Determining radioactive waste with accelerator mass spectrometry
The mineral graphite is one of the natural manifestations of pure carbon. It occurs very frequently in nature and can be easily produced and used for a variety of purposes – the best-known example is probably the graphite lead in pencils. In nuclear technology, graphite is also sometimes used in reactors; for example, it is used as a moderator in some reactor types. In Germany, graphite was mainly used in the now decommissioned prototype reactors AVR (Arbeitsgemeinschaft Versuchsreaktor Jülich) and THTR-300 (Thorium High Temperature Reactor), for example in the fuel spheres and the reflector. The reflector is a layer of material that surrounds the reactor core and backscatters those neutrons that escape from the core. In contrast to the graphite of the fuel spheres, which together with the fuel is classified as heat-generating radioactive waste, the reactor graphite of the reflector structure falls under the category of radioactive waste with negligible heat generation.
Although graphite reflectors consist mainly of carbon, impurities in the form of trace elements are always found in the material. Small air pockets are also found in pores of the material. The material of the reflector is not actually radioactive, but it can be activated by the neutron radiation prevailing in the reactor during operation. For example, elements that are only contained as traces are very strongly activated, so that they gain radiological significance. Accordingly, not all parts of a reflector or its materials can simply be reused after the reactor has been shut down.
In general, it is difficult to define an adequate disposal strategy for reactor graphite. On the one hand, this is due to the fact that a radiological analysis is difficult. For example, the reflectors, which account for more than half of the approximately 1,000 tonnes of activated reactor graphite in Germany, contain several relevant activation products for handling and disposal (carbon-14, chlorine-36 and tritium, i.e. superheavy hydrogen). On the other hand, graphite has chemical-physical properties that can cause the release of gaseous substances under repository conditions. A very careful examination is therefore indispensable.
Reactor graphite to be disposed of in the Konrad mine
In Germany, the activated reactor graphite is currently in long-term storage or in the reactors that are in various stages of decommissioning. It is planned to ship it to the Konrad repository as radioactive waste with negligible heat generation. However, according to German radiation protection law, it is also possible to clear substances if their activity is so low that it demonstrably will not lead to any relevant radiation exposure of humans and the environment and can therefore be neglected. Depending on the type of clearance, these substances can then be recycled or disposed of conventionally, for example. For this purpose, it must be reliably determined which radionuclides are contained in which quantities. The same also applies to substances that do not meet the clearance criteria and must therefore be disposed of as radioactive waste. For example, if reactor graphite is to be disposed of in the Konrad repository, it must meet this facility’s radiological acceptance criteria.
Procedures such as liquid scintillation counting (LSC) are used for corresponding examinations. The focus of the measurements is on the radioactive carbon isotope C-14, which makes up the largest part of the radioactive inventory. The measuring methods used so far with LSC require a great deal of preparation and measuring time and are associated with secondary waste – i.e. waste that results from the measurement.
Determining the radionuclide content of reactor graphite even more precisely and efficiently
In order to be able to determine the radionuclide content of reactor graphite even more precisely and, above all, more efficiently in future, GRS scientists are developing an automatable system in cooperation with researchers from the University of Cologne. For this purpose, they are using so-called accelerator mass spectrometry (AMS). The method circumvents a large part of the difficulties encountered so far and is at the same time able to prove reliably that the clearance values for C-14 have not been exceeded. In the project funded by the Federal Ministry of Education and Research, the research team is also working on making it possible in future to measure other radionuclides such as C-14, Cl-36 and H-3 from a single sample simultaneously.
This is not the first time that the research team has had the idea of using AMS for radiological characterisation in the dismantling of nuclear facilities. In a previous study published in 2019, they had already proved the suitability of AMS as an efficient and reliable measuring method. At that time, the radionuclide Ca-41, which is difficult to measure, was chosen as application case.
Matthias Dewald, who heads both the completed and the ongoing project for GRS, explains why the method is suitable for examining waste from the decommissioning of nuclear power plants: "This is because we can measure extremely precisely with AMS – we can detect a single radionuclide among up to 10 quadrillion non-radioactive atoms. Incidentally, the idea of using the method for our purposes came to us from the ice mummy "Ötzi": In the early 2000s, its age was determined with the help of an accelerator mass spectrometer by measuring the remaining carbon-14 in its remains."
How does AMS work?
In the measuring procedure, the substance of a measuring sample is first converted into an ion beam. The ions with the same mass as the radionuclide are then separated from this beam in a first mass spectrometer. The remaining ions can be single atoms or ionised molecules with the same mass. This is where the accelerator comes into play: it ensures that all molecular bonds are destroyed so that only a beam of individual ionised atoms remains. This beam is then filtered again in a second mass spectrometer on the basis of mass, so that in the end only the nuclides sought are counted in a detector.
This way, the radionuclides in the reflectors or other components can not only be detected and characterised, but also quantified so precisely that an unambiguous judgement can be made. For example, do the reflectors fulfil the acceptance criteria for the Konrad mine with regard to the nuclides under investigation? Or can they even be cleared?
First measurements taken, follow-up project in the starting blocks
The project has now been running for just over two years. During this time, the research team first developed and then built a gas system that is needed for the automated measurements. Having undergone extensive testing, this system is now ready for use. The scientists have already carried the first measurements and are currently evaluating them. They want to complete the project by mid-2022. From then on, it should be possible to routinely measure at the University of Cologne the reactor graphite waste that has already accumulated or will accumulate in the future as part of the decommissioning of nuclear installations.
However, the end of the project does not mean that there is no longer any need for research on AMS as a tool for radiological characterisation. A follow-up project, funded by the Federal Office for Radiation Protection, is already in the starting blocks. AMS will be used to simplify the clearance measurement of other radionuclides such as Ca-41. The gas system developed and validated in the current project can be used again.
Project highlights Decommissioning
When a nuclear power plant is finally shut down and no electricity is fed into the supply grid any longer, the so-called post-operational phase begins. This can take several years and ends when the licence holder receives from the competent supervisory authority a licence for the decommissioning and dismantling of the nuclear power plant. During the post-operational phase, the licence holder, in coordination with the competent supervisory authority, can already make preparations for the dismantling of the plant. Currently, there are three plants in post-operation in Germany.