SONGS Decommissioning – San Onofre Nuclear Generating Station
San Onofre Unit 1 was one of the first nuclear facilities commissioned in the USA. It went into service in 1968 and by 1992 the safety, operational, and seismic mandated upgrades which evolved with subsequent reactor designs had reached a level of sophistication which rendered Unit 1 uneconomical to operate.
The decommissioning plan prescribed burial of the reactor in a specially designed storage canister. In order to fit into its container, all 6 nozzles had to be severed from the vessel within a strict tolerance of less than 1/8”. The nozzles were each 42” in diameter with 12” thick carbon walls clad with stainless steel.
The project posed several logistical and financial challenges. The cost of decontamination and disposal dictated minimizing waste. Diamond wire must be cooled as it cuts, but this site configuration precluded recapturing the water, therefore an alternate method of cooling the wire had to be engineered. Finally, the work site was so highly radioactive our technicians could not get close enough to the nozzle to operate and maintain the saws by customary methods.
At the time water was the standard cooling agent used on wire saws to prevent overheated beads from disintegrating. Using an innovative method customized for this application by Bluegrass, CO2 was used instead of water. The CO2 evaporated, leaving only fine metal shavings to discard. * To solve the problem of proximity to the nozzles, our engineers designed and fabricated specialized tools for operating and maintaining the saws and other equipment remotely, and our technicians innovated procedures and techniques for accomplishing the cuts under these restrictive conditions.
The cutting time for each nozzle averaged 22 hours. Each cut successfully produced a cut surface within the strict 1/8” tolerance allowed. “Tabletop smooth and straight as an arrow” was the description, and a noteworthy achievement lauded by industry experts.
*Diamond wire cutting technology has since evolved and wire quality has improved, allowing for air-cooled dry cutting.