Chalk River Nuclear Reactor Explosion and Meltdown Summary

  • Last updated on November 10, 2022

Human error and mechanical failure caused an explosion in and a partial meltdown of the Chalk River reactor core. Public outcry was lacking after the accident, largely because nuclear energy was a new, if not unknown and thus insignificant, entity.

Summary of Event

On December 12, 1952, a series of events occurred at the experimental nuclear reactor at Chalk River, Ontario, about 100 miles northwest of Ottawa, which presaged the pattern of nearly every nuclear reactor disaster that followed. The reactor was not part of an electrical-power station. Rather, it had been designed as a plutonium generator, but was later used as a research tool to test nuclear fuels and materials. Chalk River nuclear accident (1952) Nuclear energy;accidents [kw]Chalk River Nuclear Reactor Explosion and Meltdown (Dec. 12, 1952) [kw]Nuclear Reactor Explosion and Meltdown, Chalk River (Dec. 12, 1952) [kw]Reactor Explosion and Meltdown, Chalk River Nuclear (Dec. 12, 1952) [kw]Explosion and Meltdown, Chalk River Nuclear Reactor (Dec. 12, 1952) [kw]Meltdown, Chalk River Nuclear Reactor Explosion and (Dec. 12, 1952) Chalk River nuclear accident (1952) Nuclear energy;accidents [g]North America;Dec. 12, 1952: Chalk River Nuclear Reactor Explosion and Meltdown[03980] [g]Canada;Dec. 12, 1952: Chalk River Nuclear Reactor Explosion and Meltdown[03980] [c]Disasters;Dec. 12, 1952: Chalk River Nuclear Reactor Explosion and Meltdown[03980] [c]Environmental issues;Dec. 12, 1952: Chalk River Nuclear Reactor Explosion and Meltdown[03980] [c]Energy;Dec. 12, 1952: Chalk River Nuclear Reactor Explosion and Meltdown[03980] Mackenzie, C. J. Lewis, W. B.

At the time of the accident, the reactor was operating at low power. One of the members of the operating crew, working in the basement of the building at another task, opened three or four valves that should have remained closed, causing three or four of the reactor control rods to lift out of the reactor. This reduced the reactor’s margin of safety to the point that warning lights went on at the control console upstairs. The supervisor there telephoned downstairs to tell the worker to stop whatever he was doing, then went downstairs himself. Once there, the supervisor saw what had happened and closed the valves, which should have returned the control rods to their fully inserted positions, but, as was later found, failed to do so. Rather, the control rods went in just far enough to turn off the warning lights, but not far enough to restore the margin of reactor safety.

This done, the supervisor called back upstairs to his assistant to tell him to punch the combination of buttons on the console that would have restored the reactor’s operations to normal. He gave the wrong number for one of the buttons, with the result that four more control rods were lifted out of the reactor. Of the twelve rapid-action shutdown rods, seven or eight were, at that point, disengaged from the reactor, and the power of the atomic reaction in the reactor began to rise. After only about twenty seconds, the assistant supervisor realized that something had gone wrong, and he activated an emergency button that should have dropped all rods back into the reactor immediately. In fact, only one rod dropped back, and that at an excruciatingly slow rate. The power continued to rise, and true emergency measures were initiated: The heavy water that maintained the reaction was dumped, and the water ran out slowly; in about thirty seconds, the power dropped to zero. The reactor, at least, was under control.

The accident was far from over. The supervisor in the basement watched the highly radioactive coolant water rush away into a room below the reactor, while another staff member reported a gush of water from the top of the reactor. In short order, alarms throughout the building began to sound, indicating radioactivity in the air. A nearby building reported the same, and within a few minutes the buildings were evacuated, leaving only the control-room personnel inside. The entire incident, from valve-closing to evacuation, lasted only a few minutes. No one was injured, although a few people were exposed to low levels of radiation.

It was later determined that the dispersal of radioactive material was caused by a leak from one or more of the aluminum tubes that contained the uranium fuel, which melted out and caused rapid boiling and eruption of cooling water. The dispersal of radioactive material also generated hydrogen gas by the reaction of both metals, at high temperature, with water and steam. Air rushing into the reactor chamber formed an explosive mixture with the hydrogen and, at the high temperature of the chamber, produced an immediate explosion that lifted up the four-ton gas-holder dome, which then dropped and caused even more damage.

The initial newspaper report of the incident cited “a tiny radioactive leak” as the source of the contamination. Radioactive contamination C. J. Mackenzie, president of the government-owned company that operated the Chalk River reactor, blamed the heavy clouds and high humidity of the day for trapping the radioactivity in the plant and causing general contamination. He expressed confidence that the decontamination teams that went in to clean up would allow the reopening of the plant in a few days. In fact, the plant’s closure was announced two days later.

This scenario of a nuclear accident was acted out again and again in years to come: Human error was compounded by design or mechanical failure and was exacerbated by its occurrence in a time frame that allowed for human control only by instinct, not reason. The public-relations damage control would become a standard element of this scenario as well.


Although the Chalk River accident was thoroughly investigated by the government corporation, Atomic Energy of Canada, Ltd. Atomic Energy of Canada, Ltd. , the final product of the investigation was, in effect, an internal report. No public outcry followed the accident, largely because the attitude of the time combined a general lack of public knowledge about nuclear reactors and their technology with a vague hope that nuclear-generated electric power was the ideal innovation. It was not until a number of better-publicized disasters had taken place—in Idaho Falls, in the United States, in 1955 and 1961; in Windscale, England, in 1957; and in Lagoona Beach, Michigan, in 1966, among others—that public concern about safety in nuclear plants began to be widely expressed. Thereafter, protest rallies outside nuclear plants were common in the United States and in many European countries.

The Union of Concerned Scientists was founded to inform the public about nuclear matters. The commission and construction of nuclear power plants, which peaked in 1973, fell off sharply thereafter in the United States.

The Canadians responded to the Chalk River accident by improving the technical engineering of the reactor. Design and materials were changed to produce an inherently more manageable type of reactor. Canadian reactors differ from those of nearly all other nations. Most reactors in the United States and Europe use enriched uranium as fuel. This is necessary because natural uranium Uranium consists of about 99.3 percent of the isotope uranium 238, which reacts slowly by alpha-decay but cannot be made to split apart and release the enormous amount of energy that fission produces. Only the remaining 0.7 percent of uranium, uranium 235 (U-235), can do that.

The Canadian system, called Canadian Deuterium-Uranium Canadian Deuterium-Uranium[Canadian Deuterium Uranium] (CANDU), works with the relatively cheap natural uranium, which is actually more abundant than silver or mercury in the earth’s crust, and overcomes the low percentage of U-235 by choice of design and materials. The uranium is contained in tubes made of an alloy containing zirconium and niobium, which absorbs few neutrons, instead of in stainless steel containers. Instead of using regular water, which absorbs neutrons, the CANDU system uses heavy water, which is virtually transparent to neutrons. These factors, combined with a careful geometry of placement of the fuel tubes, allows maximum utilization of the available neutrons by the small number of fissionable U-235 nuclei. Reactors designed to use natural uranium rather than enriched uranium also incorporate features that coincidentally improve reactor safety.

By building on what was learned from the Chalk River incident, and by utilizing zirconium alloys and the pressure-tube design rather than a vat configuration, the CANDU reactors have shown an impressive safety record over the decades. More than one dozen electrical-generating plants were built by the 1980’s; in 1984, some 37 percent of Ontario’s power was produced by nuclear reactors, and by 2005, sixteen operating reactors produced two-thirds of the province’s electricity, even as the decision was made to refurbish four decommissioned plants to meet steadily growing power needs.

In opposition to nuclear power, the Canadian Coalition for Nuclear Responsibility began mobilizing public opinion. In the 1970’s, Canadian protest was added to the growing worldwide protest, not only of power plant safety and nuclear waste disposal but also of all aspects of production and proliferation of fissionable materials, but this has not impeded the growth of the CANDU system. Chalk River nuclear accident (1952) Nuclear energy;accidents

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Flavin, Christopher. Nuclear Power: The Market Test. Washington, D.C.: Worldwatch Institute, 1983. Addresses the problems of nuclear power primarily from the standpoint of economics. Discusses some safety issues.
  • citation-type="booksimple"

    xlink:type="simple">Hayes, Denis. Nuclear Power: The Fifth Horseman. Washington, D.C.: Worldwatch Institute, 1976. A thorough discussion of the hazards of nuclear power as they were beginning to be understood in the 1970’s, when popular protest was mounting. Describes the Chalk River incident, among others.
  • citation-type="booksimple"

    xlink:type="simple">Jones, P. M. S., ed. Nuclear Power: Policy and Prospects. New York: John Wiley & Sons, 1987. An authoritative volume of essays by many experts; the chapters on safety and on Canada are particularly relevant to this article.
  • citation-type="booksimple"

    xlink:type="simple">Krenz, Kim. Deep Waters: The Ottawa River and Canada’s Nuclear Adventure. Montreal, Que.: McGill-Queen’s University Press, 2004. A firsthand account of the history of the Canadian nuclear power system (CANDU) as well as a study of Deep River, the town built around the Chalk River plant for its scientists and other employees. The author of this book worked in the nuclear-power industry in Canada.
  • citation-type="booksimple"

    xlink:type="simple">Novick, Sheldon. The Careless Atom. Boston: Houghton Mifflin, 1969. Contains a full description of the Chalk River accident, as well as many others, in support of its thesis that nuclear power plants are poorly operated and inherently highly dangerous.
  • citation-type="booksimple"

    xlink:type="simple">_______. “Ontario Atom Plant Leak Forces Employees to Flee.” The New York Times, December 14, 1952, pp. 25-27. Illustrates the minimization of nuclear accidents in the news media of the time.

World’s First Nuclear Reactor Is Activated

Atomic Energy Commission Is Established

Hanford Nuclear Reservation Becomes a Health Concern

Meltdown Occurs in the First Breeder Reactor

Price-Anderson Act Limits Nuclear Liability

Nuclear Waste Explodes in the Ural Mountains

England’s Windscale Reactor Releases Radiation

Categories: History