Decommissioning of nuclear power plants is technically demanding

Author: Dr. Klaus Hassmann, Cluster Energietechnik

Decommissioning is a building block of the energy transition

The German energy transition is primarily about building an environmentally compatible, supply-secure and affordable generation structure. It is often not considered or even underestimated that a significant component of the energy transition is the decommissioning of power plants of whatever kind, with the lengthy and expensive decommissioning of nuclear reactors being of particular importance. In this article, an outlook on the technical and economic characteristics of dismantling is attempted.

The history of nuclear energy use in Germany

For a better classification of what order of magnitude is involved in dismantling, plant numbers, outputs, operating durations are given over the entire period that nuclear power plants (NPPs) have existed in D, and some comparisons are drawn.

Tab 1: Power NPP in MW/number of plants (as of 4/2017; data source German Atomic Forum (DAtF))

The history of nuclear energy in Germany also features a "zero number" that is missing from the DAtF statistics. This is the first commercial plant of the "Fast Sodium Cooled Breeder Reactor (SNR)" with 327 MW power, which was built in the period from 1973 to 1985 in Kalkar on the Lower Rhine in NRW, but which never went into operation for safety and strategic reasons (abandonment of a fast breeder reactor generation); the plant was dismantled between 1991 and 1995 without having generated a kWh of electricity; the "excursion" cost 3.5 billion. € (source Wikipedia) - and so it looks at the site today.

Of two variants, immediate dismantling is the most common form

In immediate dismantling, the dismantling of the plants takes place directly after receipt of the decommissioning permit. The goal is to restore the "greenfield" site in the shortest possible time without leaving visible and measurable traces of the plant. The feasibility of dismantling in general has been demonstrated in Germany by three completed dismantling projects. The cooling of the fuel on site and its transfer to the transport containers alone can take 5 to 7 years. The dismantling of the plant in 10 to 15 years and the removal of the remaining buildings as well as the recultivation of the site are the next steps. 

The second variant, which according to statistics has only been implemented once, is "safe containment". The goal is to safely contain the radioactivity on site and allow it to decay. This process takes decades before the plant can be decommissioned at a much lower cost. Compared to immediate dismantling, this variant takes significantly longer, 40 to 45 years. 

The volumes to be dismantled in Germany are large

More than 99% of the radioactive inventory is located in the fuel assemblies. These are unloaded in the first step and brought to one of the interim storage facilities. After that, the radioactive inventory of the plant is only about one ten-thousandth of the original value; this residual radioactivity is mostly present bound in the reactor pressure vessel and its internals.

Depending on reactor type and power, 160,000 to 250,000 tons of material are generated; about 97% of this is returned to the economic cycle; construction waste is conventionally landfilled. For example, the radioactive material content of a 1200 MW pressurized water reactor is about 4000 tons; after removal of the fuel elements, this is low- to medium-level radioactive material. The cost of dismantling is borne by the operators; it varies between €500 and €1000 million, with power plant size, age and operating hours being the dominant cost factors.

The Stade NPP (672 MW, operated 1972-2003) was dismantled in 4 phases: 1. Dismantling of systems no longer needed in the nuclear control area; 2. Dismantling of primary coolant lines, pumps, steam generators; 3. Dismantling of reactor pressure vessel with core internals and concrete shielding; 4. Dismantling of systems and components remaining in the control area. Dismantling is not yet complete.

In Germany, experts are (still) available in sufficient numbers

A high level of expertise on NPP dismantling is available, especially in Germany - as mentioned above, 3 plants have already reached greenfield status; for some NPPs, dismantling is well advanced. These experts usually work for the owners of the NPPs to be dismantled, for the power plant manufacturers, and for subcontractors; in addition, the Gesellschaft für Nuklear-Service (GNS) should be mentioned, which is responsible for radioactive waste management.

On personnel requirements, duration, and costs - an example

The Obrigheim NPP, a pressurized water reactor with an electrical output of 357 MW, was decommissioned in 2005 after 37 years of operation. Decommissioning began in 2008 and is scheduled for completion in 2018/2020, at an estimated cost in the mid-three-digit millions. A few years before the work was completed, about 180 employees of the former operator Energie Baden Württemberg (EnBW) and another 145 employees from outside companies were on site.

Removal of the nuclear area - a guide to implementation

For a successful dismantling of the nuclear area (control area) of the plant just as for the conventional part, the following knowledge in particular is indispensable:

• The structural analysis of the structure - the construction and design documents prepared during construction are particularly helpful here
• the modifications made during service life
• the strength values of the building materials used
• the additional loads envisaged for dismantling that are to be erected on the old structure, such as scaffolding, work surfaces, partition walls (for air circulation) and lifting equipment
• the additional loads of new structures, e. g. e.g., for the storage of radioactive materials and their load-bearing capacity
• measures against the spread of radioactive contamination and its measurement/removal or reduction.
• As a rule, the structure is removed piecemeal and transported away from the plant. Also in this process, the load-bearing capacity of the residual structure changes, which is why a special attention of the structural engineers is to be directed to this transient phase.

In consideration of the above findings, the step sequence of the individual successive activities (action plan) is to be worked out; the licensing authority is to be involved. By type (pressurized or boiling water reactor), the sequences differ greatly due to the construction method, but even within the two types there are variations from plant to plant.

Removal of Nuclear Power Plants - An Outlook

The dismantling of nuclear power plants has the potential to develop into a noteworthy business field not only in Germany, but also in Europe and worldwide. The personnel requirement until 2025 is estimated at two thousand specialists throughout Europe - not a lot, but in the decades thereafter with an increasing tendency due to the growing number of dismantling projects. In addition, there is a residual team of considerable size per project. It must also be taken into account that these personnel are also highly qualified for the demolition of other types of power plants. Whoever is first on the home market also has good cards internationally.

The specialist knowledge required in the coming decades for the nuclear dismantling portion must be taught in theory and practice; as a rule, this is based on nuclear engineering courses of study. In Germany, the Karlsruhe Institute of Technology (KIT) has founded a competence center "Decommissioning of NPPs"; the central topics being developed at some institutes or chairs include theoretical as well as practical knowledge, for example, about new demolition techniques, radiological determination/removal of contamination on components, and radiation protection measures; this approach has already and will continue to have an impact on other courses. A European Decommissioning Academy (EDA) is also being established in Europe, with a degree program at technical universities providing theoretical and practical training.