Engineering Chemistry S Datta

NUCLEAR CHEMISTRY 53
6. Shielding: It is an important consideration for a reactor installation. Its purpose is
to weaken the γ-rays and neutrons coming out of the reactors so that they cannot do any harm
to the persons in immediate vicinity. In high power reactors, two shields are used.
(a) Thermal Shield: It is kept very close to the reactor core, it is a thick steel or iron
covering.
(b) Biological Shield: It is a layer of concrete of few decimeter thick surrounding the
thermal shield. It is used to absorb γ-rays and the neutrons leaking through the
inner thermal shield.
Breeder Reactor
The conversion factor is defined as “The ratio of the number of secondary fuel
atoms produced to the number of primary fuel atoms consumed”. In general type of
reactors the conversion factor is 0.9. A reactor with the conversion factor above unity is known
as breeder reactor. In the breeder reactor, 235 U is fissioned by the slow moving neutrons. In
this case, per nuclear fission, 2.5 neutrons remain in the reactor and 0.5 neutrons escape out of
the reactor. Among which one neutron is used to maintain the steady chain reaction. The
residual 1.5 neutrons is used up as follows:
(i) 238 U captures 0.9 neutrons
(ii) Moderator, coolant absorbs 0.6 neutrons.
235 U produces 239 Pu by capturing those 0.9 neutrons and it is noteworthy to mention
that 239 Pu is a manmade nuclear fuel.
Uses of Radioisotopes
Radioisotopes provide a powerful tool for the solutions of enormous problem in chemistry,
industry, physics, biology and physiology. The application of isotopes depends on the fact that
the chemical properties of the isotope of a given element are essentially identical. The detection
of a radioisotope is made on the basis of its radioactivity and a stable isotope by means of a
mass spectrometer. The radioactivity or mass of an isotope can act as a ‘tag’ or ‘label’ which
permits the fate of the element or of a compound containing this element to be traced through
a series of chemical and physical changes. The element which is ‘labelled’ or ‘tagged’ is called a
‘tracer element’. A few example of the use of radioactive tracers are given below:
(a) Photosynthesis in plants: When plants come in contact with sunlight, they can
take up CO 2
and water, which are then converted into sugar and starches. At the same time,
O 2
is liberated. The O 2
produced may come from CO 2
or H 2
O molecules or from both. By
performing experiments with labelled oxygen in CO 2
or H 2
O, it was found that O 2
came only
from water.
6CO 2
* + 10H 2
O ⎯⎯→ C 6
H 12
O 6
* + 6H 2
O * + 6O 2
6CO 2
+ 12H 2
O* ⎯⎯→ C 6
H 12
O 6
+ 6H 2
O + 6O 2
*
(b) Agriculture: During growth, plant absorbs elemental P both from the soil and from
the added fertilizer. How much of the P is consumed from the fertilizer by the plant can only be
determined by tracer technique.
(c) Medicine and diagnosis: (i) Radioisotopes are used to locate the presence of tumours,
particularly brain tumours.
(ii) 59 Fe is used to diagnose the blood disorder.
(iii) 131 I is used to diagnose malfunctioning of liver, kidney, etc.
(iv) 60 Co (γ-radiations) is used in cancer therapy.