• INDUSTRY:Nuclear
  • CLIENT:Portland General Electric
  • LOCATION:Rainier, Oregon

Portland General Electric retained Northwest Demolition as a prime contractor to undertake the final portion of the overall decommissioning of the Trojan Nuclear Power Plant. Northwest Demolition’s contract called for the removal of the reactor containment building and several support structures in addition to other improvements. NWDD also created a detailed “Legacy Document” outlining the residual below-grade features that remain at the site in addition to costing for all remaining above-grade features.

The containment dome was approximately 125-feet wide by 203-feet tall. It sat 450-feet from the Independent Spent Fuel Storage Installation, which is licensed by the Nuclear Regulatory Commission and houses the spent fuel generated from operations at the Trojan nuclear site. The dome was constructed of heavily reinforced concrete with a system of 4-inch post-tensioned tendons embedded in the concrete walls. Some of these tendons were more than 500-feet long. The miles of tendons were contained within sheathing in the concrete that was filled with a petroleum-based corrosion inhibitor.

The project required dividing the concrete rubble from the thousands of gallons of corrosion inhibitor in the tendon sheathing. Northwest Demolition created a quality assurance plan to identify and separate concrete contaminated with corrosion inhibitor. The plan was highly successful; only a small quantity of concrete impacted by corrosion inhibitor required disposal. More than 22,000 tons of concrete were recycled and used as backfill onsite. While demolition was under way, NWDD continually monitored seismic activity to ensure that vibrations experienced by the surrounding structures (including nuclear fuel canisters) were within acceptable limits.

Because the corrosion inhibitor extended to the dome’s full 203-foot height, it was not feasible to implode the structure or demolish it from the top down. Northwest Demolition engineers and superintendents devised a plan to bring the dome down from the bottom up and remove the tendons and corrosion inhibitor as it sank. The plan called for the segmentation and controlled incremental collapse of the dome inches at a time, resulting in a seismically controlled collapse over a period of several months. Painstaking as the demolition process was, the project nonetheless finished on schedule.