Shielding analysis of the ITER Collective Thomson Scattering system
Introduction
The Collective Thomson Scattering (CTS) diagnostic system will be the operating diagnostic responsible for measuring the velocity distribution of the alpha particles produced by the D-T reaction at ITER [1,2]. CTS is the only energetic particle diagnostic sensitive to alpha particles below 1.7 MeV [3]. Alpha particles above 1.7 MeV can also be diagnosed by gamma-ray spectrometry [4,5]. Integrated data analysis will allow a reconstruction of the 2D velocity distribution function [3,6]. CTS will further measure the densities of alpha particles [7,8]. Additionally, the CTS will have secondary roles, namely the measurement of the plasma rotation and the ion temperature. This diagnostic, as it was originally proposed [9,10], will operate in the 60 GHz frequency range, which is below the electron cyclotron resonance corresponding to the ITER nominal magnetic field of B0 = 5.3 T. A 1 MW, 60 GHz gyrotron will produce the CTS probing beam that is injected into the plasma through the apertures in the first wall. The in-vessel components of the CTS system will be placed in drawer #3 of the Equatorial Port Plug #12, and with the exception of the plasma-facing launcher and receiver mirrors they will be protected from the plasma by the diagnostic first wall (DFW) and will be shielded inside the drawer by the diagnostic shield module (DSM). The ITER CTS system is further described in recent publications [[11], [12], [13], [14], [15], [16]].
The current design, shown in different perspectives in Fig. 1, Fig. 2, consists of a series of waveguides and mirrors that deliver the probing beam into the plasma and collect part of the resulting scattered radiation. The system, as shown in Fig. 3, includes 11 mirrors, 2 in the high-power launcher transmission line and 9 in the low-power receiver transmission line. These mirrors, in particular the plasma-facing Launcher and Receiver mirrors, will be fully or partially exposed to plasma radiation, being bombarded by energetic neutrons and gammas, resulting in considerable nuclear heat loads that may affect their performance and durability [17].
A recent concept was proposed for the ITER Diagnostics Equatorial Port Plugs – the modular DSM [18], shown in Fig. 4. The modules in this concept consist of stacks of trays with boron carbide bricks. Fig. 5 shows how the modular DSM concept is implemented in the CTS system. Applying this concept to the CTS, the DSM will have 6 columns with a variable number of trays, each containing 120 B4C bricks (if the CTS system was not installed in the drawer). The brick dimensions are 54 mm × 40 mm × 40 mm with a 22 mm diameter hole in the centre. Fig. 6 shows the CTS system without the modular DSM, revealing the existence of a steel backfilling structure that fills all the gaps between the waveguides of the system and the modular DSM shielding trays.
This paper presents a nuclear analysis for the main components of the current design of the ITER CTS diagnostic, taking into account the DSM implementation in the CTS system. The current design has been changed compared to the previous CTS design [19]. The new model is more complex and much more detailed. Concrete differences that impact directly some of the estimations include the inclusion of a bulk shielding block around the collecting mirrors and the introduction of the already mentioned backfilling structure shown in Fig. 6. In previous analyses the shielding materials were modelled as a homogeneous mixture of boron carbide (B4C), stainless steel (SS) and water with different volume fractions, a crude approximation which did not consider the small gaps between the different components and between the CTS system, as well as the shielding structure.
In the present work, an accurate model of the modular DSM is developed, including all the trays and all the individual B4C bricks. The results obtained with this model are compared with the results obtained using a homogeneous mixture to assess the effect of the homogeneous approximation on the estimation of the neutron fluxes in the port interspace. An assessment of the shutdown dose rates (SDDR) in the port interspace is also performed for the homogeneous mixture shielding.
Section snippets
Neutronics models
In this paper, three types of neutronics models are distinguished:
- •
Reference model: standard ITER neutronics model incorporating a 40-degree toroidal section of the most up-to-date design of the ITER machine;
- •
System-specific model: neutronics model featuring the design of the CTS diagnostic to be implemented in the Reference model;
- •
Integrated model: the neutronics model resulting from the incorporating the system-specific model into the reference model.
The neutronics simulations were performed
Neutron fluxes across the CTS system
In order to understand how each of the shielding approaches affects the neutron fluxes in the back end of the system, simulations with mesh tallies were performed. Without variance reduction, however, no neutrons reach the back end of the system, even if the number of source particles is greatly increased. Therefore, a weight window mesh is generated using ADVANTG [28] to increase the efficiency of the simulations.
As an example, the weight windows obtained for 14 MeV neutrons are shown in Fig.
Conclusions
Our results show that the full implementation of the modular DSM in the neutronics model leads to significantly higher neutron fluxes in the back part of the CTS system and near the closure plate. This means that the common strategy for neutronics simulations, which consists of modelling the shielding with a homogeneous mixture, is underestimating the neutron streaming in the port interspace – typically up to a factor of 2. Therefore, even though simulations will require more resources when
CRediT authorship contribution statement
A. Lopes: Investigation, Formal analysis, Methodology, Visualization, Writing - original draft. R. Luís: Investigation, Formal analysis, Methodology, Writing - review & editing. E. Klinkby: Investigation, Writing - review & editing. Y. Nietiadi: Methodology, Visualization, Writing - review & editing. A. Chambon: Investigation, Formal analysis, Methodology, Writing - review & editing. E. Nonbøl: Investigation, Writing - review & editing. B. Gonçalves: Supervision, Project administration, Writing
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
The work in this publication has been funded partially under the framework of the Specific Grant 05 (SG05) of the Fusion for Energy (F4E) Framework Partnership Agreement 393 (F4E-FPA-393) which consists in executing the R&D of the components of the diagnostic to be installed in the equatorial port plug #12 – drawer number 3. IST activities also received financial support from Fundação para a Ciência e Tecnologia (FCT) through the individual grants PD/BD/114322/2016 and PD/BD/135230/2017 under
References (29)
Thermo-structural analyses of the in-vessel components of the ITER Collective Thomson Scattering system
Fusion Eng. Des.
(2019)Neutronics analysis of the ITER Collective Thomson Scattering system
Fusion Eng. Des.
(2018)Benchmarking of MCAM 4.0 with the ITER 3D model
Fusion Eng. Des.
(2007)The ITER tokamak neutronics reference model C-Model
Fusion Eng. Des.
(2018)D1SUNED system for the determination of decay phton related quantities
Fusion Eng. Des.
(2020)Impact of ICRH on the measurement of fusion alphas by Collective Thomson Scattering in ITER
Nucl. Fusion
(2009)Comparison of Collective Thomson Scattering signals due to fast ions in ITER scenarios with fusion and auxiliary heating
Plasma Phys. Control. Fusion
(2009)Alpha-particle velocity-space in ITER
Nucl. Fusion
(2018)Conceptual design of the radial gamma ray spectrometers system for alpha particle and runaway electron measurement at ITER
Nucl. Fusion
(2017)MeV range particle physics studies in tokamak plasmas using gamma-ray spectroscopy
Plasma Phys. Control. Fusion
(2020)
Recent progress in fast-ion diagnostics for magnetically confined plasmas
Rev. Mod. Plasma Phys.
Inference of alpha-particle density profiles from ITER Collective Thomson Scattering
Nucl. Fusion
Diagnostic of fast-ion energy spectra and densities in magnetized plasmas
J. Instrum.
ITER Fast Ion Collective Thomson Scattering, feasibility study and conceptual design
EFDA Rep.
Cited by (4)
Neutronics Simulations for DEMO Diagnostics
2023, SensorsITER collective Thomson scattering-Preparing to diagnose fusion-born alpha particles (invited)
2022, Review of Scientific InstrumentsAssessment of shutdown dose rates in the ITER Collective Thomson Scattering system and in equatorial port plug 12
2021, Journal of InstrumentationFast production of microwave component prototypes by additive manufacturing and copper coating
2021, Review of Scientific Instruments