630 MANAGEMENT OF RADIOACTIVE WASTES
a practical necessity to renew the ion exchangers after they
have developed a certain level of radiation. The net result of
these considerations is that for reasons of operator safety and
economics the presence of more than a small proportion of
ruptured fuel in a reactor will require its removal.
Fuel removed from the reactor is normally stored on site
for a considerable time to permit decay of shortlived radioac-
tivity. Storage facilities are usually deep tanks filled with water,
which acts simultaneously as coolant and radiation shield. If
defective fuel is present the water will rapidly become con-
taminated, but even if there are no defects in the cladding the
water in cooling ponds does not remain free from radioactive
material. This is because the cladding and the reactor structure
contribute neutron activation products (or corrosion products)
to the cooling water and the cladding itself always contains
minute traces of uranium, which undergoes fission in the
reactor. Hence, the pond water must be purified, usually by
resin ion exchangers, so these resins also become a waste.
If resins are regenerated, the regenerants (acids, alkalis,
or salts) will appear as a liquid waste for disposal. Otherwise,
the resin will be handled within its original container or as a
powder or slurry.
The radioactive content of gaseous effluents from reactors
depends upon the design of the reactor. If air passes through
the core very large amounts of argon-41 may be emitted
from the stack. Although^41 Ar is a hard gamma emitter it
has a short half-life (about two hours) so its effects are only
noticeable within or very near to the plant. Radioactive iso-
topes of nitrogen and oxygen decay so rapidly that they do
not reach the stack in appreciable amount and the long-lived
carbon-14 is not produced in sufficient amount to be hazard-
ous at the present scale of nuclear power generation. Some
concern has, however, been expressed that by the end of this
century the buildup of^14 C in the atmosphere might become a
significant source of radiation within the biosphere.
More concern attaches to radioactive krypton,^85 Kr, with
a half-life of 10.4 years. This, in contrast with^41 Ar and^14 C, is
a fission product. It is liberated via fuel defects and by diffu-
sion through fuel cladding. It is not a hazard from any single
plant, but with increasing numbers of nuclear power stations
it might become an ubiquitous source of low-level radiation,
though the source of most of the^85 Kr would be spent fuel
processing plants rather than power stations.
Similar concern has been expressed regarding tritium,
the radioactive isotope of hydrogen, which is produced
within the fuel and by neutron activation of the heavy hydro-
gen in ordinary water or the D 2 O coolant and moderator of
heavy-water reactors. It is also formed by neutron activation
of lithium, sometimes used as a neutralising agent in reactor
coolants, or of boron which functions as a “poison” in some
reactor control systems.
Sometimes the significance of a “source” of radioactive
waste depends on whether one is considering the safety of
people within the plant, or the public outside. For example,
ruptured fuel elements or ordinary day-to-day type mechani-
cal failures can produce air-borne radioactive iodines and
other fission products which are a nuisance to operators
because they have to work in plastic suits and respirators.The ventilation filtration system and the high dispersion
capability of the atmosphere combine to make sources of
this kind insignificant beyond the boundary of the exclusion
area. However, they may reduce efficiency and disrupt work
schedules within the station very seriously, and give rise to
significant disposals in the form of clean-up solutions, con-
taminated clothing, mopheads and metal scrap.
A noteworthy source of this nature is the tritium which
builds up in the coolant and moderator of heavy-water reactors.
In a 1000 MW (electrical) power station the equilibrium
tritium concentration in the moderator is about 50 Ci/litre.
This leads to stack discharges which are quite negligible,
but any leaks in pump seals, valves or pipe joints within the
station would produce operating problems for those respon-
sible for the radiation safety of the staff. On the other hand,
material sent for waste disposal would be no problem, partly
because heavy water is recovered for economic reasons and
partly because the maximum permissible concentrations of
tritium in air and water are much higher than those of most
other radionuclides.
In summary, in spite of the enormous potential source
of radionuclides within an operating power station the
amount of waste generated is small compared with that
arising from a research and development establishment,
and minute in comparison with a plant fuel processing
plant. This statement covers normal operation, including
the ordinary accidents and malfunctions expected in any
well-designed plant. It does not include the consequences
of the “Maximum Credible Accident” which is, in fact, so
improbable that designers of waste management systems
do not normally make provision for it.
However, the accident at the Chernobyl Nuclear Power
Station in 1986 was particularly sensational. A reactor
exploded and caught fire, releasing an estimated 30 million
Curies. Half of the resulting fallout was within 30 kilometers
of the plant. The remainder spread over much of Europe.
There was great economic loss and many cancer deaths were
attributed to the incident.Spent Fuel ProcessingWastes arising from processing of spent fuel account for
more than 99.9% of the “waste disposal problem”. Fuel
which has been enriched with^235 U must be treated for
recovery of unburned^235 U because the fission product load
of spent fuel reduces its efficiency as a source of energy. It
ceases to be economic as fuel long before the expensive^235 U
is exhausted.
After removal from the reactor, and storage for sufficient
time for decay of short-lived fission products, the fuel is
de-sheathed and dissolved, usually in strong nitric acid
(Figure 2).Uranium and plutonium are extracted into an
organic solvent, and the acid solution of fission products
left behind forms the high level or primary waste. Washing
of the organic extractant produces Medium Level wastes,
whereas Low Level waste consists of further washings,
cooling water, scrubber water and liquids from other
sources too numerous to catalogue.C013_001_r03.indd 630C013_001_r03.indd 630 11/18/2005 10:38:19 AM11/18/2005 10:38:19 AM