SNC-Lavalin Nuclear Bruce A Steam Generator Replacement

The replacement of steam generators in Units 1 and 2 at the Bruce Power Nuclear plant near Kincardine, Ontario made history as the first steam generator replacement ever at a CANDU plant.

The Bruce A plant’s unique design includes an arrangement of bulbless steam generators connected to a horizontal, integrated steam drum. The steam generators were being replaced because of corroded internal tubing. Since all the internal tubing is contained in the lower steam generator sections, the steam drums were not affected and could therefore be re-used.

Replacing the 16 steam generators required careful engineering; the most critical aspect was aligning the replacement steam generators with the steam drums and the primary heat transport piping. This work was completed using advanced laser alignment and scanning technology.

Several methods were considered for removing and replacing the steam generators. An approach to replace them horizontally through openings in the 2-metre thick boiler room walls or through the reactor building’s north wall was discarded as too risky and too complex.

The method chosen was to perform the operation vertically through the reactor building roof. This approach required sourcing one of the largest cranes in the world to reach up and over the reactor building to lift the steam generators and the mammoth steam drums.

SNC-Lavalin Nuclear used high-precision laser scanning equipment to obtain 3D images of the equipment and plant areas, which allowed them to create a representative 3D model of the work areas. The model was used by the engineering team to supplement out-of-date station drawings and then to develop the method for replacing the generators. Spatial analysis techniques were used to measure, certify, adjust, align, re-position and machine the equipment to very tight tolerances.

Method of Removal

The steam generators at Bruce A are arranged vertically in banks of four. There are eight steam generators in each Unit and each weighs 100 tonnes. Each bank of steam generators is supported by a trapeze framework, which is suspended from vertical hangers embedded in concrete.

Each bank of generators is attached to an overhead, horizontal 100-ft.-long steam drum. At 250 tonnes, the mas-sive stream drums are the size of a small submarine.

To permit the steam generators to be replaced, the steam drums had to be cut free and temporarily relocated. Instead of removing them from the reactor building, which would have been a huge undertaking, SNC-Lavalin Nuclear designed several large temporary steel structures to be placed on the floor of the reactor building next to the steam drum enclosures. The steam drums were lifted out of their enclosures and carefully set on the temporary structures high off the floor and out of the way.

Before the steam drums were relocated the roof of their enclosure had to be removed. The roof consisted of