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| Fig. 1: A spent fuel pool at the San Onofre Nuclear Generating Station near San Clemente, Calif. (Source: Wikimedia Commons. Courtesy of the NRC) |
Nuclear power accounts for approximately 10% of the world's energy, half of all low-carbon electricity generation in the U.S., and 30% of the world's low-carbon electricity. [1] Most nuclear power plants were built between 1970 and 1990. Hence, as many of these reactors reach the end of the conventional 30 to 40 year design lifetime, stakeholders are faced with a decision to either renovate and extend service life or shut the reactors down. [2] Nuclear decommissioning refers to the process of safely removing a nuclear facility from service so that it no longer requires radiation protection measures. This process is done with nuclear plants, fuel processing facilities, power stations, laboratories, reactors on ships, and more. [3] There is a backlog of civilian nuclear power reactors that have been shut down but not yet decommissioned. This number is expected to grow in the coming decades.
The decommissioning process starts with the removal of highly radioactive spent fuel. Buildings surrounding the reactor core must then be demolished. This requires careful handling of materials for safe transport, storage, and disposal. Each decommission project has its own set of technical risks and challenges based on a plant's design and specifications. [3]
There are generally three approaches to decommissioning: immediate dismantling, deferred dismantling, and entombment. [3]
Immediate dismantling requires the removal or decontamination of all equipment and structures so that a site can be used in an unrestricted or less restricted manner. A positive aspect of this approach is that knowledgeable operational staff is available. A negative aspect is that the reactor's radioactivity levels are higher than with deferred dismantling, so more precautions must be taken.
With deferred dismantling (also referred to as "safe enclosure"), all spent fuel is removed, plumbing is drained, and the facility is made safe while dismantling is left for the future. The period of deferral can range anywhere from 10 to 80 years. [3]
Entombment is a relatively new approach considered in unique instances, such as for reactors in remote locations or small research reactors. Entombment consists of encasing the structure in lasting material (such as concrete) while the radioactivity decays. This limits exposure to workers but requires long-term observation and maintenance.
Nuclear facilities are taken out of service for a variety of reasons. They can be classified into three distinct categories: [2]
Economic: early-model designs constructed before 1989 have become technically obsolete, so renovation is not cost-effective in many cases.
Accident/incident related: when a major or minor incident occurs, repairs can be costly. If there is a substantial risk of continued usage (and/or public outcry), it may lead a plant to shut down.
Political or regulatory impediment: a governing body may decide to phase out or shut down nuclear plants in a specific region or entire nation.
While there are many challenges surrounding decommissioning, one main difficulty is predicting the price. This stems from the distinctive complications and cost drivers associated with each reactor. In the United States, nuclear power plant licensees must provide financial assurance that they will have funds available for decommissioning as needed. [4] This financial assurance must be at least the legally established amount. However, there are often disparities in the financial assurance and the real cost estimates, which are generally higher. Going into further detail of how minimum financial assurance (MFA) costs are legally established for nuclear reactors, one must consider the variable P, which refers to the power of the reactor in thermal megawatts. The MFA funding amount in millions of January 1986 dollars for the pressurized water reactor of a nuclear plant is approximately MFA = (75 + 0.0088 P). Similarly the MFA for a boiling water reactor is approximately MFA = (104 + 0.00 9P). In order to account for inflation since 1986, these amounts are adjusted by multiplying the minimum financial assurance by the escalation factor which is ESC (current year) = (0.65 L + 0.13 E + 0.22 B), where L = labor, E = energy, and B = waste burial. Calculating these minimum financial assurances is critical for nuclear decommissioning because it protects the public from a licensee's bankruptcy or other means of being overcharged (which is important in the US as most of the energy is generated by private utilities). [4]
Around 1990, it became clear that predicting overall decommissioning costs using broad parameters of nuclear reactors (design, construction, or operation) was not effective. The variability made it evident that cost estimates should be based on a bottom-up approach for each facility. As detailed information on decommissioning tasks became more available, confidence and predictability in decommissioning costs grew. It is currently estimated that cost uncertainties based on detailed decommissioning estimates are no greater than 5-10%. The revised cost structure, referred to as the International Structure for Decommissioning Costing (ISDC 2012), provides guidance on developing a cost estimate for decommissioning a nuclear facility. It estimates hierarchically based on decommissioning activities, with first and second levels being aggregations of third level actives. [4] The Principal Activities, identified at the first level, are:
Pre-decommissioning actions
Facility shutdown activities
Additional activities for safe enclosure or entombment
Dismantling activities within the controlled area
Waste processing, storage, and disposal
Site infrastructure and operation
Conventional dismantling, demolition, and site restoration
Project management, engineering and support
Research and development
Fuel and nuclear material
Miscellaneous expenditures
These assessments may be put in context by considering the specific case of the San Onofre Nuclear Generating Station in Southern California. In 2012, the plant (comprised of pressurized water reactors) was shut down after a small radiation leak led to the discovery that hundreds of newly replaced steam-generator tubes were damaged. [5] The $4.4 billion decommissioning project was planned to take two decades. A large portion of the low-level nuclear waste has now been transported to a storage site near Clive, Utah. [6] The 770-ton reactor vessel was shipped by rail and a fleet of eight trucks to its final destination. The high-level waste, which consists of hundreds of spent fuel rods, remains on-site. Fig. 1 shows a photo of a San Onofre spent fuel pool from 2014. The owners are currently completing the transfer of canisters of highly radioactive spent fuel rods to over seventy concrete bunkers. These will remain on the premises until further notice since the federal government is yet to come up with a long-term storage site for high-level nuclear waste. The station's two twin domes are scheduled to be removed between 2025 and 2027. The decommissioning should be complete within the next six to eight years. [6]
© Nick Barber. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Hsu, "Nuclear Power Looks to Regain Its Footing 10 Years after Fukushima," Scientific American, 9 Mar 21.
[2] X. M. Lee, "Decommissioning Nuclear Power Plants," Physics 241, Stanford University, Winter 2015.
[3] "UNEP Yearbook 2012," United Nations Environmental Programme, 2012.
[4] M. Laraia, Nuclear Decommissioning: Its History, Development, and Current Status (Springer, 2018).
[5] A. Sewell, "San Onofre Nuclear Power Plant to Be Closed Permanently," Los Angeles Times, 7 June 17.
[6] A. St. John, "San Onofre Decommissioning Update," KPBS, 27 Jul 20.
