Contact Premier
to Learn how to
Prevent Fractures
in Your Windows!
|
|
The fracture of a window cover plate or
internal slab constitutes a serious and expensive maintenance problem.
Such fractures may occur in several different ways. Fracture modes
may be classified as pressure fracture, compression fracture, impact
fracture, electrostatic-discharge fracture, or a combination of
these. Any of these fractures may destroy the view through the
shielding window, and may also create other incidental problems
(such as in-cell oil leakage and glass shards requiring cell shutdown
and remote-handling operations for correction.)
Pressure Fracture
A pressure-induced fracture is caused by over pressurizing the oil
filled window tank while changing the oil, or over pressurizing the
tank during a dry window purge gas bottle change out. If oil
is pumped into a window with the bleed valve closed, the tank cover
plates may be broken within minutes. Also, if the window tank unit
is filled with oil and the valve to the reservoir is closed, thermal
expansion of the oil may fracture the cover plate. A pressure
induced fracture typically produces several triangle shaped
shards, with their apex near the center of the window.
To prevent this type of fracture, we recommend that all
oil-filled windows are refilled with oil by gravity feed only; no
pump is allowed in the system. The oil refill drum is placed on a
forklift and raised slightly higher than the
window, with the upper window vent valve open to the oil reservoir.
An argon gas purge (heavier than air) is maintained on the window
tank reservoir and
the oil drum, to preclude oxygen contamination of the oil. Adopting
the gravity-fill-and-drain method for window oil change out, will
preclude a broken cover plate.
Compression Fracture
A compression induced fracture results when a shielding slab is
forced against an object, causing a concentrated compressive force
on the glass, generally during a maintenance operation. The
resulting fracture has a clamshell-shaped fracture zone, the
boundaries of which follow the stress pattern induced in the glass.
This same clam-shell fracture mode occurs if a glass slab is broken
during resizing with an abrasive saw or water laser. Any mechanically induced or
internal stresses caused by improper annealing will cause fractures
in this clam-shell pattern as a stress-relief mechanism. If a cut
is made into a high stress zone, a clamshell fracture is virtually
certain.
Stress zone patterns in glass slabs can be made visible using
plastic polarizing sheets and a light source of moderate intensity.
The polarized light will form visible red stress zones where the
highest stresses are induced or trapped.
To prevent imposing concentrated compression forces when doing
maintenance work on internal slabs, it is recommended the window
housing assembly be returned to the manufacturer for maintenance, if ever
required.
Impact Fracture
An impact fracture is evidenced by a zone of fragmented glass,
with cracks radiating from the fragmented impact zone. Some
facilities have a nonbrowning, tempered glass shield plate in front
of the hot side slab to act as a sacrificial barrier and protect the
window slab behind it. A tempered glass plate will generally
fracture in thousands of small pieces rather than shards when
impacted; thereafter, visibility through the window is very limited
until the protective shield is replaced.
Electrostatic Discharge Fracture
A fracture caused by the sudden release of high potential
electrical energy, produced within the glass by intense gamma
radiation, is known as an electrostatic discharge fracture. This
phenomenon is generally initiated by an impact upon highly
irradiated glass, but such a discharge may also occur spontaneously.
When exposed to intense gamma radiation, the thick, gamma
attenuation glass slabs near the hot-side face store
electrical energy and become charged like a huge capacitor. This
charge may be released in milliseconds through a very high intensity
electric arc discharge having the appearance and destructive force
of a lightning bolt. An electrostatic discharge will virtually
destroy the usefulness of any window slab it occurs in, and is a
serious shock hazard to personnel at the window. Window work
stations near high energy gamma sources should have rubber floor
mats, or wooden steps with nonconductive carpet, for electrical
ground isolation.
Because a strike of the manipulator tongs or other
object against a window slab could initiate a discharge, all
master-slave manipulators should be grounded to the cell structure
with heavy copper cables which preclude electrical shock through a
manipulator. The fracture resulting from an electrostatic discharge
has a unique appearance, readily distinguished from other fracture
mechanisms. The fracture looks like a lightning strike frozen in
glass and, if the slab is internally stressed, may continue to
fracture over a period of time.
One hazard of working with a window that has had an electrostatic
discharge is that other parts of the same slab may still hold
unreleased charges. Such a slab may again discharge.
We recommend that any maintenance work on hot side window slabs
is started only after a 14-day (or longer) bleed-off period, during
which the window is shielded from intense gamma radiation.
Back to Topic List
Data and information contained in the tutorial was written by Dale A. Tobias, Premier Technology, Inc. and Hienz E. Hoffman and William G. Wash, Schott Glass Technologies, Inc. and may be reproduced only with written consent.
For information, contact Lyle Freeman
Vice President of Business Development
(208) 782-9129 lfreeman@ptius.net
)
)
)