Shielding Penetrations (HVAC Ducts, Electrical Boxes, Pipes and Conduit)
Penetrations made by screws/nails do not require lead “tabs” to cover the top of the screws/nails as long as the specified lead shielding is less than 2mm. If the shielding is in excess of 2 mm, the penetrations require shielding. For less than 2 mm, the steel nails/screws generally attenuate radiation equally, or more effectively than the lead displaced by the screws/nails.
The joints between lead sheets should be constructed so that their surfaces are in contact with an overlap of not less than 1 cm. This can be accomplished by using lead battens, or by using drywall that is manufactured with the lead sheet extending beyond the edge of the drywall for adequate overlap.
Air conditioning ducts, electrical conduit, plumbing, and other infrastructure will penetrate shielded walls, floors, and ceilings. The shielding of the x-ray room shall be so constructed that the protection is not impaired by these openings or by service boxes, etc., embedded in barriers. The duct/conduit shielding thickness shall be equivalent to the displaced shielding thickness. Below are general shielding details for duct/pipe penetrations into a shielded surface where H is the diameter of the penetration.
1. Flat Panel Penetrations (electrical panels, electrical boxes, etc.)
Flat panel penetrations can be shielded by either wrapping the entire structure (panel/box)
being installed or by shielding behind the panel/box. If the lead shielding is installed behind the panel, the lead must extend beyond each side of the panel (penetration) by twice the thickness of the wall. For example, if the wall is 4 inches thick, the lead shielding must extend 8 inches beyond each edge of the cut-out section of the wall.
2. Straight pipe/conduit penetration
Situations where there is a straight
pipe/conduit penetrating a lead shielded surface require lead to line (wrap) the
pipe/conduit twice the distance as the diameter/thickness of the pipe or conduit.
See Detail 2 below.
3. Curved elbow penetration
Situations where there is a curved pipe/conduit penetrating lead shielding requires lead to line (wrap) the entire elbow and pipe for 3 times the distance of the diameter of the opening. See Detail 3 below.
4. 90-degree elbow running out of the wall
Situations where there is a 90 degree elbow penetrating a lead shielded surface requires lead to line (wrap) the entire elbow up to the 90 degree turn and then the entire pipe for twice the distance of the diameter/thickness of the pipe PLUS the thickness of the wall. See Detail 4 below:
The above recommendations and details are adequate for shielding most common shielding penetrations. There may be exceptions where these methods are not appropriate for providing adequate radiation protection. Please contact your radiation physicist or county health department if you have questions on how to appropriately shield a space.
Reference: The primary reference for these recommendations is NCRP Report 147.