Oil filled windows are far superior to gas filled (dry) windows in light transmissibility because
the oil essentially eliminates reflection losses at the glass / oil interface surfaces,
thus enhancing light transmission. The coefficient of light transmittance at an oil / 2.53
density glass interface is 0.99972 (0.028% loss), whereas the coefficient for a gas / 2.53
density glass interface with low reflectance treatment is 0.9950 (0.500% loss).
This light transmittance factor is a major consideration in the design of multi-slab
shielding windows.
Premier uses an optically clear mineral oil specifically developed for use in radiation
shielding windows as an interlayer medium. This oil, Sontex LT-100 ®, has been tested to
be superior in terms of light transmission, cost and replaceability. It is a refined,
highly purified organic aliphatic hydrocarbon, whose general chemical formula is CnH2n+2.
In the normal refining process, it is distilled between gasoline and paraffin and has a
density of 0.86 gm/cc.
Mineral oil is used in the window housings primarily for the following reasons:
- It provides a high index of refraction (1.47), which increases the view angles.
- It eliminates most of the mirror reflection between the polished internal glass slab surfaces
and simulates the view of a solid glass block.
- It provides a minimal loss of light transmissibility through the oil medium.
- It provides thermal stability to all the components within the window housing.
- It provides high hydrogen content for neutron absorption.
- It improves compatibility between the diverse components in a window tank; i.e., leaded glass,
steel, lead packing and paint.
- It provides radiation shielding in proportion to its relative density, allowing some very
expensive leaded shielding glass to be replaced with inexpensive oil.
- It acts as an insulator to help prevent an electrostatic discharge.
- As a replacement for inert purge gas, it eliminates the large capital cost for a facility gas
system and substantially reduces on-going operating and maintenance costs.
Mineral oil can have some drawbacks if its quality is not maintained properly:
- Although it is radiation resistant, it is
organic and does slowly degrade at a rate dependant on the radiation field exposure. Under high gamma
radiation exposure, observable changes occur in the oil at approximately 106 R (cloudiness), which
produces a minor change in the light transmissibility of the oil. An exposure of 107 R - 108 R
initiates further observable degradation in the mineral oil. It turns brown, has a high acid content,
smells burnt and is actually undergoing further refining to form paraffin (wax) and polyethylene
through a process of chain cross linking (polymerization). Both of these materials plate out on the
glass slab surfaces and impair vision through the window. However, under the proper maintenance
conditions designated by the window manufacturer, the mineral oil should remain useable for many years.
- If water is allowed to be entrained in the oil, it may form hydrogen peroxide
(H2O2) and hydrogen gas (H2) under exposure to radiation. Bubbles
of H2 gas also form from the catalyzed breakdown of the oil. Sontex LT-100 ® oil has a maximum dose
limit of 108R, after which rapid deterioration occurs. It is somewhat hydroscopic; any major amount
of water (> 5ppm) in the oil exacerbates the formation of acid which literally stains/deteriorates
the glass surfaces.
Therefore, it is very important that the oil be maintained free of entrained water
and be changed prior to the onset of oil cloudiness at approximately 106 R integrated dose.
Five major periodic (2 - 3 year intervals) preventative maintenance tests are recommended to
be performed on the oil windows:
- IFT - Interfacial tension against water per ASTM D-971 (water in the oil)
- NEU - Neutralization per ASTM D-974
(air and moisture suspended in oil)
- H2O2 - Hydrogen Peroxide per ASTM D-1563
(breakdown of oil through exposure to radiation).
- Lead and hydrogen content analysis in the oil will
need to be requested and analyzed by a laboratory.
- Laboratory chemical test results will
mandate a window oil change if any of the following conditions occur:
| > |
5 ppm H2O |
46.7 DYNES (IFT) |
| > |
.008 mg/g |
KOH (neutralization or acid number) |
| > |
30 ppm H2O2 |
PEROXIDE |
| > |
5 ppm Pb |
LEAD |
| > |
4% H2 |
OIL RESERVOIR GAS SAMPLE |
Oil windows are actually a low maintenance facility equipment item if the proper cover gas is
maintained on the oil reservoir and a dry gas purge is maintained in the housing / liner interface
annulus. Oil may be substituted for purge gas in the annulus, which eliminates the need for a facility
purge gas system. Preventative maintenance tests determine if the oil needs to be replaced; in Premier
windows it can be removed quickly and easily from the cold side while the windows are in service,
thereby eliminating cell downtime, decontamination operations and window removal or disassembly.
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
