A new doctoral thesis improves the safeguards monitoring method for nuclear waste and contributes to Finland’s final disposal efforts. The FAME Flagship’s area of expertise served in a pivotal role.
Growing population. Accelerating urbanisation. Globally interconnected industry and trade. Ever-expanding digitalisation. These are just a few examples of how our modern-world societies are driving up the global energy demand, thus putting an increasing toll on our planet. While means for more carbon-free electricity production are urgently needed, renewable energy sources such as water or wind alone cannot satisfy our current energy needs.
For decades nuclear power has provided a steady source of greenhouse-gas-free energy. However, its use and building of new reactors have typically been subjects of passionate policy and public debates. At the same time when younger generations especially are growing more environmentally conscious, the current media landscape has plenty to offer in vivid reminders of past catastrophes, such as HBO’s hit TV show Chernobyl (2019) and Netflix’s Fukushima-centred The Days (2023) mini-series.
One of the major challenges in using nuclear energy is how to deal with nuclear waste that gets created as a byproduct. As is highly radioactive and dangerous to all living things for thousands of years, spent nuclear fuel could potentially be used by terrorist organisations and other malicious actors to produce nuclear weapons or nuclear explosive devices such as dirty bombs. Therefore, the spent nuclear fuel needs to be both securely isolated from the biosphere and closely monitored. Luckily, Finnish research and multidisciplinary development are facilitating progress on both fronts.
PGET-who?
On 4 June 2024, M.Sc. (Tech) Riina Virta defended her doctoral dissertation “Gamma tomography of spent nuclear fuel for geological repository safeguards” at the University of Helsinki’s Faculty of Science. In her research, Virta developed solutions on how to improve the usage of data collected by the so-called PGET, or passive gamma emission tomography, system during spent nuclear fuel safeguards monitoring.
“The PGET device was developed by the International Atomic Energy Agency, and there are only handful of such devices in the world”, says Virta, now working as a nuclear safeguards inspector at Finland’s national Radiation and Nuclear Safety Authority (STUK). “And while the development work goes back all the way to the 1980s, with Finland heavily involved from the start, it was only in 2017 that IAEA approved the system to be used in measuring the spent nuclear fuel.”
Nuclear fuel consists of small, processed uranium pellets that have been stacked together in a sealed metal tube called fuel rod. These rods are in turn bundled together in large numbers to form a fuel assembly. A nuclear reactor core, where the necessary reaction for energy production takes place, contains typically a couple of hundred fuel assemblies. After outrunning their usefulness and before being placed safely out of human reach deep underground – more on that later – the integrity of fuel assemblies needs to be verified to make sure that none of the rods have gone missing. By rotating 360 degrees around the fuel assembly, the PGET device collects gamma emission data to produce a cross-section image from which the number of fuel rods can be verified.
“However, nuclear fuel dampens the radiation, so getting a good-quality picture from the middle of fuel assembly especially has posed a challenge for this method”, says Virta. “So instead of trying to tweak the PGET device itself, our focus was on the data that it produces and how to utilise it more efficiently to improve the image quality.”
From theory to practice
For the energy sector to be able to rely on nuclear fuel for generations to come, a responsible way to safely storage all accumulating nuclear waste is a crucial prerequisite. As an international trailblazer, in 2025 Finland will become the first nation to implement a final disposal solution for its spent nuclear fuel by starting to deposit it in the Finnish bedrock. This further highlights the need for methods to accurately verify that all fuel rods are accounted for.
Indeed, Virta’s research has been intrinsically linked to Finland’s final disposal efforts.
During her research work, Virta’s project team had an opportunity to test its progress in field tests at Finnish nuclear power facilities in Loviisa and Olkiluoto. As a result, the PGET-driven imaging method is now mature enough to serve Finland’s national goals.
“A unique challenge here is the fact that all spent nuclear fuel on its way to final disposal has been cooling down in temporary storage for at least 20 years. So, from a measuring standpoint, the emission levels from which the desired cross-section image will be produced are quite low”, says Virta. “To further complicate things, once the material has been sealed in the bedrock, we cannot reach it anymore. We have tried to predict what the future needs regarding to our measurement data could be, documenting and storing as much reliable information as possible.”
The work continues
In essence, the goal for the research team was to map out the insides of a fuel assembly with data collected from an environment prone to uncertainties. The FAME Flagship of Advanced Mathematics for Sensing, Imaging and Modelling is based on cutting-edge research on inverse problems such as the one present in Virta’s research – and as it happens, Virta’s doctoral thesis supervisors Peter Dendooven and Samuli Siltanen both function as Principal Investigators at FAME.
According to Virta, the method described in her dissertation can be improved still, and discussions about possible future systems are already underway within science community. As the work continues, there is a need and space for FAME’s expertise.
“As it stands, the PGET device method is fully ready to support Finland’s work on spent nuclear fuel disposal. And the necessary cross-section image behind it would not be possible to construct without inverse mathematics”, Virta concludes.
Photo: Samuli Siltanen. From the right: The doctoral candidate Riina Virta, Visiting professor Peter Dendooven from University of Helsinki as Custos, and Associate professor Sophie Grape from Uppsala University as Opponent.
