Evening star., 1945-08-22
Third OF a Series.)
By HOWARD w. BLAKSLEE,
Associated pre4s Science Editnr

Associated Press Science Editor.
| NEW YORK, Aug. 22-Plutonium
makes the bombs, but the rare
uranium 235 is the key is atomic
power.
1t makes the atomic fire that pro-
duces plutonium by alchemy, and
there is no other way of getting
plutonium.
The use of plutonium to fuel
atomic power houses would multiply
by 100 times the worlds uranium
power sources. But If the supply
of rare 235 should be exhausted, it
would not be feasible by present
known methods to make any more
plutonium. Consequently, even be-
fore the war ended, scientists plan
ned to save the precious 235 for
atomic fires only.
Unlocks Chemical Secrets.
In addition to making atomic pow
er, the rare uranium is probably the
best tool for unlocking the secrets
of power resources in other chemical
elements.
Through wartime discoveries,
there are known four other kinds
of atoms which, like uranium 235,
split through their cores when hit
by neutrons, to yield about 100000
600 electron volts.
Two of these, the metals protacti-
mum and thorium, are too scarce to
be promising. The third is plutoni-
UNI. The fourth is common urani-
um 238, but only small percentage
OF these atoms split, too few to make
a chain reaction at present.
(The numbers 235 and 23S for
uranium mean, respectively, that
their atoms weigh that many
times more than an atom of hy-
drogen, the lightest chemical
element.)
The fact that there is even as
much as seven-tenths of one per
cent of 235 in the common 238 form
of uranium is one of the lucky
breaks in scientific history. Had this
tiny fraction been little smaller,
it would have been far more diff
cult, and perhaps impossible at this
time, to make atomic power or
bombs.
The tiny percentage is just about
enough, when common uranium is
piled inside graphite brick ovens, to
set up chain reaction. This means
that common uranium is a ready
made atomic fuel. 1t also means
that hotter atomic fires may be made
by using as fuel the uranium which
has been concentrated by processes
found in the war, so that the 235
percentage is a little higher, about
doubled in fact. Such concentra-
ton is possible in the plants in
Tennessee. One immediate use of
the concentrated uranium is to make
plutonium more readily.
The 235 also lends itself to con-
trolls which seem to make the atomic
uranium fires reasonably safe.
Use Steel Strips Also.
When the piles, the graphite ovens
in Washington State, are stocked with
uranium, some metal strips of
different kind also are placed in the
oven. These may be either cadmium,
or boron steel, or other elements.
The atomic explosions in the urani-
UNI 235 start automatically from
neutrons, particles from cores of
atoms, which are appearing natur-
ally in air and earth all the time.
When neutron splits a uranium
atom, 13 new neutrons come from
the broken core. AS the neutrons
multiply, the chain reaction begins.
The metallic strips in the oven
delay this start, because those metals
absorb lot of neutrons. without
themselves being split. This reduces
and controls the number of neutrons
present, so that the fire can be
started when wanted, merely by
withdrawing the strips. So accurate
is this control that the fire can be
started by pulling out single strip.
Also, after the oven is going, and
workmen may not approach because
of the rays, automatic electrical
controls can quench the fires by
thrusting strips into the oven. The
controls read the danger pointed by
the increasing amount of radioac-
tive rays in the oven.
The newly created plutonium
atoms in such an oven themselves
explode like the 235 atoms. They
add to the heat of the fire. So
also do a few of the common 238
uranium atoms. If the fire is left
too long, it will consume
all the plutonium it has created.
This is avoided by stopping the fire
from time to time, and removing
the uranium with plutonium in it.
Million Atoms Invisible.
Men probably nevtr before under
took a task as difficult as the separa-
Men probably never before under
ton of uranium 235 from the more
common metal If a million of these
uranium 235 atoms were
concentrated on the point of a pin
they still would be invisible toI