Fracture Appraisal of Large Scale Glass Blocks Under Realistic Thermal Conditions
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FRANCIS LAUDE, ETIENNE VERNAZ, and MICHEL SAINT-GAUDENS D~partment de Genie Radioactif, C.E.A. Valrho, Site de Marcoule BP 171, 30200 Bagnols sur Ceze, France.
INTRODUCTION The compositions of borosilicate glasses used to solidify fission product solutions are selected for their low leach rates. Fracturing of the glass blocks, caused primarily by thermal and residual stresses during cooling,
increases the
potential leaching surface area and the number of small particles, both of which are undesirable since long term container failure must be considered.
It
is
therefore important to know the state of fracture of the glass occurring at different stages of its
thermal history. A theoretical appraisal of the frac-
ture state of the glass is
presented,
complimented by an experimental study
using industrial scale glass blocks under realistic thermal conditions. POSSIBILITIES AND LIMITS OF A THEORETICAL APPRAISAL OF FRACTURE When a glass block cools
Stresses due to the cooling of a glass blockI. after casting,
stresses appear because the surface cools faster than the core.
The surface solidifies first
around a more dilated core, which in turn solidi-
fies later and compresses the surface. This stress can generally be avoided by annealing
(i.e. the glass is
brought to a sufficiently high temperature for a
sufficiently long period of time to remove the stresses and then is cooled).
slowly
The residual stresses in the block are directly linked to the thermal
gradients which exist at the moment of solidification. The lower the gradient ae -x the less the stress, in other words, the slower the cooling rate during solidification,
the less the stress.
These stresses can also be created by blowing cold air over the hot glass, so that the surface is
under compression
tempering).
(i.e.
In
addition to these
residual stresses there are also transitory stresses during the cooling, sign of which (tension or compression)
the local thermal gradient with time (sign of
a
The cooling history of a fission product glass is of tempering and annealing.
the
e increase or decrease of
depends
in x). intermediate between that
Independent of the technological problems posed,
total annealing of the block is
impossible,
as the heat generated by the fission
products imposes a thermal gradient between the core and the surface independent
240 of the cooling process.
It
is
however important to reduce these stresses in
order to reduce or even eliminate fracturing of the block during cooling as well as the residual stresses in the pieces which could cause ultimate fractur2
ing by stress corrosion . Thermal stresses which develop during the cooling of a glass block are extremely complex and the details difficult to analyze. The glass does not solidify at a given temperature,
but there exists a temperature range above which
the stresses are almost instantly relaxed,
and below which there is
no relaxa-
tion. The extent of this temperature range depends on the cooling rate. For example: - 5500-5000 for a temp
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