Metals, their Biological Functions and Harmful Impacts - Mining and the Environment: From Ore to Metal - page 487

fossil fuel stations (NRCC 2006). Unfortunately, nuclear waste is more hazardous, and as
yet no safe long-term nuclear waste treatment/disposal system has been implemented.
With the major isotope U-238 having a half-life equal to the age of the Earth, ura-
nium is certainly not strongly radioactive. U-235 has a half-life one-sixth of this and emits
gamma rays as well as alpha particles. Hence a lump of pure uranium would give off some
gamma rays, but less than those from a lump of granite. Its alpha radioactivity in practi-
cal terms depends on whether it is as a lump (or in rock as ore), or as a dry powder. In the
latter case the alpha radioactivity is a potential, though not major, hazard. It is also toxic
chemically, being comparable with lead in this respect. Uranium metal is commonly han-
dled with gloves as a sufi cient precaution. Uranium concentrate is handled and contained
to ensure that it is not inhaled or ingested (UIC 2006).
Uranium ores, however, also contain over a dozen radioactive materials which are all
harmful to living things. The gamma radiation detected by exploration geologists look-
ing for uranium actually comes from associated elements such as radium and bismuth,
which over geological time have resulted from the radioactive decay of uranium. The
most important decay products are thorium-230, radium-226, radon-222 (radon gas) and
the radon progeny, including polonium-210 ( Figure 11.5 ) . Having no commercial value
uranium decay products are discarded together with mine wastes when uranium is mined.
Radon gas is a decay product of radium, itself a decay product of uranium. Scientists
were bafl ed as to why this alpha-emitting gas, radon, was such a powerful cancer-causing
agent. It seemed much more damaging than other alpha emitters such as those found in
the ore dust. The mystery went unexplained for more than a decade. In the 1950s the mys-
tery was partially dispelled when it was pointed out that the radon gas, hovering in the
stagnant air of the mine, produces radioactive decay products called radon progeny (or,
formerly, 'radon daughters'). These solid radioactive by products, produced a single atom
at a time, hang in the air along with the radon gas. When radon gas is inhaled, the radon
progeny are also inhaled, resulting in a much larger dose of alpha radiation to the lungs
than would be delivered by the gas alone ( Case 11.2 ). These solid radioactive materials also
attach themselves to tiny dust particles and droplets of water vapour l oating in the air.
By itself, radon gas is exhaled as easily as it is inhaled; but when the accompanying radon
progeny are inhaled, they lodge in the lining of the lung. There they bombard the delicate
tissues with alpha particles, beta particles and gamma rays ( Figure 11.6 ) .
Three different isotopes of polonium are included among the radon progeny. They are
polonium-218, polonium-214 and polonium-210. These pernicious substances are respon-
sible for most of the biological damage attributed to radon. In particular, polonium-214
and polonium-218, when inhaled, deliver massive doses of alpha radiation to the lungs,
causing i brosis of the lungs as well as cancer. Animal studies have coni rmed that polo-
nium is extremely harmful, even in minute quantities.
Radon has a half-life of 3.8 days. This may seem short, but due to the continuous pro-
duction of radon from the decay of radium-226, which has a half-life of 1,600 years, radon
presents a long-term hazard. Further, because the parent product of radium-226, tho-
rium-230 (with a half-life of 80,000 years) is also present, there is continuous production of
radium-226 (WISE 2004). As such radon releases constitute a major hazard that continues
long after uranium mines are shut down. USEPA estimates the lifetime excess lung can-
cer risk of residents living near a bare uranium tailings pile of 80 hectares at two cases per
hundred. Since radon spreads quickly with the wind, many people receive small additional
radiation doses. Although the excess risk for the individual is small, it cannot be neglected
due to the large number of people concerned. USEPA estimates that the uranium tailings
deposits existing in the United States in 1983 would cause 500 lung cancer deaths per cen-
tury, if no countermeasures were taken (WISE 2004).

Uranium metal is commonly
handled with gloves as a
suffi cient precaution.

Radon gas is a decay product of
radium, itself a decay product of
uranium.

Radon releases constitute a
major hazard that continues long
after uranium mines are shut
down.