rN) 
E 
oS) 
“~ 
w 
c 
fe) 
i= 
= 
3 
o 
Zz 
FIG. 1. 
irradiation on various materials. 
Irradiation dose is in epithermal neutrons. 
altogether unsuitable for a proposed 
use in a radiation field, and a new de- 
sign is required using another material. 
There are often two other alterna- 
tives: the component can be moved to 
a region of lower radiation intensity, 
or it sometimes can be shielded. In 
any event the problem should be care- 
fully considered, and the more radi- 
ation-stable materials should be used 
where possible so as not to place an 
excessive burden on the designer. 
Structural Metals 
Structural metals usually are evalu- 
ated by strength, ductility, and impact 
tests. But the integrated-flux depend- 
ences of different properties are not the 
same. This is shown in Fig. 2. In 
normal structural materials the 
strengths (tensile and ultimate) invari- 
ably are increased with sufficient radi- 
ation at ordinary temperatures. This 
may not be so at higher irradiation 
temperatures. 
The strength in creep and in fatigue 
have only been determined under a few 
conditions; these properties deserve 
Sensitivity of engineering properties to radiation. 
GERMANIUM TRANSISTOR — loss of amplification 
GLASS — coloring 
POLYTETRAFLUORETHYLENE — loss of tensile strength 
POLYMETHYL METHACRYLATE & CELLULOSICS — loss of tensile strength 
WATER & LEAST STABLE ORGANIC LIQUIDS — gassing 
NATURAL & BUTYL RUBBER — loss of elasticity 
ORGANIC LIQUIDS — gassing af most stable ones 
BUTYL RUBBER — large change, softening 
POLYETHYLENE — loss of tensile strength 
MINERAL-FILLED PHENOLIC POLYMER — loss of tensile strength 
NATURAL RUBBER — large change, hardening 
HYDROCARBON OILS — increase in viscosity 
METALS — most show appreciable increase in yield strength 
CARBON STEEL — reduction of notch-impact strength 
POLYSTYRENE — loss of tensile strength 
CERAMICS — reduced thermal conductivity, density, crystallinity 
ALL PLASTICS — unusable as structural materials 
CARBON STEELS — severe loss of ductility, yield strength doubled 
CARBON STEELS — increased fracture-transition temperature 
STAINLESS STEELS — yield strength tripled 
ALUMINUM ALLOYS — ductility reduced but not greatly impaired 
STAINLESS STEELS — ductility reduced but not greatly impaired 
more attention. A strength increase 
usually is regarded as beneficial if there 
isno concurrent lossin ductility. Some 
ASTM standards specify a maximum 
ultimate strength, and irradiation can 
cause carbon steels to exceed the maxi- 
mum. Instructures where some plastic 
flow normally is expected, increased 
strength. can be objectionable—it may 
lead to high elastic stresses that nor- 
mally would be relieved by deforma- 
tion or stress relaxation. 
Ductility limits. Ductility (in the 
tension test) normally decreases after 
irradiation. The difficult problem is to 
set limits on the allowable decrease, 
because it is not yet possible to assess 
quantitatively the meaning of the duc- 
tility values. Radiation can reduce 
the ductility (in the tension test) below 
the minimum in ASTM specifications. 
The loss of energy absorption in the 
notch-impact test and the increase in 
the fracture-transition temperature has 
been some cause for concern in irradi- 
ated steels. The minimum values 
specified for the notch-impact test are 
the result of service experience; the 
There is wide variation in effect of 
Levels indicated are approximate and subject to variation. 
Indicated changes are in most cases at least 10% 
level of energy absorption required ap- 
pears to depend on structure or heat 
treatment, and radiation could well act 
to modify the necessary, safe, minimum 
level requirements. 
Changes in behavior. The be- 
havior during mechanical testing of car- 
bon and stainless steels, shown in Fig. 
3, has suggested that the response of 
these materials to stress is changed by 
radiation. For instance, stainless steels 
show a yield point, and the yield stress 
is strain-rate dependent. This has sug- 
gested that the conventional inter- 
pretation of mechanical tests may not 
be applicable to irradiated metals. 
Another consideration in evaluating 
metals is that the stress, strain, and 
time conditions in a reactor may be 
different than in other structures. The 
fatigue properties in reactor pressure 
vessels may be a major factor because 
of the thermal-stress conditions during 
startup and shutdown. 
Plastics and Elastomers 
Since they are basically organic poly- 
mers, the changes in the physical prop- 
85 
