Page 339 - Advanced Mine Ventilation

316 Advanced Mine Ventilation

handheld pump were used for this purpose, but now electrochemical cells have become
available for this purpose. When NO, NO 2 , or even CO enter a special cell; they react
with the electrolyte and create a light signal, “a photon.” A bank of photomultiplier
tubes picks up the signal and translates it into specific gas concentrations. The preci-
sion of such instruments is quite good at 1 ppm.
Earlier instruments were bulky, such as Ecolysers [2], and were difficult to carry,
but new instruments are light and can be handheld. An instrument named “Passport”
can measure five gases in a single unit as listed below.
CO (1e50 ppm)
O 2 (1%e20%)
CH 4 (1%e5%)
NO (1e25 ppm) and
NO 2 (1e5 ppm).
It is strongly advised that for conclusive results, all gas samples must be analyzed in
a laboratory by trained technicians using a GC.

19.1.5 Radon

Radon is a gaseous, chemically inert, radioactive product of the disintegration of
radium. Found primarily in uranium mines, although present in trace amounts in
several coal mines, radon diffuses from the rock strata into the mine environment,
where the decay process continues. Table 19.2 gives the disintegration process for
uranium-238 to become lead-206. Shown in this table is the type of radiation given
off by each decay process and the half-life of each element in the series. The half-
life of a radioactive substance is the time required for a given amount of that substance
to lose one half of its radioactivity. The half-life of uranium-238 is approximately 4.5
billion years; radium, 1622 years; and radon, 3.8 days.
Once radon is released into the mine environment, the decay process continues
with the formation of radium A, which decays to radium B, which produces radium
C, and so forth. The products formed by the decay of radon are referred to as radon
daughters. The radon daughter products are atoms of solid matter having relatively
short half-lives. During the decay process, either alpha or beta particles are emitted.
These emissions may also be accompanied by gamma ray activity. It is the short-
lived alpha particles and potential alpha emitters such as radon and its daughters
that are of prime concern to the ventilation engineer. Because it is a gas and has
a relatively long half-life, inhaled radon is exhaled before large amounts of alpha
particles are emitted. The daughter products, however, attach themselves to the
dust that is present in the environment and when inhaled, tend to be deposited in
the respiratory system. It has been estimated that when both radon and radon
daughters are inhaled, only about 5% of the alpha radiation received is contributed
by the radon [3].
During radioactive decay, the individual members in the series are decaying and be-
ing formed at the same time. At some point in time, equilibrium is reached, and the
quantity of each member in the series remains constant. At this time, each member