Nuclear analysis of the ITER Cryopump Ports

Fusion Engineering and Design 98–99 (2015) 1561–1565

Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Nuclear analysis of the ITER Cryopump Ports Fabio Moro a,∗ , Rosaria Villari a , Davide Flammini a , Alexander Antipenkov b , Matthias Dremel b , Bruno Levesy b , Michael Loughlin b , Rafael Juarez c , Lucia Perez c , Luigino Petrizzi d a

ENEA, Fusion Technical Unit, Nuclear Technologies Laboratory, Via Enrico Fermi 45, 00044 Frascati, Rome, Italy ITER Organization, Route de Vinon sur Verdon, 13115 Saint Paul-lez-Durance, France UNED, Energetic Engineering Department, C/Juan del Rosal 12, Madrid, Spain d European Commission, DG Research & Innovation G5, CDMA 00/030, B-1049 Brussels, Belgium b c

h i g h l i g h t s • Evaluation the shielding effectiveness of the TCPHs by means of 3-D neutrons and gamma maps. • Assessment of the nuclear heating induced by neutron and photons on the TCP and TCPHs. • Calculation of the dose rate at 12 days after shutdown in the maintenance area of the Lower Ports with the Advanced D1S method, in order to verify the design target (100 ␮Sv/h).

• Potential improvements of the shielding configuration aimed at the reduction of the dose level in the Port Cell have been proposed and discussed.

a r t i c l e

i n f o

Article history: Received 19 September 2014 Received in revised form 12 June 2015 Accepted 16 June 2015 Available online 7 July 2015 Keywords: ITER Cryopump Neutronics Design Analysis

a b s t r a c t The ITER machine will be equipped with 6 torus Cryopumps (TCP) that are positioned in their housings (TCPH) and integrated into the cryostat walls at B1 level in the port cells. A comprehensive nuclear analysis of the Cryopump Ports #4 and #12 has been carried out by means of the MCNP-5 Monte Carlo code in a full 3-D geometry, providing guidelines for the design of the embedded components. Radiation transport calculations have been performed in order to determine the radiation field inside the Lower Ports, up the Port Cell: 3-D neutrons and gamma maps have been provided in order to evaluate the shielding effectiveness of the TCPHs. Nuclear heating induced by neutron and photons have been estimated on the TCP and TCPH to assess the nuclear loads during plasma operations. The shutdown dose rate in the maintenance area of the Lower Ports has been assessed with the Advanced D1S method to verify the design limits. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Cryopump system is a fundamental component to ensure optimal plasma performance during ITER operations: its main function is to reduce the pressure inside the vessel; moreover it is designed to extract the ashes generated by the deuterium–tritium fusion reactions, that will be processed in the tritium plant. The ITER machine will be equipped with 6 torus Cryopumps (TCP) that are positioned in their Housings (TCPHs), integrated into the Cryostat walls at lower level (B1) of the ITER tokamak complex in the port cells (Fig. 1) [1]. The Housings structure depends on the port cells

∗ Corresponding author. E-mail address: [email protected] (F. Moro). http://dx.doi.org/10.1016/j.fusengdes.2015.06.077 0920-3796/© 2015 Elsevier B.V. All rights reserved.

in which it will be located: the Lower Ports #4, #10 and #16 will host the pellet injection system, consequently the relative TCPHs present dedicated cut-outs, while the TCPHs in the port cells #6, #12 and #18 have five straight penetrations for diagnostics systems. The aim of the study presented in this paper, is to perform a complete assessment of nuclear quantities in the Lower Ports #4 and #12 by means of the MCNP-5 Monte Carlo code [2] in a full 3-D geometry. Nuclear analyses have been performed to evaluate the shielding effectiveness of the TCPHs by means of 3-D neutrons and gamma maps; moreover, the nuclear heating induced by neutron and photons has been assessed on the TCP and TCPH. Dose rate at 12 days after shutdown has been calculated in the maintenance area of the Lower Ports with the Advanced D1S method [3], in order to verify the design target (<100 ␮Sv/h). Potential improvements of

1562

F. Moro et al. / Fusion Engineering and Design 98–99 (2015) 1561–1565

Fig. 1. Lower Port #4: section along its longitudinal axis.

the shielding configuration aimed at the reduction of the dose level in the Port Cell have been proposed and discussed. The results of the neutronic analysis carried out provide guidelines for the design of the embedded components, in order to guarantee their structural integrity and proper operations. 2. Modeling and calculation tools To build up the MCNP model used in the calculations, the CAD models of both the Lower Ports #4 and #12 have been pre-processed and properly simplified. Successively, the MCNP geometries, generated by means of the CAD-to-MCNP interface MCAM 4.8 [4], have been singularly incorporated in the last MCNP ITER 40◦ reference model B-lite v3 [5], including the Shielding, TCP main components (Fig. 2), the TCPHs and all the elements that are present in the area. The chemical composition of the materials defined in the MCNP inputs includes impurities according to the latest documentation [6]. The volume percentages of the materials in the mixtures as well as the densities are settled on the basis of the masses extracted from the original CATIA files. Furthermore, both the ITER B-lite v3 models have been extended beyond the Bioshield to the port door, integrating details of the Port Cells. Fig. 3 shows sections of the model integrating the Lower Port #12: the building walls, the Primary Heat Transfer System pipes structure, the Pellet Injection System and Cold Valve Boxes have been included. The analyses have been performed using the MCNP5 v.1.6 [2] Monte Carlo code with the FENDL 2.1 [7] nuclear data libraries. The calculations carried out are based on the ITER Deuterium–Tritium plasma source included in B-lite v3 model and normalized to 500 MW of fusion power. The shutdown dose rates have been calculated using the irradiation scenario SA2 [8]. MCNP simulations have

Fig. 3. ITER B-lite v3 MCNP model upgraded with the Lower Port #12: poloidal section along the Cryopump longitudinal axis (upper panel) and toroidal section along the Cryopump mid-plane (lower panel).

been run on the ENEA CRESCO1 HPC facility and variance reduction techniques (weight windows [9]) have been used to improve statistics and provide results with sufficient accuracy (statistical uncertainty within few %). 3. Nuclear analyses 3.1. Radiation streaming Total and fast (E > 100 keV) neutron fluxes in the Lower Ports area have been calculated using a mesh tally extended up to the plug of the TCP. The radiation streaming through the Lower Ports structure is well visible in 3-D maps of total neutron flux shown in Fig. 4. The neutron flux decreases of about 4/5 orders of magnitude from the Lower Port shielding to the back end of the TCPHs (∼1013 n/cm2 /s in correspondence of the Lower Port entrance, ∼108 n/cm2 /s behind the TCP plug). Moreover, 3-D maps of fast neutrons flux highlight the streaming effect through the diagnostic penetrations in the Lower Port #12 (Fig. 5): a slightly higher neutron streaming (∼a factor 2) is observed in the lower area of the TCPH. As far the neutron-induced prompt gammas are concerned, 3-D maps of the gamma flux (␥/(cm2 s)) have been carried out (Fig. 6): the gamma flux drops of about 5/6 orders of magnitude along the Lower Port (∼1012 ␥/cm2 /s in correspondence of the Lower Port entrance, ∼108 ␥/cm2 /s behind the TCP plug). 3.2. Nuclear heating Nuclear heating (i.e. energy deposited by neutrons and gammas) calculations have been performed on both the Lower Ports TCP and TCPH. The nuclear heating density (W/cm3 ) has been evaluated for neutron and secondary photons in Stainless Steel (SS) and the results are provided in 3-D maps (Fig. 7).

Fig. 2. ITER torus cryopump.

1

www.cresco.it.

F. Moro et al. / Fusion Engineering and Design 98–99 (2015) 1561–1565

Fig. 4. 3-D total neutron flux map (n/cm2 /s): poloidal section through the longitudinal axis of the Lower Port #4 (upper panel) and Lower Port #12 (lower panel).

Fig. 5. 3-D fast neutron flux map (n/cm2 /s): toroidal section through the diagnostics penetrations of the Lower Port #12.

1563

Fig. 8. 3-D nuclear heating density (W/cm3 ) map: sections of the Lower Port #12 TCP and TCPH.

The most loaded components in both cases are the TCPHs bellows and the TCP flange (∼10−4 W/cm3 ), facing the direct neutrons flux from the plasma source. Inside the TCP main body the nuclear heating decreases of two orders of magnitude, reaching ∼10−6 W/cm3 in the TCP back area. Inside the plug, an increase of the nuclear heating is observed, mainly due to the higher density SS composing this element with respect to the one used to define the inner structure of the TCP. Fig. 8 shows different sections of the Lower Port #12 TCP and TCPH: some hotspots are observed around the five diagnostics penetrations, extended along the SS collimators inside the TCPH structure. The behavior of the nuclear heating density radial profile has been evaluated along three paths (Fig. 9): one located along the TCP axis (‘Cryopump’) and two lying in the upper and lower area of the TCPH (‘Housing 1’ and ‘Housing 2’ respectively). The radial profiles inside the TCPH structure show a general reduction of the nuclear load of ∼1 order of magnitude and some peaks corresponding to the intersections of the vertical flanges inside the TCPH layout. In particular the ‘Housing 1’ path is characterized by a peak in the area close to the bellows that, according to the 3D maps showed so far, are the most heated region together with the TCP flange. The ‘Housing 2’ radial profile presents higher values of the heat load in the front region due to the radiation streaming through the diagnostics cut-outs. The radial profile inside the TCP exhibits a mitigation of ∼1.5 order of magnitude along the main body and an

Fig. 6. 3-D gamma flux map (␥/cm2 /s): poloidal section of the Lower Port #4 (upper panel) and Lower Port #12 (lower panel).

Fig. 7. 3-D nuclear heating density (W/cm3 ) map: poloidal section through the longitudinal axis of the TCP and TCPH for the Lower Port #4 (left) Lower Port #12 (right). The yellow vertical lines indicate the sections that follow in Fig. 8. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

Fig. 9. Nuclear heating density (W/cm3 ) radial profiles (Lower Port #12). Peaks correspond to the Housing SS vertical flanges.

1564

F. Moro et al. / Fusion Engineering and Design 98–99 (2015) 1561–1565

Fig. 11. Shielding block configuration for the Lower Ports.

Fig. 10. Dose rate at 12 days after shutdown 3-D map (␮Sv/h): poloidal section through the longitudinal axis of the Lower Port #12 (upper panel) and section outside the TCP plug (lower panel).

increase in the nuclear heating in the TCP plug due to the higher density SS that compose it; in the actuator, the heat load density decreases to ∼10−8 W/cm3 . 3.3. Shutdown dose rate The shutdown dose rate (effective dose) has been calculated using Advanced D1S method at different cooling times (105 , 106 , 107 s after shutdown) considering a total neutron yield of 3 × 1027 n (whole ITER lifetime [7]) for the Lower Port #12 model, being the less shielded configuration. The outcome of this analysis shows that the dose rate 12 days after shutdown, evaluated in the Port Cell area immediately out of the TCP plug, is generically around 100 ␮Sv/h, thus very close to the safety requirement for the access zone 1 (Fig. 10). However, some hot spots have been observed around the TCP plug, mainly due to the void region inside the TCPH (regeneration volume) that reduces its shielding capability. Moreover, the direct neutron flux through the diagnostics penetrations located in the lower area of the TCPH, contributes noticeably to increase the dose level in the Port Cell: this effect will be possibly reduced introducing the diagnostic devices inside the cut-outs that can provide a mitigation of the radiation streaming. The time evolution of the dose rate has been investigated at 105 and up to 107 s after the shutdown: in the first case it reaches values of ∼200 ␮Sv/h around the hot spots, while in the second one a reduction of ∼30% in the Port Cell area has been observed. 3.4. Additional shielding analysis The studies performed suggest that the present layout of the Lower Ports might need some optimization in order to meet the safety requirements in all positions. In this frame, the study of an additional shielding configuration has been carried out. The proposed option consists of three shielding blocks composed by a mixture of SS316L(N)-IG and water (volumetric relative percentage: 80% and 20% respectively), arranged inside the Lower Port #12 to create a dog-leg path inside it (Fig. 11). A systematic analysis with different combinations of the #1, #2 and #3 shielding blocks has been performed, aimed at increasing the shielding efficiency and, at the same time, at minimizing the impact of the integrated blocks on the Lower Port present design and pumping performance.

Fig. 12. Impact the shielding block #3 integration in the Lower Port #12: 3-D total neutron flux map (n/cm2 /s) poloidal section (upper panel) and dose rate at 12 days after shutdown 3-D map (␮Sv/h) section outside the TCP plug (lower panel).

The result of this study (Fig. 12) showed that the blocks #1 and #2 are almost ineffective, whereas, integrating in the structure the only shielding block #3, a significant reduction of the neutron streaming (∼33%) can be obtained. Moreover a non-negligible mitigation of the shutdown dose rate in the Port Cell has been achieved and the 100 ␮Sv/h design target could be fulfilled. Further analyses aimed at verifying the impact on the TCP efficiency are fundamental to evaluate the feasibility of this option. 4. Conclusions A comprehensive nuclear analysis of the Lower Ports #4 and #12 has been performed with the MCNP code in support of the design of the TCP and TCPHs. Detailed 3-D geometries of both the Lower Ports have been developed and integrated in the ITER B-lite v3 MCNP model that has been extended beyond the Bioshield, up to the Port Cell. Neutron and gamma fluxes and nuclear heating distribution on the TCP and TCPHs have been evaluated; global shutdown dose rate calculations at different cooling times have been performed, by means of the Advanced D1S method. The analysis performed highlighted that the void areas (regeneration volume [10]) in the inner structure of the TCPHs and the presence of diagnostics penetrations in the Lower Port #12 weaken the shielding efficiency. The evaluated dose rate in the Port Cell maintenance area is generically within the design target of 100 ␮Sv/h 12 days after shutdown, nevertheless some hot spots in critical positions are still present: an additional shielding configuration has been proposed in order to satisfy the safety requirements.

F. Moro et al. / Fusion Engineering and Design 98–99 (2015) 1561–1565

Acknowledgments The present activity is performed within the Contract ITER/CT/12/4300000730. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References [1] B. Levesy, et al., ITER lower port system integration, Fusion Eng. Des. 86 (2013) 1812–1815. [2] X-5 Monte Carlo Team, MCNP – A General Monte Carlo N-Particle Transport Code, Version 5, Los Alamos National Laboratory, Los Alamos, NM, USA, 2003.

1565

[3] R. Villari, et al., Shutdown dose rate assessment with the Advanced D1S method: development, applications and validation, Fusion Eng. Des. 89 (2014) 2083–2087. [4] Y. Wu, FDS Team, CAD-based interface programs for fusion neutron transport simulation, Fusion Eng. Des. 84 (2009) 1987–1992. [5] ITER Neutronic model: B-lite v3, ITER IDM ITER D 9KKVQR, 2012. [6] F. Moro, et al., Neutronic Analysis of the ITER Cryopump Port ITER IDM ITER D NC5N3T, 2014. [7] D. Lopez Al-dama, et al., FENDL-2.1: update of an evaluated nuclear data library for fusion applications Report INDC (NDS)-46, IAEA, Vienna, 2004. [8] Recommendation on Plasma scenarios, ITER IDM ITER D 2V3V8G, 2009. [9] A.J. van Wijk, et al., An easy to implement global variance reduction procedure for MCNP, Ann. Nucl. Energy 38 (2011) 2496–2503. [10] R.J. Pearce, et al., The ITER divertor pumping system, design evolution, simplification and performance, Fusion Eng. Des. 88 (2013) 809–813.