SHN tests 2,300 concrete samples for nuclear plant decommissioning

SHN collected and continues to test 2,300 concrete samples as part of the decommissioning of Unit 3 at PG&E's Humboldt Bay Power Plant. The decommissioning requires a way to access the non-operating nuclear reactor caisson for decontamination, demolition, and removal. However, any deep hole along the bay will quickly fill with groundwater unless an underground wall can be constructed that keeps the water out. PG&E contractors (including CB&I, DrillTech Drilling and Shoring, SHN, and many others) recently completed this circular underground wall.

The 173-foot-deep circular wall was constructed by cast-in-place panels, via cutter soil mix (CSM) technology, which aptly describes the process. Cutter wheels fixed to a boom penetrate the ground, injecting bentonite slurry so the column stands and does not cave in. Once the cutter wheels are at the right depth, the drilling operator switches to injecting a cement slurry, which mixes with the existing soil, creating the "concrete" (technically, the soil-cement) that becomes the cast-in-place panel of the wall.

SHN materials technicians collected 2,300 concrete samples, starting with the first panel that was installed on July 9, 2015. Because concrete hardens as it cures over time, we have been breaking cylinder samples since then, and will continue until the last one is broken on January 26, 2017. Recognition is due to Anson Call, Joe Aufdermaur, Dave Gonzales, and Leif Ayres for their exceptional attention to detail combined with hauling all that concrete!

The integrity of the CSM wall is critical, acting as a shoring wall and groundwater cutoff. Meticulous concrete sample collection, storage, testing, and documentation is an important but unglamorous job that is critical in the safe removal of the reactor caisson.

 

CSM Wall Complete Resized

 

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