Susanna McLeod

Susanna McLeod

Glimpses of Canadian History : Email:

ZEEP, Canada’s First Nuclear Reactor

‘Zero Energy Experimental Pile’ Had First Reaction in 1945

The research on atomic theories performed with the ZEEP reactor at Chalk River, Ontario, lead to the large scale atomic reactors providing nuclear power today.

The ‘Zero Energy Experimental Pile’ was the test model for the NRX, a large reactor in the planning stages. The NRX would produce energy from natural uranium fuel and heavy water (a neutron moderator and coolant), but the theories needed proof through experimentation.

First ZEEP Test

A joint program between Canada and the United Kingdom, physicist John Cockcroft arrived from England to lead research of the Canadian Atomic Energy Project. (Cockcroft had studied under Ernest Rutherford, a renowned scientist whose discoveries put Canada on the physics map in the early 20th century. Cockcroft’s biggest achievement was splitting the atom with his colleague, Ernest Walton.) Approval was given by the government to proceed with the design of the experimental ZEEP, “Zero Energy Experimental Pile”, in August of 1944, said R. E. Green and A. Okazaki in their article, ZEEP: the Little Reactor that Could. With the help of fellow UK physicist Lew Kaworski and many other colleagues, ZEEP’s plan was approved and construction began in October. In less than a year, the construction was complete.

September 5, 1945 was a blue-ribbon day in Canadian nuclear history. On that day, the ZEEP produced one watt of nuclear power, the first nuclear success outside of the United States. The reactor was easily reconfigured to adjust to results of the experiments, giving scientists great flexibility to find answers. “The researchers lowered fuel rods containing small amounts of uranium into the calandria,” reported the Science and Technology Museum. “As heavy water was pumped into the calandria, the fuel rods released neutrons from the radioactive uranium.” Adjusting the heavy water level permitted changeable neutron release rates. The researchers were able to reconfigure the rod placement until the most efficient arrangement was found, and shut-downs were created by using shut-off rods – special rods that had no fission capabilities.

Operator Safety

Researchers were kept from receiving radiation by graphite shielding and using electronic panels to record activity. A safety system released shut-off rods into the calandria if anything went awry, halting any nuclear reactions. Later, a room was built for the researchers with masonite and steel blocks, and tanks of plain water were placed around the reactor. Nuclear testing continued at ZEEP until 1947. Early that year, the reactor was shut down and the heavy water transferred to the newly-built NRX, adjacent to the ZEEP building at Chalk River. According to Green and Okazaki, many useful results were obtained, a few being:

  • radio-chemical studies
  • gamma ray emission during fission
  • negative proton research
  • ion chamber calibration for safety and control systems for the NRX
  • critical heights of heavy water
  • temperature coefficients of uranium and reactivity

John Cockcroft left the project in 1946, returning to a post in England as Director of the Atomic Energy Reseach Establishment, Harwell. He received the Nobel Prize in Physics in 1951 and over his life was recognized with 19 honourary degrees plus a number of other awards. Cockcroft was knighted in 1948.

NRX Reactor Opened

In 1950, the “Zero Energy Experimental Pile” program was restarted to perform power tests up to 50 watts. The NRX reactor was completed in 1947 and was in operation until 1992; a second reactor, the NRU, was constructed in 1957. The NRU produces the medical isotopes and cobalt-60 for the cancer therapy used today. While the ZEEP reactor produced one watt of power for experimentation, the new Advanced Candu Reactor (ACR-1000), with a 2016 in-service date, will create three billion thermal watts of clean power. That is enough energy for nearly two million people per day.

Now that’s a Canadian success.


ZEEP: the Little Reactor that Could, R.E. Green, A.Okazaki, CNS Bulletin, Vol. 16, No. 3, Autumn 1995

Canada Science and Technology Museum, Ottawa, Ontario

A glimpse inside the ZEEP reactor, prior to 1956.  AECL

This Canadian History article was first published on                                          ©  Susanna McLeod

Comments are closed.