Ultimote 
tensile strength 
(1,000 ps: units) 
Yield stress 
for 0.2% 
offset 
(1,000 per 
units) 
Reduction 
of area (%) 
Transition 
temperature (°F) 
Uniform 
elongation (%) 
\ 
Bee: 4 6 8 
Integrated Fost-Neutron Flux (>! Mev) (10'%n/cm? units) 
FIG. 2. Integrated-fast-neutron-flux de- 
pendence of several mechanical proper- 
ties of A-212B carbon-silicon steel 
erties of plastics and elastomers are re- 
flected in the extent of two reactions of 
the polymer—crosslinking and cleav- 
age. Some of the engineering proper- 
ties associated with cleavage and cross- 
linking are summarized on page 61. 
Chemical changes. Chemically, 
polymers are altered quite drastically. 
Aside from the crosslinking and cleav- 
age reaction, which produces most of 
the physical change in the material, 
there is usually a variety of side reac- 
tions. The production of gas is a 
direct result of crosslinking, but it also 
may come as a side reaction. Often 
unsaturation is produced, but double 
bonds may be destroyed also. Some- 
times increased chemical reactivity is 
indicated by both increased solubility 
and the production of corrosive byprod- 
ucts. Increased water absorption 
sometimes is observed. The cross- 
linked materials are harder to dissolve 
and harder to melt. 
Dimensional changes. In this class 
of materials dimensional changes can 
be extremely large. In general, cross- 
linking causes an increase in density by 
reducing the specific volume of the 
material, and many plastics do show a 
large shrinkage effect. However, due 
to the formation of gas, there is also 
very often a swelling effect and a large 
decrease in density. 
Electrical changes. Permanent 
electrical changes are usually not seri- 
ous before mechanical failure occurs. 
However, while the material is in the 
radiation field, the electrical properties 
may change drastically. The change 
is greatest for the best insulators. 
86 
Normal testing speed 
Slow testing speed 
- -Irradiated 
~ Irradiated 
Normalized 
1 
Austenitic Corbon Stee 
Stainless Steel 
Unirradiated 
' 
' 
1 
Unirradioted 
(Normal and Slow 
Testing Speeds) 
Irradiated 
‘ 
High Purity 
Iron 
Unirradioted 
Extension (%) 
FIG. 3. Behavior of steels and iron in tensile 
test. Irradiation changes shape of curves 
and rate of work hardening 
Discoloration. Many plastics dis- 
color, and transparent materials may 
transmit less light. But some ecrystal- 
line materials become more transparent 
because radiation destroys the crystal- 
linity. The color in plastics, caused 
usually by decomposition products, 
normally cannot be removed by heat. 
Darkening is sometimes prevalent in 
the presence of oxygen when it may not 
occur without oxygen. 
Mechanical changes. The early 
changes in some of the properties of 
plastics are often an improvement. 
Increased tensile strength and increased 
softening temperature that accompany 
crosslinking are desirable changes. 
However, some of the accompanying 
changes, like increased hardness and 
decreased elongation, may not be de- 
sirable. Even though some of the im- 
provements in properties may occur 
before any serious less desirable 
changes, for the engineer this is usually 
a sign of impending radiation damage. 
With further irradiation the strength 
will decrease and embrittlement may 
be followed by cracking and powdering. 
The elastomers that harden decrease 
in tensile strength to almost zero and 
then have a very large sudden increase 
in strength (Fig. 4). At this point the 
elastomer has been changed to a glass, 
and, with further irradiation, has the 
radiation-damage characteristics of a 
glass. (This rubber to glass transition 
can be brought about by other means 
too, such as lowering the temperature). 
For an application (a seal, for ex- 
ample) where a rubber need not re- 
tain its elastomeric properties, it may 
be considered for a much longer life in 
a radiation field. 
Ceramics 
Many materials that fall into the 
class of ceramics are being studied 
quite intensively from a fundamental 
standpoint, but the engineering proper- 
ties are just now beginning to receive 
attention. For reactors there is a 
great deal of interest in ceramics as 
high-temperature materials. Here 
very little data is available. 
As a class the ceramics are much 
more radiation resistant than organic 
materials, but will probably suffer more 
damage than most metals. Ceramics 
generally are not used where great 
strength is required, but not much loss 
in strength is seen for moderate irradi- 
ation periods. Swelling, or a decrease 
in density, is observed for most ceram- 
ics. But some glasses increase in den- 
sity, presumably because radiation is 
completing the annealing processes 
that was not allowed to go to comple- 
tion during manufacture. 
Disordering, coloration. Crystal- 
line materials are fairly readily dis- 
ordered by fast-neutron bombardment, 
and the same materials show an early 
decrease in thermal conductivity. 
However, the thermal-conductivity 
change may be due at least partially to 
impurity atoms introduced by transmu- 
tation by thermal neutrons. 
Coloration, particularly noticeable in 
some glasses, is caused by the formation 
of F-centers, and is readily cleared up 
by heat. Since much of the radiation 
damage in ceramics can be annealed 
out, it is often assumed that ceramics 
irradiated at high temperatures will 
suffer little damage. This has not yet 
been established experimentally. 
Fuels, Control Rods 
Fuels, of course, suffer heavy radi- 
ation effects because of fission. Most 
of the damage results from fission frag- 
ments and fast neutrons. But a large 
number of impurity atoms are intro- 
duced as a result of fission also. Since 
the fuel normally is not required to per- 
form any structural function except 
holding itself together; and since, in 
addition, the fuel does not remain in 
the reactor very long, the radiation- 
damage requirements are not too severe. 
Most troubles are encountered from 
dimensional changes and separation of 
the cladding from the fuel. 
Control or shield materials can be 
