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Transmutation characteristics of minor actinides in a low aspect ratio tokamak fusion reactor
Fusion Engineering and Design 89 (2014) 2523–2528
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Transmutation characteristics of minor actinides in a low aspect ratio tokamak fusion reactor B.G. Hong, S.Y. Moon ∗ High-enthalpy Plasma Research Center, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeollabuk-do 561-756, Republic of Korea
h i g h l i g h t s • • • •
Optimum radial build for the LAR tokamak transmutation reactor through the self-consistent calculation. Dependence of the transmutation characteristics on the aspect ratios. Additional effects of Pu239 in transmutation blanket for minor actinides destruction. Transmutation of the minor actinides using by a compact transmutation reactor based on the LAR tokamak.
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 14 April 2014 Accepted 30 May 2014 Available online 1 July 2014 Keywords: Transmutation reactor LAR tokamak Minor actinides BISON-C Systems analysis
a b s t r a c t The transmutation characteristics of minor actinides in the transmutation reactor of a low aspect ratio (LAR) tokamak are investigated. One-dimensional neutron transport and burn-up calculations coupled with a tokamak systems analysis were performed to determine optimal system parameters. The dependence of the transmutation characteristics, including the neutron multiplication factor, produced power, and the transmutation rate, on the aspect ratio A in the range of 1.5–2.0 was examined. By adding Pu239 to the transmutation blanket as a neutron multiplication material, it was shown that a single transmutation reactor producing a fusion power of 150 MWth can destroy minor actinides contained in the spent fuels for more than 38 units of 1 GWe pressurized water reactors (PWRs) while producing a power in the range of 1.8–6.8 GWth . © 2014 Elsevier B.V. All rights reserved.
1. Introduction In order to destroy high-level waste from spent pressurized water reactor (PWR) fuel, the transmutation of long-lived radioactive actinides into short-lived and stable isotopes using a fission reactor, an accelerator driven system, and D-T fusion neutrons has been studied so as to reduce the long-term toxicity levels of nuclear waste [1–5]. Thermal fission approaches to destroy Pu and minor actinide (MA) isotopes in a fusion reactor have been also explored [2–5]. The transmutation of high-level waste through the utilization of D-T fusion neutrons will provide a viable application of fusion before its essential purpose of power production. Minor actinides can be separated from other nuclear waste based on an appropriate waste management strategy and thus, a feasibility study of the transmutation of minor actinides by a fission reaction with 14 MeV fusion neutrons in a low aspect ratio (LAR) tokamak
∗ Corresponding author. Tel.: +82 632195337; fax: +82 632704861. E-mail addresses: [email protected], [email protected] (S.Y. Moon). http://dx.doi.org/10.1016/j.fusengdes.2014.05.029 0920-3796/© 2014 Elsevier B.V. All rights reserved.
is very important. In a tokamak fusion device, an aspect ratio is defined as a ratio of a major radius to a minor radius. As a 14 MeV neutron source, the LAR tokamak is a viable option since it combines a compact tokamak reactor with plasma having a large elongated shape which is a favorable shape for a transmutation reactor. In order to destroy as much of the waste as possible while minimizing its overall size, the neutron source based on the LAR tokamak must be optimized. The sub-critical reactor concept is preferred in terms of reactor safety [6]. For the optimal design of a LAR tokamak neutron source, the radial build of the reactor components must be determined in order to satisfy all plasma physics and engineering constraints. Therefore, in this work we assume that the constraints are the same as those used in the design of the international thermonuclear experimental reactor (ITER). In a transmutation reactor, a blanket should produce enough tritium for tritium self-sufficiency, while the neutron multiplication factor keff should be less than 1.0 to maintain sub-criticality. Hence, the key objective in the neutronic design is to obtain a higher transmutation rate via optimization of the neutron energy spectrum, the inventories of actinides, and the thickness
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Fig. 1. Radial build of a transmutation reactor.
of the transmutation blanket. A shield should provide sufficient protection for the superconducting toroidal field (TF) coil against radiation damage and nuclear heating induced by both fusion and fission neutrons. For this purpose, we coupled the systems analysis [7] with one-dimensional neutron transport analysis, and the concept of the transmutation reactor based on the LAR tokamak was developed with an aspect ratio A in the range of 1.5–2.0. In this work, the transmutation characteristics of minor actinides in a LAR tokamak transmutation reactor with a neutron wall loading of less than l.0 MW/m2 were investigated. In order to obtain a higher neutron flux and improve the transmutation rate, Pu239 was chosen as a neutron multiplication material, and its effect on the transmutation characteristics was studied. In Section 2, the optimal system parameters of the LAR tokamak transmutation reactor are discussed with A in the range of 1.5–2.0. The transmutation characteristics of the minor actinides are then examined for three different aspect ratios, and the results are outlined in Section 3. Finally, a summary of our findings is provided in Section 4.
2. Concept of a LAR tokamak transmutation reactor The system parameters of the transmutation reactor must satisfy all plasma physics and engineering constraints. Such constraints are in turn related to various system parameters. In this work, we assumed that the constraints were on the same technological level as those in the design of the ITER. Furthermore, we coupled the tokamak systems analysis [7] with one-dimensional neutron transport and burn-up calculations for the self-consistent determination of system parameters. BISON-C [8] with a 42 neutron group cross section library based on JENDL-3 was employed for the one-dimensional neutron transport and burn-up calculations [9], while the JENDL-3 dosimetry file was used for estimations of the local tritium breeding ratio (TBR). In the BISON-C code, the onedimensional neutron transport equation was solved to obtain the neutron flux, and the nuclide production-depletion equations were solved using the obtained flux and the burn-up library. We note that, after the scoping study based on the systems analysis and onedimensional transport calculations, more accurate computations of quantities such as keff , the radiation damage to plasma-facing components, and the tritium breeding capability of the blanket using multi-dimensional analysis must be performed with the selected concept of the transmutation reactor. The radial build of the LAR tokamak transmutation reactor was modeled in a cylindrical geometry, as shown in Fig. 1. Neutron transport calculations were performed with the same geometry under the assumption of an isotropic neutron distribution. There was no inboard blanket in the inboard region because tritium selfsufficiency could be satisfied with the proper choice of an inboard shield material in the LAR tokamak [10]. The material composition of the reactor components is listed in Table 1. Consistent with
Table 1 Material composition of a transmutation reactor. Component
Material composition (vol.%)
Vacuum vessel Shield High temperature shield Blanket 1 Blanket 2 First wall
Borated steel (60), H2 O (40) WC (80), H2 O (20) WC MA2 O3 (50), SUS316 (15), He (35) SUS316 (7), PbLi (90), SiC (3) SUS316 (60), H2 O (40)
current ITER technology, water was selected for cooling all reactor components except for the blankets, and an Nb3 Sn superconductor was used as the toroidal magnetic field coil. In addition to the current density and maximum toroidal magnetic field at the inner leg of the TF coil, neutron damage to the superconductor should be taken into account to determine the radial build. The vacuum vessel was assumed to be made of 0.15 m-thick borated stainless steel cooled by water. The shield was composed of WC cooled by water. Sufficient space for the shield should be required to protect the superconducting TF coil against nuclear heating and radiation damage [10]. With the lifetime of the neutron source assumed to be 40 years at 75% availability, the fast neutron fluence on the superconductor should be kept below 1019 n cm−2 for Nb3 Sn, the displacement damage to the Cu stabilizer should be below 5 × 10−4 dpa, and the dose should be lower than 109 rad for the organic insulators. As seen in Fig. 1, blankets 1 and 2 have different functions and thus, they can be managed separately. In blanket 1, the minor actinides are loaded for transmutation. SUS316LN coated with SiC is used as a structural material, and helium is employed as a coolant in blanket 1. Blanket 2 is used for tritium breeding, where SUS316LN coated with SiC is utilized as the structural material, PbLi as the coolant, and tritium as the breeding material. By placing the tritium breeding blanket after the transmutation blanket, tritium self-sufficiency can be easily satisfied due to the abundance of neutrons produced by fission of the minor actinides. Therefore, natural Li can be employed, while the enrichment of Li-6 is necessary to satisfy the tritium self-sufficiency requirement in the fusion reactor [10]. The first wall with a thickness of 3 cm is made of SUS316LN cooled by water. The system parameters of the transmutation reactor based on a LAR tokamak with an aspect ratio A in the range of 1.5–2.0, which allow a compact sized reactor with a maximum fusion power of 150 MWth , were found following the procedure in Ref. [11]. The plasma physics and engineering constraints of the LAR tokamak were the same as those used in Ref. [10]. With the minor actinides in the blanket, the neutron flux from the fission of the minor actinides will have an impact on the shielding requirements, and the required inboard shield thickness will increase slightly when compared to the case without minor actinides. When considering a fusion power of 150 MWth , shield thicknesses of 40.0 cm for A = 1.5, 44.5 cm for
B.G. Hong, S.Y. Moon / Fusion Engineering and Design 89 (2014) 2523–2528
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Table 2 Parameters of a LAR tokamak. Parameters
A = 1.5
A = 1.8
A = 2.0
Fusion power (MW) Major radius (m) Minor radius (m) Plasma elongation Triangularity Plasma current (MA) Plasma beta Edge safety factor Neutron wall load (MW/m2 ) Heating power (MW) H factor Shield (cm)
150 3.49 2.32 3.2 0.3 12.3 0.491 2.6 0.18 122 1.2 40.0
150 2.72 1.51 2.9 0.3 9.2 0.297 2.7 0.38 125 1.2 44.5
150 2.49 1.24 2.7 0.3 7.9 0.22 2.8 0.51 126 1.2 46.0
A = 1.8 and 46.0 cm for A = 2.0 were required to provide adequate shielding for a 40 FPY lifetime within the radiation damage limit. As the aspect ratio increases, the neutron wall loading increases due to an increase in the shield thickness and a decrease in the plasma surface area. Table 2 shows the plasma performance and system parameters of the transmutation reactor for A = 1.5, 1.8, and 2.0.
Fig. 2. Variation of (a) keff and (b) specific power as the MAs burn up for A = 1.5, 1.8, and 2.0.
3. Characteristics of minor actinide transmutation We investigated the characteristics of minor actinide transmutation in the LAR tokamak transmutation reactor for A = 1.5, 1.8, and 2.0. The source strength, i.e., the fusion power, was maintained as 150 MWth . Table 3 shows the isotope composition of the minor actinides in the spent fuel of a 1 GWe Korean Standard Nuclear Power plant (KSNP) [12]. The radial build of the outboard blankets and the concentration of minor actinides in the outboard blankets were determined to limit the maximum neutron multiplication of keff to below 0.95 and satisfy the tritium self-sufficiency constraint of TBR > 1.35. We chose the maximum value of keff to 0.95 to allow sufficient safety margin. Blanket 1 was loaded with 50% MA2 O3 , 35% He, and 15% SUS316. The radial thickness of blanket 1 to limit keff to below 0.95 was 7.5 cm for A = 1.5 and 7.9 cm for A = 1.8 and 8.2 cm for A = 2.0. Blanket 2 was loaded with 90% PbLi (Nat. Li), 7% He, and 3% SUS316; the radial thickness of blanket 2 was set as 20.0 cm. Fig. 2 shows the variation of keff and the specific power (defined as the power per unit height as the minor actinides burn up) when A is 1.5, 1.8, and 2.0. The produced power is the sum of the fusion power from the plasma and the fission power generated in blanket 1. The keff and specific power initially increase, then decrease as the minor actinides burn up. For the larger A case, the maximum values of keff and the specific power occur earlier than in the case where A is smaller. The specific power is large for the large A case since the reactor height (which is proportional to Äa, where Ä is the elongation and a is the minor radius) is small, although the fission power, which is proportional to the fusion power times k/(1 − k), is the same for identical values of k [13]. LAR tokamaks have unique physics features such as large natural elongation, and it was shown that the maximum elongation depends on the aspect ratio, A and it decreases with the large A [14]. Fig. 3 shows the variation in the TBR as the minor actinides burn up when A is 1.5, 1.8, and 2.0. The TBR initially increases, then
Fig. 3. Variation of the TBR as the MAs burn up for A = 1.5, 1.8, and 2.0.
decreases as the minor actinides burn up. For the larger A case, the maximum value of the TBR occurs earlier than in the smaller A case. In order to satisfy the TBR > 1.35 condition in the timeaveraged sense, a sufficiently long burn-up time, which depends on the aspect ratio and thickness of blanket 2, is required. Fig. 4 displays the specific transmutation rate (defined as the transmutation rate per unit height) with respect to the burn-up time of the minor actinides. Since the transmutation rate is proportional to the fission power, the specific transmutation rate (specific T.R.) also increases as A increases. Both the major radius and the reactor height decrease with increasing A values, and the amount of loaded minor actinides for the large A case was smaller than that for the small A case. On the other hand, a larger blanket height was possible and a large amount of minor actinides could be loaded when A is small, but the
Table 3 Isotope composition of MAs in the spent fuel of a 1 GWe KSNP. Nuclide
Composition (wt%)
Np237 Am241 Am243 Cm244 Cm245
31.9 57.1 10.1 0.68 0.21
Fig. 4. Specific transmutation rate as the MAs burn up.
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Table 4 Transmuted MAs (kg) with 3000 days of burn-up time. Parameters
A = 1.5
A = 1.8
A = 2.0
Minor radius (cm), a Elongation, Ä Reactor height (cm), Äa Transmuted MAs (kg)
232 3.2 742 5536
151 2.9 438 7474
124 2.7 335 11,066
transmutation rate was small. In Table 4, the transmuted minor actinides (kg) over a burn-up time of 3000 days are compared for different values of A. We assumed the reactor height to be Äa to take into account the space for the reactor maintenance. Since the case of A = 2.0 allowed the largest specific transmutation rate with a 3.35 m reactor height, 11,066 kg of minor actinides could be transmuted over 3000 days, and 30 PWRs (1.0 GWe ) would be supported with a single transmutation reactor based on the LAR tokamak. Here, we assumed that the generation rate of the minor actinides was about 35 kg per year from one PWR with a 1 GWe capacity. The neutron energy spectra in blankets 1 and 2 are shown in Fig. 5 for two different burn-up times, T = 0 and 2500 days, when A is 2.0. As evident in Fig. 5(a), the neutron flux in blanket 1 increases as the minor actinides burned up due to the increase in keff . In Fig. 5(b), the neutron spectra in blanket 2 for T = 0 and 2500 days are compared for cases with and without minor actinides. At day 2500, self-sufficiency could easily be satisfied by placing the tritium breeding blanket after the transmutation blanket because the neutron flux showed higher values with minor actinides than without minor actinides, and there are an abundance of neutrons produced by fission of the minor actinides. Therefore, natural Li can be used, while Li-6 must be enriched for tritium self-sufficiency in a fusion reactor [13]. Fig. 6 demonstrates the behavior of the nuclides during the transmutation operation for A = 2.0. The amounts of Np237, Am241,
Fig. 5. Neutron energy spectra in (a) blanket 1 and (b) blanket 2 for A = 2.0 at burn-up times of T = 0 and 2500 days.
Fig. 6. Behavior of the MA nuclides during the transmutation operation for A = 2.0.
and Am243 constituting the majority of the minor actinides decrease. On the other hand, the amounts of Pu isotopes, Am242, and Cm244 produced from neutron absorption of the other actinides increase until enough neutrons are produced due to the buildup of fissile isotopes such as Pu239 and Pu241. After keff reaches its maximum, the decreasing rates of Np237, Am241, and Am243 increase, but the Pu isotopes, Am242, and Cm244 saturate or start to decrease. In order to transmute the minor actinides effectively, a high neutron flux and high transmutation rate are required. By adding fissionable material to the blanket, efficient minor actinides transmutation will be possible due to the high neutron flux and high power density. Therefore, Pu239 was chosen as the neutron multiplication material in this work, and its effect on the transmutation rate of the minor actinides was investigated. Fig. 7 shows the keff , specific power, and TBR with the addition of 4.0 wt% (≈1.0 × 1021 /cm3 ) Pu239 when A is 2.0. The radial thickness of blanket 1 for the keff value below 0.95 was 6.0 cm, which is smaller than that for the case with minor actinides only (8.2 cm). Thus, the initial inventory is small when compared to the minor actinides only case. The specific power and transmutation rate for the initial 3000 days are larger than in the case with minor actinides only. With the addition of Pu239, the shield thickness increases by approximately 3% due to the increased radiation impact. The amount of transmuted minor actinides for a 3000-day burn-up time was found to be 13,424 kg, which was 21% higher than the case with minor actinides only. Therefore, 38 PWRs (1.0 GWe ) could be supported with one unit of the transmutation reactor based on the LAR tokamak. Fig. 8 compares the specific transmutation rate for cases with and without Pu239 when A is 2.0. With the addition of Pu239, the maximum value of the specific transmutation rate occurred earlier, and the amount of transmuted minor actinides was larger when compared with the minor actinides only case. Fig. 9 shows the neutron spectra with and without the addition of Pu239 at a burn-up time of 1400 days. The fast neutron flux increases due to the addition of Pu239. Fig. 10 depicts the behavior of the nuclides during the transmutation operation for the case with Pu239 when A is 2.0. It was shown that the decreasing rates of Np237, Am241, and Am243 were higher than in the minor actinides only case. The buildup rate of Pu isotopes (e.g., Pu238, Pu240, and Pu241) was also higher than that for the minor actinides only case because the isotopes were bred from Np237 by neutron capture reactions and the neutrons went through multiplication via the addition of Pu239.
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Fig. 10. Behavior of the MA nuclides during the transmutation operation with Pu239 for A = 2.0.
4. Conclusion
Fig. 7. Variation of (a) keff and the specific power, and (b) the TBR as the MAs burn up with the addition of 4 wt% Pu239 for A = 2.0.
The concept of a transmutation reactor was studied by coupling a systems analysis with the one-dimensional neutron transport code, BISON-C. The optimum radial build for a LAR tokamak transmutation reactor was determined through self-consistent calculations of the physics and engineering constraints, which were related to various components of the transmutation reactor. Within the limit of ITER physics and engineering constraints, a compact transmutation reactor based on the LAR tokamak with an aspect ratio < 2 can be used for the transmutation of minor actinides produced from the spent fuel of a PWR. One unit of a transmutation reactor can support more than 30 PWRs (1.0 GWe ) with the production of thermal power equal to 1.0–6.6 GW. By adding Pu239 to the transmutation blanket as a neutron multiplication material, a higher neutron flux and transmutation rate can be achieved. Since the reactor can transmute minor actinides from 38 units of 1 GWe PWRs and produce 1.8–6.8 GW of thermal power, it is expected that the LAR tokamak transmutation reactor can be a viable option to transmute minor actinides. Acknowledgments
Fig. 8. Specific transmutation rate as the MAs burn up with Pu239 for A = 2.0.
This research was supported by a National Research Foundation (NRF) of Korea grant funded by the Korean Government (MSIP) under contract nos. 2011-0009653 and 2008-0061900. This research was also supported by research facilities in Plasma Application Institute of Chonbuk National University. References
Fig. 9. Neutron energy spectra with and without the addition of Pu239 at a burn-up time of 1400 days.
[1] W.C. Sailor, C.A. Beard, F. Venneri, J.W. Davidson, Comparison of acceleratorbased with reactor-based nuclear waste transmutation schemes, Prog. Nucl. Energy 28 (1994) 359–390. [2] I. Murata, Y. Yamamoto, S. Shido, K. Kondo, A. Takahashi, E. Ichimura, et al., Fusion-driven hybrid system with ITER model, Fusion Eng. Des. 75–79 (2005) 871–875. [3] Y. Wu, S. Zheng, X. Zhu, W. Wang, H. Wang, S. Liu, et al., Conceptual design of the fusion-driven subcritical system FDS-I, Fusion Eng. Des. 81 (2006) 1305–1311. [4] W.M. Stacey, Transmutation missions for fusion neutron sources, Fusion Eng. Des. 82 (2007) 11–20. [5] Y.-K.M. Peng, E.T. Cheng, J.D. Galambos, R.J. Cerbone, Spherical Tokamak (ST) transmutation of nuclear waste, in: 16th Symposium Fusion Engineering, vol. 2, 1995, pp. 1423–1429.
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[6] First Phase P&T Systems Study: Status and Assessment Report on Actinide and Fission Product Partitioning and Transmutation, OECD/NEA, Paris, 1999. [7] B.G. Hong, D.W. Lee, S.R. In, Tokamak reactor system analysis code for the conceptual development of DEMO reactor, Nucl. Eng. Technol. 40 (2008) 87–92. [8] BISON-C, CCC-659, Oak Ridge National Laboratory. [9] T. Nakagawa, K. Shibata, S. Chiba, T. Fukahori, Y. Nakajima, Y. Kikuchi, et al., Japanese evaluated nuclear data library version 3 revision-2: JENDL-3.2, J. Nucl. Sci. Technol. 32 (1995) 1259–1271. [10] ITER General Design Requirements Document (GDRD), 10 GRD 2 95-02-10 F1.0, June 1995.
[11] B.G. Hong, Y.S. Hwang, J.S. Kang, D.W. Lee, H.G. Joo, M. Ono, Conceptual design study of a superconducting spherical tokamak reactor with a self-consistent system analysis code, Nucl. Fusion 51 (2011) 1130131–1130136. [12] H.B. Choi, B.W. Rhee, H.S. Park, Physics study on direct use of spent PWR fuel in CANDU (DUPIC), Nucl. Sci. Eng. 126 (1997) 80. [13] W.M. Stacey, Capabilities of a DT tokamak fusion neutron source for driving a spent nuclear fuel transmutation reactor, Nucl. Fusion 41 (2001) 135– 154. [14] C. Neumeyer, Y.-K. Peng, C. Kessel, P. Rutherford, Spherical Torus Design Point Studies, Princeton Plasma Physics Lab. Report PPPL-4165, 2006.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Transmutation characteristics of minor actinides in a low aspect ratio tokamak fusion reactor B.G. Hong, S.Y. Moon ∗ High-enthalpy Plasma Research Center, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju, Jeollabuk-do 561-756, Republic of Korea
h i g h l i g h t s • • • •
Optimum radial build for the LAR tokamak transmutation reactor through the self-consistent calculation. Dependence of the transmutation characteristics on the aspect ratios. Additional effects of Pu239 in transmutation blanket for minor actinides destruction. Transmutation of the minor actinides using by a compact transmutation reactor based on the LAR tokamak.
a r t i c l e
i n f o
Article history: Received 19 June 2013 Received in revised form 14 April 2014 Accepted 30 May 2014 Available online 1 July 2014 Keywords: Transmutation reactor LAR tokamak Minor actinides BISON-C Systems analysis
a b s t r a c t The transmutation characteristics of minor actinides in the transmutation reactor of a low aspect ratio (LAR) tokamak are investigated. One-dimensional neutron transport and burn-up calculations coupled with a tokamak systems analysis were performed to determine optimal system parameters. The dependence of the transmutation characteristics, including the neutron multiplication factor, produced power, and the transmutation rate, on the aspect ratio A in the range of 1.5–2.0 was examined. By adding Pu239 to the transmutation blanket as a neutron multiplication material, it was shown that a single transmutation reactor producing a fusion power of 150 MWth can destroy minor actinides contained in the spent fuels for more than 38 units of 1 GWe pressurized water reactors (PWRs) while producing a power in the range of 1.8–6.8 GWth . © 2014 Elsevier B.V. All rights reserved.
1. Introduction In order to destroy high-level waste from spent pressurized water reactor (PWR) fuel, the transmutation of long-lived radioactive actinides into short-lived and stable isotopes using a fission reactor, an accelerator driven system, and D-T fusion neutrons has been studied so as to reduce the long-term toxicity levels of nuclear waste [1–5]. Thermal fission approaches to destroy Pu and minor actinide (MA) isotopes in a fusion reactor have been also explored [2–5]. The transmutation of high-level waste through the utilization of D-T fusion neutrons will provide a viable application of fusion before its essential purpose of power production. Minor actinides can be separated from other nuclear waste based on an appropriate waste management strategy and thus, a feasibility study of the transmutation of minor actinides by a fission reaction with 14 MeV fusion neutrons in a low aspect ratio (LAR) tokamak
∗ Corresponding author. Tel.: +82 632195337; fax: +82 632704861. E-mail addresses: [email protected], [email protected] (S.Y. Moon). http://dx.doi.org/10.1016/j.fusengdes.2014.05.029 0920-3796/© 2014 Elsevier B.V. All rights reserved.
is very important. In a tokamak fusion device, an aspect ratio is defined as a ratio of a major radius to a minor radius. As a 14 MeV neutron source, the LAR tokamak is a viable option since it combines a compact tokamak reactor with plasma having a large elongated shape which is a favorable shape for a transmutation reactor. In order to destroy as much of the waste as possible while minimizing its overall size, the neutron source based on the LAR tokamak must be optimized. The sub-critical reactor concept is preferred in terms of reactor safety [6]. For the optimal design of a LAR tokamak neutron source, the radial build of the reactor components must be determined in order to satisfy all plasma physics and engineering constraints. Therefore, in this work we assume that the constraints are the same as those used in the design of the international thermonuclear experimental reactor (ITER). In a transmutation reactor, a blanket should produce enough tritium for tritium self-sufficiency, while the neutron multiplication factor keff should be less than 1.0 to maintain sub-criticality. Hence, the key objective in the neutronic design is to obtain a higher transmutation rate via optimization of the neutron energy spectrum, the inventories of actinides, and the thickness
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B.G. Hong, S.Y. Moon / Fusion Engineering and Design 89 (2014) 2523–2528
Fig. 1. Radial build of a transmutation reactor.
of the transmutation blanket. A shield should provide sufficient protection for the superconducting toroidal field (TF) coil against radiation damage and nuclear heating induced by both fusion and fission neutrons. For this purpose, we coupled the systems analysis [7] with one-dimensional neutron transport analysis, and the concept of the transmutation reactor based on the LAR tokamak was developed with an aspect ratio A in the range of 1.5–2.0. In this work, the transmutation characteristics of minor actinides in a LAR tokamak transmutation reactor with a neutron wall loading of less than l.0 MW/m2 were investigated. In order to obtain a higher neutron flux and improve the transmutation rate, Pu239 was chosen as a neutron multiplication material, and its effect on the transmutation characteristics was studied. In Section 2, the optimal system parameters of the LAR tokamak transmutation reactor are discussed with A in the range of 1.5–2.0. The transmutation characteristics of the minor actinides are then examined for three different aspect ratios, and the results are outlined in Section 3. Finally, a summary of our findings is provided in Section 4.
2. Concept of a LAR tokamak transmutation reactor The system parameters of the transmutation reactor must satisfy all plasma physics and engineering constraints. Such constraints are in turn related to various system parameters. In this work, we assumed that the constraints were on the same technological level as those in the design of the ITER. Furthermore, we coupled the tokamak systems analysis [7] with one-dimensional neutron transport and burn-up calculations for the self-consistent determination of system parameters. BISON-C [8] with a 42 neutron group cross section library based on JENDL-3 was employed for the one-dimensional neutron transport and burn-up calculations [9], while the JENDL-3 dosimetry file was used for estimations of the local tritium breeding ratio (TBR). In the BISON-C code, the onedimensional neutron transport equation was solved to obtain the neutron flux, and the nuclide production-depletion equations were solved using the obtained flux and the burn-up library. We note that, after the scoping study based on the systems analysis and onedimensional transport calculations, more accurate computations of quantities such as keff , the radiation damage to plasma-facing components, and the tritium breeding capability of the blanket using multi-dimensional analysis must be performed with the selected concept of the transmutation reactor. The radial build of the LAR tokamak transmutation reactor was modeled in a cylindrical geometry, as shown in Fig. 1. Neutron transport calculations were performed with the same geometry under the assumption of an isotropic neutron distribution. There was no inboard blanket in the inboard region because tritium selfsufficiency could be satisfied with the proper choice of an inboard shield material in the LAR tokamak [10]. The material composition of the reactor components is listed in Table 1. Consistent with
Table 1 Material composition of a transmutation reactor. Component
Material composition (vol.%)
Vacuum vessel Shield High temperature shield Blanket 1 Blanket 2 First wall
Borated steel (60), H2 O (40) WC (80), H2 O (20) WC MA2 O3 (50), SUS316 (15), He (35) SUS316 (7), PbLi (90), SiC (3) SUS316 (60), H2 O (40)
current ITER technology, water was selected for cooling all reactor components except for the blankets, and an Nb3 Sn superconductor was used as the toroidal magnetic field coil. In addition to the current density and maximum toroidal magnetic field at the inner leg of the TF coil, neutron damage to the superconductor should be taken into account to determine the radial build. The vacuum vessel was assumed to be made of 0.15 m-thick borated stainless steel cooled by water. The shield was composed of WC cooled by water. Sufficient space for the shield should be required to protect the superconducting TF coil against nuclear heating and radiation damage [10]. With the lifetime of the neutron source assumed to be 40 years at 75% availability, the fast neutron fluence on the superconductor should be kept below 1019 n cm−2 for Nb3 Sn, the displacement damage to the Cu stabilizer should be below 5 × 10−4 dpa, and the dose should be lower than 109 rad for the organic insulators. As seen in Fig. 1, blankets 1 and 2 have different functions and thus, they can be managed separately. In blanket 1, the minor actinides are loaded for transmutation. SUS316LN coated with SiC is used as a structural material, and helium is employed as a coolant in blanket 1. Blanket 2 is used for tritium breeding, where SUS316LN coated with SiC is utilized as the structural material, PbLi as the coolant, and tritium as the breeding material. By placing the tritium breeding blanket after the transmutation blanket, tritium self-sufficiency can be easily satisfied due to the abundance of neutrons produced by fission of the minor actinides. Therefore, natural Li can be employed, while the enrichment of Li-6 is necessary to satisfy the tritium self-sufficiency requirement in the fusion reactor [10]. The first wall with a thickness of 3 cm is made of SUS316LN cooled by water. The system parameters of the transmutation reactor based on a LAR tokamak with an aspect ratio A in the range of 1.5–2.0, which allow a compact sized reactor with a maximum fusion power of 150 MWth , were found following the procedure in Ref. [11]. The plasma physics and engineering constraints of the LAR tokamak were the same as those used in Ref. [10]. With the minor actinides in the blanket, the neutron flux from the fission of the minor actinides will have an impact on the shielding requirements, and the required inboard shield thickness will increase slightly when compared to the case without minor actinides. When considering a fusion power of 150 MWth , shield thicknesses of 40.0 cm for A = 1.5, 44.5 cm for
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Table 2 Parameters of a LAR tokamak. Parameters
A = 1.5
A = 1.8
A = 2.0
Fusion power (MW) Major radius (m) Minor radius (m) Plasma elongation Triangularity Plasma current (MA) Plasma beta Edge safety factor Neutron wall load (MW/m2 ) Heating power (MW) H factor Shield (cm)
150 3.49 2.32 3.2 0.3 12.3 0.491 2.6 0.18 122 1.2 40.0
150 2.72 1.51 2.9 0.3 9.2 0.297 2.7 0.38 125 1.2 44.5
150 2.49 1.24 2.7 0.3 7.9 0.22 2.8 0.51 126 1.2 46.0
A = 1.8 and 46.0 cm for A = 2.0 were required to provide adequate shielding for a 40 FPY lifetime within the radiation damage limit. As the aspect ratio increases, the neutron wall loading increases due to an increase in the shield thickness and a decrease in the plasma surface area. Table 2 shows the plasma performance and system parameters of the transmutation reactor for A = 1.5, 1.8, and 2.0.
Fig. 2. Variation of (a) keff and (b) specific power as the MAs burn up for A = 1.5, 1.8, and 2.0.
3. Characteristics of minor actinide transmutation We investigated the characteristics of minor actinide transmutation in the LAR tokamak transmutation reactor for A = 1.5, 1.8, and 2.0. The source strength, i.e., the fusion power, was maintained as 150 MWth . Table 3 shows the isotope composition of the minor actinides in the spent fuel of a 1 GWe Korean Standard Nuclear Power plant (KSNP) [12]. The radial build of the outboard blankets and the concentration of minor actinides in the outboard blankets were determined to limit the maximum neutron multiplication of keff to below 0.95 and satisfy the tritium self-sufficiency constraint of TBR > 1.35. We chose the maximum value of keff to 0.95 to allow sufficient safety margin. Blanket 1 was loaded with 50% MA2 O3 , 35% He, and 15% SUS316. The radial thickness of blanket 1 to limit keff to below 0.95 was 7.5 cm for A = 1.5 and 7.9 cm for A = 1.8 and 8.2 cm for A = 2.0. Blanket 2 was loaded with 90% PbLi (Nat. Li), 7% He, and 3% SUS316; the radial thickness of blanket 2 was set as 20.0 cm. Fig. 2 shows the variation of keff and the specific power (defined as the power per unit height as the minor actinides burn up) when A is 1.5, 1.8, and 2.0. The produced power is the sum of the fusion power from the plasma and the fission power generated in blanket 1. The keff and specific power initially increase, then decrease as the minor actinides burn up. For the larger A case, the maximum values of keff and the specific power occur earlier than in the case where A is smaller. The specific power is large for the large A case since the reactor height (which is proportional to Äa, where Ä is the elongation and a is the minor radius) is small, although the fission power, which is proportional to the fusion power times k/(1 − k), is the same for identical values of k [13]. LAR tokamaks have unique physics features such as large natural elongation, and it was shown that the maximum elongation depends on the aspect ratio, A and it decreases with the large A [14]. Fig. 3 shows the variation in the TBR as the minor actinides burn up when A is 1.5, 1.8, and 2.0. The TBR initially increases, then
Fig. 3. Variation of the TBR as the MAs burn up for A = 1.5, 1.8, and 2.0.
decreases as the minor actinides burn up. For the larger A case, the maximum value of the TBR occurs earlier than in the smaller A case. In order to satisfy the TBR > 1.35 condition in the timeaveraged sense, a sufficiently long burn-up time, which depends on the aspect ratio and thickness of blanket 2, is required. Fig. 4 displays the specific transmutation rate (defined as the transmutation rate per unit height) with respect to the burn-up time of the minor actinides. Since the transmutation rate is proportional to the fission power, the specific transmutation rate (specific T.R.) also increases as A increases. Both the major radius and the reactor height decrease with increasing A values, and the amount of loaded minor actinides for the large A case was smaller than that for the small A case. On the other hand, a larger blanket height was possible and a large amount of minor actinides could be loaded when A is small, but the
Table 3 Isotope composition of MAs in the spent fuel of a 1 GWe KSNP. Nuclide
Composition (wt%)
Np237 Am241 Am243 Cm244 Cm245
31.9 57.1 10.1 0.68 0.21
Fig. 4. Specific transmutation rate as the MAs burn up.
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Table 4 Transmuted MAs (kg) with 3000 days of burn-up time. Parameters
A = 1.5
A = 1.8
A = 2.0
Minor radius (cm), a Elongation, Ä Reactor height (cm), Äa Transmuted MAs (kg)
232 3.2 742 5536
151 2.9 438 7474
124 2.7 335 11,066
transmutation rate was small. In Table 4, the transmuted minor actinides (kg) over a burn-up time of 3000 days are compared for different values of A. We assumed the reactor height to be Äa to take into account the space for the reactor maintenance. Since the case of A = 2.0 allowed the largest specific transmutation rate with a 3.35 m reactor height, 11,066 kg of minor actinides could be transmuted over 3000 days, and 30 PWRs (1.0 GWe ) would be supported with a single transmutation reactor based on the LAR tokamak. Here, we assumed that the generation rate of the minor actinides was about 35 kg per year from one PWR with a 1 GWe capacity. The neutron energy spectra in blankets 1 and 2 are shown in Fig. 5 for two different burn-up times, T = 0 and 2500 days, when A is 2.0. As evident in Fig. 5(a), the neutron flux in blanket 1 increases as the minor actinides burned up due to the increase in keff . In Fig. 5(b), the neutron spectra in blanket 2 for T = 0 and 2500 days are compared for cases with and without minor actinides. At day 2500, self-sufficiency could easily be satisfied by placing the tritium breeding blanket after the transmutation blanket because the neutron flux showed higher values with minor actinides than without minor actinides, and there are an abundance of neutrons produced by fission of the minor actinides. Therefore, natural Li can be used, while Li-6 must be enriched for tritium self-sufficiency in a fusion reactor [13]. Fig. 6 demonstrates the behavior of the nuclides during the transmutation operation for A = 2.0. The amounts of Np237, Am241,
Fig. 5. Neutron energy spectra in (a) blanket 1 and (b) blanket 2 for A = 2.0 at burn-up times of T = 0 and 2500 days.
Fig. 6. Behavior of the MA nuclides during the transmutation operation for A = 2.0.
and Am243 constituting the majority of the minor actinides decrease. On the other hand, the amounts of Pu isotopes, Am242, and Cm244 produced from neutron absorption of the other actinides increase until enough neutrons are produced due to the buildup of fissile isotopes such as Pu239 and Pu241. After keff reaches its maximum, the decreasing rates of Np237, Am241, and Am243 increase, but the Pu isotopes, Am242, and Cm244 saturate or start to decrease. In order to transmute the minor actinides effectively, a high neutron flux and high transmutation rate are required. By adding fissionable material to the blanket, efficient minor actinides transmutation will be possible due to the high neutron flux and high power density. Therefore, Pu239 was chosen as the neutron multiplication material in this work, and its effect on the transmutation rate of the minor actinides was investigated. Fig. 7 shows the keff , specific power, and TBR with the addition of 4.0 wt% (≈1.0 × 1021 /cm3 ) Pu239 when A is 2.0. The radial thickness of blanket 1 for the keff value below 0.95 was 6.0 cm, which is smaller than that for the case with minor actinides only (8.2 cm). Thus, the initial inventory is small when compared to the minor actinides only case. The specific power and transmutation rate for the initial 3000 days are larger than in the case with minor actinides only. With the addition of Pu239, the shield thickness increases by approximately 3% due to the increased radiation impact. The amount of transmuted minor actinides for a 3000-day burn-up time was found to be 13,424 kg, which was 21% higher than the case with minor actinides only. Therefore, 38 PWRs (1.0 GWe ) could be supported with one unit of the transmutation reactor based on the LAR tokamak. Fig. 8 compares the specific transmutation rate for cases with and without Pu239 when A is 2.0. With the addition of Pu239, the maximum value of the specific transmutation rate occurred earlier, and the amount of transmuted minor actinides was larger when compared with the minor actinides only case. Fig. 9 shows the neutron spectra with and without the addition of Pu239 at a burn-up time of 1400 days. The fast neutron flux increases due to the addition of Pu239. Fig. 10 depicts the behavior of the nuclides during the transmutation operation for the case with Pu239 when A is 2.0. It was shown that the decreasing rates of Np237, Am241, and Am243 were higher than in the minor actinides only case. The buildup rate of Pu isotopes (e.g., Pu238, Pu240, and Pu241) was also higher than that for the minor actinides only case because the isotopes were bred from Np237 by neutron capture reactions and the neutrons went through multiplication via the addition of Pu239.
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Fig. 10. Behavior of the MA nuclides during the transmutation operation with Pu239 for A = 2.0.
4. Conclusion
Fig. 7. Variation of (a) keff and the specific power, and (b) the TBR as the MAs burn up with the addition of 4 wt% Pu239 for A = 2.0.
The concept of a transmutation reactor was studied by coupling a systems analysis with the one-dimensional neutron transport code, BISON-C. The optimum radial build for a LAR tokamak transmutation reactor was determined through self-consistent calculations of the physics and engineering constraints, which were related to various components of the transmutation reactor. Within the limit of ITER physics and engineering constraints, a compact transmutation reactor based on the LAR tokamak with an aspect ratio < 2 can be used for the transmutation of minor actinides produced from the spent fuel of a PWR. One unit of a transmutation reactor can support more than 30 PWRs (1.0 GWe ) with the production of thermal power equal to 1.0–6.6 GW. By adding Pu239 to the transmutation blanket as a neutron multiplication material, a higher neutron flux and transmutation rate can be achieved. Since the reactor can transmute minor actinides from 38 units of 1 GWe PWRs and produce 1.8–6.8 GW of thermal power, it is expected that the LAR tokamak transmutation reactor can be a viable option to transmute minor actinides. Acknowledgments
Fig. 8. Specific transmutation rate as the MAs burn up with Pu239 for A = 2.0.
This research was supported by a National Research Foundation (NRF) of Korea grant funded by the Korean Government (MSIP) under contract nos. 2011-0009653 and 2008-0061900. This research was also supported by research facilities in Plasma Application Institute of Chonbuk National University. References
Fig. 9. Neutron energy spectra with and without the addition of Pu239 at a burn-up time of 1400 days.
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