High density shielding glass is formulated to provide attenuation of high radioactive fields while presenting a viewing medium into a cell. Certain glasses also possess non-browning characteristics even under high radiation flux conditions.
Leaded glasses have a very high density due to their lead oxide content, and are best suited as shield glass in the RS 323 G15 and RS 520 designations. The former contains 34.10 lead oxide (by weight), while the latter contains 71% lead oxide (by weight). RS 323 R15 is also cerium stabilized to prevent browning, and is always placed in front of the RS 520 unstabilized glass if the radiation field does rate to the window is greater than 5R/hr.
The color of a leaded and cerium stabilized glass is an amber color, with thicker slabs becoming darker in color than thinner slabs. The RS 520 glass, which is unstabilized, is water white in color and perfectly clear, with a considerably better light transmittance factor than RS 323 G15.
RS 360 is a leaded, unstabilized glass formulation with 45% lead oxide (by weight), with a very clear viewing characteristic. This glass is also placed behind the RS 323 G15 glass for non-browning protection.
"Browning" means a discoloration process in the glass under the influence of radiation, which decreases the light transmission. In an extreme case, the glass will actually turn chocolate brown or black and thus offer zero percent light transmittance.
The additive cerium-oxide (CeO2) into the glass composition prevents this discoloration process, respectively effecting a recovery of the glass light transmission properties. This means that a cerium-stabilized glass will regain its initial light transmittance, or a close approximation thereto, during a down-time of the cell operation. Lead glasses also possess this ability to a somewhat lesser degree.
Cover Plates and Alpha Shields
Cover plates are in almost all cases recommended to be installed in front of the lead glass. Due to its lead oxide content, any lead glass is softer than regular window glass, and is therefore prone to scratching and chipping. Cover plates on both the hot and cold side prevent such damages to a great extent. There are various materials available.
Hot Side Cover Plates
Where hot side beta and gamma radiation doses are quite low, or where replacement is relatively easy and inexpensive, lower cost RS 253 glass is quite suitable as a hot side cover. When the dose rate becomes higher, one will have to choose a glass type with non-browning characteristics, such as RS 253 G18. Thicknesses range from 3/4 to 1 inch. Schott also produces RS 253 G25, with a cerium content of approximately 2.5%. However, RS 253 G18 is the most stable glass type available with regard to browning and electrical discharge resistance.
Alpha Shields
Alpha shields are the first cell side cover plate glass, generally attached to the embedment liner face, and function as protective shields to the window housing hot side cover plate. They also provide a physical sealed barrier between the hot cell environment and the window cavity/operating corridor if the window housing has been removed for maintenance.
They are generally RS 253 G18 glass formulations, ie, unleaded but cerium stabilized to provide a non-browning characteristic. Their thickness is generally between 1 and 2 inches in thickness. The RS 253 G18 glass is very resistant to electrostatic discharge.
Cold Side Cover Plate
Commonly, commercial float glass is utilized as cold side cover plates, typically 1/2-3/4 inch thick. This glass type has the Schott designation of RS 253. This glass contains no lead or cerium. A low iron content glass is very clear, whereas a high iron content glass has the typical faint greenish tint, although otherwise clear in vision properties.
