Measurement of large negative reactivity of an accelerator-driven system in the Kyoto University Critical Assembly

Abstract

Large negative reactivity of a subcritical system driven by a pulsed 14 MeV neutron source has been measured in the Kyoto University Critical Assembly. The subcriticality of the accelerator-driven system (ADS) ranged in effective multiplication factor roughly from 0.98 to 0.92, which corresponded to an operational range of an actual ADS proposed by Japan Atomic Energy Agency. As the measurement technique, pulsed neutron method, power spectral analysis for pulsed neutron source, accelerator-beam trip method were employed. From neutron count decay data obtained by the pulsed neutron experiment, not only the promptneutron decay constant of fundamental mode but also a higher spatial mode could be derived. The subcriticality was also determined from the fundamental decay constant. The measured cross-power spectral density consisted of a familiar correlated reactor-noise component and many uncorrelated delta-functionlike peaks at the integral multiple of pulse repetition frequency. The fundamental prompt-neutron decay constant, i.e., the subcriticality determined from the latter uncorrelated peaks was consistent with that obtained by the above pulsed neutron experiment. However, the magnitude of the former correlated component was reduced with an increase in the subcriticality and eventually this component became almost white at deeply subcritical state ranging in the multiplication factor under 0.95. Consequently, the determination of the decay constant from the correlated component was impossible under such a subcritical state. As data analysis method for the beam trip experiment, both the conventional integral count method and the least-squares inverse kinetics method (LSIKM) were employed. The LSIKM analysis led to the consistent subcriticality with that obtained by the pulsed neutron experiment, while the integral count method significantly underestimated the subcriticality. This underestimation originated from a residual background count, which was maintained after the beam trip. The LSIKM was mostly not influenced by such a slight count rate.

Determination of prompt-neutron decay constant from phase shift between beam current and neutron detection signals for an accelerator-driven system in the Kyoto University Critical Assembly

“…The prompt-neutron decay constant could be determined from the former correlated component and the latter uncorrelated peak points, respectively, and these decay constants were consistent with that obtained by a pulsed neutron experiment. Following the above study, they also showed that the magnitude of the correlated reactor-noise component was reduced with an increase in subcriticality and eventually this component could be hardly observed [20].…”

“…Adjacent to both axial sides of the active region of each fuel assembly, about 56 cm upper and 55 cm lower polyethylene reflectors were attached, respectively. A detailed view of the fuel assembly is given by Sakon et al [20]. The core was surrounded by polyethylenereflector assemblies.…”

“…A wide variety of reactor physics experiments on an ADS using the pulsed spallation neutrons is now in progress [10,11]. In the preparation for connecting the proton-beam accelerator at KUCA, a series of static and kinetic experiments on an ADS with pulsed 14 MeV neutrons has been conducted [12][13][14][15][16][17][18][19][20]. As a basic study for online monitoring of the subcriticality in the operation of an ADS, various Feynman-α and power spectral analyses for a subcritical reactor system driven by a pulsed 14 MeV neutron source have been performed.…”

Recent activities in the field of reactor physics

“…Advanced concepts of FR cores have been proposed by several authors [29,30]. On ADSs, some fundamental research works have been carried out using an experimental facility [31,32].…”

Experimental Study on Accelerator-Driven System at Kyoto University Critical Assembly

“…1. Notably, the 100 MeV protons generated from the FFAG accelerator were injected into a subcritical system under the following parameters: 30 pA beam intensity; 30 Hz repetition rate; 200 ns pulsed width; 1.0×10 7 1/s neutron yield. The calculated reaction rate was obtained by evaluating the volume tallies of activation foils.…”