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Results and Discussion

Uranium-Loaded ADS Experiments

9.3.1.1 Static Experiments

Thermal neutron flux distribution was estimated through the horizontal measurement of the 115In(n, γ)116mIn reaction rate distribution by the foil activation method using an indium (In) wire 1.0 mm in diameter. The wire was set in an aluminum guide tube, from the tungsten target to the center of the fuel region [from the position of (13, 14 – A) to that of (13, 14 – P) (Fig. 9.1)], at the middle height of the fuel assembly. The experimental and numerical results of the reaction rates were normalized using an In foil (20 x 20 x 2 mm) emitted by 115In(n, n0)115mIn at the target. In this static experiment, the subcritical state (0.77 % Δk/k) was also attained

Fig. 9.5 Comparison of measured and calculated reaction rate distributions along the horizontal from (13, 14 – A) to (13, 14 – P) in Fig. 9.1 [2]

by the full insertion of C1, C2, and C3 rods. The numerical calculation was performed by MCNPX based on ENDF/B-VII.0. The generation of the spallation neutrons was included in the MCNPX calculation bombarding the tungsten target with 100 MeV proton beams. Because the reactivity effect of the In wire is considered to be not negligible, the In wire was taken into account in the simulated calculation: the reaction rates were deduced from tallies taken in the In wire setting region. The result of the fixed source calculation for the reaction rates was obtained after 2,000 active cycles of 100,000 histories, which led to a statistical error less than 10 % in the reaction rates. As shown in Fig. 9.5, the measured and the calculated reaction rate distributions were compared to validate the calculation method. The calculated reaction rate distribution agreed approximately with the experimental results within the statistical errors in the experiments, although these experimental errors were rather larger than those of the calculations. These larger errors in the experiments were attributed to the status of the proton beams described in Sect. 9.2.1, including the weak beam intensity and the poor beam shaping at the target.

9.3.1.2 Kinetic Experiments

To obtain information on the detector position dependence of the prompt neutron decay measurement, the neutron detectors were set at three positions as shown in Fig. 9.1: near the tungsten target [position of (17, D): 1/2-in.-diameter BF3 detector]; and around the core [positions of (18, M) and (17, R): 1-in.-diameter 3He detectors]. The prompt and delayed neutron behaviors (Fig. 9.6) were

Fig. 9.6 Measured prompt and delayed neutron behaviors obtained from BF3 and 3He detectors in the A-core in Fig. 9.1 [2]

experimentally confirmed by observing the time evolution of neutron density in ADS, an exponential decay behavior and a slowly decreasing one, respectively. These behaviors clearly indicated that the neutron multiplication was caused by an external source: the sustainable nuclear chain reactions were induced in the subcritical core by the spallation neutrons through the interaction of the tungsten target and the proton beams from the FFAG accelerator. In these kinetic experiments, the subcriticality was deduced from the prompt neutron decay constant by the extrapolated area ratio method. The difference of measured results of 0.74 %Δk/k and 0.61 %Δk/k at the positions of (17, R) and (18, M) in Fig. 9.1, respectively, from the experimental evaluation of 0.77 %Δk/k, which was deduced from the combination of both the control rod worth by the rod drop method and its calibration curve by the positive period method, was within about 20 %. Note that the subcritical state was attained by a full insertion of C1, C2, and C3 control rods into the core.

 
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