expected to suffer larger radiation 
effects than the average metal or ce- 
ramic because of the secondary parti- 
cles from thermal-neutron capture. If 
the materials contain boron, they 
should be treated more as fuel elements 
than as structural materials. The B!° 
atom splits to form heavy energetic Li 
and He atoms capable of causing con- 
siderable displacement damage as do 
fission fragments. Two atoms result 
from each B! atom split by neutron 
capture, and the lattice must accom- 
modate the extra atoms. 
Most of the fuels and control-rod 
materials are metals. Thus, most of 
the radiation-damage data are on 
metallic elements. There is an increas- 
ing interest in ceramics and liquids for 
this application. 
Semiconductors * 
Components employing semiconduc- 
tors are among the most easily dam- 
aged by radiation. The amplification 
of a transistor, for example, is greatly 
reduced at a radiation level that does 
not affect even the least radiation- 
stable organic material. 
The effect of radiation on semicon- 
ductor components is eventually to 
destroy the component. The destruc- 
tion of barriers in junction devices such 
as rectifiers and transistors usually 
occurs at a low integrated flux com- 
pared to the dose required to produce a 
comparable change in most materials. 
Thus the collector leakage current of a 
transistor and the reverse saturation 
current of a rectifier will change by 
orders of magnitude before an appreci- 
able change can be detected in the 
emitter current of the transistor or for- 
ward current of the rectifier. 
There are three ways in which radi- 
ation can affect the properties of a semi- 
* J. C. Pigg, ORNL, personal communi- 
cation. 
1 
' 
' 
rubber. fe 
€ 
° 
° 
ay 
£ 
[) 
f= 
o 
C2) 
2 
a 
c 
o 
(Ss 
107 oe 
Radiation Dose (rads) 
FIG. 4. Break in plot of tensile strength vs 
radiation dose indicates rubber-to-glass 
transition of natural rubber 
How Organics are Changed by Radiation-Induced . . . 
CROSSLINKING 
® Increases Young’s modulus 
© Impedes viscous flux 
® Increases retardation of strain 
Usually causing: 
® Increased tensile strength 
® Decreased elongation 
® Increased hardness 
® Increased softening temperature 
® Decreased solubility 
© Gas formation 
© Embrittlement 
® Decreased elasticity 
conductor device: transmutation, 
photoelectric effect, and lattice dis- 
turbance. Thermal neutrons captured 
by the semiconductor produce excited 
nuclei that decay to form impurity 
atoms. The absorption cross sections, 
isotopic abundances, and decay schemes 
are well known for those semiconduc- 
tors most widely used. Hence the 
effect of thermal-neutron irradiation is 
to introduce a predictable number of 
impurity atoms. 
Neutrons having sufficient energy to 
produce displaced atoms cause regions 
of disorder in the semiconductor crys- 
tal. This changes the carrier concen- 
tration and thus the conductivity of the 
material. These displacements change 
n-type germanium to p type. Silicon 
does not change the sign of the carriers, 
but both n and p type decrease in con- 
ductivity as the number of displaced 
atoms increases. 
Liquids 
The organic liquids important to re- 
actor technology are lubricants, cool- 
ants, hydraulic fluids, moderators, and 
shields. The reactions induced by 
radiation are much the same as for 
organic solids, and the stable structures 
are of the same general type as has been 
found for organic polymers. Biphenyl 
is the most radiation-stable organic 
liquid (m.p. 70° C) ; terphenyls and sub- 
stituted biphenyls are almost as good. 
The condensed-ring structures are also 
relatively radiation resistant and for 
some applications the alkyl benzenes 
have desirable properties. 
Though a few liquids are decreased 
in viscosity, most organic liquids be- 
come more viscous by radiation-induced 
polymerization. Often a solid sludge 
is formed, and gassing is prevalent. 
Coking and the tendency to foam may 
be increased. Increased corrosion may 
result from some of the products. 
CLEAVAGE 
® Decreases Young’s modulus 
® Reduces yield stress for viscous flow 
Usually causing: 
® Decreased tensile strength 
® Increased elongation 
® Decreased hardness 
® Increased solubility 
® Decreased elasticity 
Sometimes causing: 
© Embrittlement 
® Gas formation 
In many applications where liquids 
are used they may be circulated from 
an external reservoir. This dilutes the 
effects of irradiation and often allows 
the use of a liquid that otherwise might 
be too badly damaged; it also allows 
replacement of decomposed material. 
A difficulty with circulating systems in 
a neutron flux is that radioactive mate- 
rials are pumped out of the reactor and 
require shielding. The hydrocarbon 
materials are very good from this stand- 
point because they do not become very 
radioactive. But corrosion products 
still may be bothersome. 
Water or water solutions are still the 
primary reactor coolants and moder- 
ators and are among the most used 
radiation shields. The radiation-in- 
duced reactions are being studied very 
intensely by radiation chemists. The 
principal concern of the engineer is 
corrosion. Second is gas and sludge 
formation. Most water systems can 
stand a fair amount of radiation with- 
out serious enhancement of corrosion. 
Liquid Metals 
Theoretically, none of the mecha- 
nisms of radiation damage that change 
the properties of other materials are 
operable on liquid metals, and no 
change is expected unless some new 
mechanism is discovered. No radi- 
ation-induced change has ever been 
observed in liquid metals. 
Important Variables 
Engineers are only now accumulating 
the data needed to meet problems of 
nuclear design. Much of the available 
data are incomplete because the impor- 
tance of many variables has been real- 
ized only recently. The following 
conditions must be carefully and com- 
pletely specified for any data to be 
fully useful (see Table): kind and 
amount of radiation, rate of irradiation 
87 
