Encyclopedia of Environmental Science and Engineering, Volume I and II

MANAGEMENT OF RADIOACTIVE WASTES 629

one of the most toxic of all radionuclides and presents a

serious potential hazard. Various methods of treatment, such

as co-precipitation with barium, render most of the radium

insoluble. But the water draining from tailings ponds often

contains more radium than is permissible in drinking water.

Proper design of outfalls into suitable bodies of water can

ensure adequate dilution, but vigilance is necessary to pre-

vent rupture of the tailings ponds or improper practices that

will nullify or bypass the treatment system. A monitoring

system for analysis of downstream water and fish is common

today, but in the early days of the industry the dangers were

little understood or ignored, with the result that lakes and

streams in uranium mining areas became contaminated.

In Canada the existence of a problem was recognized

in time to avert a public hazard, but the Report of a Deputy

Minister’s Committee showed that action was necessary to

protect the environment in the Elliott Lake and Bancroft

areas. This was particularly urgent as greatly increased activity

in uranium mining was anticipated within a few years.

The size of the problem can be judged from the fact that

a Congressional Hearing was told that 12,000,000 gallons of

water containing nearly 10 g of radium was discharged daily

to the tailings ponds of American uranium mills.

Processing of Uranium Oxide

The crude (70%) U 3 O 8 produced by the mills may be con-

verted to metal, to UO 2 or to UF 6. The hexafluoride is used in

separation of^235 U from^238 U. A serious waste problem would

result from nuclear fission if a critically large amount of^235 U

were to accumulate accidentally in one place. This is a rare

event, but is not impossible. Otherwise, the wastes consist

of uranium chips and fines, contaminated clothing and res-

pirators and dust accumulated in air-cleaning systems. The

uranium at this stage is practically free from radium so it is

hardly a radioactive hazard. The toxicity of natural uranium

or^238 U is that of a toxic metal rather than of a radionuclide.

Uranium metal is produced by converting the dioxide

to tetrafluoride which is then reduced to the metal at high

temperature with magnesium. The waste form this process—

magnesium fluoride slag and uranium metal fines from trim-

ming the ingots—is a normal slag disposal problem since it

is sparingly soluble in water.

Fuel Fabrication

There are many different kinds of fuel elements, but their

manufacture produces little waste beyond dust and faulty

pellets or fuel pins. This material is usually recycled, par-

ticularly if it contains added^235 U.

Reactor Wastes

An operating reactor contains a very large inventory of fission

products. A 500 MW (thermal) reactor, after operating for

180 days, contains four hundred million curies for fission

products, measured one day after shutdown. This is equivalent

to the activity of about 400 metric tons of radium. The fission

products decay rapidly at first, leaving 80 million curies at

the end of a week, and more slowly later. After a month, the

inventory is reduced to about 8 million curies.

Nuclear power stations rated at 1000 MW (electrical)—

i.e. 3000 to 5000 MW thermal—are not unusual. At first

sight it would seem that these plants would be enormous

potential sources of radioactive wastes, but in practice this

is not the case (Figure 1). In an operating power reactor the

fuel is contained within a non-corrodible cladding—usually

zirconium or stainless steel—and the fission products cannot

get out unless the cladding is ruptured.

It is possible to operate the reactor with defects in a few

fuel elements, but these sources of leakage make the primary

cooling circuit radioactive. It is impracticable to operate a

station in the presence of high radiation fields, so the primary

coolant is continually purified by ion exchangers. Again, it is

SECONDARY

CIRCUIT

TURBINE CONDENSER

WATER

COLD

WATER PRIMARY

CIRCUIT

REACTOR CORE

HOT

WATER

STEAM

HEAT EXCHANGER

FIGURE 1 Schematic diagram of processes in nuclear power station. Nearly all radioactivity remains inside

the fuel, which is inside the core, which is inside the primary circuit.

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