Measures for Progress: A History of the National Bureau of Standards

380 WORLD WAR II RESEARCH (1941-45)
initiated research in a gaseous diffusion method for the separation and con-
centration of U235. At the Bureau, Philip H. Abelson of the Carnegie Institu-
tion attempted separation of the two isotopes by thermal diffusion of
uranium hexafluoride, the only gaseous compound of uranium. Although an
exceedingly corrosive material, the hexafluoride was workable because it was
stable as a liquid at slightly elevated temperatures and moderate pressures.
Abelson's work, carried out with Navy funds, was transferred to larger
facilities at the Naval Research Laboratory in the summer of 1941.48
Efforts made in the Bureau laboratories and at Westinghouse and
General Electric to find a method for manufacturing uranium powder or
pure ingots progressed slowly and the Columbia group turned to the pro-
cessed ore, uranium oxide, which was available in small quantities from
Canada. It was evident that both Fermi's uranium pile and isotope separa-
tion depended upon obtaining uranium in a highly purified metallic form
or at least as a highly purified uranium oxide. The problem came to the
Bureau, and in the summer of 1941 a group under James I. Hoffman found
that ether extraction of uranium oxide after conversion to uranyl nitrate
removed virtually all impurities from the oxide.49
As a final step in the production of uranium metal, determination
and analysis had to be made of the residual boron content of the reconverted
oxide. Studies by Bourdon F. Scribner and J. A. Scherrer opened the way
to subsequent reduction of the boron content, by reaction with calcium
hydride. After months of experimentation, their coworker Clement Rodden
distilled an extremely pure calcium making this last step possible. The ether
extraction and boron reduction processes, as effective with pitchblende and
carnotite ore concentrates as with uranium ores, became standard procedures
in the purification of all uranium used in piles.5°
The winter of 1940—41 was a time of decision. Besides the investi-
gations in gaseous and thermal diffusion methods for isotope separation,
Jesse W. Beams at Virginia was working on a centrifuge process and
Ernest 0. Lawrence at California on electromagnetic methods of separation.
While Dr. Briggs felt that quantity separation of isotopes was important from
a military standpoint as probably the only way to a chain reaction in a mass
small enough for a bomb, separation would be difficult and expensive. Pilot
plant construction ought therefore to wait until further studies disclosed the
most promising method. Characteristically, his real interest was in power
"Both the thermal and gaseous diffusion processes were to be used in the large-scale
plants for the production of at Oak Ridge, the gaseous method eventually proving
the more efficient of the two.
Hewlett and Anderson, pp. 28—29, 86; Smyth Report, p. 93; NBS War Research, pp. 8—9.
'° Hewlett and Anderson, pp. 66, 87; NBS War Research, p. 9.