03 Apr Radioactive waste or conventional waste?
How to assess the waste from the decommissioning of nuclear facilities
Outnear the pier at Risø is a relatively unremarkable building. A building that intentionally blends in with the other buildings on the far west side of the peninsula, closest to the fjord, but which differs significantly from the others in function. Here it is decided, 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 waste or not, you first need to understand what radioactive waste is.
"Radioactive waste is waste that in one way or another consists of materials that have either been activated or contaminated. When material is activated, it means that the material itself has become radioactive and is not "just" dirty with some radioactive dust or something else sitting on the surface. The latter we call contaminated, and it can typically be handled with a wet cloth, washing down or other cleaning" - explains Sidse Lærke Lolk, Health Physicist at Danish Decommissioning.
She is in the last part of her course to be approved as authorized to take measurements for release.
Frederik Storm Mahler is authorized for the release function and thus has the authority to measure and assess whether the waste is above or below the established limit values for what we in Denmark categorize as radioactive waste.
He elaborates: "The waste that we measure here at the F-lab comes from the nuclear facilities that DD decommissions. It consists of various building components such as metal pipes, concrete blocks, insulation in walls or lead from shielding. Common to the waste is that its origin is here from Risø, as we cannot release waste that we receive from the surrounding community. To be able to measure waste for release, it is crucial to have insight into the history of the activities that have been around the material that is now treated as waste."[/vc_column_text][/vc_column][/vc_row]
From assessment to measurement to conclusion
Today, it's a collection of steel tubes on which a measurement is started. Because to measure accurately enough, especially with the relatively small amounts of radioactivity needed to categorize the waste as radioactive waste, you need to measure the waste for a longer period of time. This can be anywhere from half an hour to several days.
However, before the measurement is initiated, an assessment of whether the waste has the potential to be released has already been made earlier in the process.
Frederik explains: "We don't measure all the waste here at F-lab, as there are large parts of the waste that we decommission that we know in advance, based on its origin and location, contain too much radioactivity, and it would therefore not make sense to start measuring it for release. That's why we always start by assessing whether the waste has the potential to be released or not before it goes to the F-lab."
Sidse adds: "Part of the initial assessment is to do a surface measurement, with a handheld instrument we call a CoMo (contamination monitor), which will quickly indicate if the waste has potential for release or if it does not make sense to measure".
Only then is the waste transported to the F-lab, where the precise and more elaborate measurement can begin.
Finally, after the measurement, the conclusion is clear: If the measured radioactivity in the waste is above the established nuclide-specific limit values, the waste is categorized as radioactive waste and will be placed in the current storage facilities, then the new upgraded storage facility (NOL), and finally end its days in the final repository.
However, if the amount of radioactivity in the waste does not exceed the limit values, the waste is considered conventional waste without any restrictions from the radiation authorities, and depending on which fraction it belongs to, it will be treated like any other conventional waste in Denmark.
About the release function
About the release function
To be authorized to make release measurements, you must make two consecutive measurements without errors. The measurements are reviewed by an authorized health physicist.
Release function training is peer-to-peer training combined with a quality manual that ensures the quality of the measurements.
F-lab, where the release measurements are made, is accredited by DANAK with an ISO 17025 approval. This approval must be renewed on an ongoing basis and helps to ensure that the laboratory meets certain quality requirements.
Once a year, a number of scale numbers are updated to ensure that the measurements are accurate. Some nuclides cannot be measured directly on F-lab with gamma spectroscopy, but the activity is followed by a so-called key nuclide. The ratio between the non-measurable nuclide (the scale number) and the key nuclide (a gamma emitter) will change over time due to different half-lives.
The nuclear emergency response team has the authority to carry out spot checks and/or inspections on occasion.
The F-lab itself is located near the water, as far away from the waste as possible, while being placed on a special type of sand, all to lower the background radiation levels to a minimum to make the measurements as accurate as possible.
How to measure waste
How to measure waste
All F-lab measurements are made with germanium detectors.
First, the material is weighed and then positioned in a way that the germanium detector can best measure the activity of the item. A conservative assessment of the location of the activity is made here.
A program is used for the measurements where the geometry of the part is set up. It is not always possible to build the part 1-1 and here too, some conservative geometric shapes are created to ensure that the entire physical part is within the geometric shape set up in the program.
A germanium detector consists of a crystal in which gamma photons interact and create ionizations. This produces voltage pulses that are recorded in the device and translated into the energy of the incoming photon, allowing you to measure the different energies separately, unlike with a Geiger counter, for example. Over time, a measurement collects recordings from all these incoming photons with different energies, forming an overall spectrum that is finally analyzed. From this, it is deduced whether the measured object contains activity by comparing it with the incoming photons detected from the background radiation.
Through the weight of the item, a concentration is calculated in units of becquerels per gram (Bq/g). This indicates the decay per second per gram in the measured item. It is from 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 a lower activity concentration for each radionuclide than specified in Order 670 on the use of radioactive substances, from The Danish Health Authority, Radiation Protection.