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1 This article was downloaded by: [ ] On: 26 August 2015, At: 22:51 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: 5 Howick Place, London, SW1P 1WG Journal of Nuclear Science and Technology Publication details, including instructions for authors and subscription information: Measurement and Analysis of Neutron and Photon Flux Spectra in an ITER Shield Mockup with Open Channel and Cavity S. Unholzer a, W. Hansen a, U. Fischer b, H. Freiesleben a, D. Richter a & K. Seidel a a Technische Universität Dresden, D Dresden, Germany b Forschungszentrum Karlsruhe D Karlsruhe, Germany Published online: 27 Aug To cite this article: S. Unholzer, W. Hansen, U. Fischer, H. Freiesleben, D. Richter & K. Seidel (2000) Measurement and Analysis of Neutron and Photon Flux Spectra in an ITER Shield Mockup with Open Channel and Cavity, Journal of Nuclear Science and Technology, 37:sup1, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Supplement 1, p. 243~247 (March 2000) Measurement and Analysis of Neutron and Photon Flux Spectra in an ITER Shield Mockup with Open Channel and Cavity S. Unholzer"t, W. Hansen', U. Fischer", H. Freiesleben', D. Richter' and K. Seide1* * Technische Universitat Dresden,.. Forschungszentrum Karlsruhe The neutron performance of the ITER shielding system has been experimentally investigated on the basis of an integral experiment for a shield mock-up with open streaming channel. The mock-up simulates the first wall, the shielding blanket, the vacuum vessel, the toroidal field coil and a radial channel through the shield blanket used for the mechanical attachment of the blanket module. The aim ofthis work has been to provide experimental validation of the ITER- design parameters for a streaming path with direct sight of the D-T plasma. Neutron and photon flux spectra have been measured on the central axis in a cavity at the bottom ofthe central channel and at a deep position behind that with the 14 MeV neutron source on and off the central axis and additionally for shifted sidewise positions of the detector. The experimental results are compared with Monte Carlo calculations using nuclear data from FENDL-I and -2 data libraries. In general, the measured neutron and y-ray flux spectra can be predicted within an uncertainty margin of 30% both with FENDL-I and -2 data. A somewhat larger underestimation is found for the deep position at the back ofthe shield mock-up. I. Introduction KEYWORDS: Fusion neutronics, Mock-up experiment, ITER, MCNP-calculations, Neutron flux spectra, Photon flux spectra Different nuclear data libraries have been tested against an ITER-shield mock-up experiment performed at the Frascati Neutron Generator. The fusion neutronics experiments have been started with investigations of a compact mock-up to study the penetration of 14 Me V neutrons in thick assemblies by measurement of the neutron and photon flux spectra and they are continued with the investigation of streaming effects by an open channel. The experiments are part of the joint EU investigations on the nuclear validation of the ITER-inboard shield system(l). The spectral neutron flux is the basic quantity for the calculation of nuclear design parameters like gas-production, radiation damage, material activation and radiation shielding. The photon flux is an important functional of the neutron flux and allows the direct estimation of gamma-heating and the calculation of photon induced radiation effects on materials. In previous experiments the neutronics performance of the ITER shielding system has been investigated for a compact mock-up(2). Neutron and photon flux spectra have been measured and analysed by MCNP calculations in two selected positions deep inside the assembly corresponding to the backplate of the blanket (position A) and to the boundary of the vacuum vessel with the coils (position 8). Validity of the ITER nuclear design calculations for neutron, D-OJ062 Dresden, Germany., D-7602 J Karlsruhe, Germany t Corresponding author, Tel Fax II, unholzer@physik.tu-dresden.de and photon fluence spectra using the Fusion Evaluated Nuclear Data Library (FENDL-I) has been found to be within 90% for neutron fluences and 100% for photon fluences above the full energy range in position A. Comparable results have been found also in neutron flux calculations using the European Fusion Files ( EFF-2, EFF-3) and the Japanese Evaluated Nuclear Data Library (JENDL-FF). These are remarkable results for the simulation of multiple interaction processes of 14 MeV neutrons up to a penetration depth of 41.3 cm. The 14 MeV neutron flux is here reduced to by nuclear interactions in a very complex mock-up composed of stainless steel SS-316 and Perspex (plexiglass). Therefore the status ofthe evaluated nuclear data files for the main constituents of the mock-up like Fe, Ni and Cr is comparably for all libraries and of high quality. For the deep position 8 witn a penetration depth of 87.6 cm, where the 14 MeV neutron flux is reduced to , the validity of the nuclear design calculation is going down to about 70% for the low energy part of the neutron fluence spectra below I Me V and to about 80%. for the high energy part above 10 MeV, where EFF data give 5% better results. The validity for the photon fluence calculations is better than 95% for all data files with exception of FENDL-I data with 90%. The aim of the present work is to investigate the validity of the design parameters for a central streaming path in the mockup with a direct view to the 14 MeV neutron source, simulating a channel through first wall and blanket with direct sight ofthe D-T plasma. This channel models the mechanical attachment of a blanket module to the backplate in ITER. II. Experiment and Calculation The geometry of the mock-up experiment performed at the 243

3 244 PosItions of the 14 MeV neutron source --. ) shifted on axis ~ I o IS22l Perspex Positions of detectors B1 I BO t BCu I I Z/CM Fig. 1 Horizontal cut of the mock-up assembly with measurement positions Frascati Neutron Generator(3) is shown in Fig. 1. The mockup has a block structure with dimension 1m xlm xo.94 m composed of alternate layers from stainless steel SS316 (5 cm thick) and Perspex (2 cm thick). On the front side a 1 cm copper plate is mounted. The 14 Me V neutron source has a distance of 5.3 cm from the copper plate. A central channel with a diameter of 2.8 cm and a length of 37 cm is ending in a cavity with dimension 14.8 cm x 4.8 cm x 5.2 cm. The z-co-ordinates of the detector positions are z = 41.4 cm for A and z=87.6 cm for B. Positions AO and BO are located on the central axis, whereas AI, A2 are sidewise shifted positions by 7.5 cm and 15 cm respectively, B 1, B2 are sidewise shifted by 9 cm and 18 cm respectively. In all these cases the 14 MeV neutron source is on the central axis. For two measuring positions AOS and BOS the block is sidewise shifted by 5.3 cm, thereby the neutron source is now away from the channel axis, and the detector in position AOS is out of the direct neutron beam. Two different detector systems have been used in the measurement- a 1.5"xl.5" NE-213 scintillation detector and a set of proportional counters. The scintillation detector was used in all the measuring positions stated above, the proportional counters only in the central positions AO, BO, AOS, BOS. For the measurement of the low energy part of the neutron flux between 50 ke V and 1 Me V a set of hydrogen or methane filled proportional counters was used and for the measurement of the neutron flux between 1 MeV and 20 MeV and also for the photon flux between 0.4 MeV and 12 MeV a NE-213 scintillation spectrometer. Neutron and photon events are here distinguished by their different pulse shape and separately collected in two different pulse height spectra. This method allows the simultaneous measurement of neutron and photon spectra by proton recoil and compton spectrometry. The detector responses of the used NE-213 scintillation detector are well described by response matrices, which have been precisely estimated on an absolute scale both for neutrons and photons. The low energy part of the neutron spectra, measured with the proportional counters has been normalised in an overlapping interval around 1 MeV by adaptation to the absolute measured NE-213 spectra. The yield of the 14 Me V neutron source was absolutely estimated by counting the associated a-particles from the D-T reaction. The measured neutron and photon flux spectra are by this absolutely normalised, and in the comparison with the calculated flux spectra are no free parameters. The experiment was started with the detector in position AO followed by the other A positions to avoid stronger material activation's. Nevertheless the background contribution in positions A by material activation from previous experiments in the past was in the order of 150 counts/s, which is about 7% of the total gamma counting rate and must be corrected. The neutron yield of the target was reduced by 2-3 orders of magnitude for the measurements in A positions against measurements in position B to avoid an overloading of the complex NE-213 spectrometer. The main constituents in SS316 are Fe(65.4%), Cr(17.7%) and Ni(12.1 %),and so the following dominant radionuclides of interest are produced by 14 Me V neutron interactions _60CO ( a), 54Mn( d), 58CO( d), 51Cr( 27.7 d), 57Ni ( 36.0 h), 56Mn (2.58 h) (Half -life time ). The background spectra have been measured before and after each run in all A positions to correct the photon flux spectra measured against the background contributions. In positions B the background contribution from activated materials was smaller than 1 % ( 7 counts/s) and has been neglected. Neutron and photon flux spectra measured have been compared with calculated spectra performed by 3-dimensionalcoupled neutron-gamma transport calculations with the Monte Carlo code MCNP-4A (4). For the calculations a precise geometrical model was used, describing mock-up, targetconstruction and surrounding in detail. Calculations have been done by making use of the data files FENDL-l (5) and FENDL- 2(6). FENDL-2 is an improved FENDL version where the Fe- 56 data are taken from the corresponding EFF-3 evaluation. III. Results and Discussion The open channel measurements on the central axis (detector positions AO, BO) are compared with the compact mock-up(2) results at the same detector positions ( here designated as A,8) in Fig. 2.1 for neutrons and in Fig. 2.2 for photons to demonstrate the gap influence. Comparing with A (8) the open channel causes in the position AO (80) an increase of the integrated neutron flux for I MeV<E<1O MeV by a factor of2.8 (6.5) and for E> 10 MeV by a factor of 73.4 (37.8). The integrated photon flux is increased by a factor of 3.2 (5.8). A strong gap influence can be seen also for the measurement position 81 as expected both for the neutron and photon flux, whereas the influence in the remaining positions is more moderate. For the open channel measurement all neutron and photon JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

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5 246 position A Table 1 Integrated neutron tluence per source neutron and per cm 2 for different neutron energy ranges and calculation / experiment ratios for FENDL-I (Fl) and FENDL-2 (F2) IE-II experiment FENDL-2 E [MeV] n Fig. 4.1 Measured and calculated neutron tluence spectra per source neutron on sidewise shifted positions FENDL E [MeV] -y 10.0 Fig. 4.2 Measured and calculated photon tluence spectra per source neutron on sidewise shifted positions central axis. From AO to A I the neutron flux for E> lome V (values for IMeV<E<IOMeV in brackets) decreases to 3.2 % (55%) and fromaotoa2 to 1.5% (33%) whereas the photon flux decreases to 50% for Al and 37% for A2. For position BI and B2 the neutron flux is reduced to 31 % (80%) and to 4.6% (21 %) respectively as compared to position BO. The corresponding relations for the photon flux spectra are 79% and 22%. There is a strong decrease ofthe 14 MeV spectral part with increasing distances from the central axis up to 3.2% for AI at 7.5 cm, 1.5% for A2 at 15 cm and 4.6% for B2 at 18 cm. The energy >10 range MeV MeV MeV AO exp.xl ± ± ±1.0 c/e Fl 1.39± ± ±0.02 c/e F ± ± ±0.02 AOS exp.xl ± ± ±0.04 c/e Fl 1.13±0.1l 0.99± ±0.03 c/e F2 1.13±0.1l 0.96± ±0.03 Al exp.xl ± ±0.03 c/e Fl 0.88± ±0.03 c/e F2 0.91± ±0.03 A2 exp.xl ± ±0.16 c/e Fl 0.90± ±0.03 c/e F2 0.88± ±0.03 BO exp.xlob 3.26± ±0.1l 2.19±0.06 c/e Fl 0.72± ± ±0.02 c/e F2 0.77± ± ±0.03 BOS exp.xl ± ± ±0.03 c/e Fl 0.73± ± ±0.03 c/e F2 0.77± ± ±0.03 Bl exp.xl ± ±0.17 c/e Fl 0.64± ±0.02 c/e F2 0.71± ±0.02 B2 exp.xl ± ±0.03 c/e Fl 1.26± ±0.61 Table 2 Integrated gamma tluence per source neutron and per cm 2 for Ey > 0.4 MeV and calculation / experiment ratios for FENDL-l and FENDL-2 position experiment c/e c/e FENDL'1 FENDL'2 AO (2.24±0.06)E"5 0.81± ±0.03 AOS Cl.12±0.03)E-5 O.99± ±0.03 Al Cl.15±0.03)E ± ±0.03 A2 (8.29±0.23)E ± ±0.03 BO (6. 17±0. 17)E'B 10.79± ±0.03 BOS I (1. 7±0.05) E"B 0.9l± ±0.21 Bl (4.88±0.14)E"B 0.75± ±0.10 B2 (1.90±0.05)E"B 1. 32±0.46 decrease ofthe neutron flux between I Me V <E <lome V is more moderate with 50% for A I, 33% for A2 and 21 % for B2. B I is still in the focus ofthe streaming channel with 3 1 % of the 14 MeV spectral part comparing with BO. The most part of the photon flux is produced by inelastic scattering of neutrons from the energy range 1 MeV<E<1 OMeY. It contains discrete gamma lines superimposed on a larger continuous spectral part. The energy dependence of the continuous part is similarly in all photon flux spectra and proportional to E/ 1 The structure in the low energy spectral part is caused by inelastic neutron scattering on discrete levels. High energy photons at 7.6 MeV,7.9 MeV and 9.0 MeV JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

6 247 result from neutron capture on Cr, Fe, Ni respectively. The neutron capture on Hydrogen, producing gamma quanta with 2.22 MeV, is enhanced behind the double thick Perspex layer in all A-positions. With exception of AO the neutron flux spectra are in other A-positions well described by FENDL calculations. The experiment is slightly underestimated but no more than 15%. The photon spectra are completely described within the error limit of 3%. The strongest discrepancies are found for AO where the 14 MeV spectral part is underestimated by 22%, and the calculated spectral part for the energy range I Me V <E < lome V is larger by 46%. Here the photon fluence is underestimated by 17%. No significant differences are found between FENDL-I and FENDL-2 calculations. In position BO(BOS) the experiment is underestimated by 23%(23%) for 0.1 MeV<E<1 MeV, by 8%(17%) in the energy range I MeV<E<lO MEV and above 10 MeV by 34%(21%). The underestimation in Bl is 29% for 1 MeV<E<lO MeV and 33% for E> I OMeY. B2 is overestimated by the calculations. The photon fluence spectra are underestimated between 10 % and 16%. In B-positions the FENDL-2 calculations show a significant improvement against FENDL-l calculations both for neutron and photon spectra as demonstrated in Tables 1 and 2. Apart from the fact, that the neutron fluence spectra are stronger underestimated with increasing depth ( an observation already found in the compact mock-up experiment) it seems that the 14MeV neutron streaming cannot be satisfactorily described by the calculational tool used. For all cases it has been found that the high energy spectral part with E> lome V is relatively strong underestimated in the calculations. In particular the underestimation is surprisingly large for the AO position. Possibly the small angle elastic scattering data for 14 MeV neutrons are not described well enough. IV. Conclusions The comparison of the measured neutron and photon flux spectra with MCNP-4 calculations on the basis of the nuclear data libraries FENDL-I and FENDL-2 shows a prediction uncertainty for the ITER nuclear shielding system design within about 30%.This is true for the measuring positions in and behind the streaming channel of the investigated mock-up assembly with open streaming channel. The same result had been obtained for the corresponding compact shield mock-up system. There are no significant differences in the spectra calculated with FENDL-I and FENDL-2 data, although there is a trend for a better reproduction of the photon spectra with FENDL-2 at deep locations - REFERENCES - (I) Batistoni, P., etal.: Proc. 19th Symp. Fusion Technology, p.233 (1997). (2) Fischer, U., et al. : Proc. Intern. Conf Nuc!. Data for Science and Technology, Vo1.59, p.1215 (1997). (3) Martone, M., etal.: J Nuc!.Mater., , p.161 (1994). (4) Briesmeier, J. F.: LA (1993). (5) Ganesan, S., McLaughlin, P.: laea-nds-128,(1994) (6) Herman, M., Pashchenko, A. B. (eds) : INDC(NDS)-373 (1997). SUPPLEMENT 1, MARCH 2000