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4.3 Management of Spent Materials 353

of a repository. Since cement materials are inorganic, they are generally considered biore-
v
we
, er sistant. Ho biodegradation constitutes a threat to any organic materials contained
within the matrix. Alkaline conditions and the presence of biocidal addities limit the v
action of microorganisms. Polymers are generally very resistant to biode and gradation,
er
,
thus they are considered nonbiodegradable. Ho gradual degradation of many poly- v we
mers may take place under the long-term effect of ultraviolet radiation.
Thermal stability is another important parameter. Concerning thermal stability, there are
fect two main considerations: the heat produced during the solidif ication process and the ef
of exposure to heat (or cold) after the solidification of the waste form. If e xothermic
processes such as the polymerization of thermosetting materials are used, amounts of heat
may be released high enough to deteriorate the mechanical properties of the waste. F or
example, at temperatures around 100°C, the volatilization of any water contained may
lead to cavities and cracks in the final waste form. After the solidif freeze–tha ication, w
cycles may affect the properties of cement, hard bitumen, and thermoset polymer w aste
forms. On the other hand, the elasticity of soft bitumen and thermoplastic polymers help
these materials to resist the variations of temperature. Cement waste forms are not flam-
mable, in contrast to bitumen and polyethylene, both of which will melt producing a liq-
v
uid that may maintain comb een after the remoal of the source of the flame.
ustion,
v
Comparing thermoplastic waste forms to thermoset waste forms in terms of thermal resist-
Thermoset polymers will char when ance, the f irst type is less resistant to thermal damage.
contacted with a burning flame but do not maintain the flame if the source of the flame is
removed. Thermal testing experiments have shown that polymethyl methacrylate matrices
and polyester are adequately stable up to about 280 °C (Ghutta, 1997).
An issue of great importance is the resistance of any waste form to the action of w , ater
namely to leaching. High resistance to leaching means that the exchanged ions or the
adsorbed species of concern are retained within the waste form when it is subjected to wet
conditions. Leaching behavior relies on two mechanisms:

(a) in the case of most polymers and bitumens, a physical barrier between the contami-
nant and the environment is de and eloped, v
(b) in cement-based waste forms, the contaminant often interacts with the matrix
chemically.