Clearance during nuclear power plant decommissioning
The decommissioning of a nuclear facility produces radioactive residues and disassembled or dismantled components; there may also still be buildings on-site. The radioactive residues and components have to be recycled safely, disposed of correctly or disposed of as radioactive waste. For example, decommissioning the controlled area of a nuclear power plant, including buildings, generally produces around 200,000 megagrams (Mg, formerly tonnes T) of material. Experience has shown that around 3 percent of this material have to be stored as radioactive waste. The bulk of the material – around 97 percent – is not radioactive or the radioactivity is negligible. This bulk of material can therefore be conventionally used as non-waste outside nuclear engineering, or can be safely recovered/recycled in line with the pertinent provisions, or disposed of in a way that is in the best public interest. As the experts say, it can be cleared. Buildings, too, can be cleared. A very small share of the material from the controlled area may continue to be used in a nuclear installation provided it remains under regulatory control.
The bulk of material to be cleared undergoes a process that is legally defined, stipulated in detail, documented comprehensively and subject to multiple passes of quality control several times (clearance). This ensures that the cleared material is radiologically insignificant.
Clearance is regulated by the Radiation Protection Ordinance. The regulations are based on European directives (Council Directive 2013/59/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).
Clearance releases radioactive materials and objects from buildings, rooms and parts of rooms, soil surfaces, facilities and components from regulatory control. Since it means control from supervision, clearance is subject to particularly strict provisions. Clearance measurements must be carried out. These measurements reliably identify even the lowest levels of radioactivity. Evidence is required showing that the individual steps in the procedure and the release measurement methods used are appropriate.
The Radiation Protection Ordinance contains a detailed table of the clearance values that must be observed for all significant radionuclides. The results of the clearance measurements are used to review compliance with these values. The Radiation Protection Ordinance sets out a number of other requirements that must be met for clearance.
The nuclear or radiation protection authority of the German federal state (Bundesland) where clearance is sought is responsible for implementing the procedure. The competent authority confirms compliance with the required values and determines that the dose criterion is being met and thus no impermissible radiation exposure can occur. The authority may also consult independent experts, e.g. TÜV, in this context.
Types of clearance
The Radiation Protection Ordinance differentiates between two types of clearance.
Unrestricted clearance means that the material is free from any restriction regarding its subsequent reuse, recycling, disposal; ownership of the substances and objects in question or their transfer to third parties is also unrestricted.
Specified clearance means that the subsequent reuse, recycling, disposal is restricted, as is ownership of substances and objects in question or their transfer to third parties. Restrictions may be due to the material characteristics of the substances and objects. Construction waste, for example, cannot be used as topsoil for growing plants. Industrial liquids cannot be used to process foodstuffs. Restrictions may also be imposed due to requirements for future reuse, recycling or disposal, for example clearance for landfilling or combustion. In the end, both specified and unrestricted clearance protect from exposure to radioactive radiation.
Various clearance options are listed in the Radiation Protection Ordinance:
1. Unrestricted clearance of
· solid and liquid substances,
2. Specified clearance of
• construction waste
• soil surfaces,
• solid substances for disposal in landfill,
• substances for disposal in incineration plants,
• buildings for reuse and other uses.
• buildings for demolition,
• metal scraps for recycling (melting down, recovery).
The Radiation Protection Ordinance contains further requirements for clearance, for instance:
• The clearance criteria may not be met through deliberate blending or dilution of the materials to be cleared (dilution ban).
• Clearance measurement 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.
Decontamination removes contamination from residual substances and installation components. If this leads to radiological acceptability, clearance is then possible. The contaminated material must then be disposed of as radioactive waste. Clearance cannot be granted if a substance fails any stage of the procedure. Instead, the substance remains within the scope of regulatory control or is disposed of as radioactive waste.
Radiological acceptability - dose criterion
The precondition for clearance is radiological acceptability. 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 the 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.
Due to exposure to natural radiation, the median effective dose in Germany for one person in a calendar year adds up to around 2,100 μSv, i.e. 200 times greater than 10 μSv. An additional effective dose from clearance is therefore negligible.
In Germany, the annual median effective dose varies by about 1,000 μSv, ranging from 2,000μSv to 3,000 μSv. For individuals, the annual value varies depending on where they live, diet and lifestyle between 1,000 and 10,000 μSv. For example, staying in a different place within Germany for about one week per year may affect a change in dose by 10 μSv. Depending on the place of residence, living in a different building for two days could affect another change of the annual effective dose by another 10 μSv. Plane rides can increase the effective dose by an average of 10 μSv after one hour in the air. This great variability in natural and unavoidable exposure to radiation shows that additional doses from clearance are not only negligible but cannot be attributed with certainty. Medical treatments result in significantly larger additional annual doses.
The maximum additional effective dose to which a member of the public may be exposed as a result of clearance is therefore so low that it cannot be differentiated from fluctuations in the annual dose that an individual is exposed to due to lifestyle and natural radiation and is negligible due to its insignificance.
If an impact cannot be detected and is negligible, then there is no need for legal regulation. This expressed in the legal de-minimis principle: “de minimis non curat lex” (the law cares not for small things). That is why releasing insignificantly radioactive substances from nuclear and radiation regulation is justified. The 10 μSv criterion is the dose criterion for clearance stipulated in the Radiation Protection Ordinance.
Based on the 10 μSv principle, other EU member states have developed binding clearance rules (cf. Directive 2013/59/Euratom). The principle is also recognised in other countries, e.g. the US and Japan.
Due to their radiological acceptability, cleared substances are no longer subject to nuclear and radiation protection legislation and can be disposed of as conventional waste. The Länder are responsible for enforcing waste law. As the producers and owners of the waste, the nuclear power plant operators are responsible for its safe and appropriate disposal.
Quantities of cleared substances
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 construction waste. Compared to the annual amounts of mineral waste in Germany (including 55 million Mg construction waste every), the amounts resulting from all projects dismantling German nuclear power plants over the entire nuclear phase-out is estimated at around 5 million Mg and is thus relatively low.