Keeping a 45-year-old research reactor ready for science

Istanbul, Türkiye - Research reactors are quiet yet vital engines of science. They support nuclear education, radiation training, materials research, isotope-related studies, and experiments that cannot be conducted in ordinary laboratories. To remain useful over decades, they require careful monitoring of how their fuel is aging. 

A new study from Istanbul Technical University has experimentally assessed the full-core fuel condition of the ITU TRIGA Mark II research reactor, which has operated since 1979. The work provides a clearer picture of fuel usage across the reactor core and offers guidance for future fuel reshuffling, replacement, and long-term operational planning. The study was published in Nuclear Engineering and Technology. 

The ITU TRIGA Mark II reactor is a pool-type research reactor at Istanbul Technical University. It operates at a maximum steady-state thermal power of 250 kW and can also run in pulse mode. The current core contains 67 standard fuel elements and two instrumented fuel elements. 

After more than four decades of operation without fuel reshuffling or replacement, the reactor has had increasing difficulty reaching full power. Earlier estimates, based on logbook calculations and simulations, were not fully consistent with limited experimental observations. This created a need for a full-core experimental assessment of how much fuel had actually been consumed. To address this, the research team used a non-destructive assay method based on gamma spectroscopy. Rather than cutting, opening, or damaging the fuel, the method measures gamma radiation from fission products within the fuel elements. The study used cesium-137 as a burnup indicator because its activity increases with fission and provides a measurable signal of fuel use. 

The team used a specially designed fuel investigation system installed above the reactor pool. Each standard fuel element was safely removed from the core, transferred to the inspection system, and measured with a high-purity germanium detector. Gamma spectroscopy measurements were taken from the center of each fuel element for 10 minutes. The results produced a full-core map of fuel burnup. Fuel elements in the inner B and C rings showed the highest burnup because they had been exposed to a stronger neutron flux. Fuel elements in the outer D, E, and F rings showed lower burnup levels. The highest reported burnup was 5.41% in fuel element B6, while the lowest was 2.40% in fuel element F28. 

“This work provides an experimental map of how the fuel has actually been used across the entire reactor core,” said Dr. Tayfun Akyurek of Istanbul Technical University, the study’s corresponding author and project lead. “Such information is essential for planning fuel management strategies that support safe, efficient, and sustained research reactor operation.” 

The study also confirmed a strong correlation between cesium-137 activity and burnup, supporting its use as a practical indicator for assessing TRIGA-type fuel elements. The full-core map revealed an uneven fuel-use pattern, with higher depletion near the center and lower depletion toward the periphery. This matters because uneven burnup can reduce operational flexibility and make it harder to maintain balanced reactor performance over time. Based on the measured burnup distribution, the researchers analyzed a proposed fuel reshuffling strategy to move lower-burnup fuel elements toward higher-flux regions and improve fuel-use balance. However, the study also found that reshuffling alone may not be sufficient for optimal future operation. The authors therefore proposed a new core configuration that includes placing two new fuel elements in the reactor core. 

The findings are relevant beyond a single university reactor. Many research reactors worldwide have long operating histories and continue to support training, education, safeguards research, and scientific services. Full-core, non-destructive burnup assessment can help operators better understand fuel condition, validate reactor models, plan fuel use, and make evidence-based decisions about future operations. 

For long-running research reactors worldwide, the study offers a practical lesson: full-core, non-destructive fuel mapping can provide the evidence needed to extend scientific usefulness while supporting safety, safeguards, and future core-planning decisions. The researchers emphasize that the study supports fuel management and planning but does not replace regulatory review, formal safety analysis, or institutional approval. Any fuel reshuffling, fuel replacement, or reactor reconfiguration must comply with the required safety and licensing procedures. 

Future work will expand the gamma spectroscopy analysis and examine additional burnup indicators to further improve experimental fuel assessment. The work was conducted by Istanbul Technical University in collaboration with Missouri University of Science and Technology in the United States. 

The team welcomes collaboration with research reactor operators, nuclear engineering programs, safeguards specialists, and fuel-cycle researchers interested in non-destructive burnup assessment, research reactor fuel management, and experimental validation of reactor-core simulations.