(dose rate), environment during irradi- 
ation (including temperature), and 
initial state of the material. Tospecify 
the last, one usually should know the 
prior history. 
Kind of Radiation 
The type of radiation is a very im- 
portant consideration in evaluating 
radiation effects. In a reactor, the 
important classes of radiation that 
strike materials not in contact with 
the fuel are: thermal neutrons, epi- 
thermal neutrons (all energies above 
thermal), and gamma rays. 
For metals and most ceramics, the 
energetic neutrons usually cause the 
only important changes. To ade- 
quately describe flux conditions, flux 
spectrum should be indicated. This is 
important because radiation-damage 
reactions occur at different neutron- 
energy levels—it takes neutrons of 
1,000 ev or more to displace a metal 
atom from its lattice to produce an 
interstitial and a vacancy; it takes 
neutrons of about 100 ev to cause lat- 
tice expansion in a ceramic; and it takes 
only a few ev to break a chemical bond 
to cause changes in polymers. 
In metals, thermal neutrons cause 
transmutations that may affect the 
properties at extreme doses—about 
10?5 nvt for steel—depending on the 
elements present. 
For organic materials, the gamma 
radiation in a reactor (especially at 
locations where plastics are used) may 
be of such an intensity as to be more 
important than fast neutrons. Al- 
though organics suffer from collision 
reactions with heavy particles, they 
generally suffer much larger effects 
from the ionization and excitation pro- 
duced by the secondary particles cre- 
ated in the collision reaction and by 
the secondary electrons produced in 
stopping the gammas. For all but the 
very rigid organic solids, the change 
produced by radiation is proportional 
to the energy absorbed (regardless of 
source—whether it be gammas or neu- 
trons or another kind). Thus, the 
radiation dose is best expressed in ergs 
or rads per gram of material. 
Materials in contact with reactor 
fuel suffer greatest damage by fission 
fragments. Principally, they damage 
organics by ionization, and they dam- 
age metals by displacing atoms and by 
introducing impurity atoms into the 
lattice. 
Dose Rate 
If most data have been determined at 
radiation dose rates comparable to the 
dose rate for the expected service, it is 
reasonable to assume that the effect is 
proportional to the integrated dose. 
For large differences in dose rate an 
estimate of the radiation effect based 
on this assumption may be in error. 
Competing reactions may go at differ- 
ent rates for different levels of radiation. 
Liquids. So far the dose rate ap- 
pears to be unimportant for liquids. 
Most of the data for organic liquids is 
for dose rates of the order to 106 rad/hr 
or less, and only a few special cases of 
inorganic liquids have received higher 
dose rates than this. Since the mole- 
cules in a liquid are mobile, the dose 
rate at which the chemical reaction 
rate is slower than the formation of free 
radicals is very high. 
Solids. For organic solids, a dose- 
rate effect has been observed for a few 
materials, but very little data are 
available. One expects the dose rate 
effect to be more serious for the rigid 
materials, in which the molecules have 
decreased mobility, than for soft plas- 
tics and elastomers. The effect that 
has been observed is to increase dam- 
age per total dose for low dose rates. 
Metals. For metals, no dose-rate 
data have been obtained. In dynamic 
properties such as creep or fatigue there 
should be a rate effect during irradi- 
ation. Dose rate may be important 
where several competitive processes are 
acting simultaneously. 
Environment 
There are in a reactor the normal 
environments considered when oper- 
ating other types of machines—tem- 
perature, pressure, and surrounding 
fluid. Added to this is radiation. 
The effect of temperature on the 
radiation-induced reactions is espe- 
cially important in power reactors, 
which usually operate at high tempera- 
tures. To the engineer the most im- 
portant effect of increased temperature 
is the annealing effect, which acts to 
reduce the amount of damage in some 
metals and ceramics. However, in- 
creased irradiation temperature does 
not always result in a lesser change 
than at lower temperatures. The ulti- 
mate strength of a carbon steel may 
show greater increases at higher tem- 
peratures of irradiation although the 
amount of increase in yield stress is 
reduced by the higher temperature of 
irradiation. Plastics and elastomers, 
since they are not high-temperature 
materials, have not been studied much 
at temperatures different from room 
temperature. Since soft and elastic 
polymers became hard and brittle at 
low temperatures, the radiation effects 
at low temperatures will be different 
from those at higher temperatures. 
The effects of stress on metals during 
irradiation are still not known. For 
plastics and elastomers this effect can 
be very large. Elastomers will suffer 
“ozone damage’”’ from radiation-pro- 
duced ozone or from free radicals pro- 
duced by radiation if the elastomers are 
stressed during irradiation. Stressed 
polymers generally will cleave under 
radiation more readily than those in the 
unstressed condition. 
In addition to the permanent change 
that is caused by radiation (which is the 
topic of this article) there isa temporary 
effect on the insulation properties of 
materials caused by the presence of the 
radiation field. The resistance of a 
good insulator may be reduced 1,000- 
fold in the presence of the ionization 
produced by the radiation field, and an 
induced photo-voltage is also produced. 
Initial State 
The initial state of a material may 
have an effect on radiation-induced 
changes. Certain physical character- 
istics such as crystallinity and ordered 
arrangements, tend to be destroyed by 
radiation. In general there is a de- 
crease in density if the substance is 
originally in its most compact form. 
On the other hand, crosslinking of 
polymers tends to increase the density. 
In metals there seems to be a great 
difference in radiation effects on the 
mechanical properties of alloys of the 
same nominal composition. Whether 
this is a composition effect of certain 
elements or a function of the heat- 
treatment and metallurgical practice 
is not yet known. But the fact that 
the susceptibility to radiation effects 
in similar alloys has been observed to 
vary by a factor of 100 (in flux) points 
to the importance of the initial state. 
In plastics the rigidity of the mate- 
rial, which affects the mobility of the 
polymer molecule, will affect the rate 
of radiation damage. Unfortunately, 
the soft materials that are most desir- 
able for certain applications are more 
easily changed than a hard material of 
similar chemical structure. 
