After use, nuclear fuel becomes highly radioactive and hazardous. It contains significant amounts of uranium and plutonium, which are materials used in nuclear weapons. These materials must be intact when stored in their final repository, necessitating thorough and reliable measurement before disposal.
"This way, we can be sure of what is being deposited in the bedrock, and that all nuclear materials will remain in peaceful use," says visiting researcher Riina Virta at the University of Helsinki.
All essential information must be collected before final disposal, with measurements stored in a manner that will remain accessible and comprehensible for thousands of years.
For her doctoral thesis, Riina Virta has studied nuclear waste measurement methods at the University of Helsinki, in cooperation with the Helsinki Institute of Physics (HIP). Virta also works as an inspector in the nuclear materials safeguards section of the Radiation and Nuclear Safety Authority.
Gamma camera sees inside assembly
In her thesis, Riina Virta developed a PGET device, or passive gamma emission tomography, to measure the gamma radiation emitted by spent nuclear fuel. This method produces a detailed cross-section image of the fuel assembly, confirming that all rods are intact.
"The challenging thing with this method is that the fuel dampens the radiation very efficiently," Virta explained. "In practice, the radiation from the middle of the assembly just barely reaches the detector, i.e. the 'camera'. We wanted to fix this problem in our research."
The image quality was improved by better data collection and utilization. The method was also adapted for use in both water and air, making it suitable for Finnish plants managing final disposal. Software tools were developed to simplify the method's application.
The method's performance was validated with extensive field measurements in Finnish nuclear power plants.
"This means the method has been studied in detail and found to work well, and now we are just waiting for the operations of final disposal to start in Olkiluoto," says Virta.
Research Report:Final information of disposed spent nuclear fuel with Finnish methods
Related Links
University of Helsinki
Nuclear Power News - Nuclear Science, Nuclear Technology
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