Assessment of on-line burnup monitoring of pebble bed reactor fuel using passive gamma-ray spectrometry

An investigation was performed to assess the feasibility of passive gamma-ray spectrometry assay as an approach for on-line burnup determination for the Modular Pebble Bed Reactor (MPBR). In addition to its inherently safe design, a unique feature of this reactor is its multi-pass fuel cycle in which graphite fuel pebbles are randomly loaded and continuously cycled through the core until they reach their prescribed End-of-Life burnup limit (∼ 80,000 MWD/MTU). Therefore, unlike the situation with conventional light water reactors, depending solely on computational methods to perform in-core fuel management will be highly inaccurate. As a result, an on-line measurement approach becomes the only accurate method to assess whether a particular pebble has reached its End-of-Life burnup limit. The results of this investigation indicate that the fission products Cs-137 and Eu-154 provide accurate, and power-history-independent signatures that can be directly correlated with burnup. Furthermore, depending on the fuel manufacturing process, artificially introduced dopants (e.g., Co) can provide gamma-ray lines that are usable for burnup monitoring. In fact, it was found that the relative activity of Co-60 to Cs-134 could form an accurate burnup indicator that is independent of power history. In this case, the use of a relative indicator has several advantages, among which is the elimination of the need for absolute knowledge of the detector full energy peak efficiency curve, and the establishment of a system quality assurance figure of merit based on the peak area ratio of the Co lines.