TRADE Preliminary comparison of codes for the activation calculation of the water cooling the target

Transcription

1 TRADE Preliminary comparison of codes for the activation calculation of the water cooling the target TRADE_TS_8_TR_00_0 (February 2005) A.Stankovsky, C.Petrovich, D.Cepraga, M. Frisoni

2 TABLE OF CONTENTS page 1. Introduction 2. Codes and tools used 4. Results 4 4. Conclusions 9 References 9 2

3 1. Introduction In the TRADE project [1] (TRiga Accelerator Driven Experiment), the spallation target is cooled by ordinary water, which is used also as coolant for the TRIGA reactor. Due to the insertion of the proton beam in the sub-critical system, the water undergoes activation not only due to the fission neutrons but also due to the spallation neutrons and the protons. The activity and the formation of new elements in water have to be evaluated by means of calculation codes able to take into account particle energies up to 0 MeV, that is the energy of the primary proton beam (the current is assumed here to be 285. µa). This work presents a preliminary comparison of activity calculations in the locations close to the spallation target by means of different inventory codes: FISPACT [2], SP-FISPACT [] and ANITA-IEAF [4]. The geometrical model is based on the tantalum target solution 1-C-200 [5], shown in Fig.1. Since the main goal of this work is to compare the activation calculation tools in the neutron energy range over 20 MeV, only the water between the target and the shroud has been considered (see the zone delimited by red colour in Fig.1). This is the zone where the contribution from the spallation neutrons and (possibly) the protons is the highest. Of course, also the contribution of the fission neutrons coming from the core has been taken into account. Two simplifying assumptions were made for all these preliminary calculations: the water is considered to be still (=not circulating) and pure (=no impurities are considered). Fig.1. Configuration of the spallation target (proportions are changed). The zone where the activation has been calculated is the water between target and shroud (delimited by red colour).

4 2. Codes and tools used For the calculation of the particle fluxes and spectra the Monte Carlo particle transport code MCNPX version 2.5.d. ([6], []) was used. Where available, the proton and neutron LA0 cross-section data libraries [8] were employed (up to 0 MeV). For nuclides not present in LA0 (e.g. for tantalum), the public library ENDF60 up to 20 MeV [9] was used together with the CEM2k [10] (Cascade-Exciton Model) included in MCNPX. The calculated particle spectra and fluxes were provided to inventory codes to perform activation calculations. The following codes were applied for the purpose of comparison: FISPACT [2], an activation code designed for fusion applications for neutron energies lower than 20MeV; SP-FISPACT [], an extension of FISPACT to take into account also the evolution (accumulation and decay) of the residual nuclei formed after the nuclear interactions (protons, neutrons >20 MeV, etc.) calculated by MCNPX; ANITA-IEAF [4], an inventory code based on the library IEAF-2001 [], which contains neutron-induced reaction cross-sections for activation calculations up to 0 MeV.. Results The calculated neutron and proton spectra in the zone under investigation (the water between the target and the shroud, the water mass being 0.54 kg) are presented in Fig.2. The proton flux turns out to be less than 0.01% compared to the neutron flux. Since ANITA-IEAF does not take into account the activation due to protons (while MCNPX/SP-FISPACT does take into account proton activation), a simulation has been done with MCNPX without transporting protons in water. In this way the comparison between SP-FISPACT and ANITA-IEAF would have been at the same conditions (only neutron activation would be calculated in both cases). On the other hand, this would have changed the working conditions, since the protons in water create secondary neutrons contributing to the total flux for the 12% (see the difference between the black and the blue lines). Thus, the transport of protons in water in the MCNPX simulation has been considered to be necessary and consequently the comparison of the results between SP- FISPACT and ANITA-IEAF is not performed at the same conditions (proton activation is not taken into account in ANITA-IEAF). 4

5 Lethargy * Neutron flux (n/(cm 2 s)).0x x x x x x10 neutrons. Flux = 5.95E+12 n/(cm 2 s) neutrons with no transported protons in water. Flux= 5.25E+12 n/(cm 2 s) that is 88.2% from total neutron flux protons. Flux = 1.52E+08 p/(cm 2 s) x10 8 2x10 8 1x10 8 Lethargy * Proton flux (p/(cm 2 s)) Energy (MeV) Fig. 2. Particle spectra in the water cooling the target. There were considered 4 calculation options to estimate the activity of the water, utilizing different combinations of evaluated neutron activation cross-section libraries (EAF-2001 and IEAF-2001) and physics models (CEM2k in MCNPX): 1) Calculation using the FISPACT code. Since here the neutron activation library is up to 20 MeV, only the neutron spectra below 20 MeV is considered and the tail of the neutron spectrum above 20 MeV is not included (1.4% contribution to the total neutron flux); 2) The energy tail of the neutron spectrum (>20 MeV) was added to the highest energy group and the inventory was calculated by means of the standard FISPACT code; ) The residual nuclides from neutron interactions > 20 MeV and from proton interactions were calculated by MCNPX. This information, together with the neutron spectrum < 20 MeV (same as in option 1), was provided to SP-FISPACT; 4) The activity is calculated with the ANITA-IEAF code using the whole neutron spectrum (up to 0 MeV). For the purpose of comparison, 1 year of operation with subsequent cooling was chosen. The results are shown in Fig. for the accumulation during irradiation and in Fig.4 for the activity after shutdown. It is clearly seen from Fig. that it is necessary to take into account the high-energy part of the neutron spectrum: the results obtained with option 1 (taking into account only neutrons <20 MeV) is well below the other results. Except for the first one month of operation, the activity calculated with ANITA-IEAF code becomes higher than those calculated with the other codes. Thus, the proton activation seems to be not so relevant. 5

6 10 Total activity Activity (Bq/kg) FISPACT FISPACT (spectrum mod.) SP-FISPACT ANITA-IEAF Time (yrs) Fig.. Accumulation of activity in the water near target 10 Total activity Activity (Bq/kg) FISPACT FISPACT (spectrum mod.) SP-FISPACT ANITA-IEAF Time (yrs) Fig.4. Activity behaviour after shutdown. 6

7 The numerical values of the specific activity calculated with options 2, and 4 are summarized in Table 1 (for accumulation during 1 year of irradiation) and in Table 2 (for decay after shutdown). The discrepancy between the activity of the main isotopes calculated by MCNPX/SP-FISPACT and ANITA-IEAF is shown in Table and in Table 4. The discrepancy between the total specific activity calculated with these two codes is about 10% after 1 year of irradiation. This discrepancy reaches the value of a factor of 4.5 during the cooling process and this is due to the difference in the tritium production estimation. Finally it reduces to 0% in the long-term activity, when the C (T 1/2 =50 years) becomes dominant. Moreover, Table 1 and Table 2 show that SP-FISPACT and ANITA-IEAF provide some different nuclides, such as Be (T 1/2 =5.29d) and 12 B (T 1/2 =20.2 ms) (in SP-FISPACT only) and 18 N (T 1/2 =624 ms) (in ANITA- IEAF only). Table 1. Main contributors to the activity (accumulation up to 1 year) in 0.54 kg of water. Total activity (Bq/kg) and main contributors (%) Irradiation time FISPACT MCNPX/SP-FISPACT ANITA-IEAF 1 sec 1 day N (99.5%), O (0.25%) N (96.0%), O (.94%), H (0.06%) 1 month year N (94.2%), O (.86%), H (1.91%) N (8.42%), H (18.6%), O (.22%) 10 N (50.9%), 12 B (9.56%) C (5.5%), O (.2%) O (41.5%), N (8.52%), C (10.1%), 1 N (4.1%), 12 B (2.8%), C (1.6%) O (41.1%), N (8.%), C (10.21%), 1 N (4.06%). 12 B (2.8%), C (1.59%) O (9.50%), N (6.65%), C (9.8%), 1 N (.9%), H (.5%), 12 B (2.6%), Be (1.55%), C (1.52%) N (64.9%), 18 N (24.4%), C (5.24%), O (4.5%) O (4.6%), N (41.50%), 1 N (4.0%), C (2.91%), 18 N (2.19 %), C (1.2%) O (4.0%), N (40.95%), 1 N (4.01%), C (2.8%), 18 N (2. %), H (1.%), C (1.26%) O (41.0%), N (5.2%), H (1.96%), 1 N (.50%), C (2.50%), 18 N (1.88 %), C (1.09%)

8 Table 2. Main contributors to the activity (decay after shutdown) in 0.54 kg of water Total activity (Bq/kg) and main contributors (%) Time after shutdown FISPACT MCNPX/SP-FISPACT ANITA-IEAF 1 sec 1 day N (6.%), H (19.80 %), O (.45%) O (42.12%), N (5.65%), C (10.51%), 1 N (4.18%), H (.59%), Be (1.66%) N (99.9%), C (0.0%) 9 H (68.64%), Be (1.1%) O (42.98%), N (4.1%), H (.1%), 1 N (.69%), C (2.64%), H (99.9%) C (0.0%) 1 year C (0.05%) C (0.0%) Be (0.42%) H (99.9%), H (99.5%), H (99.9%) C (0.0%) C (0.08%) 100 years C (6.9%) C (1.9%) C (6.%) H (9.0%), H (82.21%), H (9.6%), 1000 years C(100%) C (100%) C (100%) Table. Discrepancy between the main isotopes of SP-FISPACT and ANITA-IEAF (accumulation during 1 year). Irradiation time Isotope MCNPX/SP- FISPACT (Bq/kg) ANITA-IEAF (Bq/kg) Discrepancy (ratio ANITA/SP-FISPACT) 1 sec 1 day 1 month 1 year N O H N O H N O H N O H

9 Table 4. Discrepancy between the main isotopes of MCNPX/SP-FISPACT and ANITA-IEAF (decay after shutdown). Time after shutdown Isotope MCNPX/SP- FISPACT ANITA-IEAF (Bq/kg) Discrepancy (ratio ANITA/SP-FISPACT) 1 sec 1 day 1 year 100 years 4. Conclusions (Bq/kg) 10 N O H H H H The consideration of the high-energy tail of the neutron spectrum (>20 MeV) is necessary for the activation calculation of the water close to the target. This comes from the fact that the activity obtained with FISPACT is lower than that obtained with SP-FISPACT and ANITA-IEAF. The discrepancies in the activity calculations by means of neutron-induced cross sections contained in the IEAF-2001 library or with the help of the physics model CEM2k are modest during the irradiation time but can reach values up to a factor of 5 during the cooling time because of the different tritium production evaluation. References [1] TRADE - Final feasibility report, The Working Group on TRADE, TRADE PC.0 FS 002 0, March 2002, ( [2] RA Forrest, FISPACT-2001: User manual (UKAEA FUS 450, 2001). [] C. Petrovich, SP-FISPACT2001. A Computer Code for Activation and Decay Calculations For Intermediate Energies. A Connection Of FISPACT With MCNPX, ERG/2001/10 (ENEA, 2001). [4] D.G. Cepraga, M. Frisoni, G. Cambi, Fusion Engineering and Design 69 (200) pp [5] F. Desideri, Solution 1-C-200 Cooler-1 (Brasimone-ENEA, May ). [6] J. S. Hendricks et al., MCNPX, VERSION 2.5.d, LA-UR-0-59, August 200. [] Laurie S. Waters, Editor, MCNPX User s Manual, Version 2.4.0, LA-CP , September 2002 ( [8] M. B. Chadwick, P. G. Young, S. Chiba, S. C. Frankle, G. M. Hale, H. G. Hughes, A. J. Koning, R. C. Little, R. E. MacFarlane, R. E. Prael, and L. S. Waters, Cross Section Evaluations to 0 MeV for Accelerator-Driven Systems and Implementation in MCNPX, Nuclear Science and Engineering, Number (March 1999) 29. [9] J. S. Hendricks, S. C. Frankle, J. D. Court, ENDF/B-VI Data for MCNP TM, LA-12891, December [10] S. G. Mashnik and A. J. Sierk, J. Nucl. Sci. Tech. S2, (2002). [] Yu.A.Korovin, A.Yu.Konobeyev, P.E.Pereslavtsev, A.Yu.Stankovsky, C.Broeders, I.Broeders, U.Fischer, U.von Moellendorff, Evaluated nuclear data files for accelerator driven systems and other intermediate and high energy applications, Nuclear Instruments and Methods in Physics Research A 46 (2001) pp