Page 178 - Radiochemistry and nuclear chemistry

162 Radiochemistry and Nuclear Chemistry

The exposure At (GBq hours) required for an optical density (/) = log (incident
light/transmitted light)) ~ 2 at an absorber (object) thickness x (em) using a typical
industrial X-ray film and a 6~ "y-ray source positioned at a distance of 1 m from the film
can be estimated by the expressions

log(At) = 1.068 + 0.135x for an iron absorber
log(At) = 1.068 + 0.040x for a concrete absorber

Exercise 6.14 is an example of the use of these expressions.
Gamma-radiography has been used for determining the number of reinforced iron bars in
concrete construction, cavities in various kinds of castings (as explosives, plastics or
metals), cracks or other defects in turbine blades in airplane parts, detonators in unexploded
bombs, welded joints in pressure vessels, distillation towers and pipes, corrosion inside
pipes and furnaces, and medical field X-rays, to mention only a few applications.
Gamma-radiography is used throughout the world for product control leading to improved
working safety and economy.
Because v-absorption occurs through interaction with the electrons, objects of high atomic
numbers show the strongest absorption. By using neutrons instead of "t-rays, the opposite
effect is achieved, i.e. low Z objects are most effective in removing neutrons from a beam.
This is used in neutron radiography in which both reactor neutrons and neutrons from 252Cf
sources are applied. Because of the higher neutron flux from the reactor than from 252Cf
sources of normal size (i.e. _< 1 mg) the exposure time at the reactor is much shorter. On
the other hand, the small size of the 252Cf source offers other conveniences.

6.9.3. Radionuclide power generators

The absorption of radiation leads to an increase in the temperature of the absorber. An
example of this is the absorption of the kinetic energy of fission products in nuclear reactor
fuel elements which is a main source of the heat production in reactors. The absorption of
decay energy of radioactive nuclides in appropriate absorbing material can be used in a
similar - albeit more modest - way as an energy source.
Figure 6.29 shows the principles of two different radioisotope power generators, the
larger (to the left) is of SNAP-7 type and produces - 60 W, the smaller one produces -~
10 roW. The radiation source for the larger generator consists of 15 rods (A) clad with
hastell0~ and containing approximately 7 kg of SrTiO 3 which has approximately 225 000
Ci of 9USr. This heat source is surrounded by 120 pairs of lead telluride thermoelements
(B) and a radiation shield of 8 cm of depleted uranium (C). The whole arrangement is
surrounded by a steel cover with cooling fins. The weight of this generator is 2.3 t with
dimensions of 0.85 m in length and 0.55 m in diameter. It is estimated that the lifetime of
such an energy source is at least 5 y, although the half-life of 9~ (30 y) promises a longer
period. Radionuclide generators in unmanned lighthouses, navigation buoys, automatic
weather stations, etc., in sizes up to about 100 W, have been in use in a number of
countries, e.g. Japan, Sweden, the UK, the USA, etc. Since no moving parts are involved,
these generators need a minimum of service. Their reliability makes them valuable in
remote areas like the Arctic regions where several such generators have been installed.