Abstract
In graphite-reflected thermal reactors, even a detector placed far from fuel region may detect a certain degree of the correlation amplitude. This is because mean free path of neutrons in graphite is longer than that in water or polyethylene. The objective of this study is experimentally to confirm a high flexibility of neutron detector placement in graphite reflector for reactor noise analysis. The present reactor noise analysis was carried out in a graphite-moderated and -reflected thermal core in Kyoto University Critical Assembly (KUCA). BF3 proportional neutron counters (1” dia.) were placed in graphite reflector region, where the counters were separated by about 35cm and 30cm -thick graphite from the core, respectively. At a critical state and subcritical states, time-sequence signal data from these counters were acquired and analyzed by a fast Fourier transform (FFT) analyzer, to obtain power spectral density in frequency domain. The auto-power spectral density obtained from the counters far from the core contained a significant degree of correlated component. A least-squares fit of a familiar formula to the auto-power spectral density data was made to determine the prompt-neutron decay constant. The decay constant was 63.3±14.5 [1/s] in critical state. The decay constant determined from the cross-power spectral density and coherence function data between the two counters also had a consistent value. It is confirmed that reactor noise analysis is possible using a detector placed at about 35cm far from the core, as we expected.
Highlights
Research and development of High Temperature Gas-cooled Reactor (HTGR) has been carried out in High Temperature Engineering Test Reactor (HTTR) of The Japan Atomic Energy Agency (JAEA) [1,2,3,4,5]
The graphite reflected core is expected to be able to measure the correlation component of reactor noise with a detector more than 30 cm away from the fuel region. This is because the mean free path of neutrons in graphite is longer than that of light-water or polyethylene
A series of reactor noise experiments by power spectrum method were performed on the graphite-reflected core of the Kyoto University Critical Assembly (KUCA)
Summary
Research and development of High Temperature Gas-cooled Reactor (HTGR) has been carried out in High Temperature Engineering Test Reactor (HTTR) of The Japan Atomic Energy Agency (JAEA) [1,2,3,4,5]. In light-water- or polyethylene- reflected thermal reactors, neutron detector for a reactor noise analysis must be placed closely to fuel region to observe a neutron correlation amplitude of significance. The objectives this study is to examine experimentally advantage of graphite-reflected reactor for noise analysis, and adverse effect from neutron source. To achieve these goals, a series of power spectral analysis experiments were performed in a graphite-reflected thermal core of the Kyoto University Critical Assembly (KUCA). A series of power spectral analysis experiments were performed in a graphite-reflected thermal core of the Kyoto University Critical Assembly (KUCA) This experiment was carried out in a critical state and in subcritical states driven by Am-Be neutron source.
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