03 Apr Radioactive waste or conventional waste?
How waste from the Decommissioning of nuclear facilities is assessed
Out near the pier at Risø, lies a relatively unassuming building. A building intentionally designed to blend with the other structures on the outermost western side of the peninsula, closest to the fjord, yet functionally distinct from them. Here, it is determined, once and for all, whether waste is radioactive waste or conventional waste.
What is Radioactive Waste?
To understand how to determine whether waste is radioactive or not, one must first comprehend what radioactive waste entails.
"Radioactive waste is material that has, in one way or another, either been activated or contaminated. When material is activated, it means the material itself has become radioactive, and is therefore not 'merely' soiled with radioactive dust or other surface contamination. The latter is what we refer to as contaminated, and it can typically be addressed with a wet cloth, washing down, or other decontamination methods" – explains Sidse Lærke Lolk, health physicist at Danish Decommissioning.
She is in the final phase of her training to be authorized to conduct measurements for release purposes.
Frederik Storm Mahler is authorized for the release function and thus has the authority to measure and assess whether waste is above or below the established limit values for what we categorize as radioactive waste in Denmark.
He elaborates: ”The waste we measure here at F-lab (the release laboratory, ed.) originates from the nuclear facilities that DD decommissions. It consists of various building components such as metal pipes, concrete blocks, wall insulation, or lead from shielding. Common to this waste is its origin from Risø, as we cannot release waste received from the surrounding community. To measure waste for release, it is crucial to have insight into the history of activities involving the material now treated as waste.”[/vc_column_text][/vc_column][/vc_row]
From Assessment to Measurement to Conclusion
Today, measurements are being initiated on a collection of steel pipes. This is because, to achieve sufficient precision in measurements, especially with the relatively small amounts of radioactivity required for waste to be categorized as radioactive, the waste must be measured over an extended period. This can range from half an hour to several days.
However, prior to initiating the measurement, an assessment has already been conducted earlier in the process to determine whether the waste has the potential for release.
Frederik explains: "We do not measure all waste here at F-lab, as significant portions of the material we decommission are known in advance, based on their origin and location, to contain excessive radioactivity. Therefore, it would be pointless to begin measuring them for release purposes. Consequently, before any waste arrives at F-lab, we always conduct an initial assessment to determine whether it has the potential for release."
Sidse adds: "Part of the initial assessment involves conducting a surface measurement with a handheld instrument we call a CoMo (contamination monitor), which will quickly indicate whether the waste has potential for release or if measuring it would be futile."
Only then is the waste transported to F-lab, where the precise and more elaborate measurement can be initiated.
Finally, after the completion of measurements, the conclusion is clear: If radioactivity in the waste is measured above the established nuclide-specific limit values, the waste is thereby categorized as radioactive waste. It will then be placed in the current storage facilities, subsequently in the new upgraded storage facility (NOL), and ultimately conclude its existence in the final repository.
Conversely, if no quantities of radioactivity above the limit values are detected in the waste, it is considered conventional waste without any restrictions from the radiation authorities. Depending on its specific fraction, it will be processed accordingly, like all other conventional waste in Denmark.
About the Release Function
About the Release Function
To be authorized to perform release measurements, one must successfully complete two consecutive measurements without errors. The measurements are reviewed by an authorized health physicist.
Training for the release function involves on-the-job instruction combined with a quality manual, ensuring the integrity of the measurements.
F-lab, where release measurements are conducted, is accredited by DANAK with an ISO 17025 certification. This certification requires continuous renewal and helps ensure that the laboratory adheres to established quality requirements.
Annually, a series of scaling factors are updated to ensure the accuracy of the measurements. Some nuclides cannot be directly measured at F-lab using gamma spectroscopy; instead, their activity is monitored using a so-called key nuclide. The ratio between the non-measurable nuclide (the scaling factor) and the key nuclide (a gamma-emitter) will change over time due to differing half-lives.
The nuclear emergency preparedness organization is authorized to conduct spot checks and/or inspections as needed.
F-lab itself is situated near water, as far as possible from the waste, and is also located on a specific type of sand. All these measures are implemented to minimize background radiation levels, thereby ensuring the highest possible precision in measurements.
How Waste is Measured
How Waste is Measured
All measurements at F-lab are performed using germanium detectors.
First, the material is weighed, after which it is positioned in a manner that allows the germanium detector to optimally measure the item's activity. A conservative assessment of the activity's location is then performed.
A software program is used for the measurements, where the item's geometry is configured. It is not always possible to model the item exactly 1:1; therefore, conservative geometric shapes are also employed here to ensure that the entire physical item is encompassed within the geometric form configured in the software.
A germanium detector consists of a crystal in which gamma photons interact and create ionizations. This produces voltage pulses that are registered by the instrument and translated into the energy of the incoming photon. Thus, unlike with a Geiger counter, for example, different energies can be measured separately. Over time, a measurement collects registrations from all these incoming photons with varying energies, forming a comprehensive spectrum that is subsequently analyzed. From this, it is deduced whether the measured item contains activity by comparing it with incoming photons registered from background radiation.
Based on the item's weight, a concentration is calculated, expressed in becquerel per gram (Bq/g). This indicates decays per second per gram in the measured item. It is based on this concentration that the item either passes the release measurement and is categorized as conventional waste, or fails the measurement and is categorized as radioactive waste.
To 'pass' a release measurement, the waste must contain an activity concentration for each radionuclide that is lower than specified in Executive Order 670 on the Use of Radioactive Substances, issued by The Danish Health Authority, Radiation Protection.