ATOMIC ENERGY IN 
nucleus of a radioactive isotope of 
some element ranging from atomic 
number 35 to 65. 
The two fragments fly apart with 
tremendous velocities. Their kinetic 
energy constitutes most of the energy 
released at the expense of a loss in 
mass of one-thousandth of the original 
mass. The fragments will collide 
with adjacent atoms, giving up energy 
by increasing the vibration, amplitude, 
and velocity of the surrounding atoms. 
Such atomic agitation is recognized 
as the temperature of the material. 
Thus the energy appears almost im- 
mediately as sensible heat in the fuel 
itself and within a few thousandths 
of an inch from where the fission 
occurred. 
Fission was caused by the capture 
of one neutron by the U235 nucleus. 
This neutron, when added to the 92 
protons and 143 neutrons present, 
caused the group of particles to become 
violently unstable and to break im- 
mediately into the two unequal frag- 
ments. 
The entering neutron was the trig- 
ger. It has been absorbed and now is 
lost to the process. If the reaction is 
to proceed at a constant rate, another 
neutron must trigger another U235 
nucleus, and so on, at equal intervals 
of time in a continuous chain. The 
characteristic of U235 (and also 
plutonium Pu239 and U233) which 
makes it suitable for such a chain of 
fissions is that two to three free neu- 
trons are released as part of the debris 
at fission in addition to the two main 
fragments. Thus there are available 
more than enough triggers for the 
next fission. 
The fact that more than one neutron 
is released per fission permits some 
losses or nonprofitable absorptions of 
neutrons without killing the chain 
reaction (an obvious necessity in prac- 
tice). The fact that on the average 
more than two neutrons are released 
per fission permits several choices in 
the use of the extra neutrons which are 
available in addition to the one re- 
quired for a continuous constant-rate 
INDUSTRY—WINNE 179 
chain reaction. Of course, some of 
these extra neutrons escape from the 
reactor and are wasted in the shield 
or otherwise, but some can be put to 
useful purpose. 
The extra neutrons can be used to 
expand the reaction, essentially doub- 
ling it at each generation. This may 
result in a bomb. They can be ab- 
sorbed in a suitable material to pro- 
duce radioactive isotopes for chemical, 
biological, or therapeutic use. Of 
most significance to power, however, 
is the use of these neutrons to make 
new fissionable atoms, for example, to 
transmute U238 to U239 which 
changes spontaneously to neptunium 
(N239) which, in turn, changes spon- 
taneously to Pu239. Pu239 is a suit- 
able atomic fuel. Every neutron 
which can be used in this way replaces 
the U235 atom burned with a new 
Pu239 atom and so helps to replenish 
the atomic fuel supply. Another ex- 
ample of fuel replacement is the pro- 
duction of U233 from thorium. 
The neutrons in any nuclear reactor 
are valuable. Their loss or waste on 
reactions extraneous to the purpose of 
the pile is a permanent economic loss. 
Therefore, pile design for high neutron 
economy is important. 
By controlling the neutron flux, the 
power output of the pile is regulated. 
To hold constant power level in a slow 
neutron pile, rods containing an ex- 
cellent neutron absorber such as boron 
or cadmium may be inserted to absorb 
some neutrons so that equal numbers 
are available to produce fission at each 
generation. To increase power, the 
rods are withdrawn so that a small 
fraction of the excess neutrons is avail- 
able at each generation to produce 
fission. The power then accelerates 
exponentially. It will continue to 
accelerate, except for variations in re- 
activity produced by temperature, ex- 
pansion of materials, or change of their 
nuclear properties, until the control 
rods again are pushed in to absorb the 
excess neutrons. After the control 
rods are returned to an equilibrium 
position the reaction continues at a 
