The 75th Anniversary of ZEEP

In 1943 an agreement was put in place by the UK, USA, and Canada for a heavy-water moderated, natural uranium reactor, later named National Research X-perimental (NRX), to be built in Canada.  The site was yet undetermined.  When John Cockcroft took control of the Montreal Lab and the Canadian project in April, 1944, he pushed, in addition, for the construction of a much simpler reactor with a power of only a few watts to serve as a facility where critical nuclear data could be determined for the design of NRX.  Cockcroft was able to convince General Groves, the head of the massive US Manhattan project, of the need for the test reactor and to supply the project with the heavy water required.  The test reactor became known as the Zero Energy Experimental Pile or ZEEP.

By August, 1944, Chalk River was selected as the site for NRX and the ZEEP project had been officially approved. Cockcroft recruited Lew Kowarski from the Cavendish Lab and appointed him Project Chief.  Kowarski had conducted heavy-water lattice experiments in France and at Cavendish with the heavy water spirited out of the Norsk facility in Norway. Details of the design and time-line of the ZEEP project in Montreal and Chalk River have been published in many places and will not be repeated here.  (See in particular the Article “Other Research Reactors” under the Projects tab of this website and its reference.)

To avoid interference with the design of NRX an NRC team in Ottawa, led by George Klein, assumed responsibility for the mechanical design. Construction of the building to house ZEEP began in Chalk River in March, 1945, at about the same time that blocks of nuclear-grade graphite from Welland, ON, and barrels of heavy-water from Trail, BC, began to appear.  The building was essentially complete by the end of May, 1945, and assembly of the reactor, which had at its heart a large (6” 9” diameter by 8” 6” tall) aluminum tank, continued. 

Final assembly of the controls and aluminum-clad, uranium-metal fuel rods was completed by the beginning of September and first criticality was achieved on Wednesday, September 05, 1945, at 15:45.  ZEEP became the first reactor outside the USA to achieve criticality.  It operated for many years and provided critical information for many aspects of the Canadian nuclear program. (After the fact, Kowarski, with tongue in cheek and deleting memories of extended delays, claimed to have chosen Wednesday as the day of the week for the start-up because that was the established tradition for nuclear reactors – CP-1 In Chicago went critical on Wednesday, December 2, 1942).

Following construction photographs courtesy of Atomic Energy of Canada Limited.



An overview of the design of the reactor within the building.




Anniversary Activities

The Society for the Preservation of Canada’s Nuclear Heritage Inc. has produced a stamp to celebrate the 75th anniversary year of ZEEP. 
ZEEP’s 75th anniversary year (2020) celebrated in snow. Unfortunately, unlike the original, this version suffered a severe meltdown during the spring. (Design and construction engineer – Morgan Brown, SPCNH Secretary.)