DAMAGING EFFECTS 
OF RADIATION 
On Solid 
Reactor Materials 
By J. H. KITTEL 
Argonne National Laboratory, Lemont, Illinois 
RaDIaTION HAS important effects on 
the engineering properties of solid mate- 
rials that are used in the construction 
of reactors. In fuel materials only 
moderate irradiation produces exten- 
sive changes (1-3). Changes in non- 
fissionable reactor materials tend to be 
less drastic, particularly in the case of 
metals. Nevertheless reactor design- 
ers must include in their plans pro- 
visions for expected changes. 
This article covers the important 
effects that have been found to affect 
the properties of moderators, struc- 
tural materials, control materials, and 
shielding materials. 
Moderators 
Solid materials that are used as 
moderators are principally graphite, 
beryllium, and beryllium oxide. All 
of them show effects in their mechanical 
properties. In addition, graphite dis- 
plays anisotropic dimensional changes 
and an increase in volume. 
Graphite. The most widely used 
solid moderator is graphite, and changes 
produced in it by radiation have been 
studied more closely than those in any 
other nonfissionable material. It has 
been shown that the magnitude of 
radiation-induced changes depends to 
a large extent on the source of the 
graphite and the method of manufac- 
ture (4). The extent of radiation 
damage can be obtained readily by 
determination of the interplanar spac- 
ing, Co. Investigators are using the 
change in this crystal parameter of 
graphite powder to determine total 
fast flux when conducting irradiations 
on other materials. 
Radiation produces a stronger, 
harder, and more brittle graphite. 
Compression strength and cross-break- 
ing strength are approximately tripled 
after a total neutron exposure of 2.5 
10'° n/em? (4). These changes are not 
always deleterious. 
More serious is the change in thermal 
conductivity, which may decrease as 
much as fiftyfold. As shown in Fig. 1, 
this change is markedly less when 
exposures are made at elevated 
temperatures. 
Of greatest interest to reactor engi- 
neers, perhaps, are dimensional changes. 
Most graphites have been observed to 
expand along planes transverse to the 
axis of preferred orientation and to con- 
tract along planes parallel to this axis. 
In addition to this anisotropic dimen- 
sional change there is a volumetric 
increase that proceeds steadily with 
increasing exposure as shown in Fig. 2. 
As an example of how this knowledge 
affects engineering design, the MTR 
tank is surrounded by 1-in.-dia graph- 
ite pebbles free to move so that graph- 
ite growth can be accommodated with- 
out undue stresses on the reactor 
structure (4). 
When graphite is irradiated in sealed 
containers, an additional problem may 
arise because of gas evolution. How- 
ever, the principal gases, CO and H2, 
will be dissolved in certain container 
materials, such as zirconium, so that 
undesirably high pressures may be 
avoided (6). But in at least one 
zirconium-canned graphite reactor pro- 
vision is being made to vent the gases 
from the cans (7). 
Since all observations to date show 
that damage to graphite is considerably 
less at high temperatures than at low, 
one may conclude that the Sodium Re- 
actor Experiment will produce con- 
siderably less damage in its graphite 
than would a similar nonpressurized 
water-cooled reactor operating with 
comparable neutron flux (7). 
Other Moderators. Use of solid 
moderators other than graphite has 
been quite limited, but investigations 
have been made of radiation effects in 
several suitable materials. For exam- 
ple, beryllium, used as a reflector in the 
MTR, has been irradiated with fast 
neutron nvt’s up to 1.5 & 107° n/em? 
_ wv 7) 
nm, cy N 
ces} 
Ratio of Initio! to Final Thermal Conductivity 
ce) 50 
100 
Exposure Temperature (°G) 
FIG. 1. Thermal conductivity of graph- 
ite decreases with irradiation, but the 
effect is decidedly smaller at high tem- 
peratures than at low (4) 
89 
-~-~-9.43 x'1029 nem? 
---3,71 X 102° n/cm? 
150 
