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

Abstract

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 multipass fuel cycle in which graphite fuel pebbles are randomly loaded and continuously circulated through the core until they reach their prescribed end-of-life burnup limit ( 80 000 MWD/MTU). 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 have the potential to provide accurate and power-history-resistant 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 a burnup indicator that is resistant to power-history variations. In this case, the use of a relative indicator has several advantages, among them 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.

NGNP Point Design - Results of the Initial Neutronics and Thermal-Hydraulic Assessments During FY-03, Rev. 1

“…49 This system permits pebble burnup to be measured shortly after the pebbles emerge from the core, instead of after a cooldown period of several days. Thus, the ex-core pebble inventory is greatly reduced.…”

Application of Anticoincidence Technology to Burn-Up Measurement Systems in High-Temperature Gas-Cooled Reactors

“…The HTR-10 in China uses a burnup measurement system based on an HPGe detector to measure γ-ray activity from Cs-137 [14]. However, the HTR-10 has intermittent operation, short operation times, and long shutdown periods; the burnup of two fuel elements with the same activity may be different.…”

Pebble bed reactor fuel cycle optimization using particle swarm algorithm

“…(1) MEDUL (MEhrfachDUrchLauf-"multi-pass" in German) fuel management implies discharge and re-introduction of fuel pebbles into the core several times (4-20 times) until reaching their target burnup. The reactor design includes a fuel recirculation system which detects the burnup level of the discharged pebbles (by gamma-ray spectrometry (6)) and controls the reshuffling operations. Pebbles which reach the target burnup level are discharged from the system (to spent fuel storage); otherwise they are reintroduced into the core.…”