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First wall material damage induced by fusion-fission neutron environment
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First wall material damage induced by fusion-fission neutron environment Vladimir Khripunov National Research Center “Kurchatov Institute”, Kurchatov Sq., 1, Moscow 123182, Russia
h i g h l i g h t s • The highest damage and gas production rates are experienced within the first wall materials of a hybrid fusion-fission system. • About ∼2 times higher dpa and 4–5 higher He appm are expected compared to the values distinctive for a pure fusion system at the same DT-neutron wall loading.
• The specific nuclear heating may be increased by a factor of ∼8–9 due to fusion and fission neutrons radiation capture in metal components of the first wall.
a r t i c l e
i n f o
Article history: Received 28 August 2015 Received in revised form 6 March 2016 Accepted 21 March 2016 Available online xxx Keywords: Fusion-fission First wall Material damage
a b s t r a c t Neutronic performance and inventory analyses were conducted to quantify the damage and gas production rates in candidate materials when used in a fusion-fission hybrid system first wall (FW). The structural materials considered are austenitic SS, Cu-alloy and V- alloys. Plasma facing materials included Be, and CFC composite and W. It is shown that the highest damage rates and gas particles production in materials are experienced within the FW region of a hybrid similar to a pure fusion system. They are greatly influenced by a combined neutron energy spectrum formed by the two-component fusion-fission neutron source in front of the FW and in a subcritical fission blanket behind. These characteristics are non-linear functions of the fission neutron source intensity. Atomic displacement damage production rate in the FW materials of a subcritical system (at the safe subcriticality limit of ∼0.95 and the neutron multiplication factor of ∼20) is almost ∼2 times higher compared to the values distinctive for a pure fusion system at the same 14 MeV neutron FW loading. Both hydrogen (H) and helium (He) gas production rates are practically on the same level except of about ∼4–5 times higher He-production in austenitic and reduced activation ferritic martensitic steels. A proper simulation of the damage environment in hybrid systems is required to evaluate the expected material performance and the structural component residence times. © 2016 Elsevier B.V. All rights reserved.
1. Introduction It is known from fusion system considerations that high-energy neutrons generated in the plasma by D-T fusion reactions cause atomic displacements within the materials, leading to the generation and accumulation of radiation defects and to radiogenic helium (He) and hydrogen (H) gas atom production that influence properties of materials over the projected lifetime of a fusion power plant. As shown e.g. in [1–4] even at low concentration, gas particle can have severe life-limiting consequence for structural component life-time. In hybrid fusion-fission systems [5–8] not only do the incident D-T-neutrons but the “secondary” fission neutrons appeared in
subcritical blankets may cause atomic displacements and transmutations within the materials having a significant effect on their properties. This paper describes results from integrated studies for a simplified model of a hybrid system to define a possible variation in the first wall irradiation environment caused by two component D-T fusion and fission neutron sources in a plasma region and in fission blanket behind the first wall (FW) compared to the D-T fusion irradiation. The analysis combines both fusion and fission neutron- and gamma-ray transport simulations and inventory calculations to quantify damage, transmutation and nuclear heating of various candidate structural and plasma facing materials for the first wall under identical first wall conditions.
E-mail address: Khripunov [email protected] http://dx.doi.org/10.1016/j.fusengdes.2016.03.066 0920-3796/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. Fusion and fission neutron sources in the plasma region and in the fission blanket core.
2. Combined fusion-fission neutron source in a hybrid system A one-dimensional model First Wall panel (FW) considered here was structured as cylindrical rings: 1.9 mm layer of the 316L(N)-IG austenitic steel for the structure, 4 mm H2 O-coolant layer, 1.7 mm Cu- heat sink and 4 mm Be-armour on the side facing the plasma. This model replicates a radial build of the original first wall design of DEMO-FNS [8] in a simplified bulk geometry representation. Other FW materials recommended in [3,4] are also considered for a comparison. Two types of neutron sources have to be accounted in the hybrid system [8] simultaneously: a D-T-fusion neutron source in the plasma region and a nuclear fission neutron source in a 30-cm neutron region behind the first wall, which may contain some fission materials (or lead for neutron multiplications). According to the one-point approximation, the neutron multiplication in a subcritical system is usually expressed as follows: Mn-fiss = 1/(1-keff ), where keff is the effective neutron multiplication factor in (n,f)-reactions. For the safety reasons keff has usually to be set to 0.95, that corresponds to Mn-fiss ∼20. This Mn-fiss does not include the effect of the source position and neutron energy that significantly affects the neutron multiplication and other neutron multiplication types as (n,2n), (n,3n) reactions. Nevertheless for simplicity the total fusion neutron multiplication factor in a fission blanket Mn was proposed to be equal to 20 in this consideration. (The corresponding fusion energy multiplication factor in the subcritical blanket is expected to be high, ∼90.) The energy distribution of the D-T-fusion neutron source in the plasma region apart from the first wall and the fission neutron source in the subcritical blanket behind are shown in Fig. 1. It is seen from this figure that whereas the bulk neutron energy in the fission blanket is in the 2 MeV range, the 14.1 MeV neutrons are produced in the D-T fusion plasma. 3. Neutron spectra in the FW region It is known also that atomic displacement damage and gas production rates are greatly influenced by neutron energy spectrum peculiarities [1,2].
Fig. 2. Resultant spectra in the first wall region from the two component fusionfission neutron source.
Two neutron spectra in the first wall region calculated separately for the D-T-fusion neutron source distributed in plasma chamber of DEMO-FNS model [8] and for the fission neutron source in the front zone of the subcritical blanket, as well as the resultant spectrum are shown in Fig. 2. The fusion neutron source component in this analysis is normalized to the D-T-neutron wall loading value of 0.2 MW/m2 that is almost by 10–15 times lower than for a fusion power reactor. The fission neutron yield in the blanket region behind the first wall corresponds to the Mn ∼20. 4. Neutron fluxes in the first wall surrounding Usually the neutron fluxes, damage and gas production rates and the nuclear heating rate in a fusion system reach their maximum values in different materials near the plasma facing regions. Then they are generally decrease almost in a quite exponential way along the radial direction with increasing distance from the First Wall. But it is not true in a system with a fission blanket behind the first wall.
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Fig. 3. High-energy and total (the full energy spectrum) neutron flux distributions in plasma (R<0), FW (R = 0–1.2 cm) and Blanket (R>4 cm).
Fig. 4. The E␥ − ray Yield per one 235 U fission (See ENDF/B-6).
Fig. 5. ␥-Ray flux distributions in the FW and Blanket regions: ␥-(n-fusion) − from the D-T-neutron source in plasma, ␥ − (n-fission) − from fission neutron radiation capture, ␥-fission − the prompt gammas from fissions, ␥-total − gamma-flux total.
The high-energy and total neutron flux radial distributions in the First Wall region from the D-T-neutron source in plasma (R<0) and from the fission neutron source in the subcritical fission blanket are shown in Fig. 3. As shown in this figure both the fast neutron flux (En > 0.1 MeV) and the total neutron flux (En > 0) are actually increased with distance from the plasma mainly due to neutron multiplication in the fission blanket. Neutron and gamma-flux values in the First Wall region and neutron fluence values per one operation year from the combined fusion-fission neutron source are given in Table 1. The D-T-neutron fluence value for a typical hybrid system is ∼0.2 MWa/m2 per one operation year (in the traditionally used unit of MWa/m2 as measure of 14.1 MeV neutron current through the first wall), while the total fast neutron fluence of ∼4.4 × 1021 cm−2 is approximately a factor of ∼2.2 higher than from the initial D-Tneutron source only.
In addition to ␥-ray sources from both fusion and fission neutrons (n, ␥)-radiation capture reactions, a prompt gammaradiation appear during nuclear fissions. A prompt gamma-ray spectrum is shown in Fig. 4 for 235 U fission with the average energy E␥avr of ∼1.2 MeV while the E␥ in radiation capture in materials achieves ∼ 7–8 MeV for neutrons with En below 1.4 MeV. Three components and the total ␥-flux distributions in the First Wall region are shown in Fig. 5 and ␥-flux values from these ␥source components are given above in Table 1. A factor of ∼3 gammas (E␥avr ∼ 1.2 MeV) per one fission neutron was used here for normalization of the prompt ␥-source intensity. It is seen from Fig. 5 that the fraction of the prompt gammas in the total ␥-flux in the FW is about several percents, while gammas from the fission neutron radiation capture reactions are dominat. 6. Nuclear responses
5. ␥-Fluxes from three types of ␥-sources Three types of ␥-flux components produced by the two component fusion-fission neutron source appear in the first wall region.
As shown above the presence of the fission neutron source component increases the fast neutron flux level that in turn determines the first wall material damage and activation rate.
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Table 1 Neutron and gamma-ray fluxes in the First Wall and neutron fluence per one operation year from the combined fusion-fission neutron source (D-T- neutron wall loading ∼0.2 MW/m2 , Mn = 20). Fluxes
All
Fluences
All-to-
Neutron and gamma-Sources
units n-th DT-n n-fast n-tot gamma
D-T-n
D-T-n
n-fission
␥-prompt
cm−2 s−1 7.4 × 1012 1.2 × 1013 6.4 × 1013 1.3 × 1014 3.8 × 1013
cm−2 s−1 9.2 × 1013 4.9 × 105 7.6 × 1013 2.9 × 1014 2.3 × 1014
cm−2 s−1
cm−2 s−1 9.9 × 1013 1.2 × 1013 1.4 × 1014 4.2 × 1014 2.8 × 1014
1.8 × 1013
per 1 FPY
Source
cm−2 3.1 × 1021 3.8 × 1020 4.4 × 1021 1.3 × 1022 8.9 × 1021
Ratios 13.4 1.0 2.2 3.3 7.4
Table 2 Nuclear responses in the First Wall materials from the combined fusion − fission neutron source (See Table 1.). FW component
Material
dpa per fpy
He appm per fpy
T appm per fpy
H, appm per fpy
Heat W/cm3
PFC (armour)
CFC Be W CuCrZr H2 O SS Eurofer EK-164 V-4Cr-4Ti
3.2 2.6 0.90 3.2 0 3.1 3.0 3.2 3.8
230 670 ∼40 22 25 130 130 160 12
0.03 28 0.005 0.2 ∼0.001 0.05 0.02 ∼0.01 0.01
0.2 29 ∼900 150 −20 140 90 ∼160 550
4.1 4.9 ∼100 25 5.7 20 22 21 ∼20
Heat sink Coolant Structure
Damage and gas-production in various FW relevant materials evaluated for the combined neutron source using the cross-section data libraries [9] and [10], respectively, are given in Table 2. Here in table the plasma-facing armour material candidates included CFC composite, Beryllium (Be) and Tungsten (W), CuCrZr bronze as a heat sink material. The structural materials considered are the ITER Grade austenitic steel SS316L(N)-IG, and the Eurofer as a reduced activation ferritic martensitic (RAFM) steel, the V4Cr-4Ti vanadium alloy and a heat-resistant austenitic steel EK-164 [4] which are under development for the future fusion and fission power reactors. Relative damage and gas production functions of neutron energy integrated over the total neutron flux energy range highlight two particular aspects of dpa and gas production evaluations: the radiation damage is determined mainly by the fast neutron flux level while ∼40% dpa and 80–90% radiogenic gases in materials are produced in threshold reactions by high energy neutrons (En above 10 MeV). It is immediately obvious from Table 2 that some plasma facing materials, such as Be and CFC, have significantly higher He-production rates than e.g. Fe (See below) and other transition metals, such as Cr, or V. It is seen from table also that the He production rate of up to ∼140 appm per one full power operation year (fpy) in a steel component of the FW is likely to be significant because concentrations in the range of 400 appm are known to cause a change in the fracture behavior of neutron-irradiated steels as indicated in [3]. A non-zero He production in W-armour of the water cooled FW seems to be overestimated. More detailed analysis of self-shielding effects not considered here is required for correct tungsten transmutation rate evaluation. In Table 3 the nuclear response values calculated for the twocomponent neutron source are brought into correlation with the values obtained for the fusion neutron source component. An increase of the expected damage production rate (dpa/full power operation year) of ∼1.4–2.4 is remarkable for all materials when taking into account the 20-fold neutron multiplication in a subcritical blanket. It is considered that, the gas production from iron, as the primary constituent of steels, will be a major factor in determining
Table 3 Fusion-Fission Responses-to-Fusion Response Ratios (rel. units). FW component
Material
dpa (rel.)
He (rel.)
T (rel.)
H (rel.)
Heat (rel.)
PFC (armour)
CFC Be W CuCrZr H2 O SS Eurofer EK-164 V-4Cr-4Ti
2.3 2.4 1.4 1.5
1.0 1.1 ∼60 1.0 1.0 4.8 4.9 4.0 1.3
1.0 2.7 1.0 1.0 2.0 1.0 1.0 1.2 2.1
1.3 2.7 1.0 1.1 1.0 1.3 1.1 1.2 1.0
3.8 2.8 8.3 8.1 2.5 8.3 8.6 8.2 9.5
Heat sink Coolant Structure
1.6 1.6 1.2 1.7
the lifetime of near-plasma components in fusion reactors. In addition a lower boron content (below 10 wppm B as in the ITER Grade SS316L(N)-IG is proposed to decrease further He-production from Boron impurity in steel in (n,␣) reactions. However, the He-production in EK-164 (a high-Ni, heatresistant, austenitic cast steel for structure and fuel clad) is higher due to a high Ni content (21.5%) and high 59 Ni (n,␣) reaction crosssections values both in the resonance region (∼18.4 b) and in the low energy part of the combined neutron spectrum (∼12 b). See Fig. 6. (The 59 Ni appears in Ni according to the two-steps reaction 58 Ni (n,␥) 59 Ni (n,␣)56 Fe.) It is seen from the table also that both hydrogen and helium gas production rates in V- component of the first wall remain practically on the same level except of ∼4-5 times higher He-production in steels. Almost ∼60 times higher He production in Tungsten is reveled in the neutron spectrum from combined fusion-fission neutron source in the water cooled first wall region considered here than in the case of the pure D-T fission neutron source. A more careful neutron transport and inventory analyses are required to clarify the increased He production from transmutation products of Tungsten irradiation in the “soft” neutron spectrum with ∼60% neutrons in the intermediate energy range (0.1 eV < En < 0.1 MeV) and in harder neutron spectra conditions.
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Fig. 6.
56
5
Fe(n,␣)53 Cr and 59 Ni(n,␣)56 Fe reaction cross sections, barn.
7. The specific and total first wall nuclear heating The higher neutron and gamma-ray fluxes from the two component neutron source result in a higher both neutron and gamma-heating in candidate first wall materials. The resulting specific nuclear heating values in possible first wall materials from the combined fusion-fission neutron source are indicated in the last column of Table 2. Gamma heating in steel and tungsten components dominates the total nuclear heating with ∼95% contribution, especially from the gammas from the fission neutron radiation capture in the blanket and first wall materials. The gamma-heating in “light” materials as water and beryllium is ∼40% and 60% of the total nuclear heating value, respectively. Based on the specific data from Table 2 the resulting first wall nuclear heating estimated for this particular layered first wall model [8] with the subcritical blanket behind is a factor of ∼5 higher relating to the value just only from the D-T-fusion in the plasma chamber. Such increased nuclear heating seems to be acceptable requiring however an additional thermal analysis.
8. Conclusion Neutronic and inventory analyses were performed to quantify possible damage, gas production and nuclear heating levels in candidate structural and plasma facing materials when used in a hybrid fusion-fission system. They show that as in a fusion system design the highest damage and gas production rates in materials are experienced within the first wall region. These characteristics are non-linear functions of the fission neutron source intensity and impact of this neutron source component is essential. Atomic displacement damage production rate in first wall materials of a subcritical system (at the safe subcriticality limit of ∼0.95 and the neutron multiplication factor Mn of ∼20) is almost ∼2 times higher compared to the values distinctive for a pure fusion system at the same 14 MeV neutron wall loading.
Both the hydrogen and helium gas production rates are practically on the same level for the two components neutron source except of a ∼4-5 times higher He-production rate in steels. The specific nuclear heating in some first wall materials may be even 8 times higher mainly due to gamma-heating from the gamma-source from (n, ␥)-reactions in metallic components of the first wall and surrounding structures. While the D-T-neutron first wall loading in a hybrid system is almost by one order of magnitude lower then for a fusion power reactor the nuclear heating, radiation damage and helium production values expected in the first wall and other plasma facing components of a hybrid system are high enough. Therefore a proper simulation of the damage environment in hybrid systems is required to evaluate the expected material performance and the structural component residence times. Acknowledgements The author would like to express their sincere appreciation to Drs J-Ch. Sublet (the CCFE, UK) and J.M. Galan (the NEA Data Bank) granted him the EASY-2010 version of the code system and The European Activation File EAF-2010. References [1] M.R. Gilbert, et al., An integrated model for materials in a fusion power plant: transmutation, gas production, and helium embrittlement under neutron irradiation, Nucl. Fusion 52 (2012) 083019, 12 pp. [2] M.E. Sawan, Damage parameters of structural materials in fusion environment compared to fission reactor irradiation, Fusion Eng. Des. 87 (2012) 551–555. [3] D. Stork, et al., Developing structural, high-heat flux and plasma facing materials for a near-term DEMO fusion power plant: the EU assessment, J. Nucl. Mater. 455 (2014) 277–291. [4] V.M. Chernov, M.V. Leonteva-Smirnova1, et al., Structural materials for fusion power reactors—the RF R&D activities, Nucl. Fusion 47 (2007) 839–848. [5] W.M. Stacey, et al., A subcritical, gas-cooled fast transmutation reactor with a fusion neutron source, Nucl. Technol. 150 (2005) 162–188. [6] J. Jiang, Neutronics analysis of water-cooled energy production blanket for a fusion–fission hybrid reactor, Fusion Eng. Des. 85 (2010) 2115–2119. [7] M.T. Siddique, S.-H. Hong, M.H. Kim, Physical investigation for neutron consumption and multiplication in fusion–fission hybrid test blanket module, Fusion Eng. Des. 89 (2014) 2679–2684.
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[8] B.V. Kuteev, E.A. Azizov, et al., Development of DEMO-FNS tokamak for fusion and hybrid technologies, FIP/P 7-24, The 25th IAEA Fusion Energy Conference, 13–18 October, 2014, St. Petersburg, Russia. [9] https://www-nds.iaea.org/fendl21.
[10] L.W. Packer. J.-Ch. Sublet, The European Activation File: EAF-2010 biological, clearance and transport libraries, EASY Documentation Series CCFE-R (10)04. EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK. March 2010.
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ARTICLE IN PRESS
FUSION-8690; No. of Pages 6
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
First wall material damage induced by fusion-fission neutron environment Vladimir Khripunov National Research Center “Kurchatov Institute”, Kurchatov Sq., 1, Moscow 123182, Russia
h i g h l i g h t s • The highest damage and gas production rates are experienced within the first wall materials of a hybrid fusion-fission system. • About ∼2 times higher dpa and 4–5 higher He appm are expected compared to the values distinctive for a pure fusion system at the same DT-neutron wall loading.
• The specific nuclear heating may be increased by a factor of ∼8–9 due to fusion and fission neutrons radiation capture in metal components of the first wall.
a r t i c l e
i n f o
Article history: Received 28 August 2015 Received in revised form 6 March 2016 Accepted 21 March 2016 Available online xxx Keywords: Fusion-fission First wall Material damage
a b s t r a c t Neutronic performance and inventory analyses were conducted to quantify the damage and gas production rates in candidate materials when used in a fusion-fission hybrid system first wall (FW). The structural materials considered are austenitic SS, Cu-alloy and V- alloys. Plasma facing materials included Be, and CFC composite and W. It is shown that the highest damage rates and gas particles production in materials are experienced within the FW region of a hybrid similar to a pure fusion system. They are greatly influenced by a combined neutron energy spectrum formed by the two-component fusion-fission neutron source in front of the FW and in a subcritical fission blanket behind. These characteristics are non-linear functions of the fission neutron source intensity. Atomic displacement damage production rate in the FW materials of a subcritical system (at the safe subcriticality limit of ∼0.95 and the neutron multiplication factor of ∼20) is almost ∼2 times higher compared to the values distinctive for a pure fusion system at the same 14 MeV neutron FW loading. Both hydrogen (H) and helium (He) gas production rates are practically on the same level except of about ∼4–5 times higher He-production in austenitic and reduced activation ferritic martensitic steels. A proper simulation of the damage environment in hybrid systems is required to evaluate the expected material performance and the structural component residence times. © 2016 Elsevier B.V. All rights reserved.
1. Introduction It is known from fusion system considerations that high-energy neutrons generated in the plasma by D-T fusion reactions cause atomic displacements within the materials, leading to the generation and accumulation of radiation defects and to radiogenic helium (He) and hydrogen (H) gas atom production that influence properties of materials over the projected lifetime of a fusion power plant. As shown e.g. in [1–4] even at low concentration, gas particle can have severe life-limiting consequence for structural component life-time. In hybrid fusion-fission systems [5–8] not only do the incident D-T-neutrons but the “secondary” fission neutrons appeared in
subcritical blankets may cause atomic displacements and transmutations within the materials having a significant effect on their properties. This paper describes results from integrated studies for a simplified model of a hybrid system to define a possible variation in the first wall irradiation environment caused by two component D-T fusion and fission neutron sources in a plasma region and in fission blanket behind the first wall (FW) compared to the D-T fusion irradiation. The analysis combines both fusion and fission neutron- and gamma-ray transport simulations and inventory calculations to quantify damage, transmutation and nuclear heating of various candidate structural and plasma facing materials for the first wall under identical first wall conditions.
E-mail address: Khripunov [email protected] http://dx.doi.org/10.1016/j.fusengdes.2016.03.066 0920-3796/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. Fusion and fission neutron sources in the plasma region and in the fission blanket core.
2. Combined fusion-fission neutron source in a hybrid system A one-dimensional model First Wall panel (FW) considered here was structured as cylindrical rings: 1.9 mm layer of the 316L(N)-IG austenitic steel for the structure, 4 mm H2 O-coolant layer, 1.7 mm Cu- heat sink and 4 mm Be-armour on the side facing the plasma. This model replicates a radial build of the original first wall design of DEMO-FNS [8] in a simplified bulk geometry representation. Other FW materials recommended in [3,4] are also considered for a comparison. Two types of neutron sources have to be accounted in the hybrid system [8] simultaneously: a D-T-fusion neutron source in the plasma region and a nuclear fission neutron source in a 30-cm neutron region behind the first wall, which may contain some fission materials (or lead for neutron multiplications). According to the one-point approximation, the neutron multiplication in a subcritical system is usually expressed as follows: Mn-fiss = 1/(1-keff ), where keff is the effective neutron multiplication factor in (n,f)-reactions. For the safety reasons keff has usually to be set to 0.95, that corresponds to Mn-fiss ∼20. This Mn-fiss does not include the effect of the source position and neutron energy that significantly affects the neutron multiplication and other neutron multiplication types as (n,2n), (n,3n) reactions. Nevertheless for simplicity the total fusion neutron multiplication factor in a fission blanket Mn was proposed to be equal to 20 in this consideration. (The corresponding fusion energy multiplication factor in the subcritical blanket is expected to be high, ∼90.) The energy distribution of the D-T-fusion neutron source in the plasma region apart from the first wall and the fission neutron source in the subcritical blanket behind are shown in Fig. 1. It is seen from this figure that whereas the bulk neutron energy in the fission blanket is in the 2 MeV range, the 14.1 MeV neutrons are produced in the D-T fusion plasma. 3. Neutron spectra in the FW region It is known also that atomic displacement damage and gas production rates are greatly influenced by neutron energy spectrum peculiarities [1,2].
Fig. 2. Resultant spectra in the first wall region from the two component fusionfission neutron source.
Two neutron spectra in the first wall region calculated separately for the D-T-fusion neutron source distributed in plasma chamber of DEMO-FNS model [8] and for the fission neutron source in the front zone of the subcritical blanket, as well as the resultant spectrum are shown in Fig. 2. The fusion neutron source component in this analysis is normalized to the D-T-neutron wall loading value of 0.2 MW/m2 that is almost by 10–15 times lower than for a fusion power reactor. The fission neutron yield in the blanket region behind the first wall corresponds to the Mn ∼20. 4. Neutron fluxes in the first wall surrounding Usually the neutron fluxes, damage and gas production rates and the nuclear heating rate in a fusion system reach their maximum values in different materials near the plasma facing regions. Then they are generally decrease almost in a quite exponential way along the radial direction with increasing distance from the First Wall. But it is not true in a system with a fission blanket behind the first wall.
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Fig. 3. High-energy and total (the full energy spectrum) neutron flux distributions in plasma (R<0), FW (R = 0–1.2 cm) and Blanket (R>4 cm).
Fig. 4. The E␥ − ray Yield per one 235 U fission (See ENDF/B-6).
Fig. 5. ␥-Ray flux distributions in the FW and Blanket regions: ␥-(n-fusion) − from the D-T-neutron source in plasma, ␥ − (n-fission) − from fission neutron radiation capture, ␥-fission − the prompt gammas from fissions, ␥-total − gamma-flux total.
The high-energy and total neutron flux radial distributions in the First Wall region from the D-T-neutron source in plasma (R<0) and from the fission neutron source in the subcritical fission blanket are shown in Fig. 3. As shown in this figure both the fast neutron flux (En > 0.1 MeV) and the total neutron flux (En > 0) are actually increased with distance from the plasma mainly due to neutron multiplication in the fission blanket. Neutron and gamma-flux values in the First Wall region and neutron fluence values per one operation year from the combined fusion-fission neutron source are given in Table 1. The D-T-neutron fluence value for a typical hybrid system is ∼0.2 MWa/m2 per one operation year (in the traditionally used unit of MWa/m2 as measure of 14.1 MeV neutron current through the first wall), while the total fast neutron fluence of ∼4.4 × 1021 cm−2 is approximately a factor of ∼2.2 higher than from the initial D-Tneutron source only.
In addition to ␥-ray sources from both fusion and fission neutrons (n, ␥)-radiation capture reactions, a prompt gammaradiation appear during nuclear fissions. A prompt gamma-ray spectrum is shown in Fig. 4 for 235 U fission with the average energy E␥avr of ∼1.2 MeV while the E␥ in radiation capture in materials achieves ∼ 7–8 MeV for neutrons with En below 1.4 MeV. Three components and the total ␥-flux distributions in the First Wall region are shown in Fig. 5 and ␥-flux values from these ␥source components are given above in Table 1. A factor of ∼3 gammas (E␥avr ∼ 1.2 MeV) per one fission neutron was used here for normalization of the prompt ␥-source intensity. It is seen from Fig. 5 that the fraction of the prompt gammas in the total ␥-flux in the FW is about several percents, while gammas from the fission neutron radiation capture reactions are dominat. 6. Nuclear responses
5. ␥-Fluxes from three types of ␥-sources Three types of ␥-flux components produced by the two component fusion-fission neutron source appear in the first wall region.
As shown above the presence of the fission neutron source component increases the fast neutron flux level that in turn determines the first wall material damage and activation rate.
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Table 1 Neutron and gamma-ray fluxes in the First Wall and neutron fluence per one operation year from the combined fusion-fission neutron source (D-T- neutron wall loading ∼0.2 MW/m2 , Mn = 20). Fluxes
All
Fluences
All-to-
Neutron and gamma-Sources
units n-th DT-n n-fast n-tot gamma
D-T-n
D-T-n
n-fission
␥-prompt
cm−2 s−1 7.4 × 1012 1.2 × 1013 6.4 × 1013 1.3 × 1014 3.8 × 1013
cm−2 s−1 9.2 × 1013 4.9 × 105 7.6 × 1013 2.9 × 1014 2.3 × 1014
cm−2 s−1
cm−2 s−1 9.9 × 1013 1.2 × 1013 1.4 × 1014 4.2 × 1014 2.8 × 1014
1.8 × 1013
per 1 FPY
Source
cm−2 3.1 × 1021 3.8 × 1020 4.4 × 1021 1.3 × 1022 8.9 × 1021
Ratios 13.4 1.0 2.2 3.3 7.4
Table 2 Nuclear responses in the First Wall materials from the combined fusion − fission neutron source (See Table 1.). FW component
Material
dpa per fpy
He appm per fpy
T appm per fpy
H, appm per fpy
Heat W/cm3
PFC (armour)
CFC Be W CuCrZr H2 O SS Eurofer EK-164 V-4Cr-4Ti
3.2 2.6 0.90 3.2 0 3.1 3.0 3.2 3.8
230 670 ∼40 22 25 130 130 160 12
0.03 28 0.005 0.2 ∼0.001 0.05 0.02 ∼0.01 0.01
0.2 29 ∼900 150 −20 140 90 ∼160 550
4.1 4.9 ∼100 25 5.7 20 22 21 ∼20
Heat sink Coolant Structure
Damage and gas-production in various FW relevant materials evaluated for the combined neutron source using the cross-section data libraries [9] and [10], respectively, are given in Table 2. Here in table the plasma-facing armour material candidates included CFC composite, Beryllium (Be) and Tungsten (W), CuCrZr bronze as a heat sink material. The structural materials considered are the ITER Grade austenitic steel SS316L(N)-IG, and the Eurofer as a reduced activation ferritic martensitic (RAFM) steel, the V4Cr-4Ti vanadium alloy and a heat-resistant austenitic steel EK-164 [4] which are under development for the future fusion and fission power reactors. Relative damage and gas production functions of neutron energy integrated over the total neutron flux energy range highlight two particular aspects of dpa and gas production evaluations: the radiation damage is determined mainly by the fast neutron flux level while ∼40% dpa and 80–90% radiogenic gases in materials are produced in threshold reactions by high energy neutrons (En above 10 MeV). It is immediately obvious from Table 2 that some plasma facing materials, such as Be and CFC, have significantly higher He-production rates than e.g. Fe (See below) and other transition metals, such as Cr, or V. It is seen from table also that the He production rate of up to ∼140 appm per one full power operation year (fpy) in a steel component of the FW is likely to be significant because concentrations in the range of 400 appm are known to cause a change in the fracture behavior of neutron-irradiated steels as indicated in [3]. A non-zero He production in W-armour of the water cooled FW seems to be overestimated. More detailed analysis of self-shielding effects not considered here is required for correct tungsten transmutation rate evaluation. In Table 3 the nuclear response values calculated for the twocomponent neutron source are brought into correlation with the values obtained for the fusion neutron source component. An increase of the expected damage production rate (dpa/full power operation year) of ∼1.4–2.4 is remarkable for all materials when taking into account the 20-fold neutron multiplication in a subcritical blanket. It is considered that, the gas production from iron, as the primary constituent of steels, will be a major factor in determining
Table 3 Fusion-Fission Responses-to-Fusion Response Ratios (rel. units). FW component
Material
dpa (rel.)
He (rel.)
T (rel.)
H (rel.)
Heat (rel.)
PFC (armour)
CFC Be W CuCrZr H2 O SS Eurofer EK-164 V-4Cr-4Ti
2.3 2.4 1.4 1.5
1.0 1.1 ∼60 1.0 1.0 4.8 4.9 4.0 1.3
1.0 2.7 1.0 1.0 2.0 1.0 1.0 1.2 2.1
1.3 2.7 1.0 1.1 1.0 1.3 1.1 1.2 1.0
3.8 2.8 8.3 8.1 2.5 8.3 8.6 8.2 9.5
Heat sink Coolant Structure
1.6 1.6 1.2 1.7
the lifetime of near-plasma components in fusion reactors. In addition a lower boron content (below 10 wppm B as in the ITER Grade SS316L(N)-IG is proposed to decrease further He-production from Boron impurity in steel in (n,␣) reactions. However, the He-production in EK-164 (a high-Ni, heatresistant, austenitic cast steel for structure and fuel clad) is higher due to a high Ni content (21.5%) and high 59 Ni (n,␣) reaction crosssections values both in the resonance region (∼18.4 b) and in the low energy part of the combined neutron spectrum (∼12 b). See Fig. 6. (The 59 Ni appears in Ni according to the two-steps reaction 58 Ni (n,␥) 59 Ni (n,␣)56 Fe.) It is seen from the table also that both hydrogen and helium gas production rates in V- component of the first wall remain practically on the same level except of ∼4-5 times higher He-production in steels. Almost ∼60 times higher He production in Tungsten is reveled in the neutron spectrum from combined fusion-fission neutron source in the water cooled first wall region considered here than in the case of the pure D-T fission neutron source. A more careful neutron transport and inventory analyses are required to clarify the increased He production from transmutation products of Tungsten irradiation in the “soft” neutron spectrum with ∼60% neutrons in the intermediate energy range (0.1 eV < En < 0.1 MeV) and in harder neutron spectra conditions.
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Fig. 6.
56
5
Fe(n,␣)53 Cr and 59 Ni(n,␣)56 Fe reaction cross sections, barn.
7. The specific and total first wall nuclear heating The higher neutron and gamma-ray fluxes from the two component neutron source result in a higher both neutron and gamma-heating in candidate first wall materials. The resulting specific nuclear heating values in possible first wall materials from the combined fusion-fission neutron source are indicated in the last column of Table 2. Gamma heating in steel and tungsten components dominates the total nuclear heating with ∼95% contribution, especially from the gammas from the fission neutron radiation capture in the blanket and first wall materials. The gamma-heating in “light” materials as water and beryllium is ∼40% and 60% of the total nuclear heating value, respectively. Based on the specific data from Table 2 the resulting first wall nuclear heating estimated for this particular layered first wall model [8] with the subcritical blanket behind is a factor of ∼5 higher relating to the value just only from the D-T-fusion in the plasma chamber. Such increased nuclear heating seems to be acceptable requiring however an additional thermal analysis.
8. Conclusion Neutronic and inventory analyses were performed to quantify possible damage, gas production and nuclear heating levels in candidate structural and plasma facing materials when used in a hybrid fusion-fission system. They show that as in a fusion system design the highest damage and gas production rates in materials are experienced within the first wall region. These characteristics are non-linear functions of the fission neutron source intensity and impact of this neutron source component is essential. Atomic displacement damage production rate in first wall materials of a subcritical system (at the safe subcriticality limit of ∼0.95 and the neutron multiplication factor Mn of ∼20) is almost ∼2 times higher compared to the values distinctive for a pure fusion system at the same 14 MeV neutron wall loading.
Both the hydrogen and helium gas production rates are practically on the same level for the two components neutron source except of a ∼4-5 times higher He-production rate in steels. The specific nuclear heating in some first wall materials may be even 8 times higher mainly due to gamma-heating from the gamma-source from (n, ␥)-reactions in metallic components of the first wall and surrounding structures. While the D-T-neutron first wall loading in a hybrid system is almost by one order of magnitude lower then for a fusion power reactor the nuclear heating, radiation damage and helium production values expected in the first wall and other plasma facing components of a hybrid system are high enough. Therefore a proper simulation of the damage environment in hybrid systems is required to evaluate the expected material performance and the structural component residence times. Acknowledgements The author would like to express their sincere appreciation to Drs J-Ch. Sublet (the CCFE, UK) and J.M. Galan (the NEA Data Bank) granted him the EASY-2010 version of the code system and The European Activation File EAF-2010. References [1] M.R. Gilbert, et al., An integrated model for materials in a fusion power plant: transmutation, gas production, and helium embrittlement under neutron irradiation, Nucl. Fusion 52 (2012) 083019, 12 pp. [2] M.E. Sawan, Damage parameters of structural materials in fusion environment compared to fission reactor irradiation, Fusion Eng. Des. 87 (2012) 551–555. [3] D. Stork, et al., Developing structural, high-heat flux and plasma facing materials for a near-term DEMO fusion power plant: the EU assessment, J. Nucl. Mater. 455 (2014) 277–291. [4] V.M. Chernov, M.V. Leonteva-Smirnova1, et al., Structural materials for fusion power reactors—the RF R&D activities, Nucl. Fusion 47 (2007) 839–848. [5] W.M. Stacey, et al., A subcritical, gas-cooled fast transmutation reactor with a fusion neutron source, Nucl. Technol. 150 (2005) 162–188. [6] J. Jiang, Neutronics analysis of water-cooled energy production blanket for a fusion–fission hybrid reactor, Fusion Eng. Des. 85 (2010) 2115–2119. [7] M.T. Siddique, S.-H. Hong, M.H. Kim, Physical investigation for neutron consumption and multiplication in fusion–fission hybrid test blanket module, Fusion Eng. Des. 89 (2014) 2679–2684.
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[8] B.V. Kuteev, E.A. Azizov, et al., Development of DEMO-FNS tokamak for fusion and hybrid technologies, FIP/P 7-24, The 25th IAEA Fusion Energy Conference, 13–18 October, 2014, St. Petersburg, Russia. [9] https://www-nds.iaea.org/fendl21.
[10] L.W. Packer. J.-Ch. Sublet, The European Activation File: EAF-2010 biological, clearance and transport libraries, EASY Documentation Series CCFE-R (10)04. EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK. March 2010.
Please cite this article in press as: V. Khripunov, First wall material damage induced by fusion-fission neutron environment, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.03.066