Radioisotope Shielding Windows are utilized when a direct view into a hot materials handling cell is required. The observer is able to monitor a large area of a cell with one window, and this is very advantageous for the operation of remote handling equipment. In addition to providing a superb real time, three dimensional view, a shielding window has no sudden breakdowns, no wear due to moving parts, and requires relatively little maintenance.
Shielding windows manufactured by Premier Technology, utilizing Schott shielding glass, offer a clear, distortion free view into the area to be observed. Therefore, an operator can perform his in-cell tasks comfortably with minimal eye strain. If a cell has adequate lighting, an operator can easily read a newspaper through a forty-eight inch thick shielding window. This capability is available because of Premier's engineering and window design expertise, and the use of the finest optical quality glass.
The design of a shielding window is determined by many factors. The primary consideration is adequate shielding. These requirements are based on the wall material, its thickness, type and strength of the isotopes and the desired attenuation on the cold side.
Of equal importance are the viewing requirements, pertaining to the items in-cell the operator needs to view and his observation point of view. The viewing criteria is defined as normal view, where the operator is looking directly into the window, and extreme views, where the operator looks into the edge of the window and can thus greatly expand the view angle in-cell.
There are many other important criteria that have to be considered for the design and construction of a shielding window. These criteria have been summarized on a design questionnaire in the appendix. With the help of the data from this completed form, your shielding window will be designed to fit your specific needs, all the lowest customer expense. Since each window application is typically different from prior requirements, there is little demand for standard, off-the-shelf type designs. We do recommend, that, if possible; the windows be as similar in construction and size as possible; this consideration can considerably minimize the design and fabrication costs of a new window. However, if your design requirements are unique, Premier Technology, Inc. will provide the design and fabrication for those design requirements. There are two basic design concepts in the construction of a shielding window; Dry and Oil Filled.
Dry Windows
The spaces and gaps between the individual slabs/plates in the window assembly are filled with a purged inert gas, generally either nitrogen or argon. The purge gas precludes the admittance of moisture, dust, and oxygen in the window housing, and thus maintains a "like new" condition of the glass surfaces as long as the purge is maintained. As a guideline, we generally propose a dry window design for wall types and thicknesses equivalent to four (4) inches if lead, or where the design requires only one major shielding slab of glass. Cover plates are also installed to protect the relatively soft surfaces of the high density shielding slab. AR (anti-reflective) coatings are generally applied to the glass surfaces, to minimize the light transmittance loss due to the polished, reflective surfaces.
If thick, dry windows are required, similar glass types are sometimes laminated together to minimize reflection losses at the glass interfaces. However, if the window assembly is ever radiologically overstressed, the glass interface surfaces may delaminate and create vision obstruction. The only maintenance remedy to repair a delaminated surface is to remove the glass blocks from the window housing, mechanically saw the slabs apart at the lamination surface, grind and repolish the surface and then relaminate the slabs together. This is a very expensive, labor and time intensive process.
Premier's dry window design is superior to the laminated block concept, as each individual slab has an AR coating or process applied that minimizes surface reflection without the possibility of a delamination occurrence if the window is radiologically stressed during cell operation.
Oil Filled Windows
The spaces and gaps between the individual slabs/plates in the window assembly are filled with highly refined white (clear) mineral oil. This design has many performance advantages, which are listed for review and consideration.
a) Total reflection loss is greatly improved with oil as compared to the reflection loss performance of AR coated dry windows. This occurs because there is a minimal loss of light transmissibility at the glass/oil interfaces.
b) Viewing angles within a given liner size are increased considerably by the substitution of oil for air or inert gas. This is caused by the high index of refraction of oil.
c) Oil provides an excellent light transmissibility factor through the medium.
d) Oil provides thermal stability to the window housing (tank) components, such as the large shielding slabs.
e) Oil provides a high hydrogen content medium for neutron absorption. This is especially important with higher energy neutrons, where the low atomic number elements must be used to moderate the neutrons. The oil also provides a minimal amount of gamma shielding. Neutron shielding in a dry window must be provided by an additional slab/plate of polycarbonate, in addition to the gamma attenuation glass components.
Thus, a properly designed and fabricated dry window involves basically the same expense as a properly designed and fabricated oil-filled window. However, for an identical size window, the cost to place AR surfaces on the dry window glass slabs will increase the dry window cost, and the viewing angles will be less than the oil filled window of the identical size.
Premier builds superior quality shielding windows that meet or exceed the customers design requirements, whether dry or oil-filled. Generally, we believe that an oil-filled construction is preferable for most medium to large scale windows. However, Premier has designed and built large dry windows for up to 48 inch thick concrete cell walls; the governing factor being the customer's design requirements. Either window type, oil filled or dry, has a maintenance expense associated with it, but differing in maintenance requirements.
