Recounted: Cologne researchers are working on a new research method for waste arising from decommissioning of nuclear power plants

19.09.2018

Dismantling a nuclear power plant generates several hundred thousand tonnes of debris. Only a few percent of this waste have to be disposed of as radioactive waste. Researchers from Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) and the University of Cologne are currently working on establishing a new measurement method for the analysis of waste arising from the decommissioning of nuclear power plants, which allows for a more precise and reliable determination of the type and quantity of radioactive substances in the waste.

Wanted: radionuclides that are difficult to measure

The decommissioning and dismantling of a nuclear power plant presents one of the greatest challenges in dealing with the remaining radioactivity. The operation of the plant results in certain components and structural structures being contaminated with radioactive substances, also known as radionuclides. In addition to surface contamination, a process known as activation is also important. During this process, originally non-radioactive substances inside components are converted into radionuclides by the neutron radiation prevailing during operation. During this process, originally non-radioactive substances inside components are converted into radionuclides by the neutron radiation prevailing during operation. This process leads, for example, to the formation of radionuclides such as calcium-41 in the biological shield, a concrete structure about one to two meters thick that surrounds the reactor pressure vessel. Part of the concrete has thus to be disposed of as radioactive waste after decommissioning.

GRS project manager Matthias Dewald at the detector of the AMS system of the University of Cologne. The wanted radionuclides are counted here after they have been separated from the other ions in the sample by two mass spectrometers and an accelerator. (Source: GRS)For many aspects of decommissioning, it is important to know as precisely as possible which radionuclides are present in which quantities in plant components – for example, for radiation protection planning or for the selection of suitable methods for the decontamination of components. This radioactive inventory is also decisive in determining which materials arising from dismantling can be reused or disposed of as conventional waste and which are to be classified as radioactive waste. Measurements play an essential role in determining the radioactive inventory.

Measurements of surface contamination usually do not present any problems. However, the measurement of radionuclides within activated structures is more difficult. Only those radionuclides emitting gamma radiation can be measured on site. For the determination of the remaining nuclides, so-called reference nuclides and nuclide vectors are used. This is based on the knowledge that gamma emitters and other radionuclides that are more difficult to measure are present in a certain proportion in certain cases - for example in the wall of the biological shield. The quantity of the remaining radionuclides can thus be determined from the quantity of the easily measurable reference nuclide. For the determination of the nuclide vectors, detailed investigations of samples of activated materials as well as extensive calculations based on the decay series of relevant radionuclides and their half-lives are used. This ensures that the radionuclides that are difficult to measure are also considered by the nuclide vectors, thus leading to conservative results regarding radiological relevance.

Under certain circumstances, however, the use of reference nuclides and nuclide vectors reaches its limits. If the reference nuclide has a short half-life and therefore decays faster than the other nuclides of the vector, the remaining quantity of the reference nuclide may be too small to be measured after only a few years. A similar problem arises if only one element which from the very beginning is only present in very small traces is considered as a reference nuclide.

One to 10 quadrillion

Researchers of Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) have therefore set themselves the goal of investigating the radioactive inventory of activated concrete using one of the most accurate methods currently available for determining the smallest amounts of material. Within the framework of a project funded by the Federal Ministry fort the Environment, Nature Conservation and Nuclear Safety, the experts together with scientists of the Department of Chemistry and the Institute for Nuclear Physics of the University of Cologne are using the so-called accelerator mass spectrometry (AMS). The substance of a sample is first converted into an ion beam. The ions with the same mass as the required radionuclide are then separated from this beam in a first mass spectrometer. However, the ions remaining can be single atoms or ionized molecules with the same mass. This is where the accelerator comes into play: it ensures that all molecular compounds are destroyed so that only one beam of individual ionized atoms remains. This beam is then filtered again in a second mass spectrometer using the mass, so that at the end only the wanted nuclides are counted in a detector.

This method allows extremely precise measurements to be made. This means that the AMS could be used to detect a single radionuclide in an amount of up to 10 quadrillion other non-radioactive nuclides. The idea of using this measurement method for the investigation of waste arising from the decommissioning of nuclear power plants was inspired by the history of the "Ötzi ice mummy": "An accelerator mass spectrometer was used to measure the amount of carbon-14 in Ötzi's remains. In the past, methods such as liquid scintillation, which was also used in nuclear technology, were used for such measurements. In the meantime, AMS has become the method of choice for age determination or trace analysis in climate research - the procedure is simply much more precise and less complex than the others. That's why it made sense for us to apply this method to our investigations as well," explains physicist Matthias Dewald, who manages the project at GRS.

Concrete samples such as this were first irradiated in the Mainz research reactor TRIGA and then examined in the accelerator mass spectrometer. (Source: GRS) Research reactor as time lapse

In one of the two AMS plants at the University of Cologne, the researchers are currently evaluating concrete samples that had previously been irradiated in the TRIGA research reactor in Mainz. "There, we can achieve an activation of the samples within seconds to a few minutes as it is only achieved in a nuclear power plant after many years of operation. The TRIGA is something of a time-lapse for us,” says Dewald. To generate different concentrations of the wanted radionuclides in the samples, the duration of irradiation was varied. As a concrete target, the scientists set themselves a concentration of the radionuclide calcium-41 ranging between one to 10 billion and one to one trillion. This corresponds approximately to the concentration found in the biological shield after the decommissioning of a nuclear power plant.

In addition to the AMS measurements, some of the samples are also evaluated using gamma spectroscopy. This method takes advantage of the fact that every radionuclide is emitting gamma rays in a very specific energy range. In this project, scientists can use this "energetic fingerprint" to determine which gamma emitters are present in the samples and in which proportions. In combination with the results of the AMS measurements, it will be possible, for example, to expand the knowledge base for the development of nuclide vectors. It is thus possible, for example, to clarify whether other, very long-lived radionuclides such as calcium-41 can also be considered as reference nuclides.

First new findings

According to Matthias Dewald, the first results of the analysis confirm the approach of relying on the new procedure for nuclear technology: "We have already been able to determine first differences between the radionuclide compositions and nuclide vectors measured by us, which are known to us from the literature. How significant these differences are, however, is not yet possible to determine - work still lies ahead for us." However, Dewald has no doubts that the nuclide vectors used so far are covering and conservative: "Our project contributes to improving the methodological basis and to extending the validation of nuclide vectors". In the next phase of the project, samples from the biological shields of two decommissioned reactors will be examined to further confirm the results of the previous investigations. The project is to be completed by the end of 2019. A detailed report is to be published at the beginning of 2020.

In the longer term, Dewald and his colleagues are aiming for a material change: After the concrete, activated graphite used in reactors is to be investigated using AMS. This is not only used in reactors of the Chernobyl construction line RBMK, but also in Germany in the high-temperature reactor THTR-300 in Hamm and in several research reactors.