To trace steel melts of critical composition, such as for reactor construction, 
radioisotopes offer several advantages over customary chemical analysis: time 
involved negligible, little plant space necessary, melt identification definite 
By DAVID L. DOUGLAS 
Chemistry and Chemical Engineering Sec- 
tion, Knolls Atomic Power Laboratory* 
Schenectady, New York 
THE SECRET OF SUCCESS in many in- 
dustrial fields lies in the use of-special 
steels or other alloys. With alloy 
steels, some particular property, such 
as corrosion resistance or high-tem- 
perature strength, can be essential to 
the successful operation of the device 
under consideration. Nowhere is this 
more true than in the field of reac- 
tor technology. 
The usual method of operation is to 
order the alloy steel as a special melt 
and then, by way of insuring that no 
substitutions are made, to set up an 
extensive inspection and analysis pro- 
gram to follow the steel from the mill 
to finished article. Clearly this is both 
expensive and uncertain, the latter 
since one cannot commit all the steel 
to analysis and still have a finished 
product. As an answer to this prob- 
lem, inclusion of a radioactive tracer 
in the original steel melt would appear 
to be ideal; not only would the labo- 
ratory space committed be negligible, 
but a 5-min check would suffice where a 
chemical analysis involves 15 man-hr. 
* Operated by the General Electric Co- 
for the Atomic Energy Commission. 
162 
It is axiomatic that the amount of 
activity added must be kept as small 
as is consistent with the required de- 
tection sensitivity. Because several 
tons of the steel are likely to be accu- 
mulated in a warehouse or in the final 
product assembly, the maximum per- 
missible specific activity, in consider- 
ation of the personnel hazard, is of the 
order of millicuries per ton. 
Discussion 
The important limits that constrain 
the selection of the nature and amount 
of the radioactive tracer employed are 
discussed next. 
Detector sensitivity. To obtain sat- 
isfactory levels of sensitivity with 
readily available detection equipment, 
the emitted radiations must be gamma 
rays of energy greater than about 0.3 
Mev or beta particles of energy greater 
than about 0.5 Mev. Thus low-energy 
beta-emitting and electron-capturing 
nuclides are eliminated. The discus- 
sion of detection sensitivity must refer 
to the specific problem in terms of the 
smallest fabricated part whose activity 
is to be measured. In this article, this 
will be taken as 1 gm. 
The efficiency of radiation detection 
is related directly to geometry and 
range of the radiation, i., to the 
amount of material that the detector 
can ‘“‘see.’’ Since destruction of the 
sample by dissolution is prohibited, 
gamma emitters are much to be pre- 
ferred over beta emitters on account of 
the limited range of beta particles in 
dense solids such as steel. Scintillation 
counters are the preferable detectors 
because of their high efficiencies for 
gamma rays. 
Radioisotope availability. The ra- 
dioisotope should be easy to obtain, 
e.g., sold by the AEC. 
Metallurgy. Chemical and physi- 
cal properties of the steel must not be 
altered by the tracer. (In reactor ap- 
plications, nuclear properties must be 
considered too.) 
One method of tagging is that the 
tracer be an isotope of one of the con- 
stituents of the alloy. Another is that 
the actual amount of material added 
be so small, 0.001% or less, that the 
properties of the alloy are not altered. 
The added material must mix thor- 
oughly in the melt and have no tend- 
ency to collect in a slag or on the walls 
of the crucible. 
Time. A lower limit of a few months 
on the half-life of the activity is set by 
the fact that it will usually be neces- 
sary to “follow” the steel for about a 
year. Shorter half-lives would mean 
inordinately high activity levels at the 
beginning of the period. The upper 
limit may be important if the purchaser 
is required by the steel mill to be re- 
