more, the effect may be overestimated 
in the case of metals. Its existence as 
a major effect in fission-fragment dam- 
age is probably better founded. 
The thermal spike was first utilized 
to explain the disordering effect in 
Cu3Au (4). However, it has recently 
been shown that if one uses a replace- 
ment concept instead of a displacement 
concept, even the disordering data can 
be explained without a thermal spike 
(15). 
The thermal-spike concept, while 
useful, will require more study before 
it will emerge as a quantitative tool in 
the interpretation of radiation damage. 
Variation with Material 
In any attempt to anticipate radi- 
ation damage under reactor operating 
conditions, it is difficult to make uni- 
versally applicable generalizations. 
The type of damage and its extent de- 
pend strongly on the type of solid that 
is bombarded. 
The damage process alters certain 
properties to a far greater extent than 
others. For example, in representa- 
tive metals and alloys one observes 
sharp increases in the yield strength 
after modest radiation doses (16). 
Ductility often is reduced appreciably 
as well. On the other hand, one does 
not expect or observe significant 
changes in thermal or electrical 
conductivity when metals or alloys 
are bombarded at reactor ambient 
temperatures. 
Nonmetals. Nonmetallic com- 
pounds exhibit variable behavior. 
Materials like Al,O3;, MgO, NaCl may 
show moderate electrical, thermal, and 
optical changes, but they do not show 
extensive density changes. They do 
show an increased susceptibility to 
fracture. Compounds such as quartz 
reveal large density and_ structural 
changes after moderate irradiations but 
are able to maintain macroscopic 
integrity (17). 
Semiconductors. Semiconductors 
are extremely sensitive to radiation. 
This is shown by the drastic change in 
electrical conductivity and in a change 
in the character of the conduction. 
The conduction may change from elec- 
tron to hole conduction and vice versa, 
with short exposures. The nature of 
the change depends on the original 
state of the material (18). 
Plastics, elastomers. Plastics and 
elastomers, too, are sensitive to nuclear 
radiation (19). The changes from 
moderate dosages range from complete 
disintegration and embrittlement to 
only discoloration. Quantitative data 
on threshold for damage and for ulti- 
mate failure are discussed in the fol- 
lowing article of this report. 
Thus, specification of a particular 
material and a specific property are 
required in order to make any estima- 
tion of probable damage. In addition 
to these aspects, environmental condi- 
tions will play a strong role. 
Effect of Temperature 
The temperature is most important 
in any consideration of the damage 
process in a crystal since the tempera- 
ture will determine the subsequent be- 
havior of the defects introduced by 
irradiation. Very low temperatures 
(—253° C) will cause the defects to be 
completely immobile. Thus they are 
retained, and changes in properties like 
electrical conductivity that depend 
only on the presence of defects will be 
increased at low temperature. Ther- 
mal conductivity of nonmetals at low 
temperatures is greatly decreased be- 
cause of the effect of defects on lattice 
conduction. This type of conduction 
is of chief importance at these tempera- 
tures in insulating materials. 
If the subject of irradiation is either 
a metastable metallic or nonmetallic 
alloy, enhanced diffusion effects that 
result in the conversion of the material 
to the stable phase can be observed. 
The temperature must be high enough, 
to permit diffusion to take place. At 
low temperature these effects are re- 
pressed. The critical temperature for 
these effects is a function of the tem- 
perature dependence of diffusion in the 
material under consideration. In cop- 
per beryllium, precipitation can be 
observed during room-temperature ir- 
radiation, in nickel beryllium the me- 
tastability is preserved at room tem- 
perature but can be observed at 200° C 
and higher (20). 
Irradiation of ZrO; at room tempera- 
ture results in the disappearance of the 
monoclinic phase (21). However, sub- 
sequent annealing at elevated tempera- 
tures will cause the material to revert 
to the stable configuration. Many 
additional examples of the importance 
of the temperature of irradiation are 
available in the literature (22). 
Other Influences 
Several other aspects of materials 
may influence behavior under irradi- 
ation. Particularly important are 
anisotropy, history, impurities, and 
surface conditions. 
It has been repeatedly observed that 
materials possessing highly anisotropic 
characteristics such as metallic uranium 
and graphite will show an enhancement 
of the anisotropic properties under 
irradiation. The history of a given 
material may determine the suscepti- 
bility of a material to damage. Thus 
quenched or cold worked metals and 
alloys may show anomalous density 
and dimensional changes when irradi- 
ated. Small amounts of impurities of 
high cross section material such as 
boron and lithium may result in en- 
hanced radiation effects because of the 
(n, a) reaction. The surface condition 
may influence the chemical reactivity 
induced by the irradiation. 
Implications 
The necessity for the control of the 
structure sensitivity of solids prepara- 
tory to use in a nuclear-reactor-radi- 
ation field is just as important as in any 
other usage where structure-sensitive 
properties play an important role. 
Specifications of materials for reactor 
use require criteria that are not a part 
of conventional specifications since all 
pertinent property changes found under 
irradiation are not consistent with be- 
havior in the absence of radiation. 
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