PG&E Humboldt Bay Power Plant Unit 3 Nuclear Decontamination and Decommissioning
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Project Objective

The Pacific Gas and Electric (PG&E) Humboldt Bay Unit 3 nuclear reactor went online in 1963, but was only operational until 1983, when functional, safety, and seismic concerns led to a shutdown. Decommissioning of Unit 3 was approved in 2009, and began in 2012. Unit 3 decontamination and decommissioning (D&D) is being performed by CB&I under an EPC contract with PG&E. Work required for removing the Unit 3 caisson, whose bottom is founded at Elev. -80.5, 93 ft below existing ground surface, included a deep hydraulic cutoff wall and construction of a dewatering system.

Geosyntec’s Scope of Services

Geosyntec’s scope of work included development and execution of a detailed subsurface investigation, design of the deep cutoff wall, and design of a construction dewatering system. This scope required a truly multi-organizational, cooperative effort to complete our scope of work and meet the project team’s objectives. Our staff developed and executed a subsurface investigation consisting of 200-ft deep mud rotary test borings, Cone Penetration Tests, monitoring wells, and pneumatic slug tests. Geosyntec’s field work, which required detailed work plans and work instruction sets for safe operation within the Radiation Control Area (RCA), was complicated by the need to schedule and execute the investigation within a highly congested, active demolition environment. Field sequencing delays resulted in schedule delays of nearly 7 months, during which time preliminary design activities had to start without the benefit of the detailed investigation data.

Geosyntec designed the 680-ft long cutoff wall to tie into the low permeability clay layer at depths of 165 to 180 ft. Our design for the wall included 3-D stability analysis of deep trench panels to be excavated by hydromill and hydraulic clamshell, and hydraulic modeling using steady state and transient finite element analyses to verify the wall thickness and hydraulic conductivity. Construction dewatering design was performed using closed form solutions and then refined using the results of finite element analyses to develop detailed time-drawdown plots for depressurization and dewatering of a mixed aquifer system depending on the pumping rates from a network of 4 to 6 turbine submersible deep wells. Geosyntec designed and detailed the system to meet the scheduled drawdown time in consideration of PG&E’s on-site water treatment plant, which had a finite 300 gpm capacity.

Notable Accomplishments

Geosyntec was a critical member and contributor to the success of this nuclear decontamination and decommissioning project. Even in the face of difficult working conditions causing schedule delays, and with jobsite coordination challenges, we executed our subsurface investigation safely and within the scheduled timeframe, keeping the design schedule on-track. Our design work was also submitted on-time and accepted by PG&E. CB&I asked Geosyntec to continue working on the project to provide geotechnical and construction engineering support during continued demolition activities.

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Project Summary

  • Location: Eureka, California
  • Client: CB&I and Remedial Construction Services, LP
  • Project Practice Areas: Water & Wastewater, Geotechnical
  • Type of Facility: Nuclear Power Plants
  • Services Provided: Geotechnical and Construction Engineering, Multi-Component Subsurface Investigation, Design of Deep Hydraulic Cutoff Wall, Construction Dewatering, Construction Documents, Work Plans and Work Instruction Sets
  • Type of Work: Geotechnical and Construction Engineering
  • Governing Regulation: State and local