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Validation of shutdown dose rate Monte Carlo calculations through a benchmark experiment at JET
Fusion Engineering and Design 83 (2008) 1782–1787
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Validation of shutdown dose rate Monte Carlo calculations through a benchmark experiment at JET R. Villari a,∗ , M. Angelone a , P. Batistoni a , U. Fischer b , P. Pereslavtsev b , L. Petrizzi a , S. Popovichev c , JET-EFDA Contributors1 a Associazione EURATOM ENEA sulla Fusione, Via Enrico Fermi 44, 00044 Frascati, Rome, Italy b Association FZK-EURATOM Forschungszentrum Karlsruhe, Germany c Euratom-UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3DB, UK
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Article history: Available online 9 September 2008 Keywords: Tokamak Activation Dose rate Monte Carlo Geiger–Muller tube TLDs detectors D1S method R2S method JET
a b s t r a c t A benchmark experiment has been performed during the 2005–2007 campaigns of JET in order to validate the rigorous two-step (R2S) and direct one-step (D1S) methods for the prediction of shutdown dose rates in full 3D geometry. Dose rate levels calculated using D1S and R2S methods have been compared with experimental data collected before and during off-operational periods and at the end of the last JET campaigns. Measurements have been carried out in two irradiation positions: in-vessel with high sensitivity thermoluminescent detectors and ex-vessel with an active detector of Geiger–Mueller type. Satisfying agreement between calculations and measurements has been obtained in in-vessel position, whereas both methods underestimate the experimental quantity in the external one. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Neutrons produced during DT and DD plasma operations induce the activation of the materials constituting the fusion machine components. The prediction of the induced activation and resulting dose rates is a main issue for fusion reactors: due to neutron activation, the normal operations of intervention, maintenance and control are restricted and must be planned in advance in order to guarantee the respect of dose limits. Over the past several years, two different methodological approaches have been developed in the frame of the Fusion Technology Programme for the three-dimensional (3D) calculation of the shutdown dose rate. One is the so-called rigorous two-step method (R2S) developed by FZK [1–3] and the second is the direct one-step method (D1S) [4–6] developed by the ITER team and ENEA.
∗ Corresponding author. Tel.: +39 0694005848. E-mail address: [email protected] (R. Villari). 1 See Appendix of M. Watkins et al. Fusion Energy, 2006 (Proceedings of the 21st IAEA International Conference, Chengdu 2006). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.08.001
Both methods, even if with different approaches, use MCNP Monte Carlo code [7] for transport calculations and FISPACT [8] inventory code for activation calculations. In R2S, a regular neutron transport calculation provides the spatial distribution of the neutron flux spectra (MCNP, first step); the decay gamma source distribution is obtained through a nuclide inventory calculation (FISPACT) using the MCNP neutron flux spectra and the irradiation history; finally, a decay gamma transport calculation (MCNP, second step) based on FISPACT decay gamma source distribution provides the dose rate at the specified locations. D1S is an innovative method based on the assumption that the decay gammas of the radioactive nuclides are promptly emitted; hence, the neutrons and decay gammas are transported in a single MCNP run. The time correction factors required to take into account the build-up and the decay time of each radionuclide according to the real irradiation history are calculated using FISPACT. Both methods have advantages and disadvantages. The main advantage of R2S is the full calculation of the nuclide inventory, so that all potential reaction chains are implicitly included. In this respect the method is problem independent. On the other hand, R2S requires in general a finer segmentation of the geometry cells because the neutron fluxes and the decay gamma source are spatially averaged over the cell volumes as defined in the regular MCNP
R. Villari et al. / Fusion Engineering and Design 83 (2008) 1782–1787
geometry model. The D1S main advantage is the direct coupling between the decay gammas and the neutrons; however the transport libraries must be adapted to the specific problem, general applications are complex and serious limitations are foreseen in case of high burn-up and multi-chain reactions. Several computational and experimental benchmarks of the two methods have been conducted in the past and a brief but complete review has been recently published [9]. Good results have been obtained for simple geometries under well-defined conditions (i.e. ITER oriented experiment at Frascati Neutron Generator [10]). However, in order to apply these methods to ITER a validation in a reactor-like configuration, such as JET, is necessary. JET is the world’s largest tokamak (brought into operation since 1983) and is the only European device in which DT experiments have been conducted. A first benchmark was performed in 2004 [11]. D1S and R2S dose rate calculations were compared with dose measurements collected by the JET Health Physics group during the first experiment with tritium (DTE-1). The obtained results were not satisfactory, mainly because of the rather high uncertainties of the available measurements and the inadequacy of the MCNP modeling. It was therefore decided to conduct a dedicated experiment to validate the R2S and D1S shutdown dose rate calculations. Dose rate data have been collected during off-operational periods and at the end of the 2005–2007 JET campaigns in two irradiation positions, at different decay times and compared with those calculated using D1S and R2S. The results of this dedicated benchmark are presented in this paper. 2. Detectors and measurements Gamma dose rates have been measured during JET offoperational periods in two experimental positions at different cooling times (the elapsed times from the last previous shot) with active (ex-vessel position) and passive (in-vessel position) detectors. An active Geiger–Mueller detector (GM) has been installed on the top of the main vertical port of a JET sector (Octant 1) below the vertical neutron camera. The detector has been switched-on only during off-operational periods and the output signal recorded by a remote-controlled Multi Channel Scaler (MCS). High sensitivity thermoluminescent detectors (TLDs) GR-200A (natural LiF) have been selected for in-vessel measurements. During off-operational periods, the detectors have been inserted inside an irradiation end (2 upper irradiation end) using the rabbit system. Both TLDs and GM have been calibrated in a secondary gammaray standard facility in terms of air-kerma using Cs-137 and Co-60 sources. Further details on the detector selection, calibration and response can be found elsewhere [12]. The schedule of the in-vessel (with TLDs detectors) and exvessel (with GM counter) measurements is reported in Table 1. The measurements have been performed during four off-operational Table 1 Schedule of the dose measurements ex-vessel with active detector (GM) and invessel with passive detectors (TLDs) Position
Experiment
Date
Acquisition time (s)
Ex-vessel
Ex Ex Ex Ex
25/09/2005 05/06/2006 From 15/12/2006 to 04/01/2007 From 04/04/2007 to 15/05/2007
1.50 × 105 6.00 × 103 1.70 × 106 1.04 × 106
29/09/2005 05/06/2006 (a) 20/12/2006 (b) 21/12/2006 (c) 03/01/2007
4.33 × 104 2.16 × 104 8.73 × 104 8.52 × 104 3.60 × 103
In-vessel
1 2 3 4
In 1 In 2 In 3
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Table 2 JET DD and DT neutron yields Year
DD neutron yield
DT neutron yield
From 1983 to 2004 2005 2006 2007
20
2.60 × 10 4.13 × 1016 3.52 × 1019 1.77 × 1019
2.38 × 1020 3.84 × 1014 3.18 × 1017 1.52 × 1017
Total
3.13 × 1020
2.39 × 1020
periods: before the start-up of the 2005 campaign (Ex 1 and In 1), during short off-operational periods occurring in June 2006 (Ex 2 and In 2) and at the end of 2006 (Ex-3 and In 3) and at the shutdown of the 2007 campaign (Ex 4). The acquisition time refers to the period during which the GM has been switched-on (active measurement) and to the integration time for passive detectors, i.e. the time between sending and fetching to/from the irradiation end. Fig. 1 shows the JET DD and DT daily neutron yields during the 2005–2007 campaigns. These quantities have been measured by JET neutron diagnostic systems and have a typical error of ±10%. The four off-operational periods corresponding to the dose measurements are also shown. The DD and DT neutron yields from 1983 up to the end of 2007 are reported in Table 2. 3. Calculation The D1S and R2S methods have been applied to calculate airkerma rate in both experimental positions, at various cooling times. The results presented in this paper refer to 25th September 2005 (experiment In 1, cooling time ∼1.5 year), 20th December 2006 (experiment In 3, cooling time ∼5 days) and 3rd January 2007 (experiment In 3, cooling time ∼19 days) at the in-vessel position. For the ex-vessel position, simulations refer to 22nd December 2006 (cooling time ∼1 week), 3rd January 2007 (cooling time ∼19 days) and to the JET shutdown (14th May 2007, cooling time ∼40 days). DT and DD neutron sources distributions (provided by UKAEA) have been described by a parametric representation using a subroutine linked to MCNP. It has been verified by some preliminary investigations that under JET conditions the production of radionuclides responsible of the dose rate is proportional to the neutron flux. In such a case the total dose rate can be linearly decomposed into two different source components (DD and DT). The results are thus obtained by summing the DD and DT contributions. The 3D MCNP model of the JET Octant 1 is shown in Fig. 2. The original model, provided by UKAEA, has been updated on the basis of CAD drawings in order to describe in detail the complex environment around the GM. The material chemical compositions of several components have been taken from Ref. [14]. The details of the GM detector and of the irradiation end, where TLDs have been installed, are shown on the right side of Fig. 2. The active volume of GM detector has been simulated as an air-filled cylinder (r = 1 cm, h = 32.5 cm) located at z2 = 350 cm.The TLDs have been simulated as an air-filled small pellet (r = 1.43 cm, h = 1 cm) positioned at z = 190 cm. 3.1. D1S calculation D1S has been applied to calculate the gamma dose rates in the two experimental positions at the times corresponding to the avail-
2
Poloidal distance from the mid-plane.
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Fig. 1. DD and DT daily neutron yields of the JET campaigns 2005–2007. The measurements have been performed during four off-operational periods: before the start-up of the 2005 campaign (1), during short off-operational periods (2, June 2006; 3, December 2006) and at the shutdown of the 2007 campaign (4).
able measurements using DD and DT plasma sources separately. Neutrons emitted from the plasma source, transported through the JET machine induce the activation reaction and the emitted decay gammas are transported as prompt in the same run. The decay gammas are emitted according to the reaction and each gamma is labeled according to the parent radionuclide. The emitted decay
gammas are transported down to 30 keV, as the response of the detectors is negligible below this value. Special purpose libraries have been used: the neutron crosssections from the transport library (FENDL-2.0 [13]) have been replaced with those from the activation library (FENDL-2.0/A) for selected reactions which, in a pre-analysis of the problem, resulted
Fig. 2. Left: 3D MCNP model of Octant 1 of JET. Right: details of the main vertical port and GM detector (top) and of the irradiation end where TLDs were located (bottom).
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Table 4 GM measurements
Table 3 TLDs measurements # Exp
Date
Air-kerma rate (Gy/h)
In 1 In 2
29/09/2005 05/06/2006
51.1 ± 11.1 87.7 ± 18.8
In 3
20/12/2006 21/12/2006 03/01/2007
134.8 ± 22.2 120.8 ± 21.9 117.3 ± 20.7
to contribute to the induced activation. Also, the prompt gammas emitted in these reactions have been replaced by decay gammas emitted by the produced nuclide. A modified version of MCNP has been implemented: a fictitious delay time is associated to the emitted gamma according to the parent radionuclide in order to distinguish the photon dose contributions due the different radionuclides. The photon fluence spectra for each radionuclide have been calculated with MCNP in the two positions (in-vessel and ex-vessel) and multiplied by the flux-to-air-kerma conversion coefficients for comparison with the measurements. In the ex-vessel position the fluences have been corrected for the GM efficiency. The assumption in the MCNP calculation is that irradiation and decay are instantaneous. Therefore, time correction factors are needed to take into account the production and the decay of each radionuclide dependent on the DT and DD irradiation histories. These factors have been calculated with FISPACT, using a detailed description of the JET DD and DT irradiation scenarios from 1983 to 2007. The time correction factor ti,j for the i-nuclide corresponding to the j-measurement is the ratio between the i-activity at the date of the j-measurement and the total number of i-nuclides produced at the end of a fictitious instantaneous irradiation, i.e. assuming that all the neutrons are emitted in 1 s. Except for multi-step reactions and isomeric transitions the correction factor is independent on the neutron spectrum, activation cross-section and amount of parent nuclides, but depends only on the irradiation history. The different DD and DT contributions to the dose rate have been added to obtain the total air-kerma rate. 3.2. R2S calculation The R2S code system has been applied for the calculation of the shutdown dose rate in both positions. The R2S interface files for the automated processing of the neutron flux spectra and the decay
# Exp
Date
Cooling time
Air-kerma rate (Gy/h)
Ex 1
25/09/2005
1.5 year
0.88 ± 0.36
Ex 2
05/06/2006
16 days
1.47 ± 0.17
Ex 3
15/12/2006 16/12/2006 22/12/2006 25/12/2006 03/01/2007
1000 s 1 day 1 week 10 days 19 days
198.30 ± 21.81 26.24 ± 2.89 2.72 ± 0.30 1.92 ± 0.21 1.77 ± 0.19
Ex 4
04/04/2007 05/04/2007 11/04/2007 14/04/2007 23/04/2007 14/05/2007
1000 s 1 day 1 week 10 days 19 days 40 days
41.42 ± 4.56 10.11 ± 1.11 2.48 ± 0.27 2.28 ± 0.25 2.01 ± 0.22 1.94 ± 0.21
gamma source distributions were adapted to the specific JET conditions. For the present calculations, the decay gamma source cells contributing more than 1% to the final air-kerma dose rate were accounted for in the decay gamma transport calculation. The 3D MCNP model shown in Fig. 2 has been modified for a fine spatial segmentation of these cells. The number of geometry cells thereby has been increased from ∼450 to ∼1300. The MCNP code with FENDL2.0/MC cross-section data has been used to perform both neutrons (first step) and decay gammas (second step) transport calculations. The neutron flux spectrum distribution has been calculated in the 175 VITAMIN-J energy group structure using the MCNP track length estimator. Neutron flux spectra have been passed to FISPACT through the MCFISP interface for the nuclide inventory calculation. FENDL-2.0/A activation cross-sections have been used to calculate the activation inventory and the decay gamma sources for all specified geometry source cells and materials. The DD and DT irradiation scenarios used for the inventory calculation have been described on the basis of the data provided by UKAEA (Fig. 1 and Table 2). For each cooling time a separate decay gamma source file containing the decay gamma intensities and the spectra for all considered cells has been produced by the interface FISPMC and subsequently used as decay photon source distribution for the pure gamma transport calculation with MCNP through a sampling procedure. Decay photon flux spectra have been calculated in the specified detectors
Fig. 3. Air-kerma rate versus cooling time measured by GM detector at the ex-vessel position in January 2007 (Ex 3) and at the JET shutdown (Ex 4).
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Table 5 Results of R2S and D1S calculations and comparison between the two methods Experiment
Date
Cooling time
Air-kerma rate (Gy/h) DD D1S
R2S/D1S DT
Total
R2S
D1S
R2S
D1S
R2S
In 1
29/09/2005
1.5 year
20.73
14.00
42.97
27.50
63.70 ± 10.19
41.5 ± 4.1
0.7
In 3
20/12/2006 03/01/2007
5 days 19 days
122.24 109.00
82.80 69.00
39.88 39.23
24.80 24.20
162.12 ± 28.21 148.22 ± 25.34
107.6 ± 10.7 93.20 ± 9.2
0.7 0.6
Ex 3
22/12/2006 03/01/2007
1 week 19 days
0.51 0.48
0.58 0.56
0.19 0.19
0.19 0.24
0.70 ± 0.08 0.66 ± 0.08
0.77 ± 0.08 0.80 ± 0.08
1.1 1.2
Ex 4
14/05/2007
40 days
0.46
0.45
0.18
0.22
0.63 ± 0.08
0.67 ± 0.07
1.1
cells using the track length estimator. Flux-to-air-kerma conversion coefficients have been then applied to obtain the dosimetric quantities. In the ex-vessel position photon fluences have been corrected for the GM efficiency. One MCNP run has been performed for each decay time, using the corresponding decay gamma source file.
4. Results and discussion The results of the dose rate measurements are summarized in Table 3 for the in-vessel position and in Table 4 and in Fig. 3, for the ex-vessel. Before the start-up of the 2005 campaign (at about 1.5 year from the previous shutdown), the dose rate inside the vessel exceeded 50 Gy/h, whereas close to the main vertical port the air-kerma rate was <1 Gy/h. By comparing the values corresponding to the same cooling times in Tables 3 and 4, it can be noted that the dose rate in-vessel was systematically 50–60 times higher than ex-vessel. Fig. 3 shows the air-kerma rate in the ex-vessel position versus time at the JET 2007 shutdown and at the end of 2006 campaign. Up to 1 week of cooling time the results of Ex 3 were higher than Ex 4 because more intense neutron shots occurred at the end of 2006 than May 2007 (shutdown). At longer decay times, as the shortlived nuclides decay and the medium–long radionuclides become important, the effect of the overall yield dominated and at ∼20 days of cooling time the doses were more than 20% higher at JET shutdown than in January 2007.
The results of the D1S and R2S calculations are summarized in Table 5. The C/E ratio (calculation/experiment) is shown in Fig. 4. The results of the calculation have been compared with the net air-kerma rate obtained by subtracting the natural background (0.1 ± 0.03 Gy/h) from the measured quantities. The total uncertainty on the C/E has been obtained as [(C/C)2 + (E/E)2 ]1/2 , where the uncertainty on E (E/E) is the estimated experimental error and the uncertainty on C (C/C) includes the statistical error of MCNP calculation and the neutron yield error. For the in-vessel position, as a general trend, the experimental value is reproduced by both approaches within the estimated total uncertainty, although D1S results are systematically higher and R2S ones slightly lower. For the ex-vessel position, although an optimal agreement between R2S and D1S evaluations is obtained (within ±20%), both methods underestimate the experimental values (the C/E varies between 0.3 and 0.5). As the distance from the source increases, the impact of the modeling (geometry and materials descriptions) of the interposed components, structures and surrounding environment becomes important and such effect has been previously discussed by the authors [12]. At the beginning of the benchmark a first D1S analysis was performed using the original Octant 1 model provided by UKAEA and the C/E was <0.02. With respect to the original version of the geometry of JET Octant 1, in which the external environment was not enough detailed, the updated model used in the present simulation provides better results. The resulting dose rate is mainly due to gamma emitted by cobalt isotopes (Co-60 and Co-58). These radionuclides are produced by neutrons reactions with nickel and cobalt predominantly. Therefore,
Fig. 4. Comparison between calculations and measurements in terms of C/E ratio versus cooling time at the two positions (In and Ex).
R. Villari et al. / Fusion Engineering and Design 83 (2008) 1782–1787
the observed underestimation can be due to the absence of some components containing these elements in the model’s description. Moreover, the modifications in machine assembly, introduction and/or removal of interposing structures, occurred in the past might have caused the variation of the irradiation conditions especially in the external regions. In order to resolve the observed discrepancies a further experiment will be performed at JET. Neutron flux and spectra will be measured during operation to check the prediction accuracy in neutron transport calculation; gamma spectroscopy will be performed in off-operational period with dose measurements to identify the dominant radioactive nuclides. 5. Conclusions A benchmark experiment has been conducted at JET during 2005–2007 campaigns to validate, in view of their application in ITER, shutdown dose rate calculations with D1S and R2S methods in full 3D geometry. D1S and R2S methods have been applied to calculate dose rate in two positions, at different cooling times: the results have been compared with dedicated measurements performed with active and passive detectors. The benchmarks show that for the in-vessel position the methods successfully predict the dose: an agreement within 20% has been obtained, confirming previous results at Frascati Neutron Generator [10]. However, for the ex-vessel position, both methods underestimate the experimental values. Neutron flux measurements during operation and gamma dose and spectra measurements after shutdown at the same detector location will be performed in the frame of the follow-up activity to resolve the severe discrepancies. The future experiment will be useful to check the prediction accuracy in two sequential steps: during irradiation (neutron spectra) and after shutdown (decay gamma doses and spectra). Acknowledgements The authors acknowledge Michael Loughlin for his comments, Giorgio Brolatti, Paul Carman and John Bird for their support in CAD analysis, Basilio Esposito for his helpful suggestions and JeanChistophe Sublet who kindly provided his PhD thesis. This work, supported by the Euratom Communities under the contract of Association between EURATOM and ENEA, and
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Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement and of the Implementing Agreement on a Co-operative Programme on the Nuclear Technology of Fusion Reactors of the International Energy Agency (IEA). The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Y. Chen, U. Fischer, ITER-FEAT, Shutdown dose rate analysis by rigorous method final report on Contract EFDA/00-564 FZK Internet Bericht IRS Nr. 9/01, FUSION Nr. 177, September 2001. [2] Y. Chen, U. Fischer, Rigorous MCNP based shutdown dose rate calculations: computational scheme, verification calculations and application to ITER, Fusion Eng. Des. 63–64 (2002) 107–114. [3] U. Fischer, P. Pereslavtev, Activation and shutdown dose rate calculations for JET Final Report on Task JW2-FT-5.5 FZK Part Interner Bericht IRS 07/03 Fusion Nr. 201, May 2003. [4] D. Valenza, H. Iida, R. Plenteda, R.T. Santoro, Proposal of shutdown dose estimation method by Monte Carlo code, Fusion Eng. Des. 55 (2001) 411–418. [5] H. Iida, D. Valenza, R. Plenteda, R.T. Santoro, J. Dietz, Radiation shielding for ITER to allow for hands-on maintenance inside the Cryostat, J. Nucl. Sci. Technol. (Suppl. 1) (2000) 235–242. [6] L. Petrizzi, H. Iida, D. Valenza, P. Batistoni, Improvement and benchmarking of the new shutdown dose estimation method by Monte Carlo code: advanced Monte Carlo for radiation physics, particle transport simulation and applications, in: Proceedings of the MC2000 Conference, Springer, Lisbon, Portugal, October 23–26, 2000, February 2001, pp. 865–870. [7] J.F. Briestmaister (Ed.), MCNPTM —a general Monte Carlo N-particle transport code, Version 4C3, Los Alamos Nat. Lab. Report, LA13709-M, March 2000. [8] R. Forrest, FISPACT-2007 User Manual EASY 2007 Documentation Series UKAEA FUS 534. [9] L. Petrizzi, M. Angelone, P. Batistoni, U. Fischer, M. Loughlin, R. Villari, Benchmarking of Monte Carlo based shutdown dose rate calculations applied in fusion technology: from the past experience a future proposal for JET 2005 operation, Fusion Eng. Des. 81 (2006) 1417–1423. [10] P. Batistoni, M. Angelone, L. Petrizzi, M. Pillon, Experimental validation of shutdown dose rates calculation inside ITER cryostat, Fusion Eng. Des. 58–59 (2001) 613–616. [11] L. Petrizzi, P. Batistoni, U. Fischer, M. Loughlin, P. Pereslavtsev, R. Villari, Benchmarking of Monte Carlo based shutdown dose rate calculations for applications to JET, Radiat. Prot. Dosim. 115 (2005) 80–85. [12] M. Angelone, L. Petrizzi, M. Pillon, S. Popovichev, R. Villari, A dose rate experiment at JET for benchmarking the calculation with direct one step method, Fusion Eng. Des. 82 (2008) 2805–2811. [13] H. Wienke, M. Herman, FENDL/MG-2.0 and FENDL/MC-2.0: the processed cross section libraries for neutron–photon transport calculations, IAEA-NDS-176, Rev. 1, International Atomic Energy Agency, Vienna, 1998, Data retrieved on-line from the IAEA Nuclear Data Section. [14] J.Ch. Sublet, Activation considerations relevant to the decommissioning of fusion reactors, PhD Thesis, Imperial College London, 1989.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Validation of shutdown dose rate Monte Carlo calculations through a benchmark experiment at JET R. Villari a,∗ , M. Angelone a , P. Batistoni a , U. Fischer b , P. Pereslavtsev b , L. Petrizzi a , S. Popovichev c , JET-EFDA Contributors1 a Associazione EURATOM ENEA sulla Fusione, Via Enrico Fermi 44, 00044 Frascati, Rome, Italy b Association FZK-EURATOM Forschungszentrum Karlsruhe, Germany c Euratom-UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3DB, UK
a r t i c l e
i n f o
Article history: Available online 9 September 2008 Keywords: Tokamak Activation Dose rate Monte Carlo Geiger–Muller tube TLDs detectors D1S method R2S method JET
a b s t r a c t A benchmark experiment has been performed during the 2005–2007 campaigns of JET in order to validate the rigorous two-step (R2S) and direct one-step (D1S) methods for the prediction of shutdown dose rates in full 3D geometry. Dose rate levels calculated using D1S and R2S methods have been compared with experimental data collected before and during off-operational periods and at the end of the last JET campaigns. Measurements have been carried out in two irradiation positions: in-vessel with high sensitivity thermoluminescent detectors and ex-vessel with an active detector of Geiger–Mueller type. Satisfying agreement between calculations and measurements has been obtained in in-vessel position, whereas both methods underestimate the experimental quantity in the external one. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Neutrons produced during DT and DD plasma operations induce the activation of the materials constituting the fusion machine components. The prediction of the induced activation and resulting dose rates is a main issue for fusion reactors: due to neutron activation, the normal operations of intervention, maintenance and control are restricted and must be planned in advance in order to guarantee the respect of dose limits. Over the past several years, two different methodological approaches have been developed in the frame of the Fusion Technology Programme for the three-dimensional (3D) calculation of the shutdown dose rate. One is the so-called rigorous two-step method (R2S) developed by FZK [1–3] and the second is the direct one-step method (D1S) [4–6] developed by the ITER team and ENEA.
∗ Corresponding author. Tel.: +39 0694005848. E-mail address: [email protected] (R. Villari). 1 See Appendix of M. Watkins et al. Fusion Energy, 2006 (Proceedings of the 21st IAEA International Conference, Chengdu 2006). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.08.001
Both methods, even if with different approaches, use MCNP Monte Carlo code [7] for transport calculations and FISPACT [8] inventory code for activation calculations. In R2S, a regular neutron transport calculation provides the spatial distribution of the neutron flux spectra (MCNP, first step); the decay gamma source distribution is obtained through a nuclide inventory calculation (FISPACT) using the MCNP neutron flux spectra and the irradiation history; finally, a decay gamma transport calculation (MCNP, second step) based on FISPACT decay gamma source distribution provides the dose rate at the specified locations. D1S is an innovative method based on the assumption that the decay gammas of the radioactive nuclides are promptly emitted; hence, the neutrons and decay gammas are transported in a single MCNP run. The time correction factors required to take into account the build-up and the decay time of each radionuclide according to the real irradiation history are calculated using FISPACT. Both methods have advantages and disadvantages. The main advantage of R2S is the full calculation of the nuclide inventory, so that all potential reaction chains are implicitly included. In this respect the method is problem independent. On the other hand, R2S requires in general a finer segmentation of the geometry cells because the neutron fluxes and the decay gamma source are spatially averaged over the cell volumes as defined in the regular MCNP
R. Villari et al. / Fusion Engineering and Design 83 (2008) 1782–1787
geometry model. The D1S main advantage is the direct coupling between the decay gammas and the neutrons; however the transport libraries must be adapted to the specific problem, general applications are complex and serious limitations are foreseen in case of high burn-up and multi-chain reactions. Several computational and experimental benchmarks of the two methods have been conducted in the past and a brief but complete review has been recently published [9]. Good results have been obtained for simple geometries under well-defined conditions (i.e. ITER oriented experiment at Frascati Neutron Generator [10]). However, in order to apply these methods to ITER a validation in a reactor-like configuration, such as JET, is necessary. JET is the world’s largest tokamak (brought into operation since 1983) and is the only European device in which DT experiments have been conducted. A first benchmark was performed in 2004 [11]. D1S and R2S dose rate calculations were compared with dose measurements collected by the JET Health Physics group during the first experiment with tritium (DTE-1). The obtained results were not satisfactory, mainly because of the rather high uncertainties of the available measurements and the inadequacy of the MCNP modeling. It was therefore decided to conduct a dedicated experiment to validate the R2S and D1S shutdown dose rate calculations. Dose rate data have been collected during off-operational periods and at the end of the 2005–2007 JET campaigns in two irradiation positions, at different decay times and compared with those calculated using D1S and R2S. The results of this dedicated benchmark are presented in this paper. 2. Detectors and measurements Gamma dose rates have been measured during JET offoperational periods in two experimental positions at different cooling times (the elapsed times from the last previous shot) with active (ex-vessel position) and passive (in-vessel position) detectors. An active Geiger–Mueller detector (GM) has been installed on the top of the main vertical port of a JET sector (Octant 1) below the vertical neutron camera. The detector has been switched-on only during off-operational periods and the output signal recorded by a remote-controlled Multi Channel Scaler (MCS). High sensitivity thermoluminescent detectors (TLDs) GR-200A (natural LiF) have been selected for in-vessel measurements. During off-operational periods, the detectors have been inserted inside an irradiation end (2 upper irradiation end) using the rabbit system. Both TLDs and GM have been calibrated in a secondary gammaray standard facility in terms of air-kerma using Cs-137 and Co-60 sources. Further details on the detector selection, calibration and response can be found elsewhere [12]. The schedule of the in-vessel (with TLDs detectors) and exvessel (with GM counter) measurements is reported in Table 1. The measurements have been performed during four off-operational Table 1 Schedule of the dose measurements ex-vessel with active detector (GM) and invessel with passive detectors (TLDs) Position
Experiment
Date
Acquisition time (s)
Ex-vessel
Ex Ex Ex Ex
25/09/2005 05/06/2006 From 15/12/2006 to 04/01/2007 From 04/04/2007 to 15/05/2007
1.50 × 105 6.00 × 103 1.70 × 106 1.04 × 106
29/09/2005 05/06/2006 (a) 20/12/2006 (b) 21/12/2006 (c) 03/01/2007
4.33 × 104 2.16 × 104 8.73 × 104 8.52 × 104 3.60 × 103
In-vessel
1 2 3 4
In 1 In 2 In 3
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Table 2 JET DD and DT neutron yields Year
DD neutron yield
DT neutron yield
From 1983 to 2004 2005 2006 2007
20
2.60 × 10 4.13 × 1016 3.52 × 1019 1.77 × 1019
2.38 × 1020 3.84 × 1014 3.18 × 1017 1.52 × 1017
Total
3.13 × 1020
2.39 × 1020
periods: before the start-up of the 2005 campaign (Ex 1 and In 1), during short off-operational periods occurring in June 2006 (Ex 2 and In 2) and at the end of 2006 (Ex-3 and In 3) and at the shutdown of the 2007 campaign (Ex 4). The acquisition time refers to the period during which the GM has been switched-on (active measurement) and to the integration time for passive detectors, i.e. the time between sending and fetching to/from the irradiation end. Fig. 1 shows the JET DD and DT daily neutron yields during the 2005–2007 campaigns. These quantities have been measured by JET neutron diagnostic systems and have a typical error of ±10%. The four off-operational periods corresponding to the dose measurements are also shown. The DD and DT neutron yields from 1983 up to the end of 2007 are reported in Table 2. 3. Calculation The D1S and R2S methods have been applied to calculate airkerma rate in both experimental positions, at various cooling times. The results presented in this paper refer to 25th September 2005 (experiment In 1, cooling time ∼1.5 year), 20th December 2006 (experiment In 3, cooling time ∼5 days) and 3rd January 2007 (experiment In 3, cooling time ∼19 days) at the in-vessel position. For the ex-vessel position, simulations refer to 22nd December 2006 (cooling time ∼1 week), 3rd January 2007 (cooling time ∼19 days) and to the JET shutdown (14th May 2007, cooling time ∼40 days). DT and DD neutron sources distributions (provided by UKAEA) have been described by a parametric representation using a subroutine linked to MCNP. It has been verified by some preliminary investigations that under JET conditions the production of radionuclides responsible of the dose rate is proportional to the neutron flux. In such a case the total dose rate can be linearly decomposed into two different source components (DD and DT). The results are thus obtained by summing the DD and DT contributions. The 3D MCNP model of the JET Octant 1 is shown in Fig. 2. The original model, provided by UKAEA, has been updated on the basis of CAD drawings in order to describe in detail the complex environment around the GM. The material chemical compositions of several components have been taken from Ref. [14]. The details of the GM detector and of the irradiation end, where TLDs have been installed, are shown on the right side of Fig. 2. The active volume of GM detector has been simulated as an air-filled cylinder (r = 1 cm, h = 32.5 cm) located at z2 = 350 cm.The TLDs have been simulated as an air-filled small pellet (r = 1.43 cm, h = 1 cm) positioned at z = 190 cm. 3.1. D1S calculation D1S has been applied to calculate the gamma dose rates in the two experimental positions at the times corresponding to the avail-
2
Poloidal distance from the mid-plane.
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Fig. 1. DD and DT daily neutron yields of the JET campaigns 2005–2007. The measurements have been performed during four off-operational periods: before the start-up of the 2005 campaign (1), during short off-operational periods (2, June 2006; 3, December 2006) and at the shutdown of the 2007 campaign (4).
able measurements using DD and DT plasma sources separately. Neutrons emitted from the plasma source, transported through the JET machine induce the activation reaction and the emitted decay gammas are transported as prompt in the same run. The decay gammas are emitted according to the reaction and each gamma is labeled according to the parent radionuclide. The emitted decay
gammas are transported down to 30 keV, as the response of the detectors is negligible below this value. Special purpose libraries have been used: the neutron crosssections from the transport library (FENDL-2.0 [13]) have been replaced with those from the activation library (FENDL-2.0/A) for selected reactions which, in a pre-analysis of the problem, resulted
Fig. 2. Left: 3D MCNP model of Octant 1 of JET. Right: details of the main vertical port and GM detector (top) and of the irradiation end where TLDs were located (bottom).
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Table 4 GM measurements
Table 3 TLDs measurements # Exp
Date
Air-kerma rate (Gy/h)
In 1 In 2
29/09/2005 05/06/2006
51.1 ± 11.1 87.7 ± 18.8
In 3
20/12/2006 21/12/2006 03/01/2007
134.8 ± 22.2 120.8 ± 21.9 117.3 ± 20.7
to contribute to the induced activation. Also, the prompt gammas emitted in these reactions have been replaced by decay gammas emitted by the produced nuclide. A modified version of MCNP has been implemented: a fictitious delay time is associated to the emitted gamma according to the parent radionuclide in order to distinguish the photon dose contributions due the different radionuclides. The photon fluence spectra for each radionuclide have been calculated with MCNP in the two positions (in-vessel and ex-vessel) and multiplied by the flux-to-air-kerma conversion coefficients for comparison with the measurements. In the ex-vessel position the fluences have been corrected for the GM efficiency. The assumption in the MCNP calculation is that irradiation and decay are instantaneous. Therefore, time correction factors are needed to take into account the production and the decay of each radionuclide dependent on the DT and DD irradiation histories. These factors have been calculated with FISPACT, using a detailed description of the JET DD and DT irradiation scenarios from 1983 to 2007. The time correction factor ti,j for the i-nuclide corresponding to the j-measurement is the ratio between the i-activity at the date of the j-measurement and the total number of i-nuclides produced at the end of a fictitious instantaneous irradiation, i.e. assuming that all the neutrons are emitted in 1 s. Except for multi-step reactions and isomeric transitions the correction factor is independent on the neutron spectrum, activation cross-section and amount of parent nuclides, but depends only on the irradiation history. The different DD and DT contributions to the dose rate have been added to obtain the total air-kerma rate. 3.2. R2S calculation The R2S code system has been applied for the calculation of the shutdown dose rate in both positions. The R2S interface files for the automated processing of the neutron flux spectra and the decay
# Exp
Date
Cooling time
Air-kerma rate (Gy/h)
Ex 1
25/09/2005
1.5 year
0.88 ± 0.36
Ex 2
05/06/2006
16 days
1.47 ± 0.17
Ex 3
15/12/2006 16/12/2006 22/12/2006 25/12/2006 03/01/2007
1000 s 1 day 1 week 10 days 19 days
198.30 ± 21.81 26.24 ± 2.89 2.72 ± 0.30 1.92 ± 0.21 1.77 ± 0.19
Ex 4
04/04/2007 05/04/2007 11/04/2007 14/04/2007 23/04/2007 14/05/2007
1000 s 1 day 1 week 10 days 19 days 40 days
41.42 ± 4.56 10.11 ± 1.11 2.48 ± 0.27 2.28 ± 0.25 2.01 ± 0.22 1.94 ± 0.21
gamma source distributions were adapted to the specific JET conditions. For the present calculations, the decay gamma source cells contributing more than 1% to the final air-kerma dose rate were accounted for in the decay gamma transport calculation. The 3D MCNP model shown in Fig. 2 has been modified for a fine spatial segmentation of these cells. The number of geometry cells thereby has been increased from ∼450 to ∼1300. The MCNP code with FENDL2.0/MC cross-section data has been used to perform both neutrons (first step) and decay gammas (second step) transport calculations. The neutron flux spectrum distribution has been calculated in the 175 VITAMIN-J energy group structure using the MCNP track length estimator. Neutron flux spectra have been passed to FISPACT through the MCFISP interface for the nuclide inventory calculation. FENDL-2.0/A activation cross-sections have been used to calculate the activation inventory and the decay gamma sources for all specified geometry source cells and materials. The DD and DT irradiation scenarios used for the inventory calculation have been described on the basis of the data provided by UKAEA (Fig. 1 and Table 2). For each cooling time a separate decay gamma source file containing the decay gamma intensities and the spectra for all considered cells has been produced by the interface FISPMC and subsequently used as decay photon source distribution for the pure gamma transport calculation with MCNP through a sampling procedure. Decay photon flux spectra have been calculated in the specified detectors
Fig. 3. Air-kerma rate versus cooling time measured by GM detector at the ex-vessel position in January 2007 (Ex 3) and at the JET shutdown (Ex 4).
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Table 5 Results of R2S and D1S calculations and comparison between the two methods Experiment
Date
Cooling time
Air-kerma rate (Gy/h) DD D1S
R2S/D1S DT
Total
R2S
D1S
R2S
D1S
R2S
In 1
29/09/2005
1.5 year
20.73
14.00
42.97
27.50
63.70 ± 10.19
41.5 ± 4.1
0.7
In 3
20/12/2006 03/01/2007
5 days 19 days
122.24 109.00
82.80 69.00
39.88 39.23
24.80 24.20
162.12 ± 28.21 148.22 ± 25.34
107.6 ± 10.7 93.20 ± 9.2
0.7 0.6
Ex 3
22/12/2006 03/01/2007
1 week 19 days
0.51 0.48
0.58 0.56
0.19 0.19
0.19 0.24
0.70 ± 0.08 0.66 ± 0.08
0.77 ± 0.08 0.80 ± 0.08
1.1 1.2
Ex 4
14/05/2007
40 days
0.46
0.45
0.18
0.22
0.63 ± 0.08
0.67 ± 0.07
1.1
cells using the track length estimator. Flux-to-air-kerma conversion coefficients have been then applied to obtain the dosimetric quantities. In the ex-vessel position photon fluences have been corrected for the GM efficiency. One MCNP run has been performed for each decay time, using the corresponding decay gamma source file.
4. Results and discussion The results of the dose rate measurements are summarized in Table 3 for the in-vessel position and in Table 4 and in Fig. 3, for the ex-vessel. Before the start-up of the 2005 campaign (at about 1.5 year from the previous shutdown), the dose rate inside the vessel exceeded 50 Gy/h, whereas close to the main vertical port the air-kerma rate was <1 Gy/h. By comparing the values corresponding to the same cooling times in Tables 3 and 4, it can be noted that the dose rate in-vessel was systematically 50–60 times higher than ex-vessel. Fig. 3 shows the air-kerma rate in the ex-vessel position versus time at the JET 2007 shutdown and at the end of 2006 campaign. Up to 1 week of cooling time the results of Ex 3 were higher than Ex 4 because more intense neutron shots occurred at the end of 2006 than May 2007 (shutdown). At longer decay times, as the shortlived nuclides decay and the medium–long radionuclides become important, the effect of the overall yield dominated and at ∼20 days of cooling time the doses were more than 20% higher at JET shutdown than in January 2007.
The results of the D1S and R2S calculations are summarized in Table 5. The C/E ratio (calculation/experiment) is shown in Fig. 4. The results of the calculation have been compared with the net air-kerma rate obtained by subtracting the natural background (0.1 ± 0.03 Gy/h) from the measured quantities. The total uncertainty on the C/E has been obtained as [(C/C)2 + (E/E)2 ]1/2 , where the uncertainty on E (E/E) is the estimated experimental error and the uncertainty on C (C/C) includes the statistical error of MCNP calculation and the neutron yield error. For the in-vessel position, as a general trend, the experimental value is reproduced by both approaches within the estimated total uncertainty, although D1S results are systematically higher and R2S ones slightly lower. For the ex-vessel position, although an optimal agreement between R2S and D1S evaluations is obtained (within ±20%), both methods underestimate the experimental values (the C/E varies between 0.3 and 0.5). As the distance from the source increases, the impact of the modeling (geometry and materials descriptions) of the interposed components, structures and surrounding environment becomes important and such effect has been previously discussed by the authors [12]. At the beginning of the benchmark a first D1S analysis was performed using the original Octant 1 model provided by UKAEA and the C/E was <0.02. With respect to the original version of the geometry of JET Octant 1, in which the external environment was not enough detailed, the updated model used in the present simulation provides better results. The resulting dose rate is mainly due to gamma emitted by cobalt isotopes (Co-60 and Co-58). These radionuclides are produced by neutrons reactions with nickel and cobalt predominantly. Therefore,
Fig. 4. Comparison between calculations and measurements in terms of C/E ratio versus cooling time at the two positions (In and Ex).
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the observed underestimation can be due to the absence of some components containing these elements in the model’s description. Moreover, the modifications in machine assembly, introduction and/or removal of interposing structures, occurred in the past might have caused the variation of the irradiation conditions especially in the external regions. In order to resolve the observed discrepancies a further experiment will be performed at JET. Neutron flux and spectra will be measured during operation to check the prediction accuracy in neutron transport calculation; gamma spectroscopy will be performed in off-operational period with dose measurements to identify the dominant radioactive nuclides. 5. Conclusions A benchmark experiment has been conducted at JET during 2005–2007 campaigns to validate, in view of their application in ITER, shutdown dose rate calculations with D1S and R2S methods in full 3D geometry. D1S and R2S methods have been applied to calculate dose rate in two positions, at different cooling times: the results have been compared with dedicated measurements performed with active and passive detectors. The benchmarks show that for the in-vessel position the methods successfully predict the dose: an agreement within 20% has been obtained, confirming previous results at Frascati Neutron Generator [10]. However, for the ex-vessel position, both methods underestimate the experimental values. Neutron flux measurements during operation and gamma dose and spectra measurements after shutdown at the same detector location will be performed in the frame of the follow-up activity to resolve the severe discrepancies. The future experiment will be useful to check the prediction accuracy in two sequential steps: during irradiation (neutron spectra) and after shutdown (decay gamma doses and spectra). Acknowledgements The authors acknowledge Michael Loughlin for his comments, Giorgio Brolatti, Paul Carman and John Bird for their support in CAD analysis, Basilio Esposito for his helpful suggestions and JeanChistophe Sublet who kindly provided his PhD thesis. This work, supported by the Euratom Communities under the contract of Association between EURATOM and ENEA, and
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Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement and of the Implementing Agreement on a Co-operative Programme on the Nuclear Technology of Fusion Reactors of the International Energy Agency (IEA). The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Y. Chen, U. Fischer, ITER-FEAT, Shutdown dose rate analysis by rigorous method final report on Contract EFDA/00-564 FZK Internet Bericht IRS Nr. 9/01, FUSION Nr. 177, September 2001. [2] Y. Chen, U. Fischer, Rigorous MCNP based shutdown dose rate calculations: computational scheme, verification calculations and application to ITER, Fusion Eng. Des. 63–64 (2002) 107–114. [3] U. Fischer, P. Pereslavtev, Activation and shutdown dose rate calculations for JET Final Report on Task JW2-FT-5.5 FZK Part Interner Bericht IRS 07/03 Fusion Nr. 201, May 2003. [4] D. Valenza, H. Iida, R. Plenteda, R.T. Santoro, Proposal of shutdown dose estimation method by Monte Carlo code, Fusion Eng. Des. 55 (2001) 411–418. [5] H. Iida, D. Valenza, R. Plenteda, R.T. Santoro, J. Dietz, Radiation shielding for ITER to allow for hands-on maintenance inside the Cryostat, J. Nucl. Sci. Technol. (Suppl. 1) (2000) 235–242. [6] L. Petrizzi, H. Iida, D. Valenza, P. Batistoni, Improvement and benchmarking of the new shutdown dose estimation method by Monte Carlo code: advanced Monte Carlo for radiation physics, particle transport simulation and applications, in: Proceedings of the MC2000 Conference, Springer, Lisbon, Portugal, October 23–26, 2000, February 2001, pp. 865–870. [7] J.F. Briestmaister (Ed.), MCNPTM —a general Monte Carlo N-particle transport code, Version 4C3, Los Alamos Nat. Lab. Report, LA13709-M, March 2000. [8] R. Forrest, FISPACT-2007 User Manual EASY 2007 Documentation Series UKAEA FUS 534. [9] L. Petrizzi, M. Angelone, P. Batistoni, U. Fischer, M. Loughlin, R. Villari, Benchmarking of Monte Carlo based shutdown dose rate calculations applied in fusion technology: from the past experience a future proposal for JET 2005 operation, Fusion Eng. Des. 81 (2006) 1417–1423. [10] P. Batistoni, M. Angelone, L. Petrizzi, M. Pillon, Experimental validation of shutdown dose rates calculation inside ITER cryostat, Fusion Eng. Des. 58–59 (2001) 613–616. [11] L. Petrizzi, P. Batistoni, U. Fischer, M. Loughlin, P. Pereslavtsev, R. Villari, Benchmarking of Monte Carlo based shutdown dose rate calculations for applications to JET, Radiat. Prot. Dosim. 115 (2005) 80–85. [12] M. Angelone, L. Petrizzi, M. Pillon, S. Popovichev, R. Villari, A dose rate experiment at JET for benchmarking the calculation with direct one step method, Fusion Eng. Des. 82 (2008) 2805–2811. [13] H. Wienke, M. Herman, FENDL/MG-2.0 and FENDL/MC-2.0: the processed cross section libraries for neutron–photon transport calculations, IAEA-NDS-176, Rev. 1, International Atomic Energy Agency, Vienna, 1998, Data retrieved on-line from the IAEA Nuclear Data Section. [14] J.Ch. Sublet, Activation considerations relevant to the decommissioning of fusion reactors, PhD Thesis, Imperial College London, 1989.