Burnup Measurement Study And Prototype Development In HTR-PM

In a pebble-bed core which employs the multi-pass scheme, it is mandatory to deter-mine the burnup of each pebble after the pebble has been extracted from the core in orderto determine whether its design burnup has been reached or whether it has to be reinsertedinto the core again.The burnup of the fuel pebbles can be determined by measuring the activity of¹³⁷Cswith an HPGe detector because of their good correspondence, which is independent ofthe irradiation history in the core. Compared to HTR-10and other HTGRs, the new chal-lenges in HTR-PM are the rather complex energy spectrum and high background, so thefeasibility study is called for. First, the HPGe spectrometer was set-up for running thedetector at high count rates while keeping the energy resolution adequately high to dis-criminate the¹³⁷Cs peak from other interfering peaks. Based on these settings, the ge-ometrical conditions are also settled. Next, experiments were performed with⁶⁰Co and¹³⁷Cs sources to mimic the counting rates in real applications while optimizing spectraanalysis algorithm and parameters, a precision of2.8%（1σ） can be achieved for the typ-ical high burnup fuel pebbles in the determination of the¹³⁷Cs net counting rate. Then,the relationship between the¹³⁷Cs activity and the net counting rate recorded in the HPGedetector was calibrated with a standard¹³⁷Cs source contained in the center of a graphitesphere with the same dimension as a real fuel pebble. Because the self-attenuation of thecalibration source differs with a fuel pebble, a correction factor of1.07±0.02（p=0.95） tothe calibration was derived by using the efficiency transfer method. Finally, by analyzingthe spectra generated with KORIGEN software followed by detailed Monte Carlo simu-lation, it is predicted that the relative standard deviation of the¹³⁷Cs activity can be stillcontrolled below3.0%despite of the presence of all the interfering peaks. These resultsdemonstrate the feasibility of utilizing HPGe gamma spectrometry in the non-destructiveonline determination of the pebble burnup in HTR-PM.On the foundation of the simulation and experiment research, a full size prototypewhich consists of an HPGe detector, a collimator, a shielding and a calibration sourcewas built. Two miscellaneous functions, reinspection of graphite pebbles and sorting thehigh-enriched and low-enriched fuel elements, were realized. We also implemented andtested the interface routine among the operator, HPGe detector and DCS. In summary, a complete solution for burnup measurement system in HTR-PM is provided.