materials emit penetrating, ionizing radiation that can injure living
tissues. The commonly used unit of radiation dose equivalent in humans is the
sievert. (In the United States, rems are still used as a
measure of dose equivalent. One rem equals 0.01 sievert.) Each individual in
the United States and Canada is exposed to about 0.003 sievert per year from
natural background radiation sources. An exposure to an individual of five
sieverts is likely to be fatal. A large population exposed to low levels of
radiation will experience about one additional cancer for each 10 sieverts
total dose equivalent. See Radiation Effects, Biological.
hazards can arise in most steps of the nuclear fuel cycle. Radioactive radon
gas is a colorless gas produced from the decay of uranium. As a result, radon
is a common air pollutant in underground uranium mines. The mining and
ore-milling operations leave large amounts of waste material on the ground
that still contain small concentrations of uranium. To prevent the release of
radioactive radon gas into the air from this uranium waste, these wastes must
be stored in waterproof basins and covered with a thick layer of soil.
enrichment and fuel fabrication plants contain large quantities of
three-percent uranium-235, in the form of corrosive gas, uranium
hexafluoride, UF6. The radiological hazard, however, is low, and
the usual care taken with a valuable material posing a typical chemical
hazard suffices to ensure safety.
| B2 Reactor
Safety Systems The safety of the power reactor itself
has received the greatest attention. In an operating reactor, the fuel
elements contain by far the largest fraction of the total radioactive
inventory. A number of barriers prevent fission products from leaking into
the air during normal operation. The
fuel is clad in corrosion-resistant tubing. The heavy steel walls of the
primary coolant system of the PWR form a second barrier. The water coolant
itself absorbs some of the biologically important radioactive isotopes such
as iodine. The steel and concrete building is a third barrier.