) . The cooling water boiled in some tubes connected to the temporary cooling system, and some of them ruptured; the positive void coefficient of the reactor led to a further power increase. About 14 seconds later valves were manually opened to drain the heavy water moderator from the calandria. As this took some time to become effective, power increased for 5 more seconds, peaked at an estimated 100 MW, then went down as the moderator level decreased and was at zero 25 seconds later. The whole accident, from low to high to zero power took about 108 seconds. Meanwhile, some fuel elements melted and were pierced in several places; the helium cover gas leaked and air was aspirated inside. Hydrogen and other gases evolved by radiation-induced dissociation of cooling water, and 3–4 minutes later oxyhydrogen exploded in the calandria. During the incident, some gaseous fission products were vented to the atmosphere, and heavy water in the calandria was contaminated with the cooling water and the fission products.[6]
To remove the fuel decay heat, the water cooling system was kept operating, leaking contaminated coolant to the reactor basement. About 10 kilocuries (400TBq) of radioactive materials, contained in about 4,500 cubic metres (1,200,000USgal) of water,[6] collected in the basement of the reactor building during the next few days.[7]
Clean-up of the reactor building required several months of work, assisted by 150 US Navy personnel after Admiral Rickover requested permission to send US nuclear submarine personnel to learn about cleaning up nuclear contamination. The US contingent included future US president Jimmy Carter, at the time a lieutenant in the US nuclear submarine program who was in charge of 12 men.[8] The NRX reactor core and calandria, damaged beyond repair, were removed and buried, and an improved replacement was installed; the refurbished reactor started operating 14 months and 5 days after the accident.[9] The clean up was primarily performed by 850 Atomic Energy of Canada staff, assisted by about 170 Canadian and 150 U.S. military personnel, and 20 contractors.[6][10]
The lessons learned in the 1952 accident advanced the field of reactor safety significantly,[11] and the concepts it highlighted (diversity and independence of safety systems, guaranteed shutdown capability,[11] efficiency of man-machine interface) became fundamentals of reactor design.[citation needed] The incident was the world's first severe nuclear reactor accident.[6]
The CANDU is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide moderator and its use of uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario, Canadian General Electric, and other companies.
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.
A nuclear meltdown is a severe nuclear reactor accident that results in core damage from overheating. The term nuclear meltdown is not officially defined by the International Atomic Energy Agency or by the United States Nuclear Regulatory Commission. It has been defined to mean the accidental melting of the core of a nuclear reactor, however, and is in common usage a reference to the core's either complete or partial collapse.
The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union. The name refers to its design where, instead of a large steel pressure vessel surrounding the entire core, the core is surrounded by a cylindrical annular steel tank inside a concrete vault and each fuel assembly is enclosed in an individual 8 cm (inner) diameter pipe. The channels also contain the coolant, and are surrounded by graphite.
Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing many neutrons without themselves decaying. These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors (BWR), pressurized water reactors (PWR), and heavy-water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons. Control rods have been used in nuclear aircraft engines like Project Pluto as a method of control.
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain advantages over the existing fleet of water cooled and water moderated reactors. These are:
A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's emergency core cooling system (ECCS) exists specifically to deal with a LOCA.
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. Net reactivity in a reactor is the sum total of multiple contributions, of which the void coefficient is but one. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative or positive. Reactors in which neither the moderator nor the coolant is a liquid will have a void coefficient value equal to zero. It is unclear how the definition of "void" coefficient applies to reactors in which the moderator/coolant is neither liquid nor gas.
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of thermal-neutron reactor.
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
The National Research Universal (NRU) reactor was a 135 MW nuclear research reactor built in the Chalk River Laboratories, Ontario, one of Canada’s national science facilities. It was a multipurpose science facility that served three main roles. It generated radionuclides used to treat or diagnose over 20 million people in 80 countries every year. It was the neutron source for the NRC Canadian Neutron Beam Centre: a materials research centre that grew from the Nobel Prize-winning work of Bertram Brockhouse. It was the test bed for Atomic Energy of Canada Limited to develop fuels and materials for the CANDU reactor. At the time of its retirement on March 31, 2018, it was the world's oldest operating nuclear reactor.
The Advanced CANDU reactor (ACR), or ACR-1000, was a proposed Generation III+ nuclear reactor design, developed by Atomic Energy of Canada Limited (AECL). It combined features of the existing CANDU pressurised heavy water reactors (PHWR) with features of light-water cooled pressurized water reactors (PWR). From CANDU, it took the heavy water moderator, which gave the design an improved neutron economy that allowed it to burn a variety of fuels. It replaced the heavy water cooling loop with one containing conventional light water, reducing costs. The name refers to its design power in the 1,000 MWe class, with the baseline around 1,200 MWe.
A swimming pool reactor, also called an open pool reactor, is a type of nuclear reactor that has a core immersed in an open pool usually of water.
The supercritical water reactor (SCWR) is a concept Generation IV reactor, designed as a light water reactor (LWR) that operates at supercritical pressure. The term critical in this context refers to the critical point of water, and should not be confused with the concept of criticality of the nuclear reactor.
A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated. Typically, the fuel will be low-enriched uranium contained in thousands of individual fuel pins. The core also contains structural components, the means to both moderate the neutrons and control the reaction, and the means to transfer the heat from the fuel to where it is required, outside the core.
In applications such as nuclear reactors, a neutron poison is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable effect. However, neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation, while others remain relatively constant.
The Whiteshell Reactor No. 1, or WR-1, was a Canadian research reactor located at AECL's Whiteshell Laboratories (WNRL) in Manitoba. Originally known as Organic-Cooled Deuterium-Reactor Experiment (OCDRE), it was built to test the concept of a CANDU-type reactor that replaced the heavy water coolant with an oil substance. This had a number of potential advantages in terms of cost and efficiency.
A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.
An organic nuclear reactor, or organic cooled reactor (OCR), is a type of nuclear reactor that uses some form of organic fluid, typically a hydrocarbon substance like polychlorinated biphenyl (PCB), for cooling and sometimes as a neutron moderator as well.
↑ Semeniuk, Ivan (19 March 2023). "Jimmy Carter's atomic bond with Canada". The Globe and Mail. Retrieved 17 November 2023. In the early days of cancer radiation therapy, it also provided the world's only ready source of the radioactive isotope Cobalt-60, which Canadian researchers first used to bombard tumours in 1951.
↑ Jedicke, Peter (1989). "The NRX Incident". Canadian Nuclear Society. London, Ontario: Fanshawe College. Archived from the original on 14 June 2019. Retrieved 4 June 2019.
1 2 Winfield, D.J.; Alsop, C.A. (2003), "Optimization of Safety System Test Frequencies", Research Reactor Utilization, Safety, Decommissioning, Fuel and Waste Management, International Atomic Energy Agency, p.39, CiteSeerX10.1.1.127.961
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