The Most Dangerous, Hazardous Dismantlement Job is Done. Safely.
The containment domes that housed the nuclear reactors at the San Onofre Nuclear Generating Station (SONGS) are robust structures, made of four-foot thick reinforced concrete with a quarter-inch steel plate lining the inside. But what helped give the domes their maximum strength was the matrix of embedded vertical and horizontal steel cables, known as tendons, that wrapped around each shell. When fully tensioned, the tendons were under 1.5 million lbs. of force to contain potential pressure increases in an accident scenario.
To create openings in the domes to install equipment for the cut-up and removal of the reactors and other waste prior to demolition, all of the tendons needed to be detensioned and the vertical ones removed.
What made the job so dangerous? Detensioning and removing the tendons, which are thickly coated with flammable grease, requires cutting with a torch. And much of the work is done in tight quarters below ground.
“You’ve got hot work (torch cutting). You’ve got grease everywhere, so there are potential slipping hazards all the time. The grease is really tough to manage and deal with. You’ve got confined space,” recounted Vince Bilovsky, SCE decommissioning director. “The team managed all those hazards, worked safely, and in the schedule. They just did a near flawless job.”
(Read more about fire safety during decommissioning here)
The containment domes can’t be demolished until all tendons are detensioned, and the vertical tendons removed. Accomplishing the job safely called for careful planning and execution.
Initially the work focused on the tendons surrounding the areas where equipment hatch openings would be created for each dome. These large entrances allow for equipment to be brought in for future work, and for dismantlement waste to be taken out.
The vertical tendons would have to be completely removed. Otherwise, because of gravity, they could slide uncontrolled out of their sheaths.
Similar work took place about a decade ago when SONGS replaced its steam generators. Schwager Davis, a consulting company on that project, became the lead sub-contractor for this tendon project.
“The biggest part of planning was figuring out how to do it safely. We had some lessons-learned from the (replacement steam generator) project, so that helped us this time around,” said Mike Hayes, Schwager Davis project supervisor, now with SONGS DecommissioningSolutions (SDS), SCE's decommissioning contractor.
Work began in August 2020 to detension 114 horizontal tendons and completely remove 90 vertical tendons in each dome.
Before the vertical tendons could be removed, they had to be drained of as much grease as possible. Each sheath contains about 250 gallons of grease. The “cans” at the end of the tendons were removed and workers captured the grease in 55-gallon drums for eventual disposal.
“It’s one of those jobs, because it is so dirty, and it is nasty and can be very dangerous, you hope you’re going to keep that crew intact and that they’re not going to get sick of it and say, ‘you know I’m tired of pulling grease out of my ears when I go home to take a shower,’” Hayes explained. “So, I give kudos to the union craft and (SONGS DecommissioningSolutions) craft support, from operating engineers to laborers.”
This work takes place in an area known as the Tendon Gallery, a confined space below ground. The gallery is about 11-feet wide with 11-foot ceilings.
“It’s kind of dark and greasy, and crowded. It fills up quickly with personnel. There’s grease everywhere. It’s pretty slippery,” Hayes said.
Fires are a potential hazard and lessons-learned from past, similar projects were employed. Examples include the use of sand-filled barrels to collect the grease prior to hot work, wet fire blankets during cut-up, and pausing for cool down to control any flame that pops up. A designated Fire Watch was present for each activity.
Rescue plans were put in place and new ventilation systems installed to keep the air clear from smoke created during the torch cutting.
“It was incredibly safe considering how potentially dangerous it could be,” Hayes said.
The vertical tendons that were removed were coiled and bagged for disposal. The drums of grease were sealed and any grease that attached itself to the exterior drum surface was removed prior to shipping, per the regulations. If not, a shipment could be returned to site, but that didn’t happen.
The project, at peak operation, involved 17 workers and wrapped-up last month. The nearly year-long effort recorded only one minor first aid incident.
“It’s dangerous, it’s dirty. The crew that they had, there were days they made me look like a superstar because all I had to do was report out it was another safe shift and here’s all the production they got,” said Phil Perry, SDS supervisor.
“You could tell by their pre-job briefs, looking at their work plans, and the mastery of the people in the field, it was all really impressive,” Bilovsky said.
With this project done, Bilovsky says the overall industrial safety risk for the decommissioning of SONGS is greatly reduced, but the dedicated focus on safety remains.
(Posted Aug. 25, 2021)
By the Numbers
Each tendon is made of 55 half-inch diameter cable strands and each strand is made from seven high strength wires
Approximately 5,700 miles of tendon strands in each containment dome
Each sleeve containing a tendon holds approximately 250 gallons of grease
450 feet – length of a typical vertical tendon
340 feet – length of a typical horizontal tendon
What the tendons did
The tendons were part of the prestressed concrete shell consisting of a 150-foot interior diameter prestressed, reinforced concrete cylindrical structure with a hemispherical dome used to completely enclose the reactor and reactor coolant system. The prestressed concrete shell primarily served to withstand peak pressure and thermal loads resulting from a hypothetical failure of the reactor coolant system or main steam line break. The tendons were tensioned by pulling the tendon ends through the anchorages which resulted in compression of the concrete shell. This concrete prestress was necessary to resist loads from the peak internal pressure generated in containment during a hypothetical accident.