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Characteristics of nuclear waste transmutation based on a tokamak neutron source
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Characteristics of nuclear waste transmutation based on a tokamak neutron source B.G. Hong a,*, Philyong Oh b a b
Department of Quantum System Engineering, Chonbuk National University, Jeonbuk, Republic of Korea High-enthalpy Plasma Research Center, Chonbuk National University, Jeonbuk, Republic of Korea
article info
abstract
Article history:
The optimum configuration is found of a transmutation reactor based on a tokamak
Received 14 January 2015
neutron source for an aspect ratio in the range of 1.5e4.0 and fusion power in the range of
Received in revised form
100e500 MW, by means of coupled analysis of the tokamak systems and the neutron
18 February 2015
transport. The dependence of the transmutation characteristics upon the aspect ratio is
Accepted 21 February 2015
investigated for the case in which the transmutation reactor produces constant power. For
Available online xxx
a low-aspect-ratio tokamak, the power density requirement was satisfied with less fusion power, and less radial thickness of the breeding blanket was required to meet a tritium
Keywords:
breeding requirement, relative to a high-aspect-ratio tokamak. It was shown that by
Fusion-driven system
adjusting the portion of minor actinides contained in the transuranic actinides, the nuclear
Nuclear waste transmutation
waste transmutation could be optimized with low fusion power while producing constant
Tokamak neutron source
power. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A concept has been studied of using a transmutation reactor based on a 14 MeV fusion neutron source [1e4] to destroy the nuclear waste contained in spent nuclear fuels. The transmutation reactor would not only burn up the nuclear waste, but would also produce power from its fission reactions. For economic reasons, the overall size of such a transmutation reactor based on a tokamak neutron source needs to be minimized. An optimal radial build of the reactor components can be found by optimizing both the transmutation rate and the tritium breeding ratio, while also limiting the neutron multiplication factor, keff, to less than 1.0, to allow safer subcritical operation, as well as limiting power density to less than 200 MW/m3 to satisfy reactor cooling requirements. To
allow self-consistent determination of the radial builds of the reactor components, a radiation transport calculation is necessary, and must be coupled with tokamak systems analysis [5], because not only the fusion neutrons but also the fission neutrons have impacts on the engineering constraints that must be satisfied by each component of the transmutation reactor. In the transmutation reactor, the fission of the nuclear waste provides abundant thermal neutrons, allowing tritium self-sufficiency; thus, nuclear waste can be loaded into the outboard blanket. In this case, the plasma performance and the shielding requirement mainly determine the inboard radial build, including the minimum major radius; and the neutron multiplication, the power density and the tritium breeding requirements determine the outboard radial build. The shield should provide sufficient protection of the
* Corresponding author. E-mail address: [email protected] (B.G. Hong). http://dx.doi.org/10.1016/j.ijhydene.2015.02.076 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
superconducting toroidal field (TF) coil from radiation damage and from nuclear heating induced by the neutrons. In this study, the optimum radial build is found of a tokamak neutron source with the aspect ratio, A, in the range of 1.5e2.5 and with fusion power in the range of 100e500 MW. The transmutation reactor's produced power and transmuted nuclear waste are proportional to Sfusion$keff/(1 keff), where Sfusion is the fusion neutron strength. To produce a large and a constant power, keff needs to be kept high and approximately constant during the burn-up period, or the fusion power needs to be increased as the nuclear waste burns up to compensate for the consumption of neutrons. To keep keff high and approximately constant, and thus to transmute nuclear waste effectively, the portions of Pu and of minor actinides (MAs) can be controlled. Depending on the nuclear waste management strategy, fission products can be eliminated and transuranic actinides (TRU: Pu and minor actinides Np, Am, Cm, etc.) can be transmuted by adjusting the amount loaded of the MAs. In this study, we investigate the dependence of the transmutation characteristics upon the aspect ratio, and derive requirements for the tokamak neutron source for the case in which the produced power is 3000 MW.
Optimum configuration of a transmutation reactor To determine the radial build of each component of the transmutation reactor, the physics and engineering constraints must be calculated that each component needs to satisfy. In this study, the physics and engineering constraints were adopted that are used in the ITER design [6]. Systems analysis coupled with one-dimensional radiation transport analysis was used to find the optimal configuration for the transmutation reactor. The one-dimensional radiation transport code BISON-C [7] was used, with a 42-neutron group cross-section library based on JENDL-3 [8]. We note that after the scoping study based on the one-dimensional calculation is performed and thus key characteristics of the reactor are identified, more accurate calculation of the neutronic quantities such as the neutron multiplication, keff, the fission power, and the tritium breeding capability of the blanket using multi-dimensional analysis needs to be performed with the selected concept of the reactor. The transmutation reactor was modeled by using onedimensional cylindrical geometry. Table 1 lists the reactor's radial structure and material compositions; the radial structure comprises a TF coil, a vacuum vessel, a shield, a blanket, and the plasma. There is no central solenoid coil; it is assumed that plasma current will be ramped up and maintained by current drive systems. To enable tritium self-sufficiency, the tritium breeding blanket is placed outside of the transmutation blanket to allow utilization of the thermal neutrons produced by the fission of the nuclear waste. Natural Li can therefore be used, whereas enriched Li-6 needs to be used in a fusion reactor [9]. In the proposed design, the toroidal magnetic field coil is made of Nb3Sn superconductor, the same as in the ITER specification. To determine the radial build of the TF coil, the
Table 1 e Radial build and material composition of the transmutation reactor. Component Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma Scrape-off layer First wall Transmutation blanket Breeding blanket High-temp. shield Low-temp. shield Vacuum vessel Gap Toroidal field coil
Materials e Nb3Sn, SUS316L, He Borated steel, H2O WC, H2O e D, T e SUS316L, H2O TRU, SUS316L, He, SiC SUS316L, PbLi, SiC WC WC, H2O Borated steel, H2O e Nb3Sn, SUS316L, He
current density and the maximum toroidal magnetic field at the inner leg of the TF coil must be taken into account, as well as the neutron damage to the superconductor. The vacuum vessel is assumed to be water-cooled and made of borated stainless steel 0.15 m thick. To allow determination of the radial build of the blanket and the shield, the quantities have to be calculated of keff, the power density, the tritium breeding ratio (TBR), and the radiation impacts on the TF coil. The shield is water-cooled and is made of WC. Sufficient space for the shield is necessary to protect the superconducting TF coil from nuclear heating and radiation damage, including fast neutron fluence to the superconductor of less than 1019 n$cm2 for Nb3Sn, displacement damage to the Cu stabilizer of less than 5 104 dpa, and dose to the insulators of less than 109 rad for organic insulators. The design lifetime for the tokamak neutron source is assumed to be 40 y with 75% availability. Sufficient space for the blankets should be maintained to maximize the TBR and energy multiplication, and to keep the power density below 200 MW/m3. In the transmutation blanket, the nuclear waste from the spent fuel of a 1 GWe Korea Standard Nuclear Power Plant (KSNP) is loaded for transmutation; SUS316L coated with SiC is used as a structural material and He is used as a coolant. For the breeding blanket, He-cooled lithium lead (LiPb) as the tritium breeding and neutron multiplying material, and SiC-coated SUS316L as the structural material is considered. The first wall is made of SUS316L, assumed to be 2 cm thick, and is water-cooled. Plasma performance is characterized by a beta limit, a plasma current limit imposed by a limitation on the safety factor q at the edge, and a plasma density limit. Appropriate models for plasma composition, non-inductive current drive, bootstrap current fraction, divertor heat load, etc. are also needed to calculate the plasma performance. Models for the plasma performance depend on the aspect ratio; we used the scaling laws given in Ref. 9 for the low-aspect-ratio tokamak and those given in Ref. 6 for the high-aspect-ratio tokamak. When A is small, features such as large natural elongation, improved confinement, large plasma b and high bootstrap current fraction make the physics model of the low-aspect-
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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Table 3 e Optimum inboard radial build of the transmutation reactor for A ¼ 3.0.
Fig. 1 e Dependence upon the aspect ratio of (a) minimum major radius and (b) neutron wall loading.
ratio tokamak distinct from that based on the high-aspectratio tokamak. Radial builds of the transmutation reactor for A ranging from 1.5 to 4.0 were investigated for fusion power in the range of 100e500 MW. The plasma performance was assumed to produce a given fusion power with a safety factor of qa ¼ qa,min, normalized plasma beta of bN ¼ bN,max, confinement enhancement factor of H ¼ 1.2, and plasma density n ¼ nG, where nG is the Greenwald density limit. Fig. 1 shows the effects of the aspect ratio upon the minimum major radius and the neutron wall loading. For a given fusion power, the minimum major radius, R0, decreases as A increases from 1.5 to 2.0, but increases as A increases above 2.5. This transition arises because the scaling laws for the plasma performance differ between the low- and high-aspectratio tokamaks. Also, as A and the fusion power increase, the magnetic field at the TF coil increases until it reaches the
Fusion power [MW] 100 Component (accumulated, [m])
200 300
400 500
Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma
0.57 0.97 1.12 1.7 1.8 0.57
0.76 1.18 1.33 1.93 2.03 0.76
0.4 0.81 0.96 1.51 1.61 0.4
0.67 1.08 1.23 1.83 1.93 0.67
0.82 1.25 1.4 2.01 2.11 0.82
engineering limit, which is assumed to be 13 T. Thus the bore radius of the inboard TF coil and R0 both increase. Given an aspect ratio and the plasma performance, the minimum major radius is determined based on the constraints of the shielding and the maximum magnetic field at the TF coil. Neutron wall loading increases as A increases, because this corresponds to decreasing plasma surface area; the neutron wall loading is less than 2 MW/m2 when A is in the range of 1.5e4.0 and fusion power is in the range of 100e500 MW. Tables 2 and 3 show the optimum inboard radial builds of the transmutation reactor for A ¼ 1.5 and A ¼ 3.0, assuming fusion power ranging from 100 to 500 MW. The shield thickness is increased as A and the fusion power increase because the neutron wall loading increases. When A ¼ 3.0, the space for the bore is increased as the fusion power increases to meet the constraints of the maximum magnetic field at the TF coil.
Transmutation characteristics The radial build of the outboard blankets was determined to limit the maximum keff to 0.95, to keep the power density below 200 MW/m3, and to satisfy the tritium self-sufficiency criterion of TBR > 1.35 (in the one-dimensional model with the blanket coverage factor of 80% and averaged over the burn cycle). For the case of TRU transmutation [3], keff decreases from its starting value as the TRU burns up; it decreases more rapidly for conditions of larger fusion power and larger A due to greater neutron wall loading. Thus, the radial thickness of the transmutation blanket was determined to satisfy
Table 2 e Optimum inboard radial build of the transmutation reactor for A ¼ 1.5. Fusion power [MW] 100 Component (accumulated, [m])
200
300 400
500
Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma
0.1 0.30 0.45 0.88 0.98 5.01
0.1 0.33 0.48 0.92 1.02 5.2
0.1 0.35 0.5 0.96 1.06 5.44
0.1 0.27 0.42 0.82 0.92 4.7
0.1 0.35 0.5 0.95 1.05 5.33
Fig. 2 e Variation of keff and the required fusion power when A ¼ 1.5.
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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Fig. 3 e Variation of the keff and the required fusion power when A ¼ 3.0.
keff ¼ 0.95 at the beginning of the cycle. In the case of MA transmutation [4], keff initially increases, then decreases as the minor actinides burn up; keff reaches its maximum earlier as A is increased. For the produced power of 3000 MW, a tokamak neutron source with A ¼ 1.5 and 2.0 satisfies the requirements of power density less than 200 MW/m3 with fusion power less than 100 MW, but for A ¼ 2.5, a tokamak neutron source with fusion power greater than 200 MW is required, and for A ¼ 3.0, more than 300 MW is required. As the TRU burns up, the TBR decreases due to the burn-up of Li-6. To satisfy the requirement of TBR > 1.35, the required radial thickness of the breeding blanket, DBL, increases as the fusion power and the aspect ratio increase. DBL of 20 cm is enough for A ¼ 1.5, but DBL of greater than 100 cm is required for A ¼ 3.0. Natural Li can be used for the small-A case with low fusion power and a short burn-up cycle, whereas Li-6 needs to be enriched for the large-A case with high fusion power and a long burn-up cycle because the TBR saturates for DBL greater than 100 cm. To transmute the nuclear waste effectively, keff needs to be kept large and approximately constant, because the transmutation capability and the produced power are proportional to Sfusion$keff/(1 keff). To produce constant power, the fusion power has to be adjusted as the TRUs burn up, to respond to the resulting variation of keff. We investigated the effects upon the transmutation characteristics of varying the portion of Pu and MA in the TRU. By varying the portion of Pu and MA, the neutron energy spectrum is adjusted for effective transmutation. Pu isotopes which are bred from Np by neutron capture reactions multiply the neutrons and these fast neutrons transmute the MAs by the fission reaction.
The effects of TRU composition are shown in Figs. 2 and 3 for A ¼ 1.5 and A ¼ 3.0, respectively, for the produced power of 3000 MW. The reactor height was assumed to be k$a (where k is an elongation and a is a minor radius), with k ¼ 3.2 and a ¼ 1.89 m for A ¼ 1.5, and k ¼ 2.1 and a ¼ 0.96 m for A ¼ 3.0. When the portions of Pu and MA were adjusted, the rate of decrease of keff was smaller than when the portions were not adjusted, and less fusion power was also required. keff was maximized during burn-up, as the portion of MA increased. With the tokamak system parameters selected for A ¼ 1.5 and the fusion power of 100 MW, the variation of keff becomes small and the required fusion power decreases as the portion of MA increases (Fig. 2). The burn-up period of 1000 d was found to be possible for cases in which the portion of MA is greater than 22%. For the MA portion of 48%, it was found that the required fusion power is less than 100 MW for a burn-up period of longer than 2000 d, and that a burn-up fraction of 13% could be obtained. For A ¼ 3.0 and the fusion power of 300 MW, the burn-up period of 1000 d was found to be possible for cases in which the portion of MA is greater than 28%; for the MA portion of 48%, it was found that the required fusion power is less than 300 MW up to a 1700 d burn-up period and that the burn-up fraction of 35% could be obtained. Table 4 compares the transmutation characteristics during a 1000 d burn-up period for the cases of A ¼ 1.5 and A ¼ 3.0 and various MA portions. The transmutation rate of the TRU does not change for the two different values of A, due to the fixed produced power, but the transmutation rates of Pu and MA vary depending on the MA portion and the aspect ratio. The case of greater A yielded a greater burn-up fraction due to the greater neutron wall loading. Thus, assuming that the TRU from 1 PWR is about 250 kg/y including MA of about 35 kg/y, it was found that by adjusting the portion of MA, a single transmutation reactor unit based on the tokamak neutron source can not only transmute more than 3 PWRs with 1.0 GWe capacity but could also produce 3000 MW. With the MA portion of 48%, Pu from more than 2 PWRs and MA from more than 10 PWRS could be transmuted.
Conclusions The optimum configuration was found of a transmutation reactor based on a tokamak neutron source for aspect ratios ranging from 1.5 to 4.0 and fusion power ranging from 100 to 500 MW, by means of self-consistent calculation of the physical and engineering constraints relating to the various components of the transmutation reactor. For a given fusion power, R0 decreases as A increases within the range from 1.5 to 2.0, but then increases with A for
Table 4 e Transmutation characteristics for A ¼ 1.5 and 3.0, and for various MA portions. MA portion [%] A ¼ 1.5 A ¼ 3.0
Trans. rate [kg/y] Burn-up [%] Trans. rate [kg/y] Burn-up [%]
28
35
48
TRU
Pu
MA
TRU
Pu
MA
TRU
Pu
MA
862 8 e e
698 9 e e
164 6 e e
866 7 866 21
615 8 584 22
251 6 282 19
872 6 872 19
478 6 457 19
394 6 415 19
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
A greater than 2.5, because the engineering limit on the magnetic field at the TF coil requires that both R0 and the bore radius of the inboard TF coil increase. Within the given aspect ratio and plasma performance ranges, the minimum major radius is determined by the constraints of the shielding and the maximum magnetic field at the TF coil. The dependence of the transmutation characteristics upon the aspect ratio was investigated for conditions of fixed produced power. For a low-aspect-ratio tokamak, the power density requirement was satisfied with less fusion power, and less radial thickness of the breeding blanket was required to meet the tritium breeding requirement, relative to a highaspect-ratio tokamak. It was shown that by adjusting the portion of MA, the produced power as well as the transmuted nuclear waste could be kept large and constant with low fusion power, and that a single transmutation reactor unit based on the tokamak neutron source could not only transmute the spent fuel from more than more than 3 PWRs with 1.0 GWe capacity but could also produce 3000 MW. By using the MA portion of 48%, the Pu from more than 2 PWRs and the MA from more than 10 PWRs could be transmuted.
Acknowledgments This paper was supported by the research funds of Chonbuk National University in 2014 and by the research facilities of the
5
Plasma Application Institute of Chonbuk National University. This work was also supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korean government (MSIP) under contracts 2011-0009653 and 20080061900.
references
[1] Wu Y, et al. Conceptual design of the fusion-driven subcritical system FDS-I. Fusion Eng Des 2006;81:1305. [2] Stacey WM. Transmutation missions for fusion neutron sources. Fusion Eng Des 2007;82:11. [3] Hong BG. Conceptual study of a fusion-driven transmutation reactor based on low aspect ratio tokamak as a neutron source. Fusion Sci Technol 2013;63:488. [4] Hong BG, Moon SY. Transmutation characteristics of minor actinides in a low aspect ratiotokamak fusion reactor. Fusion Eng Des 2014;89:2523. [5] Hong BG, Lee DW, In SR. Tokamak reactor system analysis code for the conceptual development of DEMO reactor. Nucl Eng Technol 2008;40:1. [6] Shimada M, et al. ITER physics basis. Nucl Fusion 2007;47:S1. [7] BISON-C, CCC-659, Oak Ridge National Laboratory. [8] Nakagawa T, et al. Japanese evaluated nuclear data library version 3 reversion-2: JENDL-3.2. J Nucl Sci Technol 1995;32:1259. [9] Hong BG, et al. Conceptual design study of a superconducting spherical tokamak reactor with a self-consistent system analysis code. Nucl Fusion 2011;51:113013.
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Characteristics of nuclear waste transmutation based on a tokamak neutron source B.G. Hong a,*, Philyong Oh b a b
Department of Quantum System Engineering, Chonbuk National University, Jeonbuk, Republic of Korea High-enthalpy Plasma Research Center, Chonbuk National University, Jeonbuk, Republic of Korea
article info
abstract
Article history:
The optimum configuration is found of a transmutation reactor based on a tokamak
Received 14 January 2015
neutron source for an aspect ratio in the range of 1.5e4.0 and fusion power in the range of
Received in revised form
100e500 MW, by means of coupled analysis of the tokamak systems and the neutron
18 February 2015
transport. The dependence of the transmutation characteristics upon the aspect ratio is
Accepted 21 February 2015
investigated for the case in which the transmutation reactor produces constant power. For
Available online xxx
a low-aspect-ratio tokamak, the power density requirement was satisfied with less fusion power, and less radial thickness of the breeding blanket was required to meet a tritium
Keywords:
breeding requirement, relative to a high-aspect-ratio tokamak. It was shown that by
Fusion-driven system
adjusting the portion of minor actinides contained in the transuranic actinides, the nuclear
Nuclear waste transmutation
waste transmutation could be optimized with low fusion power while producing constant
Tokamak neutron source
power. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A concept has been studied of using a transmutation reactor based on a 14 MeV fusion neutron source [1e4] to destroy the nuclear waste contained in spent nuclear fuels. The transmutation reactor would not only burn up the nuclear waste, but would also produce power from its fission reactions. For economic reasons, the overall size of such a transmutation reactor based on a tokamak neutron source needs to be minimized. An optimal radial build of the reactor components can be found by optimizing both the transmutation rate and the tritium breeding ratio, while also limiting the neutron multiplication factor, keff, to less than 1.0, to allow safer subcritical operation, as well as limiting power density to less than 200 MW/m3 to satisfy reactor cooling requirements. To
allow self-consistent determination of the radial builds of the reactor components, a radiation transport calculation is necessary, and must be coupled with tokamak systems analysis [5], because not only the fusion neutrons but also the fission neutrons have impacts on the engineering constraints that must be satisfied by each component of the transmutation reactor. In the transmutation reactor, the fission of the nuclear waste provides abundant thermal neutrons, allowing tritium self-sufficiency; thus, nuclear waste can be loaded into the outboard blanket. In this case, the plasma performance and the shielding requirement mainly determine the inboard radial build, including the minimum major radius; and the neutron multiplication, the power density and the tritium breeding requirements determine the outboard radial build. The shield should provide sufficient protection of the
* Corresponding author. E-mail address: [email protected] (B.G. Hong). http://dx.doi.org/10.1016/j.ijhydene.2015.02.076 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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superconducting toroidal field (TF) coil from radiation damage and from nuclear heating induced by the neutrons. In this study, the optimum radial build is found of a tokamak neutron source with the aspect ratio, A, in the range of 1.5e2.5 and with fusion power in the range of 100e500 MW. The transmutation reactor's produced power and transmuted nuclear waste are proportional to Sfusion$keff/(1 keff), where Sfusion is the fusion neutron strength. To produce a large and a constant power, keff needs to be kept high and approximately constant during the burn-up period, or the fusion power needs to be increased as the nuclear waste burns up to compensate for the consumption of neutrons. To keep keff high and approximately constant, and thus to transmute nuclear waste effectively, the portions of Pu and of minor actinides (MAs) can be controlled. Depending on the nuclear waste management strategy, fission products can be eliminated and transuranic actinides (TRU: Pu and minor actinides Np, Am, Cm, etc.) can be transmuted by adjusting the amount loaded of the MAs. In this study, we investigate the dependence of the transmutation characteristics upon the aspect ratio, and derive requirements for the tokamak neutron source for the case in which the produced power is 3000 MW.
Optimum configuration of a transmutation reactor To determine the radial build of each component of the transmutation reactor, the physics and engineering constraints must be calculated that each component needs to satisfy. In this study, the physics and engineering constraints were adopted that are used in the ITER design [6]. Systems analysis coupled with one-dimensional radiation transport analysis was used to find the optimal configuration for the transmutation reactor. The one-dimensional radiation transport code BISON-C [7] was used, with a 42-neutron group cross-section library based on JENDL-3 [8]. We note that after the scoping study based on the one-dimensional calculation is performed and thus key characteristics of the reactor are identified, more accurate calculation of the neutronic quantities such as the neutron multiplication, keff, the fission power, and the tritium breeding capability of the blanket using multi-dimensional analysis needs to be performed with the selected concept of the reactor. The transmutation reactor was modeled by using onedimensional cylindrical geometry. Table 1 lists the reactor's radial structure and material compositions; the radial structure comprises a TF coil, a vacuum vessel, a shield, a blanket, and the plasma. There is no central solenoid coil; it is assumed that plasma current will be ramped up and maintained by current drive systems. To enable tritium self-sufficiency, the tritium breeding blanket is placed outside of the transmutation blanket to allow utilization of the thermal neutrons produced by the fission of the nuclear waste. Natural Li can therefore be used, whereas enriched Li-6 needs to be used in a fusion reactor [9]. In the proposed design, the toroidal magnetic field coil is made of Nb3Sn superconductor, the same as in the ITER specification. To determine the radial build of the TF coil, the
Table 1 e Radial build and material composition of the transmutation reactor. Component Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma Scrape-off layer First wall Transmutation blanket Breeding blanket High-temp. shield Low-temp. shield Vacuum vessel Gap Toroidal field coil
Materials e Nb3Sn, SUS316L, He Borated steel, H2O WC, H2O e D, T e SUS316L, H2O TRU, SUS316L, He, SiC SUS316L, PbLi, SiC WC WC, H2O Borated steel, H2O e Nb3Sn, SUS316L, He
current density and the maximum toroidal magnetic field at the inner leg of the TF coil must be taken into account, as well as the neutron damage to the superconductor. The vacuum vessel is assumed to be water-cooled and made of borated stainless steel 0.15 m thick. To allow determination of the radial build of the blanket and the shield, the quantities have to be calculated of keff, the power density, the tritium breeding ratio (TBR), and the radiation impacts on the TF coil. The shield is water-cooled and is made of WC. Sufficient space for the shield is necessary to protect the superconducting TF coil from nuclear heating and radiation damage, including fast neutron fluence to the superconductor of less than 1019 n$cm2 for Nb3Sn, displacement damage to the Cu stabilizer of less than 5 104 dpa, and dose to the insulators of less than 109 rad for organic insulators. The design lifetime for the tokamak neutron source is assumed to be 40 y with 75% availability. Sufficient space for the blankets should be maintained to maximize the TBR and energy multiplication, and to keep the power density below 200 MW/m3. In the transmutation blanket, the nuclear waste from the spent fuel of a 1 GWe Korea Standard Nuclear Power Plant (KSNP) is loaded for transmutation; SUS316L coated with SiC is used as a structural material and He is used as a coolant. For the breeding blanket, He-cooled lithium lead (LiPb) as the tritium breeding and neutron multiplying material, and SiC-coated SUS316L as the structural material is considered. The first wall is made of SUS316L, assumed to be 2 cm thick, and is water-cooled. Plasma performance is characterized by a beta limit, a plasma current limit imposed by a limitation on the safety factor q at the edge, and a plasma density limit. Appropriate models for plasma composition, non-inductive current drive, bootstrap current fraction, divertor heat load, etc. are also needed to calculate the plasma performance. Models for the plasma performance depend on the aspect ratio; we used the scaling laws given in Ref. 9 for the low-aspect-ratio tokamak and those given in Ref. 6 for the high-aspect-ratio tokamak. When A is small, features such as large natural elongation, improved confinement, large plasma b and high bootstrap current fraction make the physics model of the low-aspect-
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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Table 3 e Optimum inboard radial build of the transmutation reactor for A ¼ 3.0.
Fig. 1 e Dependence upon the aspect ratio of (a) minimum major radius and (b) neutron wall loading.
ratio tokamak distinct from that based on the high-aspectratio tokamak. Radial builds of the transmutation reactor for A ranging from 1.5 to 4.0 were investigated for fusion power in the range of 100e500 MW. The plasma performance was assumed to produce a given fusion power with a safety factor of qa ¼ qa,min, normalized plasma beta of bN ¼ bN,max, confinement enhancement factor of H ¼ 1.2, and plasma density n ¼ nG, where nG is the Greenwald density limit. Fig. 1 shows the effects of the aspect ratio upon the minimum major radius and the neutron wall loading. For a given fusion power, the minimum major radius, R0, decreases as A increases from 1.5 to 2.0, but increases as A increases above 2.5. This transition arises because the scaling laws for the plasma performance differ between the low- and high-aspectratio tokamaks. Also, as A and the fusion power increase, the magnetic field at the TF coil increases until it reaches the
Fusion power [MW] 100 Component (accumulated, [m])
200 300
400 500
Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma
0.57 0.97 1.12 1.7 1.8 0.57
0.76 1.18 1.33 1.93 2.03 0.76
0.4 0.81 0.96 1.51 1.61 0.4
0.67 1.08 1.23 1.83 1.93 0.67
0.82 1.25 1.4 2.01 2.11 0.82
engineering limit, which is assumed to be 13 T. Thus the bore radius of the inboard TF coil and R0 both increase. Given an aspect ratio and the plasma performance, the minimum major radius is determined based on the constraints of the shielding and the maximum magnetic field at the TF coil. Neutron wall loading increases as A increases, because this corresponds to decreasing plasma surface area; the neutron wall loading is less than 2 MW/m2 when A is in the range of 1.5e4.0 and fusion power is in the range of 100e500 MW. Tables 2 and 3 show the optimum inboard radial builds of the transmutation reactor for A ¼ 1.5 and A ¼ 3.0, assuming fusion power ranging from 100 to 500 MW. The shield thickness is increased as A and the fusion power increase because the neutron wall loading increases. When A ¼ 3.0, the space for the bore is increased as the fusion power increases to meet the constraints of the maximum magnetic field at the TF coil.
Transmutation characteristics The radial build of the outboard blankets was determined to limit the maximum keff to 0.95, to keep the power density below 200 MW/m3, and to satisfy the tritium self-sufficiency criterion of TBR > 1.35 (in the one-dimensional model with the blanket coverage factor of 80% and averaged over the burn cycle). For the case of TRU transmutation [3], keff decreases from its starting value as the TRU burns up; it decreases more rapidly for conditions of larger fusion power and larger A due to greater neutron wall loading. Thus, the radial thickness of the transmutation blanket was determined to satisfy
Table 2 e Optimum inboard radial build of the transmutation reactor for A ¼ 1.5. Fusion power [MW] 100 Component (accumulated, [m])
200
300 400
500
Space Toroidal field coil Vacuum vessel Shield Scrape-off layer Plasma
0.1 0.30 0.45 0.88 0.98 5.01
0.1 0.33 0.48 0.92 1.02 5.2
0.1 0.35 0.5 0.96 1.06 5.44
0.1 0.27 0.42 0.82 0.92 4.7
0.1 0.35 0.5 0.95 1.05 5.33
Fig. 2 e Variation of keff and the required fusion power when A ¼ 1.5.
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Fig. 3 e Variation of the keff and the required fusion power when A ¼ 3.0.
keff ¼ 0.95 at the beginning of the cycle. In the case of MA transmutation [4], keff initially increases, then decreases as the minor actinides burn up; keff reaches its maximum earlier as A is increased. For the produced power of 3000 MW, a tokamak neutron source with A ¼ 1.5 and 2.0 satisfies the requirements of power density less than 200 MW/m3 with fusion power less than 100 MW, but for A ¼ 2.5, a tokamak neutron source with fusion power greater than 200 MW is required, and for A ¼ 3.0, more than 300 MW is required. As the TRU burns up, the TBR decreases due to the burn-up of Li-6. To satisfy the requirement of TBR > 1.35, the required radial thickness of the breeding blanket, DBL, increases as the fusion power and the aspect ratio increase. DBL of 20 cm is enough for A ¼ 1.5, but DBL of greater than 100 cm is required for A ¼ 3.0. Natural Li can be used for the small-A case with low fusion power and a short burn-up cycle, whereas Li-6 needs to be enriched for the large-A case with high fusion power and a long burn-up cycle because the TBR saturates for DBL greater than 100 cm. To transmute the nuclear waste effectively, keff needs to be kept large and approximately constant, because the transmutation capability and the produced power are proportional to Sfusion$keff/(1 keff). To produce constant power, the fusion power has to be adjusted as the TRUs burn up, to respond to the resulting variation of keff. We investigated the effects upon the transmutation characteristics of varying the portion of Pu and MA in the TRU. By varying the portion of Pu and MA, the neutron energy spectrum is adjusted for effective transmutation. Pu isotopes which are bred from Np by neutron capture reactions multiply the neutrons and these fast neutrons transmute the MAs by the fission reaction.
The effects of TRU composition are shown in Figs. 2 and 3 for A ¼ 1.5 and A ¼ 3.0, respectively, for the produced power of 3000 MW. The reactor height was assumed to be k$a (where k is an elongation and a is a minor radius), with k ¼ 3.2 and a ¼ 1.89 m for A ¼ 1.5, and k ¼ 2.1 and a ¼ 0.96 m for A ¼ 3.0. When the portions of Pu and MA were adjusted, the rate of decrease of keff was smaller than when the portions were not adjusted, and less fusion power was also required. keff was maximized during burn-up, as the portion of MA increased. With the tokamak system parameters selected for A ¼ 1.5 and the fusion power of 100 MW, the variation of keff becomes small and the required fusion power decreases as the portion of MA increases (Fig. 2). The burn-up period of 1000 d was found to be possible for cases in which the portion of MA is greater than 22%. For the MA portion of 48%, it was found that the required fusion power is less than 100 MW for a burn-up period of longer than 2000 d, and that a burn-up fraction of 13% could be obtained. For A ¼ 3.0 and the fusion power of 300 MW, the burn-up period of 1000 d was found to be possible for cases in which the portion of MA is greater than 28%; for the MA portion of 48%, it was found that the required fusion power is less than 300 MW up to a 1700 d burn-up period and that the burn-up fraction of 35% could be obtained. Table 4 compares the transmutation characteristics during a 1000 d burn-up period for the cases of A ¼ 1.5 and A ¼ 3.0 and various MA portions. The transmutation rate of the TRU does not change for the two different values of A, due to the fixed produced power, but the transmutation rates of Pu and MA vary depending on the MA portion and the aspect ratio. The case of greater A yielded a greater burn-up fraction due to the greater neutron wall loading. Thus, assuming that the TRU from 1 PWR is about 250 kg/y including MA of about 35 kg/y, it was found that by adjusting the portion of MA, a single transmutation reactor unit based on the tokamak neutron source can not only transmute more than 3 PWRs with 1.0 GWe capacity but could also produce 3000 MW. With the MA portion of 48%, Pu from more than 2 PWRs and MA from more than 10 PWRS could be transmuted.
Conclusions The optimum configuration was found of a transmutation reactor based on a tokamak neutron source for aspect ratios ranging from 1.5 to 4.0 and fusion power ranging from 100 to 500 MW, by means of self-consistent calculation of the physical and engineering constraints relating to the various components of the transmutation reactor. For a given fusion power, R0 decreases as A increases within the range from 1.5 to 2.0, but then increases with A for
Table 4 e Transmutation characteristics for A ¼ 1.5 and 3.0, and for various MA portions. MA portion [%] A ¼ 1.5 A ¼ 3.0
Trans. rate [kg/y] Burn-up [%] Trans. rate [kg/y] Burn-up [%]
28
35
48
TRU
Pu
MA
TRU
Pu
MA
TRU
Pu
MA
862 8 e e
698 9 e e
164 6 e e
866 7 866 21
615 8 584 22
251 6 282 19
872 6 872 19
478 6 457 19
394 6 415 19
Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
A greater than 2.5, because the engineering limit on the magnetic field at the TF coil requires that both R0 and the bore radius of the inboard TF coil increase. Within the given aspect ratio and plasma performance ranges, the minimum major radius is determined by the constraints of the shielding and the maximum magnetic field at the TF coil. The dependence of the transmutation characteristics upon the aspect ratio was investigated for conditions of fixed produced power. For a low-aspect-ratio tokamak, the power density requirement was satisfied with less fusion power, and less radial thickness of the breeding blanket was required to meet the tritium breeding requirement, relative to a highaspect-ratio tokamak. It was shown that by adjusting the portion of MA, the produced power as well as the transmuted nuclear waste could be kept large and constant with low fusion power, and that a single transmutation reactor unit based on the tokamak neutron source could not only transmute the spent fuel from more than more than 3 PWRs with 1.0 GWe capacity but could also produce 3000 MW. By using the MA portion of 48%, the Pu from more than 2 PWRs and the MA from more than 10 PWRs could be transmuted.
Acknowledgments This paper was supported by the research funds of Chonbuk National University in 2014 and by the research facilities of the
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Plasma Application Institute of Chonbuk National University. This work was also supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korean government (MSIP) under contracts 2011-0009653 and 20080061900.
references
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Please cite this article in press as: Hong BG, Oh P, Characteristics of nuclear waste transmutation based on a tokamak neutron source, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.076