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ZEEP —Canada’s First Nuclear Reactor

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Early Atomic Research in Canada

Between 1898 and 1907, Ernest Rutherford, a New Zealander working at McGill University in Montreal, discovered the source of radioactivity. This achievement would later earn him a Nobel Prize. In 1940, George Laurence, working at the National Research Council in Ottawa, initiated Canada’s first experiments on atomic energy. These evolved into the Atomic Energy Project (AEP) in Montreal. The ZEEP (Zero Energy Experimental Pile) project, led by Dr. Lew Kowarski, resulted from this in 1944.

ZEEP demonstrated that scientists’ theories on nuclear reactions were correct. This meant that the much larger NRX (National Research eXperimental) reactor, also being planned at Chalk River, would work as expected.

The ZEEP reactor building was adjacent to the much larger NRX facility, under construction.

On September 5, 1945, ZEEP sustained its first reaction, becoming the first nuclear reactor in the world designed and operated outside the United States. It produced only one watt of power by releasing a few of the neutrons — subatomic particles — found in the nucleus of a uranium atom.

The original design and layout of ZEEP

Atomic Energy Project,
National Research Council Canada

ZEEP’s nuclear reactions occurred
in the "calandria". Researchers lowered fuel rods containing small amounts of uranium into the calandria. As heavy water was pumped into the calandria, the
fuel rods released neutrons from
the radioactive uranium. Scientists could control the rate at which neutrons were released by adjusting the level of heavy water.

By moving the fuel rods around and measuring the changing neutron output, scientists found the most efficient pattern of rods.

Routine shut-down was achieved by lowering a set of shut-off rods into the core. Interlocking graphite blocks reflected neutrons into the core and provided some protection for the staff from radioactivity.

To avoid radioactive leaks, the reactor’s graphite blocks had to be staggered both horizontally and vertically. George Klein — one of Canada’s most ingenious engineers, and a member of the Canadian Science and Engineering Hall of Fame — designed the complex layout.

Because uranium is radioactive and hazardous to humans, scientists developed new ways of working. Activity in the calandria was tracked using electronic panels, which were located beyond a layer of protective shielding.If anything went wrong, safety mechanisms released the shut-off rods, which then fell into the vessel, immediately stopping all reactions.

With three upgrades (mostly to the electronics and safety systems), the ZEEP reactor continued to be used for basic research until 1970.

ZEEP proved that the basic science and engineering concepts behind the NRX reactor were correct. NRX operated from 1947 to 1992. A second research reactor, the National Research Universal (NRU), which began operations at Chalk River in 1957, produces more than half of the world’s medical isotopes as well as 80% of cobalt-60 used in cancer therapy, benefiting millions of people worldwide each year.


Using these basic research tools, Canadian scientists and engineers have developed generations of CANDU power reactors, including the latest ACRTM model.

Nuclear technology has come a long way in 60 years. ZEEP produced one thermal watt of energy; the new ACR-1000TM will produce three billion thermal watts—almost sufficient to power Vancouver, Canada's third largest city!

All photos not credited are courtesy of Atomic Energy of Canada Limited.