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Heterogeneous breeding blanket experiment with lithium titanate and beryllium
Fusion Engineering and Design 72 (2005) 327–337
Heterogeneous breeding blanket experiment with lithium titanate and beryllium A. Klixa,∗ , Yu. Verzilovb , K. Ochiaib , T. Nishitanib , A. Takahashia a
Department of Nuclear Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan b Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan Received 21 October 2003; received in revised form 23 February 2004; accepted 21 July 2004
Abstract An integral breeding blanket experiment has been done at Fusion Neutronics Source of JAERI with an assembly of three layers of lithium titanate, beryllium, and low activation ferritic steel F82H. The experimental assembly and the neutron source were enclosed in a SS316 stainless steel can and irradiated with D–T neutrons. The tritium production rate was measured with small Li2 CO3 pellet detectors. The tritium production from 7 Li was estimated by means of the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P which have effective cross-sections similar to 7 Li(n, n␣)T but are more sensitive methods. The neutron flux in the assembly was checked with activation foils. The measurement results were analyzed with the three-dimensional Monte Carlo code MCNP-4C and three different beryllium cross-section evaluations taken from FENDL/MC-2.0, FENDL/E-2.0, and EFF-3.03. The library FENDL/MC-2.0 was also used for the other materials in the assembly. All three beryllium libraries model the fast flux in the assembly well. Differences were seen for indium foils. An overestimation was observed for the tritium production. © 2004 Elsevier B.V. All rights reserved. Keywords: Breeding blanket; Lithium titanate; Tritium production; Beryllium
1. Introduction An important design issue of a fusion reactor is a sufficient tritium production rate in the breeding blanket so that the reactor can be operated in a self-sufficient mode. At Fusion Neutronics Source (FNS) of JAERI, the neutronics characteristics of advanced materials for ∗ Corresponding author. Present address: Nuclear Engineering Research Laboratory, The University of Tokyo, Tokaimura, Ibaraki-ken 319-1188, Japan. Tel.: +81 29 287 8918; fax: +81 29 287 8488. E-mail address: [email protected] (A. Klix).
0920-3796/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2004.07.019
breeding blankets are investigated. An integral experiment has been done with a thermal-type breeding blanket mock-up consisting of layers of beryllium, low activation ferritic steel F82H, and lithium titanate enriched in 6 Li. These materials are candidate materials for the future DEMO reactor. The experimental assembly and the 14 MeV D–T neutron source were enclosed in a SS316 stainless steel can to obtain a neutron field similar to that of a fusion reactor. The tritium production in the breeding blanket is based on the following two reactions: 6
Li(n, ␣)T,
Q = 4.783 MeV
(1)
328 7
Li(n, n␣)T,
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Q = −2.467 MeV
(2)
Reaction (1) is important for the thermal-type blanket due to its high cross-section for slow neutrons. Usually the lithium compound in the blanket will be enriched in 6 Li, therefore the macroscopic cross-section of this reaction will cause a strong self-shielding effect in the material and the local tritium production rate (TPR) will drop considerably towards the inside of the breeding layer. Reaction (2) contributes less than 2% to the overall tritium production in this experimental set-up as numeric simulations show and should vary only slowly with the position in the assembly due to its smaller cross-section and insensitivity to thermal neutrons. The D–T neutron source of FNS used in the experiment delivers up to 4 × 1011 neutrons per second. Despite a total source neutron fluence on the order of 1016 source neutrons during the experiment, the accumulated tritium activity in the lithium titanate is on the order of 100–400 Bq/g in the breeding layers. Liquid scintillation counting techniques are applied to measure the TPR in the breeding layers by means of small pellet detectors made of lithium carbonate. Additional detectors utilizing the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P which allow to model the tritium breeding reaction (2) and provide additional verification for the neutron spectrum calculations were distributed along the axis of the assembly to estimate the tritium production from 7 Li experimentally. In this paper, the measured tritium production profile in the breeding layers is presented and the analysis of the experiment with the three-dimensional Monte Carlo code MCNP-4C [1] and three state-of-the-art beryllium cross-section evaluations is discussed.
Fig. 1. Cross-sectional view of the experimental set-up.
can with a beryllium reflector on the source-side to simulate a fusion reactor neutron field, see Fig. 1. The composition of the assembly materials and the SS316 enclosure is shown in Tables 1 and 2. The breeding layers were made of 50 mm × 50 mm lithium titanate blocks with a thickness of 12 mm. Small pellet detectors made of lithium carbonate were inserted into holes in the blocks to measure the local tritium production. The pellets had a diameter of 12.3 mm and thicknesses of 0.5 and 2.0 mm. The pellet detectors were 5 cm off the axis of the assembly. The lithium titanate blocks were manufactured from powder by Kawasaki Heavy Industries by a cold-pressing and sintering method.
2. Experimental assembly
Table 1 Properties of the lithium titanate used in the experiments
The irradiation was done with the fixed target of FNS. A schematic view of the experimental set-up with the breeding assembly is shown in Fig. 1. The assembly consisted of layers of low activation ferritic steel F82H simulating structural material, beryllium for moderation and multiplication of neutrons, and lithium titanate enriched to 40% in 6 Li for tritium breeding. The neutron spectrum of the plain D–T source is harder than that of a fusion reactor, therefore the assembly and the neutron source were enclosed by a SS316 stainless steel
Enrichment nominal
40.33%
Analysis site Composition (wt.%) (major elements)
Pellet density (g/cm3 ) Block density (g/cm3 )
KHI Li Ti O
JAERI
10.8 9.4 ± 0.2 43.6 43.4 ± 0.5 45.5 (balance) 47.2 (balance) 2.83–2.92 2.88
The analysis from Kawasaki Heavy Industries (KHI) is based on the raw powder before sintering while the JAERI analysis is based on a finished pellet.
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Table 2 Chemical composition of the beryllium and F82H layers, and the SS316 enclosure Beryllium layer (ppm)
Be Li B Mg Al Ca Ti Cr Mn Fe Co Ni Cu Mo Cd Th U
SS316 enclosure (×10−24 atoms/cm3 )
F82H steel (wt.%)
97.9 ± 0.8 wt.% <1 <3 110 ± 5 570 ± 50 <1 100 100 96 ± 5 1300 ± 70 10, . . . , 100 250 ± 30 approximately 130 approximately 50 <1 1.5 ± 0.1 82 ± 3
Fe Cr W C Si Mn P S Cu Ni Mo V Nb Ta Ti B
89.74 7.74 1.95 0.095 0.1 0.1 0.005 0.0031 0.01 0.02 0.01 0.18 0.0001 0.04 0.005 1.496E−4
Metal foil detectors were set-up along the central axis of the assembly for neutron spectrum verification. Additional pellet-shaped detectors made from CH3 SO2 CH3 , NH4 Cl, NH4 PH2 O2 , and Li2 CO3 with a low 6 Li enrichment of 1.16% were set-up along the assembly axis to obtain more information about the neutron spectrum. The first two types of detectors utilize the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P. These reactions are sensitive for fast neutrons and can be used to estimate the tritium production per 7 Li (TPR7) experimentally [2–4]. The latter two detectors are sensitive to slow neutrons via the reactions 31 P(n, ␥)32 P and the breeding reaction (1). The tritium contribution from 7 Li in the Li2 CO3 detectors is low (<4%) in deep positions in the assembly but considerable near
C Si P S Cr Mn Fe Ni Mo
Assembly enclosure
Source reflector
7.1697 × 10−5 9.8440 × 10−4 4.3162 × 10−5 1.8780 × 10−6 1.5476 × 10−2 9.7963 × 10−4 5.7589 × 10−2 9.7128 × 10−3 1.0503 × 10−3
1.9879 × 10−4 8.1608 × 10−4 4.8895 × 10−5 4.4677 × 10−6 1.5025 × 10−2 1.3561 × 10−3 5.8332 × 10−2 9.1456 × 10−3 1.0254 × 10−3
the source-side surface (up to 25% immediately after the first breeding layer). A summary of the detectors used in this work is shown in Table 3 and the corresponding reaction cross-sections are plotted in Figs. 2 and 3. All pellet detectors were wrapped in thin aluminum foil to protect them from contamination during the irradiation. The cumulative irradiation time was 37 h with a total D–T source yield of (1.685 ± 0.034) × 1016 neutrons over a period of 4 days. This value is based on the output of a solid-state ␣-monitor mounted in the dbeam tube for counting the ␣-particles associated with the D–T reaction in the neutron source. The neutron source was (41.3 ± 0.5) cm away from the surface of the first F82H layer.
Table 3 Dosimetry reactions used in this analysis Reaction
Detector material
Energy range
Detection
Half-life time of product
Dosimetry file
␣)T 7 Li(n, n␣)T 32 S(n, p)32 P 35 Cl(n, ␣)32 P 93 Nb(n, 2n)92m Nb 27 Al(n, ␣)24 Na 115 In(n, n )115m In 197 Au(n, ␥)198 Au 31 P(n, p)32 P
Li2 CO3 Li2 CO3 CH3 SO2 CH3 NH4 Cl Nb foil Al foil In foil Au foil NH4 PH2 O2
Thermal >4 MeV >2 MeV >2 MeV >9 MeV >6 MeV >1 MeV Thermal and giant resonances Thermal
, LSC , LSC , Cherenkov , Cherenkov ␥, HPGe ␥, HPGe ␥, HPGe ␥, HPGe , Cherenkov
12.33 y 12.33 y 14.26 d 14.26 d 10.15 d 14.98 h 4.486 h 2.696 d 14.26 d
ACTXS1 FXDOSJ3 ACTXS1 ACTXS1 FXDOSJ3 FXDOSJ3 531dos FXDOSJ3 ACTXS1
6 Li(n,
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Aloka LSC-5100 Liquid Scintillation Counter. The error of the measurement is estimated to 12% as sum of the errors of the efficiency measurement (3%), the neutron source yield measurement (3%), counting error (1%), and an estimate for incomplete tritium recovery (5%). 3.2.
31 P, 32 S,
and 35 Cl detectors
The detectors containing 31 P, 32 S and 35 Cl were dissolved in water in 20 ml standard Teflon vials for Cherenkov counting with the Aloka LSC-5100. The produced 32 P emits -particles with a mean energy of 694.9 keV which is high enough to cause Cherenkov radiation in the solvent water [2]. Fig. 2. Cross-sections of the detectors for the fast neutron flux. Source JEF-2.2.
3.3. Metal foils The activation of the metal foils was acquired with a high purity germanium detector. Measurement conditions and results are published elsewhere [6]. The activation of 93 Nb(n, 2n)92m Nb, 27 Al(n, ␣)24 Na, 115 In(n, n )115m In, and 197 Au(n, ␥)198 Au were selected for comparison with calculations in this work.
4. MCNP calculations and nuclear data libraries
Fig. 3. Cross-sections of the detectors for the slow neutron flux. Source JEF-2.2.
3. Detector processing 3.1. Lithium carbonate The amount of tritium generated in the lithium carbonate pellets is on the order of 2–120 Bq per pellet in this experiment. A well-established method [5] was applied to process the lithium carbonate pellets and Clearsol (Nacalai Tesque) liquid scintillator was used for the preparation of the scintillation cocktails. The scintillation cocktails were counted in 20 ml Teflon vials in an
The experimental results were analyzed with the three-dimensional Monte Carlo code MCNP-4C. The continuous library FENDL/MC-2.0 [7] was used for most isotopes in this calculation. It is derived from FENDL-2 which is considered the best validated nuclear data library currently available for fusion applications. The objective of this analysis is to verify the applicability of three state-of-the-art beryllium cross-section evaluations together with the fusion cross-section library FENDL/MC-2.0 for breeding blanket calculations. For the first calculation the beryllium file included in the FENDL/MC-2.0 library was applied. It is based on ENDF/B-VI. For the second and third calculations the beryllium evaluations from FENDL/E-2.0 [8] which is based on the JENDL Fusion File and the European EFF-3.03 were used. The available EFF-3.03 file did not contain the (n, ␥) part in its absorption cross-
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section, it was therefore extracted from ENDF/B-VI and added to the EFF-3.03 file. The thermal scattering file be.01t from the TMCCS library [9] was applied in all three calculations for an accurate neutron transport in beryllium in the low energy range. The TPR in the breeding layers and the lithium carbonate detectors with the low 6 Li concentration was obtained from cell-averaged tallies while surface tallies were used for the activation foils, the 32 S, 35 Cl, and 31 P pellets. Reaction rates were calculated with tally multipliers and cross-sections taken from the ACTXS1 [10] library distributed by IAEA, the FXJDOS3 based on the JENDL– DOS91 [11] dosimetry file, and the library 531dos [12]. The angular and energy distribution of the D–T neutron source was described accurately in the input file. From measurements of pulse decay times [13] it was found that the thermal absorption cross-section of the beryllium applied in the experiment is increased by about 30% due to impurities. This increase was modelled by the addition of 2.5 ppm of 6 Li to the beryllium in the calculation. Such an increased thermal absorption cross-section reduces the TPR on and near the surface of the breeding layer where a considerable amount of tritium is bred by thermal neutrons. MCNP calculations of the experimental set-up with pure beryllium and beryllium with impurities modelled by 6 Li, however, show that this effect is small and on the order of 1% near the surface of the breeding layer. The composition of the breeding layers is shown in Table 1. A value of 10.8 wt.% was adopted for the lithium concentration.
5. Results 5.1. Fast, epithermal and thermal neutron flux in the breeding assembly The fast neutron flux in the assembly was monitored with Al and Nb foils and the pellet detectors containing 32 S and 35 Cl. Fig. 4 shows the obtained calculation/experiment (C/E) ratios. The C/E ratio for the assembly surface position is nearly 1.00 in case of 93 Nb(n, 2n)92m Nb, 27 Al(n, ␣)24 Na, and 115 In(n, n )115m In, and 0.96 in case of 32 S(n, p)32 P. The excellent agreement indicates an adequate calibration of the source neutron yield monitor and D–T target position relative to the assembly surface.
331
Fig. 4. C/E ratios of detectors sensitive for the fast neutron flux. The error estimates include the counting error and an allowance for detector efficiency and MCNP calculation. For orientation, the location of the three breeding layers is shown in gray bars.
Inside the assembly, the Nb foils C/E ratio indicates an overestimation of the fast neutron flux above 9 MeV in the calculation which is, however, not supported by the other fast neutron detectors. The C/E ratio differences between the three beryllium evaluations are only small. They become apparent at lower neutron energies measured with the 35 Cl, 32 S, and 115 In detectors. Especially the indium foil detectors show C/E ratio differences up to 10% between the calculations with the three beryllium evaluations at corresponding detector locations. While the C/E ratio obtained with EFF-3.03 scatters about 1.0, the C/E ratios based on the calculations with the two FENDL beryllium evaluations appear to have a rising tendency with depth in the breeding assembly indicating uncertainties in the beryllium nuclear data. Au foils, 31 P containing and lithium carbonate pellet detectors with a low 6 Li isotopic ratio (1.16%) were used to gain information about the thermal part of the neutron spectrum in the assembly. Since the cross-
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Fig. 5. Contribution from different neutron energy ranges to the tritium production in the pellet detectors with the low 6 Li enrichment.
sections of the corresponding reactions are composed of a 1/v part and giant resonances at higher neutron energies, it is necessary to estimate the contribution from different neutron energy ranges to the total reaction rate in the detector. This was done by a MCNP calculation and the results are presented in Figs. 5–7. It can be seen from these figures that most of the tritium production in the Li2 CO3 pellets comes from neutrons with energies below 100 eV while even the giant resonance at 240 keV in the cross-section contributes just a small amount. The contribution in the range 1–16 MeV
Fig. 6. Contribution from different neutron energy ranges to the 31 P(n, ␥)32 P reaction rate in the 31 P detectors.
Fig. 7. Contribution from different neutron energy ranges to the 197 Au(n, ␥)198 Au reaction rate in the gold foils.
is mainly from 7 Li. The 31 P detector is considered to model the tritium breeding reaction (1) in soft neutron fields. The giant resonances are at energies higher than 10 keV, but especially in case of the pellet attached to the first breeding layer at position 28 mm contribute a significant amount to the total 32 P production in the pellet.The situation is different for the gold foils. The giant resonance between 1 and 10 eV contributes more than
Fig. 8. C/E ratios of detectors sensitive for the slow neutron flux. The error estimates include the counting error and an allowance for detector efficiency and MCNP calculation. For orientation, the location of the three breeding layers is shown in gray bars.
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Fig. 9. Calculated reaction rates for 6 Li(n, ␣)T and 7 Li(n, n␣)T in the three breeding layers multiplied with the corresponding isotopic fraction (enrichment). The sum of the tritium production graphs is the tritium production per lithium target. The second reaction contributes less than 2% to the total tritium production in the breeding layers.
75% to the total reaction rate in all positions and more than 80% except for two positions in the last beryllium layer. The C/E ratios of these three detectors are presented in Fig. 8. A large scattering is observed for the gold foils which is attributed to the narrow neutron energy range of the giant resonance which contributes most to the reaction rate resulting in a bad calculation statistics. In case of the other two detector types, the C/E ratio does not show a clear rising or falling tendency in the assembly. The Li2 CO3 pellets show an overestimation of the calculated reaction rate by 3–15% inside the assembly. FENDL/MC-2.0 and EFF-3.03 represent best the experimental values. It should be noted that according to the MCNP calculation the contribution of 7 Li to the tritium production is approximately 16, 25, and 7% for the first three pellets, respectively, near the assembly surface. The C/E ratio was calculated for the total TPR in each detector. The underestimation of the 31 P(n, ␥)32 P reaction rate is thought to be due to uncertainties in the cross-section data of this reaction. It is concluded that all three beryllium evaluations simulate the fast neutron flux above 6 MeV (threshold of the 27 Al(n, ␣) reaction) well. Differences between them become apparent at energies below 6 MeV. Between 10 and 1 MeV (indium foils) the best results
came from the EFF-3.03 file followed by FENDL/MC2.0. The calculated reaction rate in the lithium carbonate detectors is overestimated by all three evaluations but the C/E ratio does not show a clear rising or falling tendency in the assembly, therefore it is also concluded that the thermal neutron transport in the assembly is adequately described.
Fig. 10. TPR7 obtained from measurements with 35 Cl and 32 S. For comparison, the TPR7 calculated for the detectors with the low 6 Li concentration is shown. The conversion factors used were R35 Cl /R7 Li = 2.14 and R32 S /R7 Li = 1.01.
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Fig. 11. Experimentally obtained tritium production rates in the three breeding layers from lithium carbonate detectors compared to calculation results. Distances are from the source-side surface of the breeding assembly. The values are averages over the pellet.
5.2. Tritium production rate in the breeding layers The 32 P production from 35 Cl(n, ␣) and 32 S(n, p) was used to estimate the tritium production from 7 Li (TPR7) and to obtain the TPR6 in the breeding layers, however, the TPR7 does not contribute much to the total TPR in this experiment. Fig. 9 shows calculated contributions from 6 Li and 7 Li to the tritium production per Li target (40% 6 Li enrichment). The contribution from 7 Li is always below 2%. The TPR6 can be computed from the difference of the measured total TPR and the TPR7 estimated by the model reactions mentioned above with sufficient accuracy since it is only a small correction. Fig. 10 shows the estimated TPR7 together with the calculated TPR7 by MCNP. It can be seen that the values agree within the error limits. To estimate the TPR7, a conversion factor was multiplied with the measured reaction rates of the two model reactions. In case of 35 Cl, the value R35 Cl /R7 Li = 2.14 [3] was used. For the estimation of the TPR7 from 32 S a conversion factor of R32 S /R7 Li = 1.01 was found in recent experiments at FNS. Fig. 11 shows the measured and calculated TPR6 in the breeding layers. The profile is typical for a thermaltype breeding blanket due to self-shielding. To illustrate the dependence of the TPR6 on the location in the breeding layers, calculated neutron and TPR6 spectra for pellet detectors inside and on the surface of a breeding layer are shown in Fig. 12. The high macro-
Fig. 12. Calculated neutron spectra and TPR spectra (normalized to one source neutron) in three pellets in the second breeding layer. The central pellet has clearly less low-energy neutrons available for tritium production than the pellets on the surface (source-side and back side).
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Fig. 13. Relative contributions to the TPR6 in the three breeding layers from different neutron energy ranges. Calculated with MCNP.
scopic cross-section of the tritium-producing reaction (1) causes a significant absorption of thermal neutrons in the breeding layer. Therefore a peak of thermal neutrons can be seen in the neutron spectra from the surface of the breeding layer but not in the spectrum in the middle of the layer. The calculated relative contributions from different neutron energy ranges to the TPR6 are shown in Fig. 13. Even in the center of the breeding layers more than 60% of the TPR6 come from neutrons with an energy below 130 eV. Near the surface this figure rises to more than 80%. The calculated absolute contributions of these energy ranges are presented in Fig. 14. As expected from Fig. 12, only the TPR6 from neutron energies below 1 eV exposes a strong dependence on location, whereas already the range from 10.7 to 130 eV despite a significant contribution does not much depend on location in the breeding layer anymore.
The calculated TPR in the breeding layers is overestimated locally by 2–20 and 6–12% on average, although most C/E data points are within the combined error estimate of experiment and calculation (<15%). The reaction of the main breeding cross-section 6 Li(n, ␣)T is well known, also the tritium measurement procedure is well established. The overestimation is thought to be due to inaccuracies in the computer model especially uncertainties in the nuclear data, for example for inelastic scattering and (n, 2n) reactions rather than an experimental underestimation. 14 MeV D–T neutrons from the source undergo various slow-down mechanisms. Especially at high neutron energies from several MeV to 14 MeV, inelastic scattering by (n, n ) and (n, 2n) reactions plays an important role. The energy and angular distribution data of the secondary neutrons emitted from these reactions are considered to have considerable uncertainties for some
Fig. 14. Absolute contributions to the TPR6 in the three breeding layers from different neutron energy ranges. Calculated with MCNP.
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isotopes used in this mock-up experiment. A recently conducted experiment to investigate the influence of the source reflector on the TPR is currently analyzed and indicates that the TPR overestimation in the calculation model may be caused by the SS316 steel of the source reflector. Sensitivity analyses are underway in an attempt to support this assumption. An uncertainty arises also from the lithium concentration value in the lithium titanate blocks used to build the breeding layers. Table 1 shows the chemical analysis of the raw lithium titanate powder for fabrication of the sintered blocks. It was provided by the manufacturer Kawasaki Heavy Industries. An analysis of a finished pellet (12.3 mm diameter and 1 mm thickness) was done by JAERI, however, this value is considered less representative for the blocks and a thorough analysis of the lithium titanate blocks is underway. Since a lithium loss during the manufacturing of the blocks by sintering is likely to occur, a sensitivity analysis with respect to a reduced lithium concentration has been done by correlated sampling with MCNP. An original value of 10.8 wt.% lithium in the breeding layers and a reduction by 5% was assumed for this calculation. It was found that the TPR6 response of the lithium carbonate detectors inserted into the breeding layers was increased by 1–3%.
6. Conclusion A breeding blanket mock-up consisting of candidate materials for JAERIs DEMO fusion reactor blanket design was irradiated with D–T neutrons in a SS316 stainless steel enclosure. The tritium production rate inside the breeding layers was measured with a wellestablished method applying lithium carbonate pellet detectors. Additional detectors in the form of metal foils and pellets were used to check the neutron field inside the assembly and estimate the contributions from 6 Li(n, ␣)T and 7 Li(n, n␣)T to the total tritium production in the breeding layers. The experiment was analyzed with the threedimensional Monte Carlo code MCNP-4C, the neutron cross-section library FENDL/MC-2.0 and three beryllium evaluations taken from FENDL/MC-2.0, FENDL/E-2.0 and EFF-3.03. Material activation was obtained from the FXDOSJ3, ACTXS1 and 531dos libraries. The fast neutron flux checked with aluminum
and niobium activation foils was modelled well by all three beryllium evaluations. Larger differences between the evaluations were observed for the 115 In(n, n )115m In reaction which is sensitive for neutrons with energies between 1 and 10 MeV and the 32 S(n, p)32 P and 35 Cl(n, ␣)32 P reactions (both >2 MeV). The best representation of the flux above 1 MeV came from the EFF-3.03 file. Lithium carbonate detectors with a low 6 Li concentration which are sensitive mostly to neutrons below 100 eV in this set-up show an overestimation of the calculation similar to the tritium production in the breeding layers where the local C/E ratio varies between 1.02 and 1.20. The tritium production is a crucial parameter of a breeding blanket design. The MCNP calculations with state-of-the-art nuclear data libraries done in the present work overestimated the tritium production in a breeding assembly of beryllium, lithium titanate, low activation ferritic steel F82H which was surrounded by a SS316 stainless steel can by up to 20% locally and 6–12% on average. It is assumed that uncertainties in the nuclear cross-section data libraries used in the MCNP calculation are responsible for the overestimation. Recent experiments indicate that this overestimation is caused by the calculated neutron transport in the SS316 stainless steel source reflector. Further study of this issue is therefore necessary in order to improve the quality of the numerical modelling of the breeding blanket designs.
Acknowledgements The authors would like to thank C. Kutsukake, S. Tanaka, Y. Abe, M. Seki, and Y. Oginuma for their excellent operation of the FNS accelerator system and Dr. U. Fischer of Forschungszentrum Karlsruhe for kind support with the EFF-3.03 file.
References [1] J.F. Briesmeister (Ed.), MCNP™ — A General Monte Carlo N-particle Transport Code, Version 4C, LA-13709-M, LANL, 2000. [2] Yu.M. Verzilov, et al., Measurements of the 32 S(n, p)32 P and 35 Cl(n, ␣)32 P reaction cross-sections in the energy range of 13.3–14.9 MeV and their integral test below 14 MeV, Fusion Eng. Des. (1997) 37.
A. Klix et al. / Fusion Engineering and Design 72 (2005) 327–337 [3] Yu.M. Verzilov, et al., Measurement of the effective crosssection for the reaction 35 Cl(n, ␣)32 P in neutron spectra within the Fusion Blanket Assembly, Vopr. At. Nauk. Tech. (1989) 4 (in Russian). [4] F. Maekawa, et al., Data collection of fusion neutronics benchmark experiment conducted at FNS/JAERI, JAERI-Data/Code 98-021, JAERI, 1998. [5] Yu.M. Verzilov, et al., A new method of extracting tritium produced in neutron-irradiated lithium-containing pellets for liquid scintillation counting, JAERI-Research 94-042, JAERI, 1994. [6] Y. Terada, et al., Experimental study of material activation of reduced activation ferritic steel F82H by D–T neutron irradiation, JAERI-Research 2002-019, JAERI, 2002 (in Japanese/English). [7] H. Wienke, M. Herman, FENDL/MG-2.0 and FENDL/MC2.0, The Processed Cross-section Libraries for Neutron–Photon Transport Calculations, Version 1, IAEA-NDS-176 Rev. 0, IAEA, 1998.
337
[8] A.B. Pashchenko, H. Wienke, FENDL/E-2.0, Evaluated nuclear data library of neutron nuclear interaction cross-sections and photon production cross-sections and photon–atom interaction cross-sections for fusion applications, released on May 15, 1998, IAEA-NDS-175 Rev. 1, IAEA, 1998. [9] R.C. Little, New Thermal Library for MCNP (Update), Internal Memorandum, X-6:RCL-86-400, LANL, 1986. [10] A.B. Pashchenko, H. Wienke, FENDL2/A-MCNP, FENDL2/ A-VITJ-E and FENDL2/A-VITJ-FLAT, The Processed FENDL-2 Neutron Activation Cross-section Data Files, IAEA-NDS-174, IAEA, 1997. [11] M. Nakazawa, et al., JENDL Dosimetry File, JAERI 1325, JAERI, 1992. [12] R.C. Little, R.E. Seamon, Dosimetry/Activation Cross Sections for MCNP, Internal Memorandum, LANL, 1984. [13] Yu. Verzilov, et al., Non-destructive analysis of impurities in beryllium affecting evaluation of the tritium breeding ratio, J. Nucl. Mater. 329–333 (2004) 1337.
Heterogeneous breeding blanket experiment with lithium titanate and beryllium A. Klixa,∗ , Yu. Verzilovb , K. Ochiaib , T. Nishitanib , A. Takahashia a
Department of Nuclear Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan b Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan Received 21 October 2003; received in revised form 23 February 2004; accepted 21 July 2004
Abstract An integral breeding blanket experiment has been done at Fusion Neutronics Source of JAERI with an assembly of three layers of lithium titanate, beryllium, and low activation ferritic steel F82H. The experimental assembly and the neutron source were enclosed in a SS316 stainless steel can and irradiated with D–T neutrons. The tritium production rate was measured with small Li2 CO3 pellet detectors. The tritium production from 7 Li was estimated by means of the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P which have effective cross-sections similar to 7 Li(n, n␣)T but are more sensitive methods. The neutron flux in the assembly was checked with activation foils. The measurement results were analyzed with the three-dimensional Monte Carlo code MCNP-4C and three different beryllium cross-section evaluations taken from FENDL/MC-2.0, FENDL/E-2.0, and EFF-3.03. The library FENDL/MC-2.0 was also used for the other materials in the assembly. All three beryllium libraries model the fast flux in the assembly well. Differences were seen for indium foils. An overestimation was observed for the tritium production. © 2004 Elsevier B.V. All rights reserved. Keywords: Breeding blanket; Lithium titanate; Tritium production; Beryllium
1. Introduction An important design issue of a fusion reactor is a sufficient tritium production rate in the breeding blanket so that the reactor can be operated in a self-sufficient mode. At Fusion Neutronics Source (FNS) of JAERI, the neutronics characteristics of advanced materials for ∗ Corresponding author. Present address: Nuclear Engineering Research Laboratory, The University of Tokyo, Tokaimura, Ibaraki-ken 319-1188, Japan. Tel.: +81 29 287 8918; fax: +81 29 287 8488. E-mail address: [email protected] (A. Klix).
0920-3796/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2004.07.019
breeding blankets are investigated. An integral experiment has been done with a thermal-type breeding blanket mock-up consisting of layers of beryllium, low activation ferritic steel F82H, and lithium titanate enriched in 6 Li. These materials are candidate materials for the future DEMO reactor. The experimental assembly and the 14 MeV D–T neutron source were enclosed in a SS316 stainless steel can to obtain a neutron field similar to that of a fusion reactor. The tritium production in the breeding blanket is based on the following two reactions: 6
Li(n, ␣)T,
Q = 4.783 MeV
(1)
328 7
Li(n, n␣)T,
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Q = −2.467 MeV
(2)
Reaction (1) is important for the thermal-type blanket due to its high cross-section for slow neutrons. Usually the lithium compound in the blanket will be enriched in 6 Li, therefore the macroscopic cross-section of this reaction will cause a strong self-shielding effect in the material and the local tritium production rate (TPR) will drop considerably towards the inside of the breeding layer. Reaction (2) contributes less than 2% to the overall tritium production in this experimental set-up as numeric simulations show and should vary only slowly with the position in the assembly due to its smaller cross-section and insensitivity to thermal neutrons. The D–T neutron source of FNS used in the experiment delivers up to 4 × 1011 neutrons per second. Despite a total source neutron fluence on the order of 1016 source neutrons during the experiment, the accumulated tritium activity in the lithium titanate is on the order of 100–400 Bq/g in the breeding layers. Liquid scintillation counting techniques are applied to measure the TPR in the breeding layers by means of small pellet detectors made of lithium carbonate. Additional detectors utilizing the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P which allow to model the tritium breeding reaction (2) and provide additional verification for the neutron spectrum calculations were distributed along the axis of the assembly to estimate the tritium production from 7 Li experimentally. In this paper, the measured tritium production profile in the breeding layers is presented and the analysis of the experiment with the three-dimensional Monte Carlo code MCNP-4C [1] and three state-of-the-art beryllium cross-section evaluations is discussed.
Fig. 1. Cross-sectional view of the experimental set-up.
can with a beryllium reflector on the source-side to simulate a fusion reactor neutron field, see Fig. 1. The composition of the assembly materials and the SS316 enclosure is shown in Tables 1 and 2. The breeding layers were made of 50 mm × 50 mm lithium titanate blocks with a thickness of 12 mm. Small pellet detectors made of lithium carbonate were inserted into holes in the blocks to measure the local tritium production. The pellets had a diameter of 12.3 mm and thicknesses of 0.5 and 2.0 mm. The pellet detectors were 5 cm off the axis of the assembly. The lithium titanate blocks were manufactured from powder by Kawasaki Heavy Industries by a cold-pressing and sintering method.
2. Experimental assembly
Table 1 Properties of the lithium titanate used in the experiments
The irradiation was done with the fixed target of FNS. A schematic view of the experimental set-up with the breeding assembly is shown in Fig. 1. The assembly consisted of layers of low activation ferritic steel F82H simulating structural material, beryllium for moderation and multiplication of neutrons, and lithium titanate enriched to 40% in 6 Li for tritium breeding. The neutron spectrum of the plain D–T source is harder than that of a fusion reactor, therefore the assembly and the neutron source were enclosed by a SS316 stainless steel
Enrichment nominal
40.33%
Analysis site Composition (wt.%) (major elements)
Pellet density (g/cm3 ) Block density (g/cm3 )
KHI Li Ti O
JAERI
10.8 9.4 ± 0.2 43.6 43.4 ± 0.5 45.5 (balance) 47.2 (balance) 2.83–2.92 2.88
The analysis from Kawasaki Heavy Industries (KHI) is based on the raw powder before sintering while the JAERI analysis is based on a finished pellet.
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Table 2 Chemical composition of the beryllium and F82H layers, and the SS316 enclosure Beryllium layer (ppm)
Be Li B Mg Al Ca Ti Cr Mn Fe Co Ni Cu Mo Cd Th U
SS316 enclosure (×10−24 atoms/cm3 )
F82H steel (wt.%)
97.9 ± 0.8 wt.% <1 <3 110 ± 5 570 ± 50 <1 100 100 96 ± 5 1300 ± 70 10, . . . , 100 250 ± 30 approximately 130 approximately 50 <1 1.5 ± 0.1 82 ± 3
Fe Cr W C Si Mn P S Cu Ni Mo V Nb Ta Ti B
89.74 7.74 1.95 0.095 0.1 0.1 0.005 0.0031 0.01 0.02 0.01 0.18 0.0001 0.04 0.005 1.496E−4
Metal foil detectors were set-up along the central axis of the assembly for neutron spectrum verification. Additional pellet-shaped detectors made from CH3 SO2 CH3 , NH4 Cl, NH4 PH2 O2 , and Li2 CO3 with a low 6 Li enrichment of 1.16% were set-up along the assembly axis to obtain more information about the neutron spectrum. The first two types of detectors utilize the reactions 32 S(n, p)32 P and 35 Cl(n, ␣)32 P. These reactions are sensitive for fast neutrons and can be used to estimate the tritium production per 7 Li (TPR7) experimentally [2–4]. The latter two detectors are sensitive to slow neutrons via the reactions 31 P(n, ␥)32 P and the breeding reaction (1). The tritium contribution from 7 Li in the Li2 CO3 detectors is low (<4%) in deep positions in the assembly but considerable near
C Si P S Cr Mn Fe Ni Mo
Assembly enclosure
Source reflector
7.1697 × 10−5 9.8440 × 10−4 4.3162 × 10−5 1.8780 × 10−6 1.5476 × 10−2 9.7963 × 10−4 5.7589 × 10−2 9.7128 × 10−3 1.0503 × 10−3
1.9879 × 10−4 8.1608 × 10−4 4.8895 × 10−5 4.4677 × 10−6 1.5025 × 10−2 1.3561 × 10−3 5.8332 × 10−2 9.1456 × 10−3 1.0254 × 10−3
the source-side surface (up to 25% immediately after the first breeding layer). A summary of the detectors used in this work is shown in Table 3 and the corresponding reaction cross-sections are plotted in Figs. 2 and 3. All pellet detectors were wrapped in thin aluminum foil to protect them from contamination during the irradiation. The cumulative irradiation time was 37 h with a total D–T source yield of (1.685 ± 0.034) × 1016 neutrons over a period of 4 days. This value is based on the output of a solid-state ␣-monitor mounted in the dbeam tube for counting the ␣-particles associated with the D–T reaction in the neutron source. The neutron source was (41.3 ± 0.5) cm away from the surface of the first F82H layer.
Table 3 Dosimetry reactions used in this analysis Reaction
Detector material
Energy range
Detection
Half-life time of product
Dosimetry file
␣)T 7 Li(n, n␣)T 32 S(n, p)32 P 35 Cl(n, ␣)32 P 93 Nb(n, 2n)92m Nb 27 Al(n, ␣)24 Na 115 In(n, n )115m In 197 Au(n, ␥)198 Au 31 P(n, p)32 P
Li2 CO3 Li2 CO3 CH3 SO2 CH3 NH4 Cl Nb foil Al foil In foil Au foil NH4 PH2 O2
Thermal >4 MeV >2 MeV >2 MeV >9 MeV >6 MeV >1 MeV Thermal and giant resonances Thermal
, LSC , LSC , Cherenkov , Cherenkov ␥, HPGe ␥, HPGe ␥, HPGe ␥, HPGe , Cherenkov
12.33 y 12.33 y 14.26 d 14.26 d 10.15 d 14.98 h 4.486 h 2.696 d 14.26 d
ACTXS1 FXDOSJ3 ACTXS1 ACTXS1 FXDOSJ3 FXDOSJ3 531dos FXDOSJ3 ACTXS1
6 Li(n,
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Aloka LSC-5100 Liquid Scintillation Counter. The error of the measurement is estimated to 12% as sum of the errors of the efficiency measurement (3%), the neutron source yield measurement (3%), counting error (1%), and an estimate for incomplete tritium recovery (5%). 3.2.
31 P, 32 S,
and 35 Cl detectors
The detectors containing 31 P, 32 S and 35 Cl were dissolved in water in 20 ml standard Teflon vials for Cherenkov counting with the Aloka LSC-5100. The produced 32 P emits -particles with a mean energy of 694.9 keV which is high enough to cause Cherenkov radiation in the solvent water [2]. Fig. 2. Cross-sections of the detectors for the fast neutron flux. Source JEF-2.2.
3.3. Metal foils The activation of the metal foils was acquired with a high purity germanium detector. Measurement conditions and results are published elsewhere [6]. The activation of 93 Nb(n, 2n)92m Nb, 27 Al(n, ␣)24 Na, 115 In(n, n )115m In, and 197 Au(n, ␥)198 Au were selected for comparison with calculations in this work.
4. MCNP calculations and nuclear data libraries
Fig. 3. Cross-sections of the detectors for the slow neutron flux. Source JEF-2.2.
3. Detector processing 3.1. Lithium carbonate The amount of tritium generated in the lithium carbonate pellets is on the order of 2–120 Bq per pellet in this experiment. A well-established method [5] was applied to process the lithium carbonate pellets and Clearsol (Nacalai Tesque) liquid scintillator was used for the preparation of the scintillation cocktails. The scintillation cocktails were counted in 20 ml Teflon vials in an
The experimental results were analyzed with the three-dimensional Monte Carlo code MCNP-4C. The continuous library FENDL/MC-2.0 [7] was used for most isotopes in this calculation. It is derived from FENDL-2 which is considered the best validated nuclear data library currently available for fusion applications. The objective of this analysis is to verify the applicability of three state-of-the-art beryllium cross-section evaluations together with the fusion cross-section library FENDL/MC-2.0 for breeding blanket calculations. For the first calculation the beryllium file included in the FENDL/MC-2.0 library was applied. It is based on ENDF/B-VI. For the second and third calculations the beryllium evaluations from FENDL/E-2.0 [8] which is based on the JENDL Fusion File and the European EFF-3.03 were used. The available EFF-3.03 file did not contain the (n, ␥) part in its absorption cross-
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section, it was therefore extracted from ENDF/B-VI and added to the EFF-3.03 file. The thermal scattering file be.01t from the TMCCS library [9] was applied in all three calculations for an accurate neutron transport in beryllium in the low energy range. The TPR in the breeding layers and the lithium carbonate detectors with the low 6 Li concentration was obtained from cell-averaged tallies while surface tallies were used for the activation foils, the 32 S, 35 Cl, and 31 P pellets. Reaction rates were calculated with tally multipliers and cross-sections taken from the ACTXS1 [10] library distributed by IAEA, the FXJDOS3 based on the JENDL– DOS91 [11] dosimetry file, and the library 531dos [12]. The angular and energy distribution of the D–T neutron source was described accurately in the input file. From measurements of pulse decay times [13] it was found that the thermal absorption cross-section of the beryllium applied in the experiment is increased by about 30% due to impurities. This increase was modelled by the addition of 2.5 ppm of 6 Li to the beryllium in the calculation. Such an increased thermal absorption cross-section reduces the TPR on and near the surface of the breeding layer where a considerable amount of tritium is bred by thermal neutrons. MCNP calculations of the experimental set-up with pure beryllium and beryllium with impurities modelled by 6 Li, however, show that this effect is small and on the order of 1% near the surface of the breeding layer. The composition of the breeding layers is shown in Table 1. A value of 10.8 wt.% was adopted for the lithium concentration.
5. Results 5.1. Fast, epithermal and thermal neutron flux in the breeding assembly The fast neutron flux in the assembly was monitored with Al and Nb foils and the pellet detectors containing 32 S and 35 Cl. Fig. 4 shows the obtained calculation/experiment (C/E) ratios. The C/E ratio for the assembly surface position is nearly 1.00 in case of 93 Nb(n, 2n)92m Nb, 27 Al(n, ␣)24 Na, and 115 In(n, n )115m In, and 0.96 in case of 32 S(n, p)32 P. The excellent agreement indicates an adequate calibration of the source neutron yield monitor and D–T target position relative to the assembly surface.
331
Fig. 4. C/E ratios of detectors sensitive for the fast neutron flux. The error estimates include the counting error and an allowance for detector efficiency and MCNP calculation. For orientation, the location of the three breeding layers is shown in gray bars.
Inside the assembly, the Nb foils C/E ratio indicates an overestimation of the fast neutron flux above 9 MeV in the calculation which is, however, not supported by the other fast neutron detectors. The C/E ratio differences between the three beryllium evaluations are only small. They become apparent at lower neutron energies measured with the 35 Cl, 32 S, and 115 In detectors. Especially the indium foil detectors show C/E ratio differences up to 10% between the calculations with the three beryllium evaluations at corresponding detector locations. While the C/E ratio obtained with EFF-3.03 scatters about 1.0, the C/E ratios based on the calculations with the two FENDL beryllium evaluations appear to have a rising tendency with depth in the breeding assembly indicating uncertainties in the beryllium nuclear data. Au foils, 31 P containing and lithium carbonate pellet detectors with a low 6 Li isotopic ratio (1.16%) were used to gain information about the thermal part of the neutron spectrum in the assembly. Since the cross-
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Fig. 5. Contribution from different neutron energy ranges to the tritium production in the pellet detectors with the low 6 Li enrichment.
sections of the corresponding reactions are composed of a 1/v part and giant resonances at higher neutron energies, it is necessary to estimate the contribution from different neutron energy ranges to the total reaction rate in the detector. This was done by a MCNP calculation and the results are presented in Figs. 5–7. It can be seen from these figures that most of the tritium production in the Li2 CO3 pellets comes from neutrons with energies below 100 eV while even the giant resonance at 240 keV in the cross-section contributes just a small amount. The contribution in the range 1–16 MeV
Fig. 6. Contribution from different neutron energy ranges to the 31 P(n, ␥)32 P reaction rate in the 31 P detectors.
Fig. 7. Contribution from different neutron energy ranges to the 197 Au(n, ␥)198 Au reaction rate in the gold foils.
is mainly from 7 Li. The 31 P detector is considered to model the tritium breeding reaction (1) in soft neutron fields. The giant resonances are at energies higher than 10 keV, but especially in case of the pellet attached to the first breeding layer at position 28 mm contribute a significant amount to the total 32 P production in the pellet.The situation is different for the gold foils. The giant resonance between 1 and 10 eV contributes more than
Fig. 8. C/E ratios of detectors sensitive for the slow neutron flux. The error estimates include the counting error and an allowance for detector efficiency and MCNP calculation. For orientation, the location of the three breeding layers is shown in gray bars.
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Fig. 9. Calculated reaction rates for 6 Li(n, ␣)T and 7 Li(n, n␣)T in the three breeding layers multiplied with the corresponding isotopic fraction (enrichment). The sum of the tritium production graphs is the tritium production per lithium target. The second reaction contributes less than 2% to the total tritium production in the breeding layers.
75% to the total reaction rate in all positions and more than 80% except for two positions in the last beryllium layer. The C/E ratios of these three detectors are presented in Fig. 8. A large scattering is observed for the gold foils which is attributed to the narrow neutron energy range of the giant resonance which contributes most to the reaction rate resulting in a bad calculation statistics. In case of the other two detector types, the C/E ratio does not show a clear rising or falling tendency in the assembly. The Li2 CO3 pellets show an overestimation of the calculated reaction rate by 3–15% inside the assembly. FENDL/MC-2.0 and EFF-3.03 represent best the experimental values. It should be noted that according to the MCNP calculation the contribution of 7 Li to the tritium production is approximately 16, 25, and 7% for the first three pellets, respectively, near the assembly surface. The C/E ratio was calculated for the total TPR in each detector. The underestimation of the 31 P(n, ␥)32 P reaction rate is thought to be due to uncertainties in the cross-section data of this reaction. It is concluded that all three beryllium evaluations simulate the fast neutron flux above 6 MeV (threshold of the 27 Al(n, ␣) reaction) well. Differences between them become apparent at energies below 6 MeV. Between 10 and 1 MeV (indium foils) the best results
came from the EFF-3.03 file followed by FENDL/MC2.0. The calculated reaction rate in the lithium carbonate detectors is overestimated by all three evaluations but the C/E ratio does not show a clear rising or falling tendency in the assembly, therefore it is also concluded that the thermal neutron transport in the assembly is adequately described.
Fig. 10. TPR7 obtained from measurements with 35 Cl and 32 S. For comparison, the TPR7 calculated for the detectors with the low 6 Li concentration is shown. The conversion factors used were R35 Cl /R7 Li = 2.14 and R32 S /R7 Li = 1.01.
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Fig. 11. Experimentally obtained tritium production rates in the three breeding layers from lithium carbonate detectors compared to calculation results. Distances are from the source-side surface of the breeding assembly. The values are averages over the pellet.
5.2. Tritium production rate in the breeding layers The 32 P production from 35 Cl(n, ␣) and 32 S(n, p) was used to estimate the tritium production from 7 Li (TPR7) and to obtain the TPR6 in the breeding layers, however, the TPR7 does not contribute much to the total TPR in this experiment. Fig. 9 shows calculated contributions from 6 Li and 7 Li to the tritium production per Li target (40% 6 Li enrichment). The contribution from 7 Li is always below 2%. The TPR6 can be computed from the difference of the measured total TPR and the TPR7 estimated by the model reactions mentioned above with sufficient accuracy since it is only a small correction. Fig. 10 shows the estimated TPR7 together with the calculated TPR7 by MCNP. It can be seen that the values agree within the error limits. To estimate the TPR7, a conversion factor was multiplied with the measured reaction rates of the two model reactions. In case of 35 Cl, the value R35 Cl /R7 Li = 2.14 [3] was used. For the estimation of the TPR7 from 32 S a conversion factor of R32 S /R7 Li = 1.01 was found in recent experiments at FNS. Fig. 11 shows the measured and calculated TPR6 in the breeding layers. The profile is typical for a thermaltype breeding blanket due to self-shielding. To illustrate the dependence of the TPR6 on the location in the breeding layers, calculated neutron and TPR6 spectra for pellet detectors inside and on the surface of a breeding layer are shown in Fig. 12. The high macro-
Fig. 12. Calculated neutron spectra and TPR spectra (normalized to one source neutron) in three pellets in the second breeding layer. The central pellet has clearly less low-energy neutrons available for tritium production than the pellets on the surface (source-side and back side).
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Fig. 13. Relative contributions to the TPR6 in the three breeding layers from different neutron energy ranges. Calculated with MCNP.
scopic cross-section of the tritium-producing reaction (1) causes a significant absorption of thermal neutrons in the breeding layer. Therefore a peak of thermal neutrons can be seen in the neutron spectra from the surface of the breeding layer but not in the spectrum in the middle of the layer. The calculated relative contributions from different neutron energy ranges to the TPR6 are shown in Fig. 13. Even in the center of the breeding layers more than 60% of the TPR6 come from neutrons with an energy below 130 eV. Near the surface this figure rises to more than 80%. The calculated absolute contributions of these energy ranges are presented in Fig. 14. As expected from Fig. 12, only the TPR6 from neutron energies below 1 eV exposes a strong dependence on location, whereas already the range from 10.7 to 130 eV despite a significant contribution does not much depend on location in the breeding layer anymore.
The calculated TPR in the breeding layers is overestimated locally by 2–20 and 6–12% on average, although most C/E data points are within the combined error estimate of experiment and calculation (<15%). The reaction of the main breeding cross-section 6 Li(n, ␣)T is well known, also the tritium measurement procedure is well established. The overestimation is thought to be due to inaccuracies in the computer model especially uncertainties in the nuclear data, for example for inelastic scattering and (n, 2n) reactions rather than an experimental underestimation. 14 MeV D–T neutrons from the source undergo various slow-down mechanisms. Especially at high neutron energies from several MeV to 14 MeV, inelastic scattering by (n, n ) and (n, 2n) reactions plays an important role. The energy and angular distribution data of the secondary neutrons emitted from these reactions are considered to have considerable uncertainties for some
Fig. 14. Absolute contributions to the TPR6 in the three breeding layers from different neutron energy ranges. Calculated with MCNP.
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isotopes used in this mock-up experiment. A recently conducted experiment to investigate the influence of the source reflector on the TPR is currently analyzed and indicates that the TPR overestimation in the calculation model may be caused by the SS316 steel of the source reflector. Sensitivity analyses are underway in an attempt to support this assumption. An uncertainty arises also from the lithium concentration value in the lithium titanate blocks used to build the breeding layers. Table 1 shows the chemical analysis of the raw lithium titanate powder for fabrication of the sintered blocks. It was provided by the manufacturer Kawasaki Heavy Industries. An analysis of a finished pellet (12.3 mm diameter and 1 mm thickness) was done by JAERI, however, this value is considered less representative for the blocks and a thorough analysis of the lithium titanate blocks is underway. Since a lithium loss during the manufacturing of the blocks by sintering is likely to occur, a sensitivity analysis with respect to a reduced lithium concentration has been done by correlated sampling with MCNP. An original value of 10.8 wt.% lithium in the breeding layers and a reduction by 5% was assumed for this calculation. It was found that the TPR6 response of the lithium carbonate detectors inserted into the breeding layers was increased by 1–3%.
6. Conclusion A breeding blanket mock-up consisting of candidate materials for JAERIs DEMO fusion reactor blanket design was irradiated with D–T neutrons in a SS316 stainless steel enclosure. The tritium production rate inside the breeding layers was measured with a wellestablished method applying lithium carbonate pellet detectors. Additional detectors in the form of metal foils and pellets were used to check the neutron field inside the assembly and estimate the contributions from 6 Li(n, ␣)T and 7 Li(n, n␣)T to the total tritium production in the breeding layers. The experiment was analyzed with the threedimensional Monte Carlo code MCNP-4C, the neutron cross-section library FENDL/MC-2.0 and three beryllium evaluations taken from FENDL/MC-2.0, FENDL/E-2.0 and EFF-3.03. Material activation was obtained from the FXDOSJ3, ACTXS1 and 531dos libraries. The fast neutron flux checked with aluminum
and niobium activation foils was modelled well by all three beryllium evaluations. Larger differences between the evaluations were observed for the 115 In(n, n )115m In reaction which is sensitive for neutrons with energies between 1 and 10 MeV and the 32 S(n, p)32 P and 35 Cl(n, ␣)32 P reactions (both >2 MeV). The best representation of the flux above 1 MeV came from the EFF-3.03 file. Lithium carbonate detectors with a low 6 Li concentration which are sensitive mostly to neutrons below 100 eV in this set-up show an overestimation of the calculation similar to the tritium production in the breeding layers where the local C/E ratio varies between 1.02 and 1.20. The tritium production is a crucial parameter of a breeding blanket design. The MCNP calculations with state-of-the-art nuclear data libraries done in the present work overestimated the tritium production in a breeding assembly of beryllium, lithium titanate, low activation ferritic steel F82H which was surrounded by a SS316 stainless steel can by up to 20% locally and 6–12% on average. It is assumed that uncertainties in the nuclear cross-section data libraries used in the MCNP calculation are responsible for the overestimation. Recent experiments indicate that this overestimation is caused by the calculated neutron transport in the SS316 stainless steel source reflector. Further study of this issue is therefore necessary in order to improve the quality of the numerical modelling of the breeding blanket designs.
Acknowledgements The authors would like to thank C. Kutsukake, S. Tanaka, Y. Abe, M. Seki, and Y. Oginuma for their excellent operation of the FNS accelerator system and Dr. U. Fischer of Forschungszentrum Karlsruhe for kind support with the EFF-3.03 file.
References [1] J.F. Briesmeister (Ed.), MCNP™ — A General Monte Carlo N-particle Transport Code, Version 4C, LA-13709-M, LANL, 2000. [2] Yu.M. Verzilov, et al., Measurements of the 32 S(n, p)32 P and 35 Cl(n, ␣)32 P reaction cross-sections in the energy range of 13.3–14.9 MeV and their integral test below 14 MeV, Fusion Eng. Des. (1997) 37.
A. Klix et al. / Fusion Engineering and Design 72 (2005) 327–337 [3] Yu.M. Verzilov, et al., Measurement of the effective crosssection for the reaction 35 Cl(n, ␣)32 P in neutron spectra within the Fusion Blanket Assembly, Vopr. At. Nauk. Tech. (1989) 4 (in Russian). [4] F. Maekawa, et al., Data collection of fusion neutronics benchmark experiment conducted at FNS/JAERI, JAERI-Data/Code 98-021, JAERI, 1998. [5] Yu.M. Verzilov, et al., A new method of extracting tritium produced in neutron-irradiated lithium-containing pellets for liquid scintillation counting, JAERI-Research 94-042, JAERI, 1994. [6] Y. Terada, et al., Experimental study of material activation of reduced activation ferritic steel F82H by D–T neutron irradiation, JAERI-Research 2002-019, JAERI, 2002 (in Japanese/English). [7] H. Wienke, M. Herman, FENDL/MG-2.0 and FENDL/MC2.0, The Processed Cross-section Libraries for Neutron–Photon Transport Calculations, Version 1, IAEA-NDS-176 Rev. 0, IAEA, 1998.
337
[8] A.B. Pashchenko, H. Wienke, FENDL/E-2.0, Evaluated nuclear data library of neutron nuclear interaction cross-sections and photon production cross-sections and photon–atom interaction cross-sections for fusion applications, released on May 15, 1998, IAEA-NDS-175 Rev. 1, IAEA, 1998. [9] R.C. Little, New Thermal Library for MCNP (Update), Internal Memorandum, X-6:RCL-86-400, LANL, 1986. [10] A.B. Pashchenko, H. Wienke, FENDL2/A-MCNP, FENDL2/ A-VITJ-E and FENDL2/A-VITJ-FLAT, The Processed FENDL-2 Neutron Activation Cross-section Data Files, IAEA-NDS-174, IAEA, 1997. [11] M. Nakazawa, et al., JENDL Dosimetry File, JAERI 1325, JAERI, 1992. [12] R.C. Little, R.E. Seamon, Dosimetry/Activation Cross Sections for MCNP, Internal Memorandum, LANL, 1984. [13] Yu. Verzilov, et al., Non-destructive analysis of impurities in beryllium affecting evaluation of the tritium breeding ratio, J. Nucl. Mater. 329–333 (2004) 1337.