Narrator: Less than one percent of naturally
occurring uranium consists of the isotope
U-235.
Only this isotope of uranium is “fissile”
and suitable for an atomic weapon.
Most uranium consists of U-238, a nearly identical
but very stable isotope.
The challenge was to separate the two isotopes
and produce high concentrations of U-235 or
enriched uranium.
As Manhattan Project veteran Bill Wilcox explains,
project director General Leslie Groves was
impressed by Ernest O. Lawrence and the potential
of his approach developed at the University
of California.
Bill Wilcox: During those first two months,
General Groves had the job of going around
talking to these university scientists about
how this incredibly difficult job of separating
uranium-235 from U-238 could possibly be done. 
Then he goes to Berkeley, California and he
talks to Professor E.O. Lawrence, who is Mr.
Enthusiasm.
“By gosh, I’m quite sure we can do this
using a modification of the cyclotron that
I developed.”
He explains this to Groves.
I’m sure Groves, when he saw this huge,
huge cyclotron, these big magnets and these
tubes and devices— I’d love to know what
he thought. 
He [Lawrence] hadn’t separated anything
to amount to micrograms of U-235, but was
enthusiastic.
So at the end of December [1942], Groves made
that decision: “We’re going to go with
Lawrence’s process, that’s probably the
best bet.”
And that’s what turned into the Y-12 plant. 
Narrator: Designed at the University of California,
the electromagnetic separation plants were
called “Calutrons.”
“Cal” stood for “California,” “U”
for “University,” “Tron” for “cyclotron”:
“Cal-U-Tron.”
