180 
constant but higher rate. Power is 
reduced by the reverse of this pro- 
cedure. 
The time between one generation of 
the neutrons emitted instantaneously 
at fission (prompt neutrons) and the 
next generation is extremely short. 
This is true even in slow-neutron piles 
where each neutron is forced to lose 
almost all of its energy by a multitude 
of collisions with the light nuclei of 
the moderator before the next fission. 
The time is so short that acceleration, 
even with small changes in reactivity, 
would be too fast for the control 
mechanism to be effective were it not 
that about 1 percent of the neutrons 
is not emitted promptly at fission, but 
is delayed from 0.01 second to a min- 
ute or more. The delayed neutrons 
provide a time constant of expo- 
nential acceleration long enough to 
permit control, provided the excess 
reactivity never is allowed to exceed 
the margin of delayed neutrons. 
The fission products include many 
radioactive isotopes which give off 
quantities of very intense radiation, 
and some of them continue to do so 
for thousands of years. Provision may 
be made to contain them within the 
fuel. If some are allowed to emigrate 
from the fuel they must be prevented 
from entering the heat-transfer fluid, 
or else the fluid system must be de- 
signed to handle the accumulated 
radioactive isotopes. 
All the materials in the pile are sub- 
ject to high neutron and gamma or 
X-rays which may affect chemical and 
physical properties. Furthermore, al- 
most all pile materials will become 
radioactive in greater or lesser degree 
after extended operation. 
DEVELOPMENT OF AN ATOMIC POWER 
PILE 
A scientist or engineer, observing the 
principles and associated properties of 
a nuclear reactor, is forced to recognize 
a number of fields in which extensive 
research and development work must 
be done to facilitate the design and 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
construction of a high-performance 
atomic power system. 
First, perhaps, a high-performance 
system should be defined. Consider- 
ing land-based plants, proposed _pri- 
marily for power, a prime consider- 
ation is economic use of fissionable 
materials, including replacement of 
burned fuel by transmutation. Sec- 
ond, losses in the transformation of 
heat to electric energy, for instance, 
should be as low as the current 
knowledge of materials and processes 
will allow. This requires temperatures 
as high as practicab! 
Considering how the energy is re- 
leased, one recognizes that the heat 
first must be removed from the body 
of the fuel to its surface. This may 
entail thermal stresses. It certainly in- 
volves a knowledge of the thermal 
conductivity of the material, and its 
stability and strength after a reason- 
able percentage of the atoms have 
disintegrated into a wide variety of 
pairs of new atoms. The percentage 
of the fuel which can be consumed 
before it must be removed and re- 
processed will depend on these factors 
and also on the neutron physics of the 
pile because, in general, as the fuel 
undergoes fission, and the fission 
products accumulate, the pile reactiv- 
ity will decrease. The frequency of 
reprocessing, including its cost and the 
accompanying losses, may have a 
profound effect on the economic use of 
fissionable material. 
Such fuel problems cannot be solved 
by simply consulting a handbook. 
The metallurgy of fissionable ma- 
terials is a new and only partially 
investigated field. Possible alloys and 
mixtures include many metallic ele- 
ments which may be valuable because 
of low absorption of neutrons but 
which are comparatively new to the 
metal-processing industries. Con- 
versely, many well-known alloy or 
mixture components have high neu- 
tron absorption and are not suitable. 
The fuel designer may wish to consider 
ceramics to obtain very high temper- 
ature. If he decides to prevent fission 
