Idaho National Laboratory Cultural Resource Management Plan

Because the reactor would operate at a power level of 175 megawatts, it generated considerably more
heat than the MTR. The primary coolant loop contained demineralized water. To keep it from boiling, it
had to be kept pressurized. Pressure was maintained by pumping the water through the core and
withdrawing it at a rate that would maintain the desired pressure. A secondary loop discharged the heat to
the atmosphere. Exhaust gases were filtered and vented to a new stack. Because the coolant accumulated
radionuclides, the pipes between the reactor building and the heat exchanger building were shrouded with
concrete shielding.
ETR Work—The typical life of a fuel element was eighteen days, in which time about 27% of the
uranium fissioned. Like the MTR, the ETR required a water-filled canal where spent fuel elements could
cool down before transport elsewhere. 113 ETR operators, like their colleagues at MTR, where the cycle
also was 18 days, lived a cyclical lifestyle, taking three days to unload and refuel the reactor. Using
remote manipulators, an operator could lift a fuel assembly part way up the side of the tank, tilt it, and
slide it through an opening and down a chute. The element “flopped” into the 18-foot-deep canal, where
technicians used grappling poles to guide the element to a resting place on a rack. Here, the fuel sat for
several months to cool off, its radioactive constituents continuing to decay. With the help of a 30-ton
crane, it would be maneuvered into a special shielded transport cask, called a “coffin,” and shipped down
the road to the Gamma Facility or the ICPP to recover the valuable U-235 remaining in the fuel
element. 114
The ETR went critical for the first time at its full power level of 175 megawatts on April 19, 1957; the
ETR Critical Facility (ETRC), on May 20, 1957. 115 This low-power reactor did the same for ETR as did
the MTR Critical Facility. In order to run the reactor safely and efficiently, operators had to know how the
experiments would affect power distribution, whether the reactivity effects of experiments would impact
the reactor or generate potential hazards. This information had to be available before each new cycle was
begun. It used fuel and control rods like the ETR and had the same type of beryllium-beryllium oxide
reflector. 116
The ETR mission was to evaluate proposed reactor fuels, coolants, and moderators. It was designed
especially to simulate environments like those expected in civilian nuclear power reactors. ETR had more
test space and more flexibility than the MTR. Over 20% of the head volume over the vessel was filled
with test voids—like a “large cake of Swiss cheese,” as one writer put it. 117
During its lifetime, the ETR had less on-stream time than the MTR because its experiments were
more elaborate and required more time to plan, pre-test, and install. They were more expensive, too.
Various test “sponsors” invested over $17 million to adapt 18 of the test loops for their experiments. 118
Fabricating the tests required the services of welders, pipe fitters, heavy equipment operators, carpenters,
mechanics, and many other specialists. These craft specialties explain the numerous shop buildings
erected at TRA and at CFA to support these activities.
Demand for test space kept growing, calling for more than the MTR and ETR could supply. Use of
space was prioritized and allocated by the Washington Irradiation Board. Military and AEC priorities
came first. After that, the rule was “first come, first served.” If private test space were available
113. Bush, p. 41-56. See also 1965 Thumbnail Sketch, p. 15.
114. R. H. Dempsey, “ETR: Core and Facilities,” Nucleonics (March 1957), p. 54.
115 . R. L. Doan, “MTR-ETR Operating Experience,” Nuclear Science and Engineering (January 1962), p. 23.
116. 1965 Thumbnail Sketch, p. 15.
117. Bush, p. 43.
118. Doan, p. 24.
227