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Policy Department D: Budgetary Affairs ____________________________________________________________________________________________ The effect that usually non-radioactive materials become radioactive by neutron-capturing is called activation. Activated materials are not only radioactive on their surfaces but the whole material that was exposed to the neutron flux is more or less homogenously radioactive. Figure 7 illustrates that activation process for cobalt in steel as an example for the many other activation processes. Figure 7: Example: the activation of cobalt in steel and subsequent decay of activated cobalt Source: Authors A second mechanism that spreads radioactivity to non-radioactive materials in a nuclear reactor is a contamination process. During reactor operation certain fission products such as cesium can leave the fuel rods to a certain extent (e.g. in a gaseous form through micro fissures in the metal enclosure called cladding, during cladding failure events), dissolve in the cooling liquid and are removed by filtering and ionexchangers. Irradiated metal surfaces corrode or are mechanically abrased. Both types, fission products as well as abrased activated particles from surfaces either dissolve or are suspended in the cooling liquid and are also removed by filters. As a result the liquid transferring the heat (normally the cooling water) is contaminated with dissolved radioactive ions and activated particles. All surfaces with which such contaminated cooling water gets in contact (pipes, valves, heat exchangers, etc.) can adsorb dissolved and suspended contamination on their surface and so themselves become more and more contaminated. Normally the contamination remains on the surface or in the outer oxide layer. A deeper penetration can only take place in materials with a rough surface (e.g. concrete) or when the base material has surface defects (e.g. on metal surface with scratches). Figure 8 illustrates that as a simplified example. 38
Nuclear Decommissioning: Management of Costs and Risks ____________________________________________________________________________________________ Figure 8: Example contamination mechanisms of materials in contact with cooling water Source: Authors 2.2.2. The need for decommissioning of nuclear reactors Reactors at the end of their operating time consist, in certain parts of the facility, of activated and contaminated materials, even after removal of the spent fuel. The removal of these materials is necessary because: People who might come into contact with contamination or come near to activated items or remove/re-use them in an uncontrolled way can receive high radioactive doses (by direct radiation, by inhalation of aerosols or dust particles, etc.) and it is impossible to ensure that, over very long time periods of hundreds of years, no one enters the facility. Even if thoroughly enclosed (with concrete, door locks or by other means) ageing, corrosion and erosion over a very long period degrades buildings, reactor internals and contaminated equipment with time. Over very long times of hundreds of years activated and contaminated material can thus leave the facility and could cause large doses in the vicinity. Any re-use of remaining buildings as well of the reactor site as a whole would not be possible because of the necessary protection measures to prevent people from entering the facility. Like all facilities where dangerous materials are or were handled and where these materials remain as items, liquids or as contamination on equipment or their surfaces, nuclear reactors have to be decommissioned, and the activated or contaminated materials completely removed and either decontaminated or disposed under circumstances where the dangerous materials cannot re-enter the biosphere. AF-Colenco 2008 defines decommissioning: 'Decommissioning [...] covers all of the administrative and technical actions associated with cessation of operation and withdrawal from service and it is the final phase in their lifecycle after siting, design, construction, commissioning and operation. The purpose of these actions (activities 39
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