Nuclear Transmutation of Minor Actinide

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.

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″).

Fig. 6.1

Top view of the KUCA core (reference core) for critical irradiation experiments on MA (Ref. [1])

Fig. 6.2

Schematic drawing of fuel assemblies in the KUCA A-core (Fig. 6.1) (Ref. [1])

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.

Fig. 6.3

BTB fission chamber (Ref. [1])

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.

Fig. 6.4

Core configuration of critical irradiation experiments on MA (Ref. [1])

Fig. 6.5

Calculated neutron spectra (MCNP6.1 with ENDF/B-VII.1) of HEU-PE (15, K) and HEU-Pb (15, O) fuel zones shown in Figs. 6.2a, b, respectively (Ref. [1])

Table 6.1 Positions [mm] of control and safety rods at critical state in HEU-Pb and HEU-PE cores in Figs. 6.4a, b, respectively (Ref. [1])

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.

Table 6.2 Number of atoms of ²³⁷Np, ²⁴¹Am, and ²³⁵U foils (Ref. [1])

Table 6.3 Main characteristics of ²³⁷Np and ¹⁹⁷Au capture reactions (Ref. [1])

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.

Fig. 6.6

Measured pulsed heights of ²³⁷Np and ²³⁵U fission reaction rates in HEU-Pb zone (15, O; Fig. 6.4a) at critical state (Ref. [1])

Fig. 6.7

Measured pulsed heights of ²⁴¹Am and ²³⁵U fission reaction rates in HEU-Pb zone (15, O; Fig. 6.4a) at critical state (Ref. [1])

Fig. 6.8

Measured pulsed heights of ²⁴¹Am and ²³⁵U fission reaction rates in HEU-PE zone (15, K; Fig. 6.4b) at critical state (Ref. [1])

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.

Fig. 6.9

Measured γ-ray spectrum of ²³⁷Np capture reaction rates in HEU-Pb zone (15, O; Fig. 6.4a) at critical state (Ref. [1])

Fig. 6.10

Measured γ-ray spectrum of ²³⁷Np capture reaction rates in HEU-PE zone (15, K; Fig. 6.4b) at critical state (Ref. [1])

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.

Table 6.4 Comparison of the results between experiments and calculations (MCNP6.1 with ENDF/B-VII.1) of fission reaction rate ratios at critical state (Ref. [1])

Table 6.5 Comparison of the results between experiments and calculations (MCNP6.1 with ENDF/B-VII.1) of capture reaction rate ratios at critical state (Ref. [1])

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.

Fig. 6.11

Region-wise contribution of energy of ²³⁷Np and ²³⁵U fission reaction rates by MCNP calculations in HEU-Pb zone at critical state (Ref. [1])

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.

Fig. 6.12

Region-wise contribution of energy of ²⁴¹Am and ²³⁵U fission reaction rates by MCNP calculations in HEU-Pb zone at critical state (Ref. [1])

Fig. 6.13

Region-wise contribution of energy of ²⁴¹Am and ²³⁵U fission reaction rates by MCNP calculations in HEU-PE zone at critical state (Ref. [1])

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.

Fig. 6.14

Region-wise contribution of energy of ²³⁷Np capture reaction rates by MCNP calculations in HEU-Pb zone at critical state (Ref. [1])

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.

Fig. 6.15

Top view of the KUCA A-core for MA irradiation experiments by ADS (Ref. [5])

Fig. 6.16

Schematic drawing of “12” partial fuel rod (Fig. 6.15) (Ref. [5])

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.

Fig. 6.17

Scanning of Gafchromic film after proton irradiation at location of (15, H; Pb–Bi target) in Fig. 6.15 (Ref. [5])

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.

Table 6.6 Measured core condition during injection of high-energy neutrons (Ref. [5])

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).

Fig. 6.18

Calculated neutron spectra (PHITS) at locations of (15, H) and (15, O) in Fig. 6.15 (Ref. [5])

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.

Fig. 6.19

Measured pulsed heights of ²³⁷Np, ²⁴¹Am, and ²³⁵U fission reaction rates at subcritical state (Ref. [5])

Table 6.7 Comparison of the results of fission reaction rate ratio at subcritical and critical states (Ref. [5])

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.

Fig. 6.20

Measured γ-ray spectrum of ²³⁷Np capture reaction rates at subcritical state (Ref. [5])

Table 6.8 Main characteristics of ²³⁷Np and ¹⁹⁷Au capture reactions (Ref. [5])

Table 6.9 Comparison of the results of capture reaction rate ratio at subcritical and critical states (Ref. [5])

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.