Clearance during nuclear power plant decommissioning
The decommissioning of a nuclear facility produces radioactive residues and de-installed or dismantled components; there may also still be buildings on-site. They must be recycled safely, reused or disposed of correctly as radioactive waste. For example, decommissioning of the controlled area of a nuclear power plant, including buildings, generally produces around 200,000 tonnes of material. However, experience has shown that only around 3% of it must be disposed of as radioactive waste. The bulk of the material – around 97% – is not radioactive or is only lightly contaminated and can therefore be used as non-waste, or be properly and safely recovered/recycled, or disposed of in a way that is in the best public interest. This process, known as clearance, applies to buildings as well. A small proportion of the material from the controlled area may be reused while remaining under regulatory control and is not integrated into material flows.
For clearance purposes, even material which is not radioactive or only slightly contaminated must undergo a clearance procedure defined in law. Detailed regulations apply to the clearance procedure, which must be fully documented and includes several quality checks. This ensures that the cleared material is radiologically insignificant.
Clearance is regulated by the Radiation Protection Ordinance. The regulations are based inter alia on European directives (Council Directive 96/29/Euratom) and the Recommendations of the German Commission on Radiological Protection (SSK Recommendations of 12 February 1998, published in Federal Gazette, 15 October 1998; SSK Recommendations of 6 December 2006, Federal Gazette, 22 June 2007). The effect of clearance is to release radioactive materials and movable objects, buildings, soil surfaces, facilities or components from regulatory control. Release measurements must therefore be taken.
Annexed to the Radiation Protection Ordinance is a detailed list, in tabular form, of the clearance values for all the significant radionuclides, which must be adhered to. It also specifies the general conditions applicable to clearance. Various types of clearance are defined.
Types of clearance
Two basic types of clearance are identified. Unrestricted clearance means that material is free from any restriction regarding its subsequent reuse or recycling. Restricted clearance means that the cleared material may only be used for a specified purpose. For example, it may be sent to landfill, burned or, in the case of metals, melted down. In every case, its radiological safety must be demonstrated. Various clearance options are listed in the Radiation Protection Ordinance:
1. Unrestricted clearance of
- solid and liquid substances,
- rubble, excavated soil,
- soil surfaces,
- buildings for reuse and other uses.
2. Clearance of
- solid substances for disposal in landfill,
- solid and liquid substances for disposal in incineration facilities,
- buildings for demolition,
- scrap metal for recycling (melting down, recovery).
The nuclear or radiation protection authority of the German federal state (Land) where clearance is sought is responsible for implementing the procedure. The authority may also consult experts, e.g. TÜV, in this context. Evidence is required that the individual steps in the procedure and the release measurement methods used are appropriate. The conditions stipulated in the Radiation Protection Ordinance must also be adhered to and include the following:
- The clearance criteria may not be met through deliberate blending or dilution of the materials to be cleared (dilution ban).
- Release measuring of buildings must generally be performed on the standing structure, i.e. prior to demolition, so that potentially contaminated components can be kept separate from non-activated components.
Residues and components are decontaminated if necessary and can then be cleared once they are found to be radiologically insignificant. The contaminated material must then be disposed of as radioactive waste. Clearance may not be granted if a substance fails any stage of the procedure. Instead, the substance continues to fall within the scope of regulatory control or is disposed of as radioactive waste.
Dose limits for cleared materials
The precondition for clearance is radiological safety. The key criterion applicable here is the internationally accepted 10 microsievert (μSv) principle (IAEO Safety Series No. 89, ISBN 92-0-123888-6). In accordance with this principle, release of a radioactive substance from regulatory control is justified only if, in a worst case, the additional exposure, expressed as effective dose, occurring for a member of the public will not exceed a range of 10 μSv per year. This additional dose has been recognised as insignificant by international experts. This dose is 200 times less than the natural background radiation exposure of the population (e. g. by cosmic radiation and naturally occurring radioactivity) – which amounts to around 2100 μSv per year in Germany – and can thus be regarded as trivial. Based on the 10 μSv principle, other EU member states have developed binding clearance rules. The principle is also recognised in other countries, e.g. the US and Japan.
As they are radiologically insignificant, cleared substances can be disposed of as conventional waste. As a consequence of clearance, they are no longer subject to nuclear and radiation protection law but are governed by conventional waste management legislation, whose implementation is a matter for the German federal states (Länder). As the producers and owners of the waste, the nuclear power plant operators are responsible for its safe and appropriate disposal.
Volumes of cleared substances arising
As the dismantling of a nuclear power plant is a stepwise process, the waste stream is produced over a number of years. Much of it consists of mineral waste, such as rubble. Compared with the total annual volume of mineral waste in Germany, the amount likely to be generated from all the German decommissioning projects is small.