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July 21, 2023

3-D printed ZEEP reactor model

Before you is a model of the Zero Energy Experimental Pile (ZEEP).

ZEEP was a zero-power, critical assembly nuclear reactor built at Chalk River Laboratories in the 40s to conduct reactor physics experiments. The reactor first went critical on Sept. 5, 1945, at which point it became the first controlled nuclear chain reaction constructed outside of the United States.

ZEEP was a landmark in global nuclear history, going on to make numerous contributions to nuclear science and technology in Canada and the world – including producing vital data needed to design and operate the National Research Experimental (NRX) reactor, which first went critical in 1947, and the pressurized heavy water reactors that were to come.

ZEEP would eventually shut down on July 27, 1970 and be fully decommissioned in 1997.

This model was born from a 2022-2023 SEED project seeking to share Canada’s rich nuclear history through capturing physical and digital models of iconic Canadian reactors. It was created using both 3D printing and materials from artistic modelling suppliers.

The team who worked to make this happen includes Anil Prasad and Luke Yaraskavitch from Advanced Fuels & Reactor Physics, Brittany Saurette and Kurt Jensen from Mechanical Equipment Development, and Morgan Brown, researcher emeritus from Thermalhydraulics & Safety Analysis. The model was built by Kurt Jensen. 

ZEEP Model Highlights and ‘Easter Eggs’

The dirt below the reactor is modelled based on what was actually found at this location, recorded in a borehole log. From 0-3.4 m below surface, there was light yellowish-brown sand, and from 3.4 m and deeper, there was granite gneiss, or rock.

Based on design drawings of the control panel, indicators that would be expected to be lit during normal operating conditions were identified and, where practically feasible, illuminated on the ZEEP model.



The water level in the calandria vessel of the model was poured to an equivalent height of approximately 165 cm. This was representative of critical moderator heights used for ZEEP fuel rod experiments.



The top of reactor (TOR) was modelled with two scientists crouched down, pointing at fuel rods. This is a nod to the commemorative stamp produced by the Society for the Preservation of Canada’s Nuclear Heritage in 2020, depicting researchers Bill Dickerson and Dave Hone. Project team members acted as models for the image capture.



The lattice configuration in the model is based on an actual lattice configuration that would have been used in the ZEEP reactor.




Various models and external publications offered some conflicting information on the shielding configuration. There appears to have been a period where both water tanks and concrete biological shielding were in use. However, the best documented configuration (the final configuration) was chosen for the model – the 1956 shielding reconfiguration, with no water tanks.

 ZEEP as a “Parent” Reactor

Considered by some as “The Little Reactor that Could”, ZEEP was not a high-power reactor like its successor, NRX. However, it is a mistake to correlate “power” with “usefulness” because both of these reactors were designed for very different purposes.

NRX was used for isotope production, fuel and material performance testing, and neutron production for beamlines – all of which were pursuits where higher power is superior, with a fixed configuration required. ZEEP, on the other hand, was built to be a reconfigurable experiment, as well as a very sensitive reactor physics instrument, that would provide insight into the physics that made NRX work.

In addition to creating the reactor-physics foundation for NRX, ZEEP was also the parent of the ZED-2 reactor, which still operates at Chalk River Laboratories since first going online in 1960.

ZED-2 was originally referred to as Z-2, which stood for ZEEP-2, but would eventually be named Zero Energy Deuterium, or ZED-2. Some say that the name “ZED-2” was meant to emphasize the Canadian pronunciation of the letter Z, as well.

ZEEP’s design of using natural uranium was passed down to its descendants all the way up to today’s CANDU reactors, including ZED-2, NRX, and for the first seven years of the National Research Universal (NRU) reactor’s operation before it was converted over to use enriched fuel in 1964. 

ZEEP’s stats:

  • Moderator: heavy water
  • Reflector: graphite
  • Fuel: various; natural uranium, plutonium, enriched uranium, in metal and oxide forms
  • Maximum Power (Heat): 30 W
  • Maximum Neutron Flux: 108 n/cm2/s (max.)
  • Maximum Core Size: 200 cm diameter, max. 250 cm high. 

Further reading:

  • The 75th Anniversary of ZEEP” from The Society for the Preservation of Canada’s Nuclear Heritage provides an in-depth explanation of the “why” and “how” ZEEP was constructed.
  • ZEEP: The Little Reactor that Could” from R.E. Green and A. Okazaki for Canadian Nuclear Society Bulletin, Vol. 16, No. 3, Autumn 1995 showcases ZEEP’s operations and many accomplishments.

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