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The Diablo Canyon nuclear plant: assessing the seismic risks of extended operation

Since the NRC has sole authority over the radiological safety aspects of Diablo Canyon, this means that the plant owner will not have to spend a penny to strengthen its seismic protection, no matter what the state of California wants

Arguably, however, the NRC is not doing enough to reduce the risk that a severe earthquake could cause a Fukushima-like core meltdown and radiation release at Diablo Canyon.

Nature, By Edwin Lyman | August 15, 2022, In 2016, Pacific Gas & Electric (PG&E) announced a historic agreement with labor and environmental groups to shut down the two-unit Diablo Canyon nuclear plant in California by 2025 and replace its roughly 2,200 megawatts of electricity with low- and zero-carbon renewable energy, energy efficiency, and storage. Today, that agreement is in serious jeopardy after an academic study (underwritten in part by the nuclear industry), combined with a sustained and vocal public relations campaign waged by Diablo Canyon supporters (including a Tik-Tok influencer), have succeeded in raising doubts about the viability of the power replacement plan.

Growing concerns about climate change-related impacts on the reliability of the electrical grid have also prompted California governor Gavin Newsom to reconsider his position and seek to keep the plant open, at least in the short term. The US Energy Department’s Office of Nuclear Energy is also doing its part to keep Diablo Canyon open by relaxing the original financial qualification criteria and extending the application deadline by more than three months for its recently established Civil Nuclear Credit Program. This will make it possible for PG&E to apply for a first round of federal subsidies aimed at helping utilities keep nuclear power plants open.

Although there is some basis for the criticism that PG&E and the State of California are not acting quickly enough to ensure that enough carbon-free power will be available to replace all of Diablo Canyon’s output, the California Public Utilities Commission’s historic decision last year to procure 11,500 megawatts of clean energy resources by 2026, along with 4,000 megawatts of new capacity (mostly battery storage) added to the grid in the last year, should help address that concern. A recent analysis by Gridlab and Telos Energy also found that renewable energy could replace Diablo Canyon and supply 85 percent of California’s electricity by 2030, while keeping the power on for its 40 million residents—even under stressful conditions such as low hydropower generation,  retirements of fossil fuel-fired plants, and heatwaves similar to what caused rolling power outages in August 2020.

Nevertheless, the disagreement over the plant’s future has become a proxy for the larger debate over what role nuclear power should play in addressing climate change, given its safety and security risks. If PG&E’s original plan were to succeed, after all, it could undermine the nuclear advocates’ argument that nuclear power is an irreplaceable asset in all circumstances.

But if Diablo Canyon is to remain open beyond 2025, PG&E will have to address a number of difficult issues. First, the company will have to prepare a new 20-year license renewal application and submit it to the US Nuclear Regulatory Commission (NRC) before the expiration of Unit 1’s operating license in 2024. PG&E will also have to undertake extensive inspections and equipment upgrades that were indefinitely postponed after it made the decision to shut the plant, as discussed in a June 2022 meeting of the Diablo Canyon Independent Safety Committee. And finally, it must take a hard look at the vulnerability of the plant to earthquakes and consider the need to make seismic upgrades to minimize the risk to the public over the period of extended operation.

Conflicting information on the seismic question has been reported. A spokesperson for the California Public Utility Commission was quoted as saying that if PG&E were to resume the license renewal proceeding for the plant, it would need to make seismic upgrades. However, this statement is not consistent with the NRC’s current position. Following a review conducted in the aftermath of the 2011 Fukushima accident in Japan, the agency concluded that no seismic upgrades at Diablo Canyon or any other US nuclear plants were necessary, because the health and safety risks to the public were acceptable. Since the NRC has sole authority over the radiological safety aspects of Diablo Canyon, this means that the plant owner will not have to spend a penny to strengthen its seismic protection, no matter what the state of California wants.

Arguably, however, the NRC is not doing enough to reduce the risk that a severe earthquake could cause a Fukushima-like core meltdown and radiation release at Diablo Canyon (or, for that matter, other seismically vulnerable nuclear plants in the country). The agency, as part of its drive to transform into a more “risk-informed” regulator, cites the low calculated radiological risk to the public from nuclear plant accidents to justify not taking action to increase safety across a wide range of areas, including seismic protection. But there’s a major problem with this approach: Assessing the seismic risk involves understanding both the uncertainties associated with nuclear accidents and the even larger unknowns encountered in trying to predict earthquake behavior. These uncertainties raise doubts whether the seismic risks can be calculated with sufficient precision to support the NRC’s complacency.

Although other nuclear plants are also seismically vulnerable, according to current information, the potential peak ground motion that the Diablo Canyon site may experience from an earthquake occurring every 10,000 years on average is far higher than any other US plant…………………………………………………

NRC requirements for protection against earthquakes.……………………………………………

The complicated history of Diablo Canyon’s seismic evaluations……………………………………..

Fukushima seismic reevaluations. The 2011 Fukushima Daiichi accident showed the world what can happen when a nuclear plant experiences a natural disaster more severe than it was designed to handle. In response to the accident, the NRC convened a task force to evaluate whether its nuclear safety requirements needed to be strengthened. In its report, the task force noted that “available seismic data and models show increased seismic hazard estimates for some operating nuclear power plant sites” and recommended that the NRC “order licensees to reevaluate the seismic and flooding hazards at their sites against current NRC requirements and guidance, and if necessary, update the design basis and SSCs important to safety to protect against the updated hazards.”

Although the NRC did accept part of the task force recommendation by directing all nuclear plants to “reevaluate the seismic and flooding hazards at their sites using present-day NRC requirements and guidance,” it did not adopt the task force’s proposed remedy: ……………………………………………………………..

Likelihood of earthquake-induced core damage at Diablo Canyon……………………………………………………………………………

The best way to reduce the risk of a spent fuel pool fire is to transfer most of the densely packed stored spent fuel in the pools to dry storage casks. If the Diablo Canyon units are decommissioned, this will be accomplished within several years after the units are shut down. But the NRC insists that the risks of densely packed spent fuel pool storage are acceptable and has refused to require licensees to expedite spent fuel transfer to dry casks. If the reactors continue to operate, PG&E will have no regulatory mandate to procure and load the additional dry casks needed to thin out the pools, prolonging the period at which the spent fuel pools will pose undue risks.

Despite the fairly high risk of core damage that PG&E’s seismic probabilistic risk assessment found, after reviewing the study the NRC took the position that the risks are acceptable and that “no modifications are warranted … because a potential cost-justified substantial safety improvement was not identified.” …………………………………………………………………..

Consequences of a severe accident at Diablo Canyon. What is at stake if there were a seismically induced core damage accident at Diablo Canyon?…………………………………………………………………

Potential modifications to reduce seismic risk.………………………………………………………………….
more https://thebulletin.org/2022/08/the-diablo-canyon-nuclear-plant-assessing-the-seismic-risks-of-extended-operation/

August 14, 2022 - Posted by | safety, USA

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