Page 294 - Radiochemistry and nuclear chemistry

278 Radiochemistry and Nuclear Chemistry

homogenization of oil products. Other uses have been the addition of vitamins to flour, coal
powder to rubber, water and gas to oil fields, etc.

9.6.2. Liquid volumes and flows

The liquid volume of a closed system may be difficult to calculate from external container
dimensions, particularly if there is a mixing action, either by external circulation or by
internal stirring. For example, sulfuric acid volume is desired in an alkylation plant where
extensive intermixing between acid and hydrocarbon prevents a well--defined level from
forming. 137Cs is added to the sulfuric acid and from the dilution of the added tracer the
total volume of acid is calculated.
Another method is applicable to determining volumes of tanks through which there is a
known constant flow. A tracer batch is put into the incoming line. Mathematical treatment,
assuming complete immediate mixing of the incoming stream with the vessel contents,
predicts that the tracer concentration in the tank falls off exponentially with a rate
determined by throughput F and volume V according to the equation

R = R 0 e -Ft/v (9.24)

where R 0 is the counting rate at zero time (break through of radioactivity at tank exit) and
t is the time when the count rate R is obtained at the same point.
The flow rate F of rivers and streams can be measured by injection of a radionuclide and
measurement of the time for its arrival at detectors placed downstream. Because of
turbulence the radioactive "cloud" becomes quite diffuse. Therefore a more efficient
technique called "total count" is used. A known amount (A0) of the radionuclide is injected
into the fiver and a downstream detector registers a total count (Riot) as the radioactivity
passes; the faster it passes, the lower is the measured radioactivity. Thus

F = ~k A0 / Rtot (9.25)

is the counting efficiency which has to be determined under known conditions. The
technique takes into consideration both longitudinal and transversal mixing.

9.6.3. Wear and corrosion

Wear and materials transfer are easily followed if the material undergoing wear is made
radioactive. This has been used for studying wear of parts in automobile engines, cutting
tools, ball-bearings, furnace linings, paint abrasions, etc. In this case it is important that the
surface undergoing wear has a high specific radioactivity. If the material cannot be tagged
by adding (e.g. plating) a radionuclide, the material has to be activated by irradiation (e.g.
by accelerator) to produce the radioactive species.
Corrosion in gas and oil pipe-lines on the sea floor is monitored by welding patches
containing a series of long-lived radionuclides at different material depth into the pipe wall
during construction of the pipe-line. Wall corrosion will remove one isotope after the other