MEASUREMENT OF FLUX SPECTRA AND TRITIUM PRODUCTION ...

MEASUREMENT OF FLUX SPECTRA AND TRITIUM PRODUCTION RATES IN AN ITER TBM MOCK-UP IRRADIATED WITH 14 MEV NEUTRONS

A. Klix1,a , P. Batistonib , U. Fischerc , H. Freieslebena , D. Leichtlec , K. Seidela , S. Unholzera a

) Technische Universit¨ at Dresden, Institut f¨ ur Kern- und Teilchenphysik, D-01062 Dresden, Germany b ) Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, I-00044 Frascati, Italy c ) Association FZK-Euratom, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany

A mock-up of the European Helium-Cooled Pebble Bed TBM was irradiated with DT neutrons in pulsed and continuous mode at the Fusion Neutronics Laboratory of the University of Technology Dresden. The aim was to measure fast neutron and gamma-ray flux spectra as well as time-of-arrival spectra of the slow neutron flux. The results of the experiments were analysed by the Monte Carlo code MCNP and nuclear data from the European Fusion File (EFF-3), and the Fusion Evaluated Nuclear Data Library (FENDL-2.0/2.1). It was found that the calculation of the fast neutron flux above 3 MeV tends to overestimate while the gammaray flux and slow neutron flux in two measurement positions in the mock-up was underestimated. The mock-up was also irradiated at FNG/ENEA Frascati to measure tritium breeding rates by means of small Li2 CO3 pellet detectors inserted into the breeding layers. The breeding experiment was analysed at FZ Karlsruhe with emphasis on determining sensitivities of the TPR to relevant cross section uncertainties of all materials in the mock-up. It was found that the TPR calculation shows a tendency to underestimate. From the sensitivity analysis it was found that the total TPR is most sensitive to the elastic scattering in Be and the 7 Li(n,αT) reaction. I. INTRODUCTION The HCPB Test Blanket Module (TBM) is a breeding blanket based on a lithium ceramics enriched in 6 Li and beryllium as neutron moderator and mul-

tiplier. Nuclear design parameters such as tritium production rate (TPR), shielding capability, nuclear heating, and material activation can be estimated with the available nuclear data files and radiation transport codes. However, the uncertainties of such calculations depend on the uncertainties of the data files used for transport and activation calculation. It is therefore necessary to validate codes and data in mock-up experiments which simulate the neutronics performance of the TBM. An experimental neutronics program has been established in the framework of the European Fusion Technology Programme to validate the TBM design in a collaboration of ENEA Frascati, FZ Karlsruhe (FZK), JSI Ljubljana, and TU Dresden (TUD) with contributions from FNS of JAEA Tokaimura. A blanket mock-up consisting of Li2 CO3 , beryllium and stainless steel was assembled at Frascati Neutron Generator (FNG). In a first experiment TPR measurements were conducted at FNG/ENEA using small pellet detectors inserted into the breeding layers which were prepared at Fusion Neutronics Source (FNS) Tokaimura.1,2 After irradiation, the detectors were processed in a wet chemistry process and the accumulated tritium activity was measured by liquid scintillation counting. The tritium breeding experiment was analyzed with the codes MCNP-4C (Ref. 3) and MCSEN (Ref. 4) at FZK.2,5 In a second series of experiments, the mock-up was

1 [email protected]

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FLUX SPECTRA AND TRITIUM PRODUCTION RATES IN ITER TBM MOCK-UP

shipped to the Fusion Neutronics Laboratory of TUD and irradiated with DT neutrons in continuous mode to obtain fast neutron spectra inside the assembly with a NE 213 detector, and in pulsed mode for the measurement of time-of-arrival spectra of the decay of the slow neutron population inside the assembly with a 3 He counter. The experiment was analysed with the neutron transport code MCNP-4C and nuclear data from the Fusion Evaluated Nuclear Data Library (FENDL) and the European Fusion File (EFF).6,7 II. EXPERIMENTS A cross-sectional view of the mock-up assembly for the breeding experiment is shown in Fig. 1. It consisted of a box made of stainless steel filled with beryllium and two layers of Li2 CO3 as breeding material. Four cylindrical sample holders were prepared which could be inserted into holes in the mock-up and carry the pellet detectors as well as activation foils for the measurement of integral neutron flux spectra inside the assembly. The backside was shielded by a stainless steel box filled with Li2 CO3 .

Figure 1: Cross-sectional view of the TBM mock-up for the TPR measurements at FNG/ENEA. An important aim of the experiment was the verification of the tritium production rate predicted by calculations with the ITER reference code MCNP and state-of-the-art nuclear data libraries such as FENDL2. To achieve the lowest experimental uncertainty, the TPR was measured with well-established wet chemistry methods applying pellet detectors of the same iso-


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topic and chemical composition as the breeding layer thereby reducing uncertainties from inhomogeneities introduced into the breeding layers by the detectors themselves. For the TPR experiment, a total of 96 lithium carbonate pellet detectors had been placed in 8 positions inside the two breeding layers of the mockup. The samples from the upper layer were processed at TU Dresden while the pellet detectors in the lower breeding layer were processed at FNG. For comparison, some detectors from both layers were analysed at FNS/JAEA Tokaimura. A second experiment was conducted at the Fusion Neutronics Laboratory of TUD with the aim to measure fast neutron and gamma-ray flux spectra as well as time-of-arrival spectra of the slow neutron population in the rear block of the assembly, see Figure 2. Fast neutron and gamma-ray fluxes are of interest because they determine the shielding capabilities of the TBM while the slow neutron flux is responsible for most of the tritium breeding on 6 Li.

Figure 2: Cross-sectional view of the TBM mock-up for the neutron and gamma-ray flux spectra measurements at TUD-FNL. The middle section had a channel of 5cm x 5cm for inserting the NE-213/3He detectors. Measurement positions P1 and P2 were changed by turning the middle section by 180 degrees about its vertical axis. For the spectra measurement the space for the pellet detectors in the inserts was filled with Li2 CO3 powder. Fast neutron and gamma-ray flux spectra were measured simultaneously with a NE-213 detector while a 3 He tube was used to obtain the slow neutron flux. Both detectors were placed in a channel in a stainless

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steel box which was inserted between the TBM box and the back-side shield. This box was also filled with Li2 CO3 . The measurement positions P1 and P2 were realized by turning this box by 180 degrees about its vertical axis. The sides of the mock-up were shielded with polyethylene blocks of 8 cm thickness. The 3 He counter had an active volume of 2.4 cm diameter and a length of 15.1 cm while the NE-213 detector had dimensions of 3.8 cm diameter and length. It was coupled to the photomultiplier by a light guide of 50 cm length. The DT neutron source was 15.8 cm away from the assembly and monitored with a Si semiconductor detector for the associated α-particle, and a 238 U fission chamber. For the fast neutron and gamma-ray flux measurement the neutron source was operated in continuous mode. The signal from the NE-213 detector was processed in a pulse shape discrimination stage and the recorded raw data were unfolded to obtain fast neutron and gamma-ray flux spectra. Time-of-arrival spectra for the slow neutron flux were measured with the DT neutron source operated in pulsed mode (pulse width of 10 μs, repetition rate of 1 kHz). The signals from the 3 He counter as well as that of the source monitors were recorded with a multichannel scaler. III. RESULTS AND DISKUSSION

17% in Position 1. The integral flux from 1 MeV up to the fusion peak energy is overestimated by 12 and 18% for EFF and FENDL-2.1, respectively, in Position 1 and 25 and 31% for EFF and FENDL-2.1, respectively, in Position 2. The results indicate that neutron shielding capability estimates for a HCPB-type blanket will be conservative.

Figure 3: Fast neutron spectra in detector positions 1 and 2 measured with a NE-213 scintillator compared with calculations.

III.A. Neutron and gamma-ray flux spectra The experiments were analysed with the coupled neutron and gamma transport code MCNP and a three-dimensional model of the assembly including the room walls and the geometry of the table on which the assembly was located.8 For the DT neutron source, an accurate angle and energy distribution of the neutrons was used. Nuclear data were taken from the EFF library (EFF-3.05 for 9 Be, EFF-2.4 for 6,7 Li and 16 O, 12 C), from the FENDL-2.0 library (origin of the files: JENDL-FF for 9 Be and 16 O, ENDF/B-VI for 6,7 Li) and from the FENDL-2.1 library (origin ENDF/BVI.8 for 9 Be, 6,7 Li and 16 O). Measured and calculated spectra of the fast neutron flux are shown in Figure 3. Good agreement of calculation and experiment was found for the energy range 1 MeV – 3 MeV. At neutron energies above 3 MeV the calculation overestimates significantly. The overestimation is larger at Position 2. The overestimation in the energy range 5 MeV – 10 MeV is largest with up to 40% and similar in both detector positions . Above 10 MeV, the overestimation is larger deeper in the assembly (Position 2) with 25% compared to 15–

Figure 4: Gamma-ray flux spectra in detector positions 1 and 2 measured with a NE-213 scintillator compared with calculations.

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Gamma-ray flux spectra are shown in Figure 4. Unlike the fast neutron flux the photon flux was found to be underestimated by the calculation by about 20% in Position P1 and 10% in Position P2.

Figure 5: Time-of-arrival spectra in the two 3 He detector positions. The signal from the detector for the associated α-particle which monitors only the generation rate of DT neutrons is also shown in this picture. The pulse length was set to 10 μs.

sults were analysed with MCNP-4C and the sensitivity/uncertainty code MCSEN at FZK.5 . From such an analysis, the sensitivity of a nuclear response of interest with respect to the uncertainty of nuclear cross section data relevant to the nuclear response of interest can be obtained.

Figure 6: Measured tritium production rates in Position 1 in the assembly. The decrease of the TPR towards Pellet 6 is due to the strong shielding of thermal neutrons inside the breeding layer.

Time-of-arrival spectra from the 3 He counter and MCNP calculations are shown in Figure 5. The calculated values were obtained from folding the neutron flux with the 3 He(n,p) cross section. The signal from the α-particle detector for monitoring the DT neutron generation rate is also shown in the diagram. 50 μs after the initial DT neutron pulse, the energies of the neutrons in the assembly are already less than 1 keV as detailed calculations suggest. It takes about 500 μs for the slow neutron population to disappear at Position 1 while the absorption proceeds more rapidly at Position 2 since the detector is completely surrounded by lithium carbonate which is a strong absorber for slow neutrons. It was found that the slow neutron flux at Position 1 is underestimated by about 10% and even more at Position 2. This results follow the same tendency as the TPR measurments.

A tendency of underestimation was found for most of the measurement points indicating that the design calculations for a HCPB-type blanket will be conservative with respect to the TPR providing an additional margin for compensation of other uncertainties. Comparing the results with similar experiments recently done at FNS of JAEA, it should be noted that the orientation of the breeding layers with respect to the assembly’s main axis and DT neutron source and hence the effect of angle-dependent cross sections on the TPR was different. The uncertainty calculations show that the TPR from 6 Li is most sensitive to the elastic scattering cross section of 9 Be (2%/%) and the (n,2n) reaction (0.5– 0.7%/%) while the TPR due to 7 Li is most sensitive to the 7 Li(n,nα)t reaction (about 1%/%).

III.B. Tritium production measurements

IV. CONCLUSION

The pellet detectors for the TPR measurement were processed following Diercks method (Ref. 9) at VKTA Rossendorf and ENEA, and a method developed by Verzilov (Ref. 10) at FNS. The re-

A mock-up of the European Helium-cooled Pebble Bed TBM was investigated at FNG/ENEA and at TUD-FNL for its neutronics characteristics with respect to tritium production and shielding performance.


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The measurements were analyzed with the reference neutron transport code MCNP and various state-ofthe-art neutron cross section data libraries. It was found that the tritium production tends to be underestimated while the fast neutron flux was overestimated by the calculation. Both results indicate that design calculations for TPR and shielding will be conservative and provide additonal margins to compensate for other uncertainties. ACKNOWLEDGEMENTS The authors would like to thank I. Sch¨ afer of VKTA Rossendorf, Yu. Verzilov and K. Ochiai of FNS/JAEA, and M. Angelone and M. Pillon of FNG/ENEA for their collaboration. This work, supported by the European Communities under the contract of Association between EURATOM/FZK(TUD), TW5-TTMN-002/D02, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. REFERENCES 1. P. BATISTONI, M. ANGELONE, L. BETTINALI, P. CARCONI, U. FISCHER, I. KODELI, et al. ”Neutronics experiment on a HCPB breeder blanket mock-up”. Proc. 24th Symposium On Fusion Technology, Warsaw, September 2006. 2. U. FISCHER, H. FREIESLEBEN, D. LEICH¨ TLE, R. PEREL, I. SCHAFER, and K. SEIDEL. ”Measurement and Analysis of Tritium Production Rates in Ceramic Breeder of the Test Blanket Module (TBM) Mock-up”. TUD-IKTP/01-06, TU Dresden, 2000. 3. J. F. BRIESMEISTER et al. ”MCNPT M - A General Monte Carlo N-Particle Transport Code, Version 4C”. LA-13709-M, LANL, 2000.

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4. U. FISCHER and R. PEREL. ”Estimation of uncertainties in neutronics calculations: Testing of the MCSEN code for track length detectors for the TBM mock-up neutronics experiment”. Final report on the EFDA task TW5-TTMN-001, Deliverable 2, Forschungszentrum Karlsruhe, January 2006. 5. U. FISCHER, D. LEICHTLE, I. KODELI, R. L. PEREL, M. ANGELONE, P. BATISTONI, et al. ”Sensitivity and uncertainty Analyses of the tritium production in the HCPB breeder blanket mock-up experiment”. Proc. 24th Symposium On Fusion Technology, Warsaw, September 2006. 6. H. WIENKE and M. HERMAN. ”FENDL/MG2.0 and FENDL/MC-2.0, The processed crosssection libraries for neutron-photon transport calculations, version 1”. IAEA-NDS-176 Rev. 1, IAEA, 1998. 7. H. HENRIKSSON et al. ”The EFF Project Status and the Nuclear Data Services”. Proc. 24th Symposium On Fusion Technology, Warsaw, September 2006. 8. K. SEIDEL, P. BATISTONI, U. FISCHER, H. FREIESLEBEN, A. KLIX, D. LEICHTLE, et al. ”Measurement and analysis of the neutron and gamma-ray flux spectra in a neutronics mockup of the HCPB Test Blanket Module”. Proc. 24th Symposium On Fusion Technology, Warsaw, September 2006. 9. R. DIERCKS. ”Direkt tritium production measurements in irradiated lithium”. Nuclear Instruments and Methods, 107:397, 1973. 10. 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.


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