1952 NRX (National Research Experimental) accident

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 * ''Note: for the "NRX" series of nuclear thermal rocket designs, see NERVA rocket. For the Neoreactionary movement (NRx), see Dark Enlightenment.

Overview
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 is operating its nuclear chain reaction generates many free neutrons, and 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, being shut down permanently on 8 April 1993.[2] It is currently undergoing decommissioning 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, NRU, which went critical in 1957.

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 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 rods 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opened valves were immediately closed, but some of the control rods did not reenter the core and stuck in almost withdrawn positions, but still low enough for their status lights to indicate them as lowered.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. 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 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 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. As this 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 and the calandria 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 exploded in the calandria. 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, the water cooling system was kept operating, leaking contaminated coolant to the floor. About 10 kilocuries (400 TBq) 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.[5]

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.[6] The NRX reactor core and calandria, damaged beyond repair, were removed and buried, and an improved replacement was installed; the refurbished reactor was operating again within two years.[7]

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.