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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