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 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.

Computational investigation of on-line interrogation of pebble bed reactor fuel

“…In fact, the results show that the impact of power history variations is expected to be negligible in the case of the Np-239/I-132 activity ratio (see Fig. 4), and to remain below 2% for Cs-137 and 4% for Cs-134/Co-60 [13], [16]. Furthermore, variations due to the neutron energy spectrum are expected to manifest themselves in the one-group cross-sections that are used in the ORIGEN simulations.…”

“…3 (detectability was defined using the criterion introduced by Currie [12]). Earlier work showed that Cs-137 can be used as an absolute indicator of burnup [13]. This is due to its long half-life ( 30 years) and small absorption cross-section ( 0.04 barns, see Table I), which make it resistant to power history variations.…”

“…In this case, an engineered indicator based on the ratio of the activity of Cs-134 to Co-60 was explored [13]. Co-60 is produced through doping the fuel with 50-100 ppm of Co-59.…”

Advanced Measuring (Instrumentation) Methods for Nuclear Installations: A Review

“…Das et al described the development of a spectrophotometric method for trace level determination of U(VI) during the reprocessing and postprocessing of nuclear fuel fabrication in clear industries [36]. Hawari et al assessed the feasibility of passive gamma-ray spectrometry assay as an approach for on-line burnup determination for nuclear fuel of the Modular Pebble Bed Reactor (MPBR) [37]. The sketch is shown in Figure 4.…”

Assessment of on-line burnup monitoring of pebble bed reactor fuel by passive neutron counting

“…If an acceptable correlation exists between radiation emission and burnup, the burnup of a pebble can then be determined indirectly by measuring its gamma or neutron emission. On-line burnup determination by gamma spectrometry has been studied (Hawari et al, 2002;Chen et al, 2003;Chen, 2004) and actually was applied in sampling of pebbles for German AVR reactor. This work investigates the feasibility of using passive neutron counting to perform on-line fuel burnup determination for pebble bed reactor fuels.…”