Keywords

1 Introduction

The accelerator-driven system (ADS) has been considered as an innovative system for the nuclear transmutation of minor actinides and long-lived fission products with the use of spallation neutrons obtained from the injection of high-energy protons into a heavy metal target. At the Kyoto University Critical Assembly (KUCA), a series of ADS experiments [1–5] was carried out by coupling with the fixed-field alternating gradient (FFAG) accelerator [6–8], and the spallation neutrons generated by 100 MeV protons from the FFAG accelerator were successfully injected into uranium- [1, 2, 4] and thorium-loaded [5, 7] cores.

In the ADS facility at KUCA, reactor physics experiments are being carried out to study the neutronic characteristics through the measurements of reactor physics parameters, including reaction rates, neutron spectrum, neutron multiplication, subcriticality, and neutron decay constant. Among these, neutron multiplication was considered as an important index to recognize the number of fission neutrons in the core induced by the external neutron source.

The mockup experiments [5] of thorium-loaded ADS carried out by varying the neutron spectrum and the external neutron source were aimed at investigating the influence of different neutron profiles on thorium capture reactions and the prompt and delayed neutron behaviors in the subcritical system. The results provided important effects of the neutron spectrum and the external neutron source on both static and kinetic parameters: the effect of the neutron spectrum was investigated by varying the moderator material in the fuel region, and that of external neutron source by injecting separately 14 MeV neutrons and 100 MeV protons into the thorium-loaded core varying the moderator. Before the subcritical experiments, a thorium plate irradiation experiment was carried out in the KUCA core to analyze the thorium capture and fission reactions in the critical system as a reference of the subcritical system, although the feasibility of 232Th capture and 233U fission reactions could be examined in the subcritical state.

In this chapter, experimental results of the uranium- and thorium-loaded ADS are shown. Accuracy was evaluated through the comparison between the experiments and the calculations of the Monte Carlo analyses through the MCNPX [9] code with ENDF/B-VII.0 [10], JENDL/HE-2007 [11], and JENDL/D-99 [12] libraries. The ADS static and kinetic experiments at KUCA are presented in Sect. 9.2, the results and discussion of the experiments and calculations in Sect. 9.3, and the conclusion of the study in Sect. 9.4.

2 Experimental Settings

2.1 Uranium-Loaded ADS Experiments

KUCA comprises two solid polyethylene-moderated thermal cores designated A and B and one water-moderated thermal core designated C. The A-core is mainly used for experiments of ADS basic research. The three cores are operated at a low mW power in the normal operating state; the maximum power is 100 W. The constitution and the configuration of the cores can be altered easily, and the coupling with the conventional Cockcroft-Walton type accelerator and with the FFAG accelerator has allowed conducting experiments separately with the use of 14 MeV neutrons from deuteron–tritium fusion reactions and 100 MeV protons with the heavy metal target, respectively.

The A-core (Fig. 9.1) employed in the ADS experiments was essentially a thermal neutron system composed of a highly enriched uranium fuel and the polyethylene moderator/reflector. In the fuel region, a unit cell is composed of the highly enriched uranium fuel plate 1/16 in. thick and polyethylene plates 1/4 in. and 1/8 in. thick (Fig. 9.2). The SV assembly is composed of a 5.08 × 5.08 × 5.08 cm center void region, 32 fuel unit cells, and the polyethylene blocks. In these ADS experiments, three types of fuel rods designated as the normal, partial, and special fuel SV were employed. For reasons of the safety regulations for KUCA, the heavy metal target was located not at the center of the core but outside the critical assembly. As in the previous ADS experiments with 14 MeV neutrons, the introduction of a neutron guide and a beam duct is requisite to lead the high-energy neutrons generated from the heavy metal target to the center of the core as much as possible. In the uranium-loaded ADS experiments, the proton beam parameters were 100 MeV energy, 0.01 nA intensity, 30 Hz pulsed frequency, 100 ns pulsed width, and 80-mm diameter spot size at the tungsten target (100 mm diameter and 9 mm thick). The level of the neutron yield generated at the target was around 1.0 × 106 1/s by the injection of 100 MeV protons onto the tungsten target.

Fig. 9.1
figure 1

Top view of the configuration of the A-core in the accelerator-driven system (ADS) experiments with 100 MeV protons

Full size image
Fig. 9.2
figure 2

Side view of of 3/8P″36EU fuel assembly (F, Fig. 9.1) in the A-core

Full size image

2.2 Thorium-Loaded ADS Benchmarks

In the ADS with 100 MeV protons (Fig. 9.3), the fuel rod was composed of a thorium metal plate and a polyethylene (PE), graphite (Gr), or beryllium (Be) moderator arranged in the A-core. Other components were selected from HEU and natural uranium (NU; 2 × 2 × 1/8 in.) plates. The cores were composed of Th-PE (Fig. 9.4), Th-Gr, Th-Be, Th-HEU-PE, and NU-PE, according to a selection of moderator materials: PE, Gr, Be, HEU-PE, and NU-PE, respectively, and spallation neutrons were generated outside the core after injection onto the tungsten target. The thorium-loaded ADS experiments were conducted especially to investigate the relative influence of different neutron profiles on capture reactions of 232Th and 238U: the reaction of 238U was taken as reference data for evaluating the validity of 232Th capture cross sections.

Fig. 9.3
figure 3

Top view of thorium-loaded ADS core with 100 MeV protons

Full size image
Fig. 9.4
figure 4

Side view of Th-PE fuel assembly (TP) in thorium-loaded ADS core in Fig. 9.3

Full size image

The main parameters of the proton beams were 100 MeV energy, 0.3 nA intensity, 20 Hz pulsed frequency, 100 ns pulsed width, and 40-mm-diameter spot size at the tungsten target (50 mm diameter and 9 mm thick). The level of the neutron yield generated at the target was around 1.0 × 107 1/s by the injection of 100 MeV protons onto the tungsten target.

Prompt and delayed neutron behavior was monitored by placing three 3He detectors (20 mm diameter and 300 mm long) at three locations. Throughout the time evolution of the prompt and delayed neutrons, the prompt neutron decay constant was deduced by least-squares fitting to an exponential function over the optimal duration. Subcriticality was deduced by the extrapolated area ratio method [13] on the basis of prompt and delayed neutron behaviors. For 100 MeV protons, neutron detectors (3He detectors: #1, #2, and #3) were set at three locations.

3 Results and Discussion

3.1 Uranium-Loaded ADS Experiments

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 × 20 × 2 mm) emitted by 115In(n, n′)115mIn at the target. In this static experiment, the subcritical state (0.77 % Δk/k) was also attained 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.

Fig. 9.5
figure 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]

Full size image

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 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.

Fig. 9.6
figure 6

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

Full size image

3.2 Thorium-Loaded ADS Experiments

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 – A′) 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 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: k eff = 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.

Fig. 9.7
figure 7

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

Full size image

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 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 (k eff = 0.6577) was different from that for 100 MeV protons (k eff = 0.7319); remarkably, the discrepancy was also observed between the experiments and calculations (k eff = 0.5876), although the calculated value was evaluated with the use of MCNPX eigenvalue calculations (total number of histories, 1 × 108).

Fig. 9.8
figure 8

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

Full size image
Table 9.1 Measured results of prompt neutron decay constant (α [1/s]) in Th-HEU-PE core
Full size table
Table 9.2 Measured results of subcriticality (dollar units) in Th-HEU-PE core by the extrapolated area ratio method
Full size table

4 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.