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Thorium-Loaded ADS Experiments

9.3.2.1 Static Experiments

the profile of neutron flux for the 232Th capture reactions was estimated through the horizontal measurement of 115In(n, γ)116mIn reaction rate distribution, as well as described in Sect. 9.3.1.1. 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 – A0) to that of (13, 14 – I); Fig. 9.3], at the middle height of the fuel assembly. The absolute values of the measured reaction rates (Fig. 9.7) revealed differently the variation of

Fig. 9.7 Measured 115In(n, γ)116mIn reaction rates obtained from the thorium-loaded ADS experiments with 100 MeV protons [5]

reaction rates attributed to varying the neutron spectrum in the core, when the spallation neutrons generated by 100 MeV protons were injected into the core. The moderating effect of the high-energy neutrons in some cores (Th-PE, Th-HEU-PE, and NU-PE: keff ¼ 0.00613, 0.58754, and 0.50867, respectively) was observed around the boundary between the core and polyethylene regions. The 115In(n, γ)116mIn reaction rates in the NU-PE core were higher than in other cores, demonstrating, that the reaction rates of 238U in the NU-PE core were larger than those of 232Th in the thorium cores with the use of 100 MeV protons. Additionally, the effect of the neutron spectrum on the reaction rates was observed with 100 MeV protons by comparing the measured results of reaction rates shown in Fig. 9.5. Thus, an expected physical effect was indeed observed as a result of the neutron spectrum change obtained by varying the moderator materials in the fuel assembly. Additionally, the accuracy [5] of experimental and numerical analyses was compared successfully with the ratio (C/E) of calculations and experiments around the relative difference of 10 %, through the subcritical parameter of neutron multiplication M.

9.3.2.2 Kinetic Experiments

The time evolution of prompt and delayed neutron behaviors was examined through the injection of an external neutron source (Fig. 9.8). In the Th-HEU-PE core, the prompt neutron decay constant (Table 9.1) at 3He detector #1 was different from those at the others by the least-squares fitting, regardless of the kind of external neutron source and the position of neutron detection. It was considered overestimated, especially at detector #1, which was located near the external

Fig. 9.8 Measured prompt and delayed neutron behaviors obtained from the thorium-loaded ADS experiments with 100 MeV protons (Th-HEU-PE)

Table 9.1 Measured results of prompt neutron decay constant (α [1/s]) in Th-HEU-PE core

Source

3He #1

3He #2

3He #3

14 MeV neutrons

5,735 ± 5

5,155 ± 4

5,161 ± 4

100 MeV protons

5,788 ± 5

5,338 ± 5

5,229 ± 5

Table 9.2 Measured results of subcriticality (dollar units) in Th-HEU-PE core by the extrapolated area ratio method

Source

3He #1

3He #2

3He #3

14 MeV neutrons

12.36 ± 0.51

29.70 ± 0.03

61.28 ± 0.09

100 MeV protons

31.19 ± 0.15

26.00 ± 0.10

43.14 ± 0.24

neutron source. The two different neutron sources provided different delayed neutron backgrounds. Subcriticality in dollar units was deduced by the extrapolated area ratio method with the use of prompt and delayed neutron components, and experimentally evaluated according to the kind of external neutron source and the location of neutron detection. As is well known, these results revealed subcriticality dependence on both the kind of external neutron source and the location of neutron detection, although the value of subcriticality was theoretically unchanged, regardless of the external neutron source and the location of the detector. Consequently, the experimental results (Table 9.2) showed that the subcriticality in pcm units for 14 MeV neutrons (keff ¼ 0.6577) was different from that for 100 MeV protons (keff ¼ 0.7319); remarkably, the discrepancy was also observed between the experiments and calculations (keff ¼ 0.5876), although the calculated value was evaluated with the use of MCNPX eigenvalue calculations (total number of histories, 1 x 108).

Conclusions

At KUCA, the ADS experiments with 100 MeV protons were carried out with the combined use of the KUCA A-core and the FFAG accelerator. The neutronic characteristics of ADS were investigated through experimental and numerical analyses of reaction rate distribution and subcriticality.

The thorium-loaded ADS study was conducted as observed by prompt neutron behavior and reaction rates through kinetic and static experiments, respectively. Further, experiments of thorium-loaded ADS were successfully carried out in the subcritical states with the use of an external neutron source (14 MeV neutrons and 100 MeV protons, respectively).

In the future, upcoming ADS experiments with 100 MeV protons could be carried out at the highly enriched uranium-fueled and Pb-Bi-zoned core of KUCA to investigate the neutronic characteristics of Pb-Bi solid materials used in the core and at the target. Also, irradiation experiments of 237Np and 241Am could be conducted in the hard-spectrum core at KUCA to examine the feasibility of reaction rate ratio (capture/fission) conversion analyses of nuclear transmutation.

Acknowledgments This work was supported by the KUR Research Program for Scientific Basis of Nuclear Safety from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The authors are grateful to all the technical staff of KUCA for their assistance during the experiments.

 
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