On Dec. 12, 1952, the world witnessed its first-ever nuclear reactor accident at Chalk River Laboratories right here in Ontario. Located roughly 180 kilometres northwest of Ottawa, Chalk River was a cutting-edge nuclear research facility.

The incident, which occurred with the National Research Experimental (NRX) Reactor, was caused by a combination of mechanical problems and operator error, which led to overheating fuel rods and significant damage to the reactor core.

On that fateful date, at approximately 3 p.m., the last experiment of the day was about to begin. The cooling system was modified, and the water flow to the reactor was reduced for the test, which was expected to require only very low power.

First public photo of the NRX reactor. Photo: Toronto Star Archives. 

However, the modifications led to unforeseen consequences. The reactor experienced overheating, which caused damage to the reactor core, and a series of hydrogen gas (or steam) explosions followed, resulting in the four-tonne gas holder dome being hurled four feet through the air, where it became jammed in the superstructure. This released thousands of curies of fission products into the atmosphere.

Following the incident, roughly a million gallons of radioactively contaminated water had to be pumped out of the basement and disposed of in shallow trenches not far from the Ottawa River, according to the Canadian Coalition for Nuclear Responsibility. The contaminated water was seven times more radioactive than the total world production of radium at the time.

Specialists from the Royal Canadian Engineers in 1952. Photo: National Department of Defence. 

The cleanup efforts involved over 800 Atomic Energy of Canada Ltd. (AECL) employees, Canadian and U.S. servicemen, including one you may have heard of — former U.S. President Jimmy Carter. At the time, Carter was serving as a U.S. Navy lieutenant on a nuclear submarine project in Schenectady, New York.

The cleanup operation involved shutting down the reactor, dismantling it, and replacing the damaged parts. An exact replica of the reactor was even constructed nearby for training purposes, where Carter and his team practiced the necessary procedures.

Due to the high levels of radiation, the men could only work in shifts of 90 seconds. Carter later recalled the after effects of the exposure, including radioactive urine for weeks and concerns that he might not be able to have children. Despite this, Carter went on to have four children.

Official reports from AECL stressed that few of the workers involved were overexposed to radiation, and most of the recorded doses were within permissible limits for atomic workers at the time. However, no medical follow-up studies were conducted to determine whether those involved in the cleanup later exhibited higher-than-normal incidence of cancer or other health problems.

Chalk River in 1958. Photo: Toronto Star Archives.

This accident was not the only one to occur at Chalk River. In 1958, another serious accident occurred at the NRU reactor, when several metallic uranium fuel rods overheated and ruptured. One of the rods caught fire and was torn in two as it was being removed from the reactor core.

As a remote-controlled crane was passing overhead, a three-foot portion of the rod burning uranium fuel broke off and fell into a maintenance pit. A team of scientists and technicians were able to extinguish the fire by wearing protective gear and dumping buckets of wet sand onto the pit.

The Chalk River incidents were pivotal in the history of nuclear energy, and these early accidents brought attention to the need for stringent safety protocols, including effective risk-based communication. Today, Chalk Nuclear Laboratories is a world leader in developing nuclear technology for peaceful and innovative applications.