Characterization of Mechanical Properties of Nuclear Waste Glasses
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"**Hahn-Meitner 1.
INTRODUCTION Part of nuclear fuel cycle waste is
isolated from people.
highly dangerous,
and must be safely
Although the site of the final waste disposal must be
the main safety barrier, the form of the waste and its properties are also important considerations. Glass and glass ceramics are regarded as suitable nuclear waste solidification products; mechanical stability is material.
Such mechanical stability is
one of the important properties of the necessary on the one hand for the safe
handling and transportation of nuclear waste glass, relevance to leaching processes.
and on the other for its
Leaching by water or by aqueous solutions
must be regarded as the most dangerous process by which radioactive materials can be set free from the glass.
Because the leaching rate is
the total surface area of the materials exposed to the liquid,
proportional to it
is
of utmost
importance to keep this area as small as possible and to prevent any enlargement by cracking processes which can arise due to mechanically or thermally induced stresses.
2. TEST TECHNIQUES It
is
known that flaws or cracks in glass can increase in size at stresses 1 Further, it has been shown that instantaneous failure.
which do not result in
the velocity v of such crack extension can be measured as a single-valued function of the fracture mechanics parameter KI which characterizes the stress intensity at the crack tip.2 intensity factor,
appropriately called the stress flaw size and
has been shown that at low crack velocities the function 3 has the empirical form
specimen geometry. v(KI)
This parameter,
can be written in terms of applied load, It
v = A-KIn
(1)
where A and n are empirical constants which depend mainly on material and environment.
These constants characterize
the susceptibility of the material
to slow crack growth. Given the crack velocity function v(KI)
for a given material and environ-
ment and the stress to which the most severe flaw will be subjected, one can in 3 principle, calculate the increase in material surface area with time.
230 Another widely used fracture mechanics parameter is intensity factor" K1C,
the "critical stress
called the "fracture toughness".
It
characterizes
the
KI value at the onset of rapid crack growth and gives the upper loading limit at which spontaneous failure of the loaded specimen can be expected. present purpose the onset of rapid crack growth is
For the
assumed to lie at about
v = 1/mms. In
addition to these fracture mechanics parameters it
is
important to have a
measure of the strength which reflects the flaw size distribution at the stressed surface.
The strength or the fracture stress of a material
characterizes the average load capability of a given glass sample. Young's modulus,
E,
and the coefficient of thermal expansion,
e, are among
other important parameters required, to estimate the build up of residual stresses due to temperature gradients which can develop during intermediate storage and final disposal of glass cylind
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