During the course of 42 years of irradiated operations in the Hot Fuel Examination Facility at the Idaho National Laboratory (INL), a hot cell window had never been replaced. Recently, slow deterioration of a window seal resulted in mineral oil leaking at a rate of over a liter per month from the window tank unit on through the protective A-slab seal and into the hot cell. A hot cell window consists of both a steel tank unit with five slabs of glass of varying thicknesses with the remaining free space filled with clear mineral oil, and a thinner protective interior A-slab of glass. The repair solution was to remove and replace the A-slab window followed by replacing the window tank unit in two distinct phases.

The facility original A-slab design was a leak tight barrier and a frame that was “L” shaped with a gasket between the glass and the window flange. Problems with the gasket adhering to the glass and the window flange resulted in pulling the glass from the frame during initial installation activities. Due to the adhesion problem, the gasket was changed to a dust seal during commissioning the facility. Over time, the window tank unit mineral oil leak flowed through this dust seal. Replacing the leaking tank unit necessitated the need for a new leak tight boundary as well as provide a method to drain the accumulated oil behind the A-slab until the tank unit could be replaced. These criteria led to a new A-slab design to be installed.

Initially, removal and replacement of the A-slab was performed in the main cell (hot side) to reestablish a leak tight barrier. Transfers of the windows and removal of the bolts/reinstallation of new bolts were all performed with specialized equipment designed for remote operations in a hazardous environment using remote manipulators and cranes. Removal and replacement of the window tank unit via the operating corridor (cold side) was scheduled during a facility outage to accommodate availability of contract service personnel who specialize in hot cell windows.

Due to the complexity of the replacement task, approximately 30% of the personnel on site were involved in the window replacement. Engineering, facility operations and radiation control personnel were primary contributors with electricians, carpenters and the analytical laboratory personnel contributing, as well. The multi-year installation program was safely concluded allowing the facility to resume full operations with the window properly sealed.

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