60 
Impact Energy (in.-Ib) 
-100 0 
Unirradiated~- 
- 1029 nfem® 
100 200 
Test Temperature (°F) 
be 
FIG, 4. 
ing impact as a function of temperature. 
Impact energy absorbed by normalized ASTM A-212 grade-B steel in break- 
Temperature of ductile-to-brittle fracture 
(break in curve) increases with increasing irradiation (15) 
metal that has been studied extensively 
after irradiation is zirconium (12, 16). 
The metal has been studied in the an- 
nealed state and with various degrees 
of cold work. Experiments with arc- 
melted Bureau of Mines sponge zir- 
conium and crystal-bar material show 
that hardness changes are greatest in 
annealed material and are progressively 
less with increasing levels of cold work. 
The greater changes are found in 
the sponge zirconium (1/6). Zirconium 
appears to recrystallize and exhibit 
grain growth under irradiation at tem- 
peratures not exceeding 250° C (12). 
Control Materials 
Only a limited amount of informa- 
tion has been published on the effects 
of radiation on control materials. Haf- 
nium shows changes in its mechanical 
properties. Boron shows extensive 
damage. Cadmium shows none. 
Hafnium. Hafnium, which is _ re- 
ceiving increasing use for eontrol rods, 
has been found by Westinghouse in- 
vestigators to show a 9.6% increase in 
electrical resistivity after three months’ 
reactor exposure, hardness increases up 
to 12 DPH (diamond pyramid hard- 
ness) numbers, and density decreases 
up to 0.25%. No significant dimen- 
sional changes are found. 
Boron. Boron carbide, on the other 
hand, is badly damaged by radiation 
(17); 16.6% burnup of the B!° caused 
extensive cracking. B,C control rods 
appear to have been principally used 
in the USSR, but no details are avail- 
able on their construction (18). 
Preliminary data on extruded stain- 
less steel containing up to 5% boron, 
irradiated to average total atom burn- 
ups up to 1.2%, show no appreciable 
change in dimensions, although the 
specimens were highly embrittled (19). 
Cadmium. Clad plates of silver con- 
taining 25 % cadmium have been irradi- 
ated for periods up to a year. Densi- 
ties and dimensions show negligible 
changes. 
Shielding Materials 
For shielding purposes investigators 
have studied principally concrete, iron, 
and boral. The effects in iron are 
those already discussed as occurring in 
carbon steels. Conflicting reports have 
been issued concerning concrete, and 
boral is relatively unaffected. 
Concrete. The most widely used 
solid shielding material for nuclear re- 
actors has been concrete. Effects of 
irradiation on this material at the expo- 
sure levels prevailing in external bio- 
logical shields have not been significant 
(20). Some gas, mainly hydrogen, 
evolves under neutron irradiation. 
Reports differ on the effect of radiation 
on mechanical properties. Some ex- 
periments indicate a decrease up to 30% 
in crushing strength while others show 
an even larger increase. Several ex- 
periments have shown decreases up to 
20% in the thermal conductivity. 
Boral. An aluminum-boron carbide 
sheet material, boral, is uniquely suited 
for shielding where production of hard 
gamma rays must be avoided (21). 
Irradiation of this material to 0.9 X 
107° n/em< has caused no change in its 
strength (22). 
Attention in this article is limited to 
materials of greatest current interest. 
Untried reactor materials always should 
be irradiation tested. Experimenters 
conducting such tests should include 
with their specimens suitable fast-flux 
monitors so that the integrated fast- 
neutron exposure can be measured and 
reported. One of the difficulties in 
comparing and evaluating present re- 
sults is that many neutron exposures 
are reported in terms of thermal-neu- 
tron exposure, or simply by the length 
of exposure in a reactor. 
* * * 
This paper is based on work done under 
the auspices of the AEC. 
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91 
