634 MANAGEMENT OF RADIOACTIVE WASTES
For this reason, limitations must be made in
Curies per unit time, not in micro-curies per mil-
lilitre, and account must be taken of volume of river
flow if this is seasonally variable. Regulations set on
the basis of concentration at the point of discharge
only protect people close to the discharge point.DISPOSAL PRACTICESGasesRadioactive gases arise mainly in reactors, spent fuel pro-
cessing, isotope production, and research and development
facilities. The general principles are the same for all procedures
that depend upon dispersion into the atmosphere.
If we have a stack that is emitting Q Curies/sec., the con-
centration C at a given distance downwind will be KQ. The
parameter K is a very complex function which depends upon
wind speed and direction, weather conditions, stack height,
topographical features, variability of temperature with height,
velocity and buoyancy of the effluent and other conditions.
Values of K for a range of conditions can be calculated from
equations proposed by Sutton (1947), Pasquill (1961) and
Holland (1953). These equations have been used to calculate
the permissible emissions from stacks by inserting appropri-
ate numbers and parameters applying to unfavourable weather
conditions likely to obtain at the site. The permissible emission
rate has been set at a value which would ensure that popula-
tions downwind would not be exposed to more than an agreed
maximum radiation dose rate.
The classical equations have been based on statistical
theory with empirical values for the diffusion parameters
being obtained from experimental work which has some-
times had little relation to real emissions from actual stacks.
Returning to the superficially simple equation C KQ,
it is apparent that if we could observe, over a long period
of time, the maximum value of C ever attained per unit
emission rate, we could define a figure K max which was not
likely to be exceeded. With a sufficient number of obser-
vations of C and Q, extended over a sufficient variety of
weather conditions, we could estimate the probability that
our value K max could ever be exceeded.
When a maximum permissible concentration is set for a
noxious substance the decision really depends upon a belief
that the probability of damage is so low that it is acceptable.
If, then, C is set at the MPC at a given distance from the
stack, and K max is known for that distance, then Q p , the max-
imum permissible release rate, is determined.
It has been shown by Barry that K max is not very depen-
dent upon topography or climate, because it depends mainly
on rather large-scale behaviour of the atmosphere, and the
frequency of most adverse conditions normally experienced
do not vary grossly from one place to another.
The maximum permissible emission rate—or in some
cases the MPC at the stack mouth—is given in the regula-
tions governing the plant or laboratory. It is then the respon-
sibility of the operator to ensure that emissions are kept as far
below the permissible level as may be practicable. Numerousmethods are available, other than variation of stack height,
for achieving this end (Figure 4).
Filtration It is advisable to filter contaminated air near
to the source of the activity. This reduces the amount of air to
be filtered and also cuts down the “plating-out” of radionu-
clides on the duct-work, which can be a source of radiation
fields with the plant.
Filters must be suitable for the job they are supposed to
do. They should be made of non-flammable material such
as glass or other fibre and should be tested before and after
installation. If fine (e.g. “Absolute”) filters are used it is
often necessary to precede them with a coarse filter to avoid
rapid clogging with dust.
Filters must be very efficient to be adequate for fuel
processing plants and incinerators burning highly active
waste. For example, a sand filter at Hanford capable of pass-
ing 10,000 m^3 /min had an efficiency of more than 99.5%, but
this was inadequate. The necessary efficiency of 99.99% was
attained with a bed of glass fibers 100 cm thick.
Electrostatic Precipitators Small airborne particles are
usually electrically charged. The charge can be increased
by passing the air through a corona discharge, or through
a charged fabric screen. The particles are attracted to a sur-
face carrying the opposite charge, from which they can be
removed mechanically. It is possible to use the same prin-
ciple by imposing a charge on filters.
Steam Ejector Nozzles The most efficient air clean-
ing device other than “Absolute” filters consists of a nozzle
in which the air is mixed with steam and expelled into an
expansion chamber where the steam condenses on the par-
ticles. After passing through a second construction into
another expansion chamber, where the air is scrubbed with
water jets, removal efficiency for 0.3 micron particles is
99.9%.
Incinerator Off-gases The hot gas from an incinerator
carriers with it fly ash, tars and water vapour as well as particles.
Tars may be removed and the gases cooled by water scrubbing
devices. Water droplets must then be eliminated by reheating or
passage through a “cyclone”. This is a cylinder with a conical
bottom. Gas injected tangentially at the top sets up a vortex
which causes deposition of particles on the sides.
In smaller incinerators the gases are cooled and some fly
ash is removed by passage through a cooling chamber fitted
with baffles. After this stage a roughing or “bag” filter is
used, followed if necessary by Absolute or charcoal filters.
Processing Plant Gases The devices required for clean-
ing gaseous effluent depend on the nature of the process.
Off-gas from boiling high level wastes must be passed
through condensers and scrubbers to recover nitric acid as
well as to remove volatile radionuclides. However, these
and other air cleaning equipment previously mentioned
will not remove gases such as^85 Kr, nor hold back all of the
radioactive halogens.
Radioactive iodine in molecular form is fairly easily
absorbed by alkaline scrubbers and copper or silver mesh
filters, but in the form of methyl iodine it can only be arrested
by an activated charcoal filter. These filters have to be kept
cool, not only to remove the decay-heat of adsorbed halogensC013_001_r03.indd 634C013_001_r03.indd 634 11/18/2005 10:38:20 AM11/18/2005 10:38:20 AM