Ed. Note: Ralph Green, a former vice-president of AECL Research, gave an oral version of this paper as
one of the three invited talks for the Nuclear Heritage celebrations held at Chalk River, August 4-6, 1995,
to mark the 50th anniversary of ZEEP, Canada's first nuclear reactor. He repeated the talk at
AECL Sheridan Park in September. Co-author Al Okazaki is a former senior scientist at AECL Chalk
River Laboratories. Both worked with ZEEP in the 1950s.
On September 5, 1945 the ZEEP reactor went critical for the first time at the Chalk River Laboratories
of Atomic Energy of Canada Limited. ZEEP (for Zero Energy Experimental Pile) was the first reactor to
operate outside the USA. In this paper we recall some of the events that led to the construction of ZEEP,
and briefly describe the role it played in the development of the Canadian Nuclear Program.
ZEEP: conception to criticality
The first attempt to achieve a self-sustained nuclear chain reaction in Canada was made by
George Laurence, assisted by B.W. Sargent, working at NRC during the years 1940-42. Their pile consisted
of sacks of uranium oxide interspersed with sacks of powdered coke. Their attempt failed mainly because
of impurities in the materials they were using, although it would have been very difficult to achieve
a critical assembly using natural uranium oxide and graphite, even with pure materials.
In 1942 it was decided to move the UK Nuclear Energy Program to Canada, and a joint Canada-UK
laboratory was set up in Montreal in the fall of 1942. The work in Montreal, described in a pamphlet
entitled Early years of nuclear energy research in Canada, by
George Laurence, led to the decision, in mid-April, 1944, to build a natural uranium fuelled,
heavy-water moderated reactor, what we know today as NRX. The design of NRX was based on theoretical
calculations backed up by subcritical experiments in the Montreal laboratory with lattice
arrangements of natural uranium metal and heavy water.
In late April 1944, John Cockcroft came to Canada to lead the Canada-UK program. In May 1944,
Cockcroft decided it would be desirable to have some operating experience with a low-power reactor like
NRX before the latter was built, and to have the capability to alter the reactor core to investigate the
effect of changes to the lattice arrangement.
The main reasons for building such a reactor were that it could be constructed quickly and the experience
gained during the construction and operation would be valuable for NRX. It could also be used to measure
some materials properties, and to test control, safety and radiation-protection equipment.
So, in July 1944, Cockcroft asked two of his staff to look at the possibility of building a low-power
reactor without seriously impeding the NRX project. In August 1944, approval was received to proceed with
the design, and Lew Kowarski, newly arrived from the UK, was asked by Cockcroft to manage the
project. Charles Watson-Munro was Kowarski's second in command, and they were assisted by A.M. Allan,
F.W. Penning, G.J. Fergusson, C.W. Gilbert, E.P. Hincks, H.F. Freundlich and
H. Carmichael. The chief designer was George Klein from the NRC Mechanical Engineering
Division at Ottawa. He was ably assisted by Don Nazzer, also of NRC.
During the design phase there was pressure from the research staff for a reactor power of 1 kilowatt,
rather than 1 watt, because this would provide neutron fluxes high enough for good cross-section
measurements for the chemists to prepare good radioisotope sources, for the engineers to study material
properties and for radiation protection work to be done. However, such a power level would require
more shielding to protect the operators, and would preclude the rapid rearrangement of the core to
study different lattice configurations. So, the power level was kept at 1 watt.
Final approval for the construction of ZEEP was given on October 10, 1944. Construction was
complete by September 4, 1945, and the reactor went critical on September 5, 1945 at 3:45 pm, only 16
months after conception and only 11 months after approval of construction.
ZEEP Building on April 10, 1945