|Fig. 1: Rancho Seco Nuclear Plant in the decommissioning process, with temporary storage of HLW in dry casks on-site. (Source: X.-M. Lee)|
Nuclear energy developed rapidly in the 1970s and 1980s. There are 434 operational nuclear power reactor worldwide as of 2013, generating 10.9% of the world's electricity. [1,2] As many of these reactors approach the end of the typical 30 to 40-year design lifetime, reactor operators have to face the decision of refurbishing and extending the service life of the reactor, or shutting it down. Decommissioning is the process of removing a commercial nuclear power plant from service so that the land can be made available for other uses. This involves the removal of spent fuel, dismantling of components containing activation products (have been made radioactive by neutron activation) and decontamination of the site to reduce residual radioactivity to safe levels.
The decision to permanently withdraw a nuclear facility from service is typically the result of a combination of cost, safety and political reasons. World Nuclear Association classifies the 127 decommissioned reactors in its database (as of 2010) into the following 3 categories: 
Most of the facilities (95 out of the 127 reactors) were shutdown because there was no longer any economic justification to continue its operation.  Of these, many were early-model designs constructed before the 1989s, and hence became technically obsolete relatively quickly and were not economical to refurbish to prolong their service lives.
Secondly, 7 facilities were closed following an accident or serious incident, which rendered repairs uneconomical.  For example, Fukushima Reactors 1-4 (not yet included in the 7) in Japan were shutdown after being hit by a tsunami in 2011 which caused partial core meltdown in the reactors 1-3, and spent fuel damage in pond of unit 4.
Lastly, 25 facilities were closed prematurely by political decision or due to regulatory impediment.  In Germany and Italy for example, the government decided that the facilities are no longer required to support national interest or priorities. More reactors are planned for decommissioning in Germany as the government proposes a strategy to replace nuclear power with renewable energy sources in the wake of the Fukushima nuclear disaster. 
The International Atomic Energy Agency has defined three standard decommissioning strategies as follows: 
Immediate dismantling involves a continuous decommissioning programme soon after termination of normal operations. Decommissioning commences with post operation clean-out (POCO) and/or fuel removal, so as to reduce radiological hazards before proceeding with dismantling. This option allows for the relatively early release of the site from regulatory control and thus be available soon for future uses. However, immediate dismantling also calls for suitable technology to work in the high radiation fields, and waste disposable capabilities to deal with the decommissioning wastes. 
Deferred dismantling introduces at least one extended period during which the decommissioning programme is limited to surveillance and maintenance of the facility in a safe state. After the initial POCO and/or fuel removal, full dismantling and final removal of control is postponed for a longer period that can range from 40 to 60 years typically, up to 100 years.
Entombment is an alternative to full dismantling, and entails leaving part of the facility in-situ in an encasement. The size of the area where the radioactive material is located is usually reduced by POCO and/or fuel removal, and some dismantling or modifications. The facility is then encapsulated in a long-lived structure such as concrete and/or earth, which will last for a period of time to ensure the remaining radioactivity is within acceptable limits. This option reduces the volumes of material sent off site and thus the usage of possibly limited national waste disposal capacity.
One pressing issue of decommissioning is the lack of a feasible, long-term management and disposal strategy for nuclear waste, particularly for high-level waste (HLW). Spent nuclear fuel is the only HLW generated by a commercial nuclear plant, accounting for less than 1% of the volume, but more than 99.9% of the radioactivity of commercial nuclear waste.  Temporary HLW storage options, such as in onsite dry casks, are currently employed until a final depository is made available. This represents a major uncertainty in the decommissioning process.
While the international experience on decommissioning is growing, the development of a proper long-term strategy for nuclear waste disposal remains a missing key point. It is disconcerting to learn that there are so many nuclear facilities established when there is no solution yet for the waste they will produce, suggesting the attitude that the problem is left for future scientists and engineers to solve.
© Xin Min Lee. 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.
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 "Aging Nuclear Power Plants: Managing Plant Life and Decommissioning," U.S. Office of Technology Assessment, OTA-E-575, September 1993.