tud experimental benchmarks of fe nuclear data

E L S E V I E R Fusion Engineering and Design 37 (1997) 31-37

Fusion Engineen.'ng and Destgn

TUD experimental benchmarks of Fe nuclear data

H. Freiesleben, W. Hansen, D. Richter, K. Seidel *, S. Unholzer h~stitute of Nuclear and Particle Physics, Technical University g[' Dresden, D-01062 Dresden, Germany

Abstract

Spectral neutron and photon fluxes from iron assemblies irradiated with 14 MeV neutrons were measured. The geometry of the experiment was chosen so as to be sensitive to shield penetration problems of fusion reactor design. The results are compared with Monte Carlo calculations (code MCNP) based on the data libraries EFF-I , EFF 2 and FENDI-I. A solid iron slab, as well as a slab with a vertical gap at different positions, were investigated. Systematic tendencies are discussed. © 1997 Elsevier Science S.A.

Keywords: FENDL-1; Fusion reactor design; Neutron flux; Monte Carlo

1. Introduction

In the field of neutronics for fusion reactors, such as the International Thermonuclear Experi- mental Reactor (ITER), one of the main objec- tives is the efficient shielding of the superconducting magnets from radiation [1]. The

design limits defined for the fluence of fast neutrons, the atomic displacements, and the nuclear heating in the magnet coils are determined by the neutron and photon fluxes penetrating and leaking blanket and vacuum vessel of the reactor. The validation of the tools for calculating the neutron and photon transport (i.e. codes and data libraries) in these components is a necessary step in reactor design.

* Corresponding author. Present address: TU Dresden, Aussenstelle Pirna Copitz, Pratzschwitzer Strasse 15, D-01796 Pirna, Germany. Tel.: +49 3501 530040; fax: +49 3501 530011.

Iron is the main element of the blanket and vessel materials. Several benchmark experiments were performed for testing its evaluated neutron nuclear data. The neutron leakage spectra from spherical shells with a 14 MeV neutron source in the centre, analysed by Fischer and Santamarina et al. [2,3] with data libraries which are used for fusion neutronics calculations, showed an underestimation of the total flux by about 10%, and especially an underestimation of the flux for energies E < 1 MeV. The angular neutron flux spectra from Fe slabs irradiated with 14 MeV neutrons at JAERI [4] were underestimated by up to 50% for energies 0.05 < E < 0.1 MeV and slightly so for E > 10 MeV.

Within bulk Stainless-Steel 316 assemblies con- sisting of up to about 70% Fe, irradiated with 14 MeV neutrons at ENEA Frascati [5] and JAERI [6,7], underestimations of the total neutron flux by about 10% were observed, which increase with depth. Gamma-ray flux and gamma heating were

0920-3796/97/$17.00 ~ 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00028-8

32 H. Freiesleben et al./Fusion Engineering and Design 37 (1997) 31-37

relatively well described, with some exceptions at deep positions.

At the Technical University of Dresden (TUD) a Fe slab experiment [8] was performed with a geometry which is sensitive to shield penetration problems. The analysis of the spectral neutron and photon fluxes penetrating and leaking the slab, with the data of the European Fusion File version l (EFF 1, [9]), showed in the case of the neutron spectra a significant underestimation for energies below 0.2 MeV and a ratio of calculated/ experimental flux (C/E) for the energy range 5 < E < 13 MeV of about 2/3. The latter was also confirmed by the neutron time-of-arrival spec- trum. For the photon flux a C/E ratio of 0.62 was found in the measured range of 0.2 < E < 8 MeV.

These unacceptable discrepancies were discussed in Ref. [10] in connection with the high sensitivity of the benchmark geometry to the angular distribution of the secondary neutron emission. Inelastic scattering of 14 MeV neutrons is dominated for neutron emission energies 5 < E < 14 MeV by pre-equilibrium processes (multi-step direct interactions) which are forward-peaked. In E F F - 1 the angular distribution of these neutrons (with the exception of those from the first level of 56Fe) is assumed to be isotropic.

In the improved version, E F F - 2 , and in the Fusion Evaluated Nuclear Data Library (FENDL, [11]) the angular distribution of the neutron emission is taken into account through double-differential cross sections. With the availability of E F F - 2 [12] and F E N D L - 1 files [13] processed for Monte carlo calculations and with the code M C N P - 4 A [14] the analyses were repeated using these data. The results are presented in the present paper.

Because a reactor shield blanket has ducts and gaps, benchmark experiments were performed not only with a solid Fe slab but also with slabs in which a vertical gap was inserted. The results for two slab assemblies are also given in this paper.

2. Experiment

tance to the neutron source of 19 cm and to the detector position of 300 cm; with the slab thickness of 30 cm the distance Q - D was 349 cm. The detectors were positioned in a collimator shaped in such a way that all neutrons and photons leaking from the slab in direction of the detector could be observed. Taking into account the vertical and horizontal slab dimension of + 50 cm, neutrons single-scattered in detector direction have scattering angles between 0 ° and about 65 °. Contrary to this relatively narrow range of forward angles, in benchmark experiments with spherical shells around the neutron source all scattering angles up to 80 ° contribute to the spec- trum observed, which represents for thin shells the angle-averaged emission cross section. Bulk assemblies are also sensitive to a broad range of scattering angles.

The neutron source was operated in pulsed mode, and for each detector event the time-of-arrival t after generation of the 14 MeV neutron was recorded. These spectra were used for neutrons as an independent test of the nuclear data, and for photons to select those which were pro- duced in the slab and not in the collimator, floor, detector itself, etc. The energy distributions of the neutron and photon flux at the detector position were obtained by unfolding the pulse-height spectra from organic scintillators (NE213, stilbene) and hydrogen-filled proportional counters. Details can be found in Ref. [8].

Q 10 0 O.

jlf

%¸¸4 ¸ " ' , D

In Fig. 1 the principle of the experimental arrangement is outlined. The Fe slab had a dis-

Fig. 1. Benchmark geometry; Q, posxuon of the 14 MeV neutron source, D, detector position.

H. Freiesleben et al . / Fusion Engineering and Design 37 (1997) 31-37 33

I

I

1 0 -~

10 -8!

1 0 -9 l

L L

I

10 -~°t

0 ..5 10 15

E [M V]

Fig. 2. Spectral neutron fluence per one source neutron from the slab assemblies (gap width b in cm/gap position × in cm) 0/0 (©), 5/10 (,) and 5/20 ( + ) .

In the time-of-arrival fluence (Fig. 3) the increase by the gap is evident from the 14-MeV peak up to the detector threshold. For the peak region, 0 n s < t < 8 0 . 6 ns, the ratio is ~b(A2)/ ~b(A0) = 1.07 -+ 0.03 and ~b(A1)/~b(A0) = 1.32 + 0.03, respectively; in agreement with the above results from the energy spectra of the neutrons. For time range 80.6 ns < t < 200 ns the ratios are ~b(A2)/~b(A0) = 1.18 -+ 0.12 and q~(A1)/q~(A0) = 1.35 _+ 0.12.

The greatest differences are observed for the photon fluence (Fig. 4). In the measured energy range (0.2 < E < 8 MeV) the ratios are q~(A2)/ q~(A()) = 1.32 + 0.02 and ~b(A1)/~b(A0) = 1.45 -+ 0.02, respectively.

3. Calculation and comparison to experiment

The neutron and the photon fluences were calculated with the three-dimensional Monte Carlo code M C N P [14]. The detector collimator, walls,

Experimental results are presented in Figs. 2 - 4 for three assemblies, AI (gap with b = 5 cm, gap position, x = 10 cm), A2 (b = 5 cm, x = 20 cm) and A0 (no gap, b = 0 cm, x = 0 cm).

The neutron fluences ~b per one source neutron (Fig. 2) agree for the three different arrangements between E = 0.04 and 1 MeV within the experimental uncertainties. By the gap the fluences are significantly increased for energies where forward- scattering is present. For instance, ~b(A2)/ ~b(A0) = 1.21 + 0.02 and ~b(A1)/~b(A0) = 1.37 + 0.02 for 5 < E < 10 MeV. For the energy range 10 < E < 15 MeV where the 14-MeV peak is dom- inant, the ratios are ~b(A2)/~b(A0)= 1.03 +0.03 and ~b(A1)/0(A0) = 1.27 + 0.03; probably as a re- sult of the more forward-peaked elastic scattering than in the inelastic case. (For instance: the ratio for the differential cross sections at 0 ° and at 90 °, cr(0°)/a(90°), is about 150 for elastically scattered neutrons, but only 2 - 5 for inelastic scattering from low-lying levels [15]).

1 0 - 9 r ~ - r ~ ~ 1 I , I I I ] I I [ I i i i i , ~ 1 i i i i i i i i i i i i

7 I 0 - 1 e L

10 -'~

10 -12 ,

I , J L l l L I - I , I I I I I I ~ i J I I I L , _ ~ I , I I

0 1 0 0 2 0 0 3 0 0

t

Fig. 3. Neutron time-of-arrival fluence per one source neutron from the slab assemblies (gap width b in cm/gap position × in cm) 0/0 (©), 5,/10 (*) and 5/20 ( + ) .

34 H. Freiesleben et al./Fusion Engineering and Design 37 (1997) 31-37

5 . 0 °7-

I

4 . 0 T

3 . 0

% F - f aa 2 . 0

1 . 0 !

T r v ' ~ ~ r T r T - I . 7 n ~ r • ~ r T [ T ~ ~ r n r • ~ , r e r~T r ~ * ~ C

0 . 0 1 2 3 4

E [M V]

Fig. 4. Spectral photon fluence per one source neutron from the slab assemblies (gap width b in cm/gap position x in cm) 0/0 (©), 5/10 (*) and 5/20 ( + ).

floor, assembly racks, air in the experimental hall, source-target backing etc. were taken into account, as well as the neutron detector efficiency in the case of the time-of-arrival spectra, such that the calculated fluences per one source neutron could be compared directly with the measured fluences. Identical runs were carried out with three different data libraries: E F F - 1 , [9]; E F F - 2 , [12] and F E N D L - I [13], respectively.

The results are presented in Fig. 5 as C/E ratio for the three assemblies investigated.

The error bars (one standard deviation) include both, the statistical and systematic uncertainties of the experimental results and the statistical uncertainties of the Monte Carlo calculations. Be- cause the Monte Carlo code and the geometry model used enable calculations without significant approximations, differences between calculated and experimentally obtained fluences should mainly be caused by the nuclear data files. Trends of the C/E for the three assemblies have smaller uncertainties as indicated by the error bars of Fig.

5, because the systematic experimental uncertainties are also included and may be assumed to be same for the series of measurements.

The neutron energy range 0.04 < E < 1.0 MeV was not included in the figure, because the differences between the C/E values obtained with the three data libraries and also the differences between the C/E ratios for the three assemblies are within the experimental uncertainties. Generally, the neutron fluence is underestimated in this energy range by 10-15%.

A significant improvement is achieved for the neutron fluence in the energy range 1.0 < E < 5.0 MeV (Fig. 5a) with the data from EFF 2 and F EN D L 1. As for lower energies, an underestimation of about 10% remains.

The greatest improvement compared to the C/E ratios obtained with E F F - 1 calculations is observed for neutrons in the energy range 5.0 < E < 10.0 MeV (Fig. 5b). For all three assemblies the fluence calculated with E F F - 2 and FENDI-1 , respectively, agrees well with the experimental results.

The C/E ratios for the range 10 < E < 15 MeV (Fig. 5c) are also improved when E F F - 2 or F E N D L - 1 data are used as compared to calculations with the E F F - 1 library. Whereas the fluence from the solid slab is underestimated with the newer libraries by 13%, the agreement with the experimental data is the better the greater the increase of the fluence by the gap, i.e. the contri- butions from elastic scattering processes in the gap.

This tendency is confirmed by the C/E ratios from the time-of-arrival spectra in the region of the 14 MeV peak, 0 < t < 80.6 ns (Fig. 5d).

For time range 80.6 < t < 200 ns (Fig. 5e) the underestimation of the neutron fluence is reduced with E F F - 2 and F E N D L - 1 data to about the half of the value obtained with EFF l data.

As expected [10], the photon fluence (Fig. 5f) is much better described if the forward-oriented character of the neutron transport is taken into account in more detail. Because of the small absorption length of the photons compared to the slab thickness, the photons observed arise from a thin layer of the slab facing the detector. With the double-differential neutron emission cross sections

H. Freiesleben et al./'Fusion Engineering and Design 37 (1997) 31-37 35

1 . 5 0 1 .50

bJ@tJtror] f l u e r ~ c e Net~ t r :Jn f l u e n ¢ ~ ! <- i I . b t,,4e',,,' . c . ~" '.7 k.4e'v 5.( b,~,4m,",/ - :- - ~c' ',,ie','

= I , ~ i • - -

',I.,

i

... ,.ooc : • , . . + <

,' I l 0 . 5 0 ,_ i 0 . 5 0 i ~ ~ _

o/o 5/~oo 5/lO o/o s/2o O/lO

(a) Gap (b) Gap

@

'7__,

Q °~

1 . 5 0 - , . . . . . . . . ~ . . . . .

i Ne,~f ror~ l ichen<,- i O. MeV : E[ ': ! 5 . MeV

t 1 . 0 0 I- - ~ " " - = ~ - :

i

0 . 5 0 / . . . . . . . . . . . . . . . . . . . .

0 / 0 5 / 2 0 5 / 1 0

, , , , .a

@

@

o

1 .50

1.0 0

0.50

N e u t r o n f l u e r a c e (). rlS ~ f : 8 1 , ns

¢ -

0 / 0 5,/~0 5 / 1 0

(c) Gap (d) Gap

Fig. 5, (a) Ratio of calculated to experimental fluence from the solid slab (0/0) and from the assembly with a gap of (width/position in cm) 5/20 and 5/10, respectively, obtained with data of the library E F F - I (A), E F F - 2 (1 ) , and F E N D L - 1 ( * ) , respectively, for the neutron energy range 1 5 MeV. (b) The same as Fig. 5(a) for the neutron energy range 5 10 MeV. (c) "l-he same as Fig. 5(a) for the neutron energy range 10-15 MeV. (d) The same as Fig. 5(a) for the neutron time-of-arrival range 0-81 ns. (e) The same as Fig. 5(a) for the neutron time-of-arrival range 81-200 ns, (f) The same as Fig. 5(a) for the photon energy range 0.2-8 MeV.

36 H. Freiesleben et al. /Fusion Engineering and Design 37 (1997) 31 -37

.,w

©

= ,2 ©

©

o

cv,

1 . 5 0

1 . 0 0

$/. ~ ! 1 . 5 0 1

&'eutro,q flupFce ] ! r~ . . . . 200 . r~::;

t ~tlF - - - r I 'i

0 , 5 0 L.___

± i i

.-- __1- . * r

o/o 5,/ o 5/ o

(e) Gap

Fig .

in E F F - 2 and F E N D L 1 the C/E ratio for the photon fluence in the measured energy range (0.2 < E < 8.0 MeV) are improved by about 20%. It is significant for the trend for C/E ratio to be closer to unity the greater the increase of the photon fluence by the gap, as in the case of the 14 MeV neutron peak region.

4. Conclusion

Benchmark experiments were performed with Fe, the main element of the structural materials used for the shield blanket and vacuum vessel design of fusion reactors. The geometry of the experiments was chosen such as to be sensitive to the radiation penetrating the shield. The spectral neutron flux as well as the spectral photon flux were investigated.

The fluxes were calculated with the three-dimensional Monte Carlo code M C N P - 4 A apply- ing data from the European Fusion files EFF 1 and E F F - 2 and from the reference library of the ITER design F E N D L - 1 .

j .4_.:

©

t "

N..

©

;.)

( f )

5. ( c o n t d . )

P h r , t c r ~ I ' ~ e q , i , '

...... !v'!,:~ v L 3 . t~ ,'v' o V

1 . 0 0

0 . 5 0

~. ~ -

[ . i 'l

1

o/o 5/20 5/lo

G a p

The ratios of calculated/experimental flux showed, besides discrepancies for neutron energies below 0.2 MeV, unacceptable underestimations, if E F F - 1 data were used, in the case of the neutron flux for 5 < E < 13 MeV and in the corresponding range of the neutron time-of-arrival spectra, which were determined independently of the energy distributions. Significant improvements were achieved utilizing data of the libraries E F F - 2 and F E N D L ! where the angular distributions of the secondary neutron emission are more precisely taken into account. That holds true even more for the photon fluxes which are, in conjunction with the small absorption length of the photons compared to the thickness of the benchmark arrangement, determined to a large extent by the neutron transport.

Generally, these comparisons revealed the great sensitivity of shield penetration problems to the secondary angular distributions of the neutrons.

The quality of the libraries E F F - 2 and F E N D L - 1 applied to the Fe slab benchmark is comparable and, in general, good. In the present work underestimations of the neutron flux at

H. Freiesleben et al./Fusion Engineering and Design 37 (1997) 31 37 37

lower energies and of the total photon flux re- main. Hogenbirk et al. [16] included in the EFF- library fluctuations also of the partial cross sections, based on high-resolution measurements at Geel, and obtained excellent agreement with the experimental data for the solid Fe slab in idealized geometry [8].

These conclusions can be drawn for the solid slab as well as for the two gap assemblies. The increase of the fluxes from the assemblies with a gap compared to those from the solid slab was pronounced for photons in the whole measured energy range (0.2 < E < 8 MeV) and for neutrons in the range 3 < E < 1 3 MeV and in the corresponding time range 80.6< t <200 ns. As expected from geometry the gap 5/10 which is located closer to the source-detector-axis en- hanced the fluxes significantly more than the gap 5/20.

For energy and time-of-arrival ranges where elastically scattered neutrons dominate and for the photon flux the agreement of the calculated values with the experimental results was the better the more the gap contributed additionally to the fluxes from the solid slab. As the geometry was described with the code MCNP without approximations, the increasing contribution of neutron streaming through the gap by only a few elastic scattering processes could cause this tendency.

Acknowledgements

This work was supported by the European Fu- sion Technology Programme, Subtask NDB 2-2.

References

[1] W. Daenner, ITER Expert Meeting on Shielding Experi- ments and Analysis, Garching (Germany), February 12- 24, Report ITER-IL 5-0-5, 1990.

[2] U. Fischer, E. Wiegner, FENDL data testing for beryl- lium, lead, iron and copper, Fusion Eng. Des. 28 (1995) 437-445.

[3] A. Santamarina, I. Abidi, B. Gastaldi, Validation and improvement of Fe, Cr, Ni nuclear data in bulk shield benchmark experiments, Fusion. Eng. Des. 28 (1995) 762-768.

[4] Y. Oyama, K. Kosako, H. Maekawa, Measurement and calculation of angular neutron flux spectra from iron slabs bombarded with 14.8 MeV neutrons, Nucl. Sci. Eng. 115 (1993) 24 37.

[5] P. Batistoni, M. Angelone, M. Martone, M. Pillon, V. Rado, A. Santamarina, I. Abidi, B. Gastaldi, M. Martini, J.P. Marquette, The bulk shielding benchmark experiment at the Frascati Neutron Generator (FNG), Fusion Eng. Des. 28 (1995) 504 514.

[6] F. Maekawa, C. Konno, K. Kosako, Y. Oyama, Y. lkeda, H. Maekawa, Bulk shielding experiment on large SS316 assemblies bombarded by d t neutrons, Report, Japan Atomic Energy Research Institute, JAERI-Re- search 94-044. 1994.

[7] M.Z. Youssef, A. Kumar, M.A, Abdou, C. Konno, F. Maekawa, A. Maekawa, Benchmarking Fendl library through analysis of existing benchmark experiments-- Part (1): Analysis of bulk shielding experiments on large SS-316 assemblies bombarded by d t neutrons, Report, UCLA- FNT 90, University of California, Los Angeles, 1994.

[8] H. Freiesleben, W. Hansen, H. Klein, T. Novotny, D. Richter, R. Schwierz, K. Seidel, M. Tichy, S. Unholzer, Experimental results of an iron slab benchmark, Report, Technische Universitaet dresden, TUD- PHY 94/2, February 1995.

[9] H. Gruppelaar, Processing of the EFF 1 and EFF 2 data files, Report, ECN Pettern, EFF Doc 46, May 1990.

[10] H. Freiesleben, W. Hansen, D. Richter, K. Seidel, S. Unholzer, Experimental investigation of neutron and photon penetration and streaming through iron assemblies, Fusion Eng. Des. 28 (1995) 545-550.

[11] S. Ganesan, P.K. McLaughlin, FENDL/E Evaluated nuclear data library of neutron interaction cross-sections and photon production cross-sections and photon-atom interaction cross-sections for fusion applications, Version 1.0, Report, International Atomic Energy Agency, IAEA- NDS-128, May 1994.

[12] L. Petrizzi, Final report on task NDB 1.2: Processing EFF 2.4 with ACER of N JOY to produce an MCNP working library, Report, ENEA Frascati, EFF Doc- 412, July 1995.

[13] R.E. MacFarlane, Status of processing for FENDL 1, IAEA Advisory Group Meeting on Improved Evalua- tions and Integral Data Testing for FENDL, Garching, Germany, September 12-16, 1994.

[14] JF . Briesmeister (Ed.), MCNP--a general Monte Carlo n-particle transport code, Version 4A, Report, Los Alamos National Laboratory, LA-12625-M, November, 1993.

[15] D. Schmidt, W. Mannhart, H. Klein, R. Nolte, Neutron scattering on natural iron at incident energies between 9.4 and 15.2 MeV, Report, Physikalisch-Technische Bunde- sanstalt Braunschweig, PTB-N-20, November 1994.

[16] A. Hogenbirk, A.J. Koning, H. Gruppelaar, Validation of the EFF-3.0 evaluation for 56Fe, Report, ECN Penen F!CN-R-95-019/EFF-Doc-382, July 1995.

tud experimental benchmarks of fe nuclear data

Documents