California’s Diablo Canyon nuclear power plant is scheduled for decommissioning by 2025. Photo courtesy of ©Pacific Gas and Electric.
A FLEX building was built for the Diablo Canyon NPP after 2011 Fukushima disaster. Image courtesy of Anderson Burton.
Dry Cask storage facility was installed for the Diablo Canyon NPP in 2006.

The 2,256MW Diablo Canyon nuclear power plant (NPP) is the only remaining nuclear power plant in operation in California, US. The two-reactor power plant is scheduled for decommissioning in two phases, in 2024 and 2025, after being in service for 40 years.

Built on a 1,000-acre site near Avila Beach, along the Pacific Coast of California, Diablo Canyon NPP is owned and operated by Pacific Gas and Electricity Company (PG&E).

Construction of the power plant began in 1968 and the first reactor unit was commissioned in November 1984, while the second unit was connected to the grid in October 1985.

The plant has been producing approximately 18,000GWh of clean electricity a year, meeting roughly 10% of California’s energy requirement, since beginning commercial production in 1985.

The announcement of the Diablo Canyon NPP closure was made by PG&E in June 2016.

Diablo Canyon nuclear power plant decommissioning plan details

PG&E completed a major refurbishment programme at Diablo Canyon involving the replacement of all steam generators of the NPP by 2009 and submitted the license renewal application to the US National Regulatory Commission (NRC) in November 2009, seeking to extend the operational life of both the reactors by 20 more years beyond the expiry of current operating licenses.

After the Fukushima accident in 2011, the company requested a delay in the process to ensure safety measures and diligently worked on the necessary safety compliance requirements at the site until October 2015.

However, in June 2016, PG&E decided to phase out the Diablo Canyon NPP after the expiry of the current NRC operating licenses for reactor units on 2 November 2024 (Unit 1) and on 26 August 2025 (Unit 2), and replace it with a cost-effective portfolio of efficient renewable energy generation and storage.

PG&E prepared a joint proposal for the same with the support labour and environmental groups and submitted it for approval to the California Public Utilities Commission (CPUC) in August 2017.

CPUC approved the retirement proposal for Diablo Canyon power plant, in January 2018.

Financing for Diablo Canyon decommissioning

PG&E has estimated the Diablo Canyon decommissioning cost to be $3.77bn in 2014 prices, which will be substantially met through the accumulated Diablo Canyon decommissioning trust fund contributed by electricity bill payers.

Along with the approval for shutdown, CPUC has also authorised PG&E to recover in rates $241.2m towards costs associated with permanent shutdown of the plant, which includes $211.3m to retain PG&E employees until 2025, $11.3m for retraining the workers and $18.6m for the license renewal expenses incurred by PG&E.

The CPUC, however, declined PG&E’s request for $85m to support a Community Impact Mitigation Program (CIMP) and instructed that the company may choose to use shareholder funds for the same.

Diablo Canyon nuclear power plant make-up

The Diablo Canyon nuclear power plant consists of two pressurised water reactors (PWR) units designed by Westinghouse.

Each four-loop PWR has a reactor core comprising 193 fuel assemblies and 264 fuel rods, enclosed in a cylindrical stainless steel reactor vessel. Each reactor unit, along with the reactor coolant system (RCS) and steam generators, is contained within a massive steel-reinforced concrete containment structure.

Each reactor building is centred between a turbine building, a spent-fuel handling building, and security facilities.

Other facilities at the plant include water intake and discharge systems, and an independent spent fuel storage installation (ISFSI) with dry storage casks.

The plant uses sea water for cooling, which is sourced from the Pacific Ocean at Diablo Cove.

Electricity transmission from Diablo Canyon

Electricity generated by the NPP is transmitted to electrical grid via three 500kV transmission lines connected to Gates and Midway substations.

Post Fukushima safety measures at Diablo Canyon

After the Fukushima disaster, PG&E carried out advanced seismic studies of earthquake faults in the region and adopted the “diverse and flexible mitigation capability”(FLEX) strategy for the Diablo Canyon NPP, as part of USNRC’s Fukushima task force recommendations.

A FLEX building housing additional pumps, generators, batteries and chargers was erected at Diablo Canyon NPP to avoid loss of cooling capability, electrical power and monitoring capability during unexpected natural events.

The plant’s Independent Spent Fuel Storage Installation (ISFSI) was inspected by the USNRC team in 2016.

Key contractors involved with Diablo Canyon NPP

Fluor has been working as the operation and maintenance contractor for the Diablo Canyon NPP since 2010.

Anderson Burton built the FLEX building for the plant. The Steam Generating Team (SGT), a joint venture of Areva NP (now Framatome) and URS Corporation’s Washington Division (now AECOM), was responsible for the Diablo Canyon steam generators replacement project completed in 2009.

GE had earlier retrofitted the steam turbine cylinders for both the generating units of the NPP.

Westinghouse Nuclear provided the reactors, nuclear steam supply system (NSSS) as well the turbine-generators for the plant.

Bechtel Power Corporation was the lead consultant for project management, engineering, construction, procurement, project costing, scheduling, and quality assurance.

The key construction and installation contractors for the power plant commissioned in 1985 were Bechtel Construction, Promatec, Pullman Power Products, Westinghouse Electric Corporation, A. J. Diani Construction, American Bridge, Ames Associates, Arrowhead Industrial Wate, Bigge Crane & Rigging, Bigge Drayage and Bostrom-Bergen.