Basic Mechanisms 
By D. S. BILLINGTON 
Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 
A NUCLEAR REACTOR can be considered, 
among other ways, as a copious source 
of high-energy radiation. This radia- 
tion can induce pronounced changes in 
reactor components. Under unfavora- 
ble conditions a reactor has the poten- 
tial for self destruction through damage 
to its structural materials, even though 
its behavior from a nuclear standpoint 
is ultra safe. 
Why “Damage’’? 
Radiation-induced changes in a solid 
may be deleterious from the standpoint 
of continued use as part of a reactor 
structure. This degradation has been 
popularly termed ‘‘radiation damage.” 
The term “‘ Wigner effect” is also often 
used, and most properly. E.P. Wigner 
was the first to point out the possibility 
of damage before it was observed ex- 
perimentally in a reactor (1). 
It should be noted, however, that 
not all radiation-induced changes in 
solids are harmful. Electron bombard- 
ment, for example, improves the tem- 
perature stability of polyethylene. 
And the increase in yield strength of 
a metal is not harmful, though the 
attendant decrease in ductility may be. 
In the main, damage is the principal 
result because materials that go into 
a reactor are normally in an optimal 
condition. Thus any change is usually 
deleterious. 
Information Needed 
The future development of nuclear 
reactors must depend strongly on the 
designers’ ability to compensate for 
potential radiation damage. Unfor- 
tunately our state of knowledge is such 
80 
that one cannot readily compile hand- 
book-type information. There are sev- 
eral reasons. 
First, the experimental difficulties 
are such that obtaining data is a slow, 
tedious, and potentially hazardous 
undertaking. 
Second, the behavior of reactor ma- 
terials in a radiation environment is 
sufficiently different from behavior in 
conventional environments to forbid 
strict interpretation in terms of the 
conventional environments. We are 
faced with a long period of difficult 
experimentation before we can ac- 
cumulate sufficient data to permit an 
engineering “‘feel”’ for the subject. We 
do not have the insight or information 
to permit us to write specifications for 
materials in terms of potential radia- 
tion damage. 
Third, the present availability of 
suitable research reactors is not suffi- 
cient to meet the demands of a compre- 
hensive program. This situation will 
remain for several years. 
Better Flux Measurements 
A primary need is for better flux 
measurements. One expects damage 
to be influenced by the rate of introduc- 
tion of defects; that is, the neutron 
flux. Most materials are obviously 
affected by the total dosage, but 
understanding in regard to flux is still 
lacking in most materials. The lack 
of suitable facilities has an important 
bearing on this point. 
A qualitative theory for the radiation 
damage process has been developed 
over the past several years, that gives 
some insight into the mechanisms and 
a feel for the scope of the field. Buta 
detailed description is not yet possible. 
Quantitative understanding is lacking 
and awaits progress in defect-solid- 
state knowledge. This undoubtedly 
will be aided by appropriate experi- 
ments in the radiation-damage field. 
Optimistic Outlook 
The above points, while pessimistic 
in tone, should not be interpreted as 
implying hopelessness; on the contrary 
the over-all situation is optimistic for 
most potential reactor designs. We 
have come a long way not only in under- 
standing but in practical results. We 
have in operation a wide variety of re- 
actors. We understand, for example, 
that neutron irradiation does not result 
in a catastrophic increase in creep rate 
at low fluxes as was earlier supposed. 
Radiation damage 7s a very complex 
subject. In its simplest terms it is a 
merger of solid-state physics with nu- 
clear physics and chemistry. Few if 
any of the complexities of each are left 
out. 
Breadth of Field 
The worker in the field must not only 
understand the object of study. He 
must be well informed in many aspects 
of reactor engineering, including shield- 
ing, production of isotopes and radio- 
activity, criticality studies, and flux 
measurements. 
Problems of shielding and radiation 
damage are related in a complementary 
manner. Those who study shielding 
are concerned with the attenuation of 
nuclear radiation in its passage through 
solid matter. The student of radiation 
