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i20

Cleanup, decommissioning and dismantling

Radioactive structures after closedown

Each nuclear power plant has to be decommisioned and dismantled after closedown. The main part of the buildings and equipment (e.g. turbines and generators) are not radioactive, if the plant has operated nominally during its technical life. The reactor vessel and associated equipment, piping, pumps, etcetera, have become highly radioactive as a result of neutron radiation and contamination with radioactive materials (CRUD: corrosion residuals and unidentified deposits). Restoration of the site of a given nuclear power plant to habitable greenfield conditions again requires a sequence of very costly activities over a period of a 100 years or even more.

Other nuclear facilities are also to be dismantled at a given time. Due to ageing, cracking, wear, corrosion and other deteriorating mechanisms any facility processing radioactive material will become increasingly contaminated with radioactive material. When the radiation doses for the personel become irresponsible or when the safety is at issue as a result of unreliable equipment, a nuclear plant has to be closed down.

Nuclear power plants

The activities related to the decommissioning and dismantling of a nuclear power plant can be divided into four stages, known under different names in the literature:

• plant cleanout, decontamination or decommissioning,

• safe-guarded cooling period, safe enclosure or 'safestor',

• dismantling, demolition of the structures,

• site clearance, including packing of the debris and scrap,

• definitive storage of the waste containers in a repository.

The radioactive inventory of the reactor with connected systems and shielding materials increases with operational lifetime of the reactor, by activation reactions, depending on the neutron flux. Based on model computations the radioactive inventory of a light-water reactor (LWR) after 20 full-power years is estimated at some 0.1 - 0.6 EBq (exabecquerel, 1 EBq = 1 billion times 1 billion radioactive desintegrations per second), one year after final shutdown, excluding the spent fuel and control rods.

The radioactive inventory also rises during the operational years by progressive contamination of the system, despite of chemical and mechanical decontaminating activities during operation of the reactor. Not only the radioactivity measured in becquerel units, but also the kind of the radionuclides contaminating the system has consequences for the way of demolition and handling the wastes. With longer operating times, the chances on contamination with fission products and actinides increase.

Reprocessing plants

Contamination is extensive in a reprocessing plant. Dismantling of the huge buildings will generate large volumes of heavily contaminated wastes. Costs will be high, because large parts of the construction are contaminated with dangerous long-living radio-nuclides and alpha-emitting wastes. Experiences with dismantling the relatively small West Valley reprocessing plant in the USA, which operated during the period 1966-1972, and some minor DOE plants, are not encouraging. Forty years after closedown the cleanup activities will still take at least another decade and investments of billions of dollars. The final decommissioning and dismantling costs are estimates at 40 times (!) the construction cost of the West Valley plant.

Dismantling of a reprocessing plant will be an exceedingly demanding task. Reprocessing plants are among the largest industrial complexes in the world. The hot areas, the compartments in which radioactive materials are processed, are strongly contaminated with radionuclides representing almost the entire Periodic Table of the Elements, including the transuranic actinides. The volume and mass of the radioactive debris and scrap resulting from the dismantling of a reprocessing plant will be a multiple of those from a nuclear power plant.

Health risks

The radioactive structures of the nuclear power plants and reprocessing plants have to be cut in small pieces, packed in steel or concrete containers and definitively stored in a safe geologic repository. How much dust and leaked liquids containing radionuclides will be dispersed into the environment? Who controls which piece is not radioactive and which one is? The demolition debris contain large amounts of many different long-lived radionuclides and not all radionuclides are easily detectable. As pointed out before, there is no relationship between the biomedical activity of a radionuclide and its detectability by common radiation counters. As yet little actual measurements, if any, of the radioactive content of dismantling debris have been published.

The health risks of decommissioning and dismantling may seem remote, for they are not very visible at this moment. No commercial nuclear power plant nor any reprocessing plant has completed the dismantling sequence. By the year 2050 more than 500 nuclear reactors are awaiting the final cleanup. The large volumes of waste and the long lead times involved greatly enhance the chances of inadvertent releases of considerable amounts of radioactive materials into the environment.

Radioactive debris and scrap

Surges in illegal trade of radioactive scrap metal may be expected in the future when large nuclear power plants are dismantled, for massive amounts of high-value metals at various levels of radioactivity are released during dismantling. Preliminary estimates - empirical evidence is still not existent - point to the following figures of radioactive and contaminated materials for one 1 GWe nuclear power plant (figures in metric tonnes):

• 1600 tonnes high-grade steel, stainless steel and special alloys

• 10000 tonnes steel

• 500 tonnes non-ferrous metals

• 3000 tonnes other materials

• 30000 tonnes concrete rubble.

In addition some 5000 Mg decontamination waste is generated, which is highly radioactive and likely will have no commercial value.

Regulations and economics

All these wastes have to packed in appropiate containers, which should be permanently stored in a geologic repository [more i11]. The high and continually rising costs of radioactive waste management may easily provoke undesirable and hazardous situations. Regulations are relaxed to admit higher concentrations of radioactiviy in materials from nuclear installations for clearance, for economic reasons [more i26, i27]. The IAEA proposes to dilute radioactive materials, such as scrap originating from dismantling nuclear-related installations, with non-radioactive materials, to be used for 'special purposes'. This is a hazardous proposal for it is inherently uncontrollable.

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