Abstract
Integral experiments on critical irradiation of neuptium-237 (237Np) and americium-241 (241Am) foils are carried out in a hard spectrum core at KUCA with the use of the back-to-back fission chamber, and Monte Carlo calculations together with a reference nuclear data library are conducted for confirming the precision of numerical simulations. Subcritical irradiation of minor actinide (MA) by ADS is a very important step, before operating actual ADS facilities, in a critical assembly at zero power, such as KUCA, which is an exclusive facility for ADS that comprises a uranium-235 (235U) fueled core and a 100 MeV proton accelerator. The first significant attempt is made to demonstrate the principle of nuclear transmutation of MA by ADS through the injection of high-energy neutrons into the KUCA core at a subcritical state. Here, the main targets of nuclear transmutation of MA by the ADS experiments are fission reactions of 237Np and 241Am, and capture reactions of 237Np.
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6.1 Integral Experiments at Critical State
6.1.1 Critical Irradiation Experiments
6.1.1.1 Core Configuration
Critical irradiation experiments [1] were carried out in the A-core (reference core; Fig. 6.1) that has polyethylene (PE) moderator and reflector rods, and two fuel assemblies: normal “F” and special “f” (Fig. 6.2a, b, respectively). The normal fuel assembly “F” is composed of 60 unit cells, and upper and lower polyethylene blocks about 23″ and 21″ long, respectively, in an aluminum (Al) sheath (2.1 × 2.1 × 60″).
A unit cell is composed of two highly enriched uranium (HEU) fuel plates 2 × 2″ square and 1/8″ thick (1/16″ × 2), polyethylene plates 2 × 2″ square and 1/8″ thick, for the normal fuel plate “F” (HEU-PE). The special fuel assembly “f” (HEU-Pb) is composed of 40 unit cells with two HEU fuel plates 1/8″ (1/16″ × 2) thick, lead (Pb) plate 2 × 2″ square and 1/8″ thick, 20 unit cells of HEU and polyethylene plates as well as in the normal fuel assembly.
6.1.1.2 Back-to-Back Fission Chamber
Critical irradiation experiments for measuring the fission reaction rate ratio were carried out with the back-to-back (BTB) fission chamber (Fig. 6.3) set inside a special void element, containing two foils: a test foil neptunium-237 (237Np) or americium-241 (241Am) and a reference one uranium-235 (235U). The reference foil 235U was a normalization factor that has the fission reactions 235U(n, f), to obtain two fission reaction rate ratios: 237Np/235U and 241Am/235U. The main characteristic of the BTB chamber is to obtain uniquely an original signal of fission event of one foil attached at the position of sample deposit, and a different signal of fission event of the other foil attached at the opposite side without any disturbance of the foil on the front side. The chamber has a structure of separating it from two electro-deposited foils that receive independently pulsed signals of fission events accumulated as electric pulses generated by fission fragments through the ionization process of filling gas (97% argon and 3% nitrogen). For measuring capture reaction rate ratio, a test foil 237Np was set in the BTB fission chamber, and a reference foil gold-197 (197Au) was attached to the Al sheath containing the chamber. The reference foil 197Au was a normalization factor that has the capture reactions 197Au(n, γ) 198Au, to obtain capture reaction rate ratio: 237Np/197Au.
The BTB fission chamber was set at the special void element position in HEU-Pb zone (15, O; Fig. 6.4a), whereas in HEU-PE zone the position was (15, K; Fig. 6.4b). The hard and intermediate spectra are shown in Fig. 6.5. As shown in Fig. 6.4a, b, one and five normal fuel assemblies were added to the cores, respectively, when comparing with reference core shown in Fig. 6.1. Further fuel assemblies were added to reach criticality for two irradiation experiments at the critical state shown in Table 6.1. A series of critical irradiation experiments on 237Np and 241Am foils was conducted under the following core conditions: irradiation time 1 h; reactor power 3.5 W; neutron flux (4.73 ± 0.24) × 107 s−1 cm−2.
6.1.1.3 237Np and 241Am Foils
Two 237Np and 241Am thin test foils (both 99.99% purity, 20 mm diameter, and 4 nm thick), electro-deposited on Al backing (28 mm diameter and 0.2 mm thick), had an isotopic mass, respectively, equal to 89 and 15 μg as shown in Table 6.2. Thin 235U reference foils (20 mm diameter and 0.6 nm thick, 5 or 10 μg weight), used to determine the fission reaction rate ratios, had a 99.91% enrichment. Test and reference 237Np and 197Au foils (8 mm diameter and 0.05 mm thick) were used to determine the capture reaction rate ratio. After the irradiation at critical state, 237Np and 197Au capture reaction rates were deduced by the saturated radioactivity obtained from the γ-ray spectra of irradiated test and reference foils, respectively, with the use of a high-purity germanium (HPGe) detector. The γ-ray energy generated by the capture reactions was as follows: 984.45, 1025.87, and 1028.53 keV in 237Np; 411.80 keV in 197Au, as shown in Table 6.3.
6.1.2 Experimental Analyses
6.1.2.1 Experimental Analyses
During the irradiation experiments, the pulsed-height signals (voltage) induced by fission reactions were stored by the use of a multi-channel analyzer to discriminate the signals originated from fission products generated by fission reactions. In the HEU-Pb zone, pulsed-height distributions were clearly observed at two peaks of 237Np (2.0 V and 5.0 V) and 235U (1.6 V and 3.4 V) foils attributed to light and heavy fragments in fission products, as shown in Fig. 6.6, demonstrating the counts by their fission reactions. For 241Am, one peak (0.1 V) was observed in the pulsed-height distribution shown in Fig. 6.7, and the counts by fission reactions were obtained by making lower-level discrimination about 1.0 V and integrating the count per channel over 1.0 V. Also, the difference in pulsed heights between 5 and 10 μg of 235U for 241Am fission reaction rates was clearly observed by comparing the HEU-Pb and HEU-PE zones shown in Figs. 6.7 and 6.8, respectively.
In addition to the fission reaction rates, capture reaction rates were obtained by measuring the γ-ray of 237Np foils in HEU-Pb and HEU-PE zones with the HPGe detector after the irradiation experiments, as shown in Figs. 6.9 and 6.10, respectively. For the 237Np capture reaction rates, three peaks (Table 6.3) were found to be clearly in the γ-ray measurements of two irradiation experiments in both HEU-Pb and HEU-PE zones, and 237Np capture reaction rates were experimentally deduced through the saturated radioactivity.
6.1.2.2 Numerical Analyses
Numerical analyses were carried out with the use of MCNP6.1 [2] together with ENDF/B-VII.1 [3]; analyses of 197Au capture reaction rates were done with JENDL/D-99 [4]. The comparison between measured and calculated results, as shown in Table 6.4, shows a good agreement demonstrating a relative difference within 5% in the C/E (calculation/experiment) value for 237Np fission reaction rates ratio in the HEU-Pb core. For the 241Am fission reaction rates ratio, the C/E discrepancy was about 10% at HEU-Pb and HEU-PE zones, demonstrating a difference between the neutron spectra of the two zones shown in Fig. 6.5. Here, the 237Np capture reaction rate ratio revealed notably good agreement between the experiments and the calculations, as shown in Table 6.5, with a relative difference of about 5% in C/E values.
From the results in Tables 6.4 and 6.5, MCNP6.1 simulations with ENDF/B-VII.1 demonstrated a fairly agreement with the experimental data of 237Np and 241Am fission and capture reaction rates.
6.1.3 Discussion
6.1.3.1 Fission Reaction Rate Ratio
The region-wise contribution of energy was examined by MCNP6.1 and ENDF/B-VII.1, for 237Np and 241Am fission reaction rates ratios to 235U, with the use of reaction rates normalized by summarizing entire reaction rates over the whole energy. For the sake of comparison in Fig. 6.11, the energy distribution of 237Np and 235U fission reaction rates are shown for the critical irradiation in the HEU-Pb zone shown in Fig. 6.4a, demonstrating that 237Np fission rate has high sensitivity to fission reactions around a few MeV region as well as in the neutron spectrum of the HEU-Pb zone shown in Fig. 6.5. Also, 235U fission reactions were found to be dominant over the thermal and epi-thermal neutron regions for critical irradiation in the HEU-Pb zone, as shown in Fig. 6.11.
Fission reaction rates of 241Am were acquired mainly in a few MeV region in HEU-Pb and HEU-PE zones shown in Figs. 6.12 and 6.13, respectively, although striking peaks of 241Am fission reactions were observed in a wide range of the thermal and epi-thermal neutron regions. Moreover, by comparing the results in Figs. 6.12 and 6.13, a difference in the reaction rates energy distribution between HEU-Pb and HEU-PE zones was clearly observed over the entire energy regions, demonstrating the important effect of the neutron spectrum variation on 241Am fission reaction rates, as shown in Table 6.4.
Through numerical analyses of the region-wise contribution of energy to 237Np and 241Am fission reactions, the effect of neutron spectrum variation (Fig. 6.5) between the HEU-Pb and HEU-PE zones was soundly confirmed on the irradiation of 237Np and 241Am foils at a critical state.
6.1.3.2 Capture Reaction Rate Ratio
Capture reaction rates of 237Np were successfully obtained by the measurement of γ-ray spectra in HEU-Pb and HEU-PE zones after critical irradiation, and used for the evaluation of capture reaction rate ratio by comparison of 197Au capture reaction rates. From the calculated capture reaction rate results in Fig. 6.14, two main peaks can be noticed around the thermal neutron region, and 237Np capture reaction rates were found to be highly sensitive to the thermal neutron spectrum field even in the HEU-Pb zone.
As shown in Table 6.5, the selection of 197Au as reference foil was experimentally meaningful for the evaluation of the 237Np/197Au capture reaction rate ratio, even if no significant differences were observed between the results in the HEU-Pb and HEU-PE zones.
6.2 ADS Irradiation at Subcritical State
6.2.1 Experimental Settings
6.2.1.1 Core Configuration
Subcritical irradiation experiments [2] of MA by ADS were carried out in the solid-moderated and -reflected (A-core) at KUCA by the injection of high-energy protons onto lead-bismuth (Pb–Bi) target. As shown in Fig. 6.15, the ADS core consists of normal and partial fuel assemblies, and polyethylene moderators and reflectors. Normal fuel assembly “F” is composed of 60 unit cells, as shown in Fig. 6.2a. Numeral “12” (Fig. 6.16b) is a partial fuel assembly for reaching a critical mass, and represents the number of unit cells in a normal fuel assembly.
6.2.1.2 Proton Beams
High-energy neutrons generated by the interaction between high-energy protons and Pb–Bi target that set at the location of (15, H) shown in Fig. 6.15 were injected into the KUCA A-core. The main characteristics of proton beams were as follows: 100 MeV energy, 0.5 nA intensity (1 nA at most), 30 Hz pulsed frequency, and 100 ns beam width. The Pb–Bi target was 50 mm in diameter and 18 mm thick; the thickness was determined by taking into account the range and the full stoppage of 100 MeV protons inside the Pb–Bi target [6]. During the injection of 100 MeV protons (no vacuum; (15, A-H) in Fig. 6.15) onto the Pb–Bi target, the size of the proton beam spot was about 40 mm in diameter (circle line), as demonstrated by the scan of the Gafchromic film [7], at the front side (end of proton beams) of Pb–Bi target (15, H; Fig. 6.15), that is highly sensitive to the charged particles shown in Fig. 6.17. Furthermore, the neutron yield was (1.33 ± 0.04) × 108 s−1, as measured through 115In (n, n′) 115mIn reaction rates with the use of the saturated activity that was deduced by the foil activation method [8] (over 0.3 MeV threshold energy; In foil; 10 × 10 × 1 mm) obtained at the location of the Pb–Bi target.
6.2.1.3 Neutron Characteristics
For carrying out the ADS experiments, subcriticality state was made by inserting control rod C2 into the lower limit position (0.00 mm) and withdrawing other rods (C1, C3, S4, S5, and S6) fully (1200.00 mm) from the core. Subcriticality was then deduced experimentally by combining control rod C2 reactivity worth by the rod drop method and control rod C2 calibration curve by the positive period method: about 225 ± 10 pcm as a reference value, as shown in Table 6.6. Additionally, the supplemental result obtained by the α-fitting method [9] was 215 ± 9 pcm. During the injection of high-energy neutrons into the core, the reactor power and the neutron flux were experimentally obtained by the foil activation method [8] with the use of two gold foils (bare and cadmium-covered) irradiated at the location between (15, M) and (15, O) in Fig. 6.15 as follows: 1.35 ± 0.07 W and (1.82 ± 0.09) × 107 s−1cm−2, respectively, for four hours of irradiation.
The neutron spectrum was numerically attained by the PHITS code [11], for the locations of the Pb–Bi target and the BTB fission chamber during the injection of high-energy neutrons, as shown in Fig. 6.18a, b, respectively. At the location of Pb–Bi target, high-energy neutrons showed a sharp peak around 2 MeV region and a unique distribution ranging between 10 and 100 MeV, as shown in Fig. 6.18a. In spite of small effect of high-energy neutrons over 10 MeV in ADS, as shown in Fig. 6.18b, no significant difference between the neutron spectra in ADS and critical cores was found at the location of the BTB fission chamber (15, O; Fig. 6.15).
6.2.1.4 237Np and 241Am Foils
To measure fission and capture reactions of 237Np and 241Am (both 99.99% purity, 89 μg and 15 μg weight, respectively), the BTB fission chamber was used in subcritical irradiation experiments and set at the location of (15, O; Fig. 6.15) of a special void element. The main function of the BTB chamber is to obtain simultaneously two signals from specially installed test (237Np or 241Am) and reference (235U; 99.91% enrichment, 10 μg weight) foils. For fission reaction rates, the two main signals from the test and reference foils come from electric ones of fission fragments caused by fission events. Capture reaction rates of 237Np and 197Au were deduced by the saturated radioactivity [8] obtained from two signals of the γ-ray emission of the 237Np test foil in the BTB chamber and the 197Au reference one that set at the location of (15, O) of the Al sheath (special void element), with the use of a germanium detector. From the two signals of test and reference foils, the fission reaction rate ratio was experimentally acquired by 237Np/235U or 241Am/235U, and the capture one by 237Np/197Au, and finally, validated by comparison with previous experimental results [1] at a critical state.
6.2.2 Demonstration of Nuclear Transmutation
Fission reaction rates were experimentally obtained by the pulsed-height distributions (voltage) of 237Np and 241Am fission events, as shown in Figs. 6.19a, b, respectively, demonstrating original electric signals of test and reference foils. As shown in Fig. 6.19a, the fission events of two 237Np and 235U foils were clearly observed over entire pulsed heights. Moreover, for 241Am and 235U shown in Fig. 6.19b, discrimination between the signals of fission fragments and α-ray induced by 241Am was made at 1.2 V, determining the small fission events by 241Am over 1.2 V and the same fission events by 235U over the entire pulsed heights, as confirmed in the critical irradiation [1]. Meanwhile, the difference between pulsed-height distributions of 235U shown in Figs. 6.19a, b was mainly attributable to reproducibility of the gain in the BTB fission chamber when changing the 235U sample installed. The fission reaction rate ratio determined by the experimental results was found to be a notable 0.048 ± 0.001 and 0.035 ± 0.003 for 237Np/235U and 241Am/235U, respectively, the same as at critical irradiation shown in Table 6.7, although no data of 237Np/235U were obtained at critical irradiation due to invalid data acquisition of two electric signals (237Np and 235U) actuated in the BTB fission chamber.
The capture reaction rate ratio was deduced by the saturated radioactivity on the basis of the γ-ray emission of 237Np and 197Au capture reactions (Fig. 6.20 and Table 6.8) was found to be a remarkable 1.88 ± 0.28, almost the same as at critical irradiation shown in Table 6.9.
From the results of fission and capture reaction rate ratios shown in Tables 6.7 and 6.9, respectively, fission and capture reaction events by ADS were successfully confirmed, and as a conclusion, nuclear transmutation of 237Np and 241Am by ADS was experimentally achieved and demonstrated in the KUCA core.
6.3 Conclusion
The integral experiments on irradiation of 237Np and 241Am in cores at critical conditions were carried out in the KUCA A-core with a neutron hard spectrum. The fission reaction rate ratios (237Np/235U and 241Am/235U) and the capture reaction rate ratio (237Np/197Au) were measured by a BTB (back-to-back) fission chamber. To investigate the behavior of 237Np and 241Am fission and capture reaction rates in the KUCA A-core, results were deeply analyzed by both the experimental signals from the BTB fission chamber and the numerical results of MCNP6.1 calculations with ENDF/B-VII.1. The calculated decomposition in energy of the fission and capture reaction rates allowed us to isolate the most important energy regions in the responses. From the results of experimental and numerical analyses, integral experiments on the irradiation of MA in cores at critical conditions were successfully carried out in the KUCA-A hard spectrum cores. In future studies, nuclear transmutation of MA by ADS is foreseen for implementation at KUCA, with the combined use of a hard spectrum core and 100 MeV protons, on the basis of measured and calculated methodologies obtained in the current at critical core conditions irradiation experiments on MA.
100 MeV proton beams were injected onto the Pb–Bi target, and subcritical irradiation experiments of MA (237Np and 241Am) by ADS were carried out with the use of high-energy neutrons generated by the interaction of 100 MeV protons and the Pb–Bi target. In the subcritical irradiation experiments, fission reaction rates of 237Np and 241Am were acquired by the electric signals of fission fragments obtained from the BTB fission chamber, and capture reaction rates of 237Np were obtained by the measurement of the γ-ray spectrum after the irradiation. Here, for the first time, nuclear transmutation of 237Np and 241Am was soundly implemented by ADS that comprises a subcritical core and a 100 MeV proton accelerator with Pb–Bi target, and demonstrated at KUCA through the experimental results of the reaction rate ratio obtained by combining the test (fission: 237Np and 241Am; capture: 237Np) and the reference (fission: 235U; capture: 197Au) foils.
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Pyeon, C.H. (2021). Nuclear Transmutation of Minor Actinide. In: Pyeon, C.H. (eds) Accelerator-Driven System at Kyoto University Critical Assembly. Springer, Singapore. https://doi.org/10.1007/978-981-16-0344-0_6
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